Re: [spfbis] RFC 4408 to draft-ietf-spfbis-4408bis-08 difference

[Scott Kitterman <[e-mail]>]

 rfc4408.txt 

 draft-ietf-spfbis-4408bis-08.txt 



 








Network Working Group                                            M. Wong
 
Network Working Group                                       S. Kitterman


Request for Comments: 4408                                    W. Schlitt
 
Internet-Draft                              Kitterman Technical Services


Category: Experimental                                        April 2006
 
Obsoletes: 4408 (if approved)                           October 22, 2012



 
Intended status: Standards Track


                   Sender Policy Framework (SPF) for
 
Expires: April 25, 2013


            Authorizing Use of Domains in E-Mail, Version 1
 




 








Status of This Memo
 
 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,



 
                               Version 1



 
                    draft-ietf-spfbis-4408bis-08.txt



 








   This memo defines an Experimental Protocol for the Internet
 
Abstract


   community.  It does not specify an Internet standard of any kind.
 



   Discussion and suggestions for improvement are requested.
 



   Distribution of this memo is unlimited.
 




 








Copyright Notice
 
   Email on the Internet can be forged in a number of ways.  In



 
   particular, existing protocols place no restriction on what a sending



 
   host can use as the "MAIL FROM" of a message or the domain given on



 
   the SMTP HELO/EHLO commands.  This document describes version 1 of



 
   the Sender Policy Framework (SPF) protocol, whereby an ADMD can



 
   explicitly authorize the hosts that are allowed to use its domain



 
   names, and a receiving host can check such authorization.



 








   Copyright (C) The Internet Society (2006).
 
   This document obsoletes RFC4408.



 








IESG Note
 
Status of this Memo



 








   The following documents  (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
 
   This Internet-Draft is submitted in full conformance with the


   are published simultaneously as Experimental RFCs, although there is
 
   provisions of BCP 78 and BCP 79.


   no general technical consensus and efforts to reconcile the two
 



   approaches have failed.  As such, these documents have not received
 



   full IETF review and are published "AS-IS" to document the different
 



   approaches as they were considered in the MARID working group.
 




 








   The IESG takes no position about which approach is to be preferred
 
   Internet-Drafts are working documents of the Internet Engineering


   and cautions the reader that there are serious open issues for each
 
   Task Force (IETF).  Note that other groups may also distribute


   approach and concerns about using them in tandem.  The IESG believes
 
   working documents as Internet-Drafts.  The list of current Internet-


   that documenting the different approaches does less harm than not
 
   Drafts is at http://datatracker.ietf.org/drafts/current/.


   documenting them.
 




 








   Note that the Sender ID experiment may use DNS records that may have
 
   Internet-Drafts are draft documents valid for a maximum of six months


   been created for the current SPF experiment or earlier versions in
 
   and may be updated, replaced, or obsoleted by other documents at any


   this set of experiments.  Depending on the content of the record,
 
   time.  It is inappropriate to use Internet-Drafts as reference


   this may mean that Sender-ID heuristics would be applied incorrectly
 
   material or to cite them other than as "work in progress."


   to a message.  Depending on the actions associated by the recipient
 



   with those heuristics, the message may not be delivered or may be
 



   discarded on receipt.
 




 








   Participants relying on Sender ID experiment DNS records are warned
 
   This Internet-Draft will expire on April 25, 2013.


   that they may lose valid messages in this set of circumstances.
 



   aParticipants publishing SPF experiment DNS records should consider
 



   the advice given in section 3.4 of RFC 4406 and may wish to publish
 



   both v=spf1 and spf2.0 records to avoid the conflict.
 




 








   Participants in the Sender-ID experiment need to be aware that the
 
Copyright Notice


   way Resent-* header fields are used will result in failure to receive
 



   legitimate email when interacting with standards-compliant systems
 



   (specifically automatic forwarders which comply with the standards by
 



   not adding Resent-* headers, and systems which comply with RFC 822
 



   but have not yet implemented RFC 2822 Resent-* semantics).  It would
 



   be inappropriate to advance Sender-ID on the standards track without
 



   resolving this interoperability problem.
 




 








   The community is invited to observe the success or failure of the two
 
   Copyright (c) 2012 IETF Trust and the persons identified as the


   approaches during the two years following publication, in order that
 
   document authors.  All rights reserved.


   a community consensus can be reached in the future.
 




 








Abstract
 
   This document is subject to BCP 78 and the IETF Trust's Legal



 
   Provisions Relating to IETF Documents



 
   (http://trustee.ietf.org/license-info) in effect on the date of



 
   publication of this document.  Please review these documents



 
   carefully, as they describe your rights and restrictions with respect



 
   to this document.  Code Components extracted from this document must



 
   include Simplified BSD License text as described in Section 4.e of



 
   the Trust Legal Provisions and are provided without warranty as



 
   described in the Simplified BSD License.



 








   E-mail on the Internet can be forged in a number of ways.  In
 
   This document may contain material from IETF Documents or IETF


   particular, existing protocols place no restriction on what a sending
 
   Contributions published or made publicly available before November


   host can use as the reverse-path of a message or the domain given on
 
   10, 2008.  The person(s) controlling the copyright in some of this


   the SMTP HELO/EHLO commands.  This document describes version 1 of
 
   material may not have granted the IETF Trust the right to allow


   the Sender Policy Framework (SPF) protocol, whereby a domain may
 
   modifications of such material outside the IETF Standards Process.


   explicitly authorize the hosts that are allowed to use its domain
 
   Without obtaining an adequate license from the person(s) controlling


   name, and a receiving host may check such authorization.
 
   the copyright in such materials, this document may not be modified



 
   outside the IETF Standards Process, and derivative works of it may



 
   not be created outside the IETF Standards Process, except to format



 
   it for publication as an RFC or to translate it into languages other



 
   than English.



 



Table of Contents
 
Table of Contents



 








   1. Introduction ....................................................4
 
   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6


      1.1. Protocol Status ............................................4
 
     1.1.  Protocol Status  . . . . . . . . . . . . . . . . . . . . .  6


      1.2. Terminology ................................................5
 
     1.2.  Experimental History . . . . . . . . . . . . . . . . . . .  7


   2. Operation .......................................................5
 
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7


      2.1. The HELO Identity ..........................................5
 
       1.3.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . .  7


      2.2. The MAIL FROM Identity .....................................5
 
       1.3.2.  Imported Definitions . . . . . . . . . . . . . . . . .  7


      2.3. Publishing Authorization ...................................6
 
       1.3.3.  Mail From Definition . . . . . . . . . . . . . . . . .  7


      2.4. Checking Authorization .....................................6
 
       1.3.4.  HELO Definition  . . . . . . . . . . . . . . . . . . .  8


      2.5. Interpreting the Result ....................................7
 
       1.3.5.  Deprecated . . . . . . . . . . . . . . . . . . . . . .  8


           2.5.1. None ................................................8
 
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  9


           2.5.2. Neutral .............................................8
 
     2.1.  The "HELO" Identity  . . . . . . . . . . . . . . . . . . .  9


           2.5.3. Pass ................................................8
 
     2.2.  The "MAIL FROM" Identity . . . . . . . . . . . . . . . . .  9


           2.5.4. Fail ................................................8
 
     2.3.  Publishing Authorization . . . . . . . . . . . . . . . . .  9


           2.5.5. SoftFail ............................................9
 
     2.4.  Checking Authorization . . . . . . . . . . . . . . . . . . 10


           2.5.6. TempError ...........................................9
 
     2.5.  Interpreting the Result  . . . . . . . . . . . . . . . . . 11


           2.5.7. PermError ...........................................9
 
       2.5.1.  None . . . . . . . . . . . . . . . . . . . . . . . . . 12


   3. SPF Records .....................................................9
 
       2.5.2.  Neutral  . . . . . . . . . . . . . . . . . . . . . . . 12


      3.1. Publishing ................................................10
 
       2.5.3.  Pass . . . . . . . . . . . . . . . . . . . . . . . . . 12


           3.1.1. DNS Resource Record Types ..........................10
 
       2.5.4.  Fail . . . . . . . . . . . . . . . . . . . . . . . . . 12


           3.1.2. Multiple DNS Records ...............................11
 
       2.5.5.  Softfail . . . . . . . . . . . . . . . . . . . . . . . 13


           3.1.3. Multiple Strings in a Single DNS record ............11
 
       2.5.6.  Temperror  . . . . . . . . . . . . . . . . . . . . . . 13


           3.1.4. Record Size ........................................11
 
       2.5.7.  Permerror  . . . . . . . . . . . . . . . . . . . . . . 13


           3.1.5. Wildcard Records ...................................11
 
   3.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . . 14


                                                                         
 
     3.1.  DNS Resource Records . . . . . . . . . . . . . . . . . . . 14


   4. The check_host() Function ......................................12
 
     3.2.  Multiple DNS Records . . . . . . . . . . . . . . . . . . . 15


      4.1. Arguments .................................................12
 
     3.3.  Multiple Strings in a Single DNS record  . . . . . . . . . 15


      4.2. Results ...................................................13
 
     3.4.  Record Size  . . . . . . . . . . . . . . . . . . . . . . . 15


      4.3. Initial Processing ........................................13
 
     3.5.  Wildcard Records . . . . . . . . . . . . . . . . . . . . . 15


      4.4. Record Lookup .............................................13
 
   4.  The check_host() Function  . . . . . . . . . . . . . . . . . . 17


      4.5. Selecting Records .........................................13
 
     4.1.  Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 17


      4.6. Record Evaluation .........................................14
 
     4.2.  Results  . . . . . . . . . . . . . . . . . . . . . . . . . 17


           4.6.1. Term Evaluation ....................................14
 
     4.3.  Initial Processing . . . . . . . . . . . . . . . . . . . . 17


           4.6.2. Mechanisms .........................................15
 
     4.4.  Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 18


           4.6.3. Modifiers ..........................................15
 
     4.5.  Selecting Records  . . . . . . . . . . . . . . . . . . . . 18


      4.7. Default Result ............................................16
 
     4.6.  Record Evaluation  . . . . . . . . . . . . . . . . . . . . 18


      4.8. Domain Specification ......................................16
 
       4.6.1.  Term Evaluation  . . . . . . . . . . . . . . . . . . . 19


   5. Mechanism Definitions ..........................................16
 
       4.6.2.  Mechanisms . . . . . . . . . . . . . . . . . . . . . . 19


      5.1. "all" .....................................................17
 
       4.6.3.  Modifiers  . . . . . . . . . . . . . . . . . . . . . . 20


      5.2. "include" .................................................18
 
       4.6.4.  DNS Lookup Limits  . . . . . . . . . . . . . . . . . . 20


      5.3. "a" .......................................................19
 
     4.7.  Default Result . . . . . . . . . . . . . . . . . . . . . . 21


      5.4. "mx" ......................................................20
 
     4.8.  Domain Specification . . . . . . . . . . . . . . . . . . . 21


      5.5. "ptr" .....................................................20
 
   5.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 22


      5.6. "ip4" and "ip6" ...........................................21
 
     5.1.  "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 23


      5.7. "exists" ..................................................22
 
     5.2.  "include"  . . . . . . . . . . . . . . . . . . . . . . . . 23


   6. Modifier Definitions ...........................................22
 
     5.3.  "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 25


      6.1. redirect: Redirected Query ................................23
 
     5.4.  "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25


      6.2. exp: Explanation ..........................................23
 
     5.5.  "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 25


   7. The Received-SPF Header Field ..................................25
 
     5.6.  "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 27


   8. Macros .........................................................27
 
     5.7.  "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 27


      8.1. Macro Definitions .........................................27
 
   6.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 29


      8.2. Expansion Examples ........................................30
 
     6.1.  redirect: Redirected Query . . . . . . . . . . . . . . . . 29


   9. Implications ...................................................31
 
     6.2.  exp: Explanation . . . . . . . . . . . . . . . . . . . . . 30


      9.1. Sending Domains ...........................................31
 
   7.  Recording The Result . . . . . . . . . . . . . . . . . . . . . 32


      9.2. Mailing Lists .............................................32
 
     7.1.  The Received-SPF Header Field  . . . . . . . . . . . . . . 32


      9.3. Forwarding Services and Aliases ...........................32
 
     7.2.  SPF Results in the Authentication-Results Header Field . . 34


      9.4. Mail Services .............................................34
 
   8.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36


      9.5. MTA Relays ................................................34
 
     8.1.  Macro Definitions  . . . . . . . . . . . . . . . . . . . . 36


   10. Security Considerations .......................................35
 
     8.2.  Expansion Examples . . . . . . . . . . . . . . . . . . . . 39


      10.1. Processing Limits ........................................35
 
   9.  Implications . . . . . . . . . . . . . . . . . . . . . . . . . 41


      10.2. SPF-Authorized E-Mail May Contain Other False
 
     9.1.  Sending Domains  . . . . . . . . . . . . . . . . . . . . . 41


            Identities ...............................................37
 
       9.1.1.  DNS Resource Considerations  . . . . . . . . . . . . . 41


      10.3. Spoofed DNS and IP Data ..................................37
 
       9.1.2.  Administrator's Considerations . . . . . . . . . . . . 42


      10.4. Cross-User Forgery .......................................37
 
       9.1.3.  Bounces  . . . . . . . . . . . . . . . . . . . . . . . 43


      10.5. Untrusted Information Sources ............................38
 
     9.2.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . 43


      10.6. Privacy Exposure .........................................38
 
       9.2.1.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . 43


   11. Contributors and Acknowledgements .............................38
 
       9.2.2.  Forwarding Services and Aliases  . . . . . . . . . . . 44


   12. IANA Considerations ...........................................39
 
       9.2.3.  Mail Services  . . . . . . . . . . . . . . . . . . . . 46


      12.1. The SPF DNS Record Type ..................................39
 
       9.2.4.  MTA Relays . . . . . . . . . . . . . . . . . . . . . . 46


      12.2. The Received-SPF Mail Header Field .......................39
 
     9.3.  Receivers  . . . . . . . . . . . . . . . . . . . . . . . . 47


   13. References ....................................................39
 
       9.3.1.  Policy For SPF Pass  . . . . . . . . . . . . . . . . . 47


      13.1. Normative References .....................................39
 
       9.3.2.  Policy For SPF Fail  . . . . . . . . . . . . . . . . . 47


      13.2. Informative References ...................................40
 
       9.3.3.  Policy For SPF Permerror . . . . . . . . . . . . . . . 48


                                                                         
 
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 49


   Appendix A.  Collected ABNF .......................................42
 
     10.1. Processing Limits  . . . . . . . . . . . . . . . . . . . . 49


   Appendix B.  Extended Examples ....................................44
 
     10.2. SPF-Authorized Email May Contain Other False Identities  . 49


      B.1.  Simple Examples ..........................................44
 
     10.3. Spoofed DNS and IP Data  . . . . . . . . . . . . . . . . . 50


      B.2.  Multiple Domain Example ..................................45
 
     10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 50


      B.3.  DNSBL Style Example ......................................46
 
     10.5. Untrusted Information Sources  . . . . . . . . . . . . . . 50


      B.4.  Multiple Requirements Example ............................46
 
       10.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 50



 
       10.5.2. External Explanations  . . . . . . . . . . . . . . . . 51



 
       10.5.3. Macro Expansion  . . . . . . . . . . . . . . . . . . . 51



 
     10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 51



 
   11. Contributors and Acknowledgements  . . . . . . . . . . . . . . 52



 
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 53



 
     12.1. The SPF DNS Record Type  . . . . . . . . . . . . . . . . . 53



 
     12.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 53



 
     12.3. SPF Modifier Registration  . . . . . . . . . . . . . . . . 53



 
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54



 
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 54



 
     13.2. Informative References . . . . . . . . . . . . . . . . . . 55



 
   Appendix A.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 57



 
   Appendix B.  Extended Examples . . . . . . . . . . . . . . . . . . 60



 
     B.1.  Simple Examples  . . . . . . . . . . . . . . . . . . . . . 60



 
     B.2.  Multiple Domain Example  . . . . . . . . . . . . . . . . . 61



 
     B.3.  DNSBL Style Example  . . . . . . . . . . . . . . . . . . . 62



 
     B.4.  Multiple Requirements Example  . . . . . . . . . . . . . . 62



 
   Appendix C.  Change History  . . . . . . . . . . . . . . . . . . . 63



 
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 66



 



1.  Introduction
 
1.  Introduction



 








   The current E-Mail infrastructure has the property that any host
 
   The current email infrastructure has the property that any host


   injecting mail into the mail system can identify itself as any domain
 
   injecting mail into the system can use any DNS domain name it wants


   name it wants.  Hosts can do this at a variety of levels: in
 
   in each of the various identifiers specified by [RFC5321] and


   particular, the session, the envelope, and the mail headers.
 
   [RFC5322].  Although this feature is desirable in some circumstances,


   Although this feature is desirable in some circumstances, it is a
 
   it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka


   major obstacle to reducing Unsolicited Bulk E-Mail (UBE, aka spam).
 
   spam).  Furthermore, many domain owning ADMDs (ADministrative


   Furthermore, many domain name holders are understandably concerned
 
   Management Domains, see [RFC5598]) are understandably concerned about


   about the ease with which other entities may make use of their domain
 
   the ease with which other entities can make use of their domain


   names, often with malicious intent.
 
   names, often with malicious intent.



 








   This document defines a protocol by which domain owners may authorize
 
   This document defines a protocol by which ADMDs can authorize hosts


   hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
 
   to use their domain names in the "MAIL FROM" or "HELO" identities.


   Compliant domain holders publish Sender Policy Framework (SPF)
 
   Compliant ADMDs publish Sender Policy Framework (SPF) records in the


   records specifying which hosts are permitted to use their names, and
 
   DNS specifying which hosts are permitted to use their names, and


   compliant mail receivers use the published SPF records to test the
 
   compliant mail receivers use the published SPF records to test the


   authorization of sending Mail Transfer Agents (MTAs) using a given
 
   authorization of sending Mail Transfer Agents (MTAs) using a given


   "HELO" or "MAIL FROM" identity during a mail transaction.
 
   "HELO" or "MAIL FROM" identity during a mail transaction.



 



   An additional benefit to mail receivers is that after the use of an
 
   An additional benefit to mail receivers is that after the use of an


   identity is verified, local policy decisions about the mail can be
 
   identity is verified, local policy decisions about the mail can be


   made based on the sender's domain, rather than the host's IP address.
 
   made based on the sender's domain, rather than the host's IP address.


   This is advantageous because reputation of domain names is likely to
 
   This is advantageous because reputation of domain names is likely to


   be more accurate than reputation of host IP addresses.  Furthermore,
 
   be more accurate than reputation of host IP addresses.  Furthermore,


   if a claimed identity fails verification, local policy can take
 
   if a claimed identity fails verification, local policy can take







   stronger action against such E-Mail, such as rejecting it.
 
   stronger action against such email, such as rejecting it.



 



1.1.  Protocol Status
 
1.1.  Protocol Status



 



   SPF has been in development since the summer of 2003 and has seen
 
   SPF has been in development since the summer of 2003 and has seen


   deployment beyond the developers beginning in December 2003.  The
 
   deployment beyond the developers beginning in December 2003.  The


   design of SPF slowly evolved until the spring of 2004 and has since
 
   design of SPF slowly evolved until the spring of 2004 and has since


   stabilized.  There have been quite a number of forms of SPF, some
 
   stabilized.  There have been quite a number of forms of SPF, some


   written up as documents, some submitted as Internet Drafts, and many
 
   written up as documents, some submitted as Internet Drafts, and many







   discussed and debated in development forums.
 
   discussed and debated in development forums.  The protocol was



 
   originally defined in [RFC4408], which this document replaces.



 








   The goal of this document is to clearly document the protocol defined
 
   [RFC4408] was designed to clearly document the protocol defined by


   by earlier draft specifications of SPF as used in existing
 
   earlier draft specifications of SPF as used in existing


   implementations.  This conception of SPF is sometimes called "SPF
 
   implementations.  This updated specification is intended to clarify


   Classic".  It is understood that particular implementations and
 
   identified ambiguities in [RFC4408], resolve techincal issues


   deployments may differ from, and build upon, this work.  It is hoped
 
   identified in post-RFC 4408 deplyment experience, and document widely


   that we have nonetheless captured the common understanding of SPF
 
   deployed extensions to SPF that have been developed since [RFC4408]


   version 1.
 
   was published.



 








1.2.  Terminology
 
1.2.  Experimental History



 




 
   This document updates and replaces RFC 4408 that was part of a group



 
   of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,



 
   RFC 4407, and RFC 4408) in 2006.  At that time the IESG requested the



 
   community observe the success or failure of the two approaches



 
   documented in these RFCs during the two years following publication,



 
   in order that a community consensus could be reached in the future.



 




 
   SPF is widely deployed by large and small email providers alike.



 
   There are multiple, interoperable implementations.



 




 
   For SPF (as documented in RFC 4408) a careful effort was made to



 
   collect and document lessons learned and errata during the two year



 
   period.  The errata list has been stable (no new submissions) and



 
   only minor protocol lessons learned were identified.  Resolution of



 
   the IESG's experiment is documented in [RFC6686].



 




 
1.3.  Terminology



 
                                                                         



 
1.3.1.  Keywords



 



   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",







   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and


   document are to be interpreted as described in [RFC2119].
 
   "OPTIONAL" in this document are to be interpreted as described in



 
   [RFC2119].



 
                                                                         



 
1.3.2.  Imported Definitions



 




 
   The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].



 




 
   The token "local-part" is defined in [RFC5321].



 




 
   "dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are



 
   defined in [RFC5322].



 




 
1.3.3.  Mail From Definition



 



   This document is concerned with the portion of a mail message
 
   This document is concerned with the portion of a mail message


   commonly called "envelope sender", "return path", "reverse path",
 
   commonly called "envelope sender", "return path", "reverse path",







   "bounce address", "2821 FROM", or "MAIL FROM".  Since these terms are
 
   "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom.


   either not well defined or often used casually, this document defines
 
   Since these terms are either not well defined or often used casually,


   the "MAIL FROM" identity in Section 2.2.  Note that other terms that
 
   this document uses "MAIL FROM" for consistency.  This means the


   may superficially look like the common terms, such as "reverse-path",
 
   RFC5321.MailFrom as defined in [RFC5598].  Note that other terms that


   are used only with the defined meanings from normative documents.
 
   might superficially look like the common terms, such as "reverse-



 
   path", are used only with the defined meanings from normative



 
   documents.



 








2.  Operation
 
1.3.4.  HELO Definition



 








2.1.  The HELO Identity
 
   This document also makes use of the HELO/EHLO identity.  The "HELO"



 
   identity derives from either the SMTP HELO or EHLO command (see



 
   [RFC5321]).  Since HELO and EHLO can, in many cases, be used



 
   interchangeably, they are identified commonly as "HELO" in this



 
   document.  This means RFC5321.HELO/.EHLO as defined in [RFC5598].



 
   These commands supply the identity of the SMTP client (sending host)



 
   for the SMTP session.



 








   The "HELO" identity derives from either the SMTP HELO or EHLO command
 
1.3.5.  Deprecated


   (see [RFC2821]).  These commands supply the SMTP client (sending
 



   host) for the SMTP session.  Note that requirements for the domain
 



   presented in the EHLO or HELO command are not always clear to the
 



   sending party, and SPF clients must be prepared for the "HELO"
 



   identity to be malformed or an IP address literal.  At the time of
 



   this writing, many legitimate E-Mails are delivered with invalid HELO
 



   domains.
 




 








   It is RECOMMENDED that SPF clients not only check the "MAIL FROM"
 
   There are [RFC4408] features that are marked "deprecated".  In the



 
   context of this document, deprecated means that senders SHOULD NOT



 
   publish SPF records that make use of such features because they might



 
   be removed entirely in future updates to the protocol.  Such features



 
   do, however, remain part of the SPF protocol and receiving systems



 
   MUST support them unless this document explicitly says otherwise.



 




 
2.  Operation



 




 
2.1.  The "HELO" Identity



 
                                                                         



 
   It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"


   identity, but also separately check the "HELO" identity by applying
 
   identity, but also separately check the "HELO" identity by applying


   the check_host() function (Section 4) to the "HELO" identity as the
 
   the check_host() function (Section 4) to the "HELO" identity as the







   <sender>.
 
   <sender>.  Checking "HELO" promotes consistency of results and can



 
   reduce DNS resource usage.  Additionally, since SPF records published



 
   for "HELO" identities refer to a single host, when available, they



 
   are a very reliable source of host authorization status.



 








2.2.  The MAIL FROM Identity
 
   Note that requirements for the domain presented in the EHLO or HELO



 
   command are not always clear to the sending party, and SPF verifiers



 
   MUST be prepared for the "HELO" identity to be malformed or an IP



 
   address literal.  This SPF check can only be performed when the



 
   "HELO" string is a valid fully qualified domain.



 








   The "MAIL FROM" identity derives from the SMTP MAIL command (see
 
2.2.  The "MAIL FROM" Identity


   [RFC2821]).  This command supplies the "reverse-path" for a message,
 



   which generally consists of the sender mailbox, and is the mailbox to
 



   which notification messages are to be sent if there are problems
 



   delivering the message.
 




 








   [RFC2821] allows the reverse-path to be null (see Section 4.5.5 in
 
   SPF verifiers MUST check the ""MAIL FROM" identity if a completed


   RFC 2821).  In this case, there is no explicit sender mailbox, and
 
   "HELO" check has not reached a definitive policy result by applying



 
   the check_host() function to the "MAIL FROM" identity as the



 
   <sender>.



 




 
   [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in



 
   [RFC5321]).  In this case, there is no explicit sender mailbox, and


   such a message can be assumed to be a notification message from the
 
   such a message can be assumed to be a notification message from the


   mail system itself.  When the reverse-path is null, this document
 
   mail system itself.  When the reverse-path is null, this document


   defines the "MAIL FROM" identity to be the mailbox composed of the
 
   defines the "MAIL FROM" identity to be the mailbox composed of the







   localpart "postmaster" and the "HELO" identity (which may or may not
 
   local-part "postmaster" and the "HELO" identity (which might or might


   have been checked separately before).
 
   not have been checked separately before).


                                                                         
 



   SPF clients MUST check the "MAIL FROM" identity.  SPF clients check
 



   the "MAIL FROM" identity by applying the check_host() function to the
 



   "MAIL FROM" identity as the <sender>.
 




 



2.3.  Publishing Authorization
 
2.3.  Publishing Authorization



 








   An SPF-compliant domain MUST publish a valid SPF record as described
 
   An SPF-compliant domain MUST have valid SPF records as described in


   in Section 3.  This record authorizes the use of the domain name in
 
   Section 3.  These records authorize the use of the relevant domain


   the "HELO" and "MAIL FROM" identities by the MTAs it specifies.
 
   names in the "HELO" and "MAIL FROM" identities by the MTAs specified



 
   therein.



 








   If domain owners choose to publish SPF records, it is RECOMMENDED
 
   SPF results can be used to make both positive (source is authorized)


   that they end in "-all", or redirect to other records that do, so
 
   and negative (source is not authorized) determinations.  If domain


   that a definitive determination of authorization can be made.
 
   owners choose to publish SPF records and want to support receivers



 
   making negative authorization determinations, then they MUST publish



 
   records that end in "-all", or redirect to other records that do,



 
   otherwise, no definitive determination of authorization can be made.



 
   Potential issues and mitigations associated with negative



 
   determinations are discussed in Section 9.



 








   Domain holders may publish SPF records that explicitly authorize no
 
   ADMDs can publish SPF records that explicitly authorize no hosts for


   hosts if mail should never originate using that domain.
 
   domain names that are neither used in the domain part of email



 
   addresses nor expected to originate mail.



 








   When changing SPF records, care must be taken to ensure that there is
 
   When changing SPF records, care has to be taken to ensure that there


   a transition period so that the old policy remains valid until all
 
   is a transition period so that the old policy remains valid until all


   legitimate E-Mail has been checked.
 
   legitimate email can reasonably expect to have been checked.  This



 
   can be as much as 30 days.



 



2.4.  Checking Authorization
 
2.4.  Checking Authorization



 



   A mail receiver can perform a set of SPF checks for each mail message
 
   A mail receiver can perform a set of SPF checks for each mail message


   it receives.  An SPF check tests the authorization of a client host
 
   it receives.  An SPF check tests the authorization of a client host


   to emit mail with a given identity.  Typically, such checks are done
 
   to emit mail with a given identity.  Typically, such checks are done


   by a receiving MTA, but can be performed elsewhere in the mail
 
   by a receiving MTA, but can be performed elsewhere in the mail


   processing chain so long as the required information is available and
 
   processing chain so long as the required information is available and


   reliable.  At least the "MAIL FROM" identity MUST be checked, but it
 
   reliable.  At least the "MAIL FROM" identity MUST be checked, but it


   is RECOMMENDED that the "HELO" identity also be checked beforehand.
 
   is RECOMMENDED that the "HELO" identity also be checked beforehand.



 



   Without explicit approval of the domain owner, checking other
 
   Without explicit approval of the domain owner, checking other


   identities against SPF version 1 records is NOT RECOMMENDED because
 
   identities against SPF version 1 records is NOT RECOMMENDED because


   there are cases that are known to give incorrect results.  For
 
   there are cases that are known to give incorrect results.  For


   example, almost all mailing lists rewrite the "MAIL FROM" identity
 
   example, almost all mailing lists rewrite the "MAIL FROM" identity







   (see Section 9.2), but some do not change any other identities in the
 
   (see Section 9.2.1), but some do not change any other identities in


   message.  The scenario described in Section 9.3, sub-section 1.2, is
 
   the message.  The scenario described in Section 9.2.2, sub-section


   another example.  Documents that define other identities should
 
   1.2, is another example.  Documents that define other identities will


   define the method for explicit approval.
 
   have to define the method for explicit approval.



 



   It is possible that mail receivers will use the SPF check as part of
 
   It is possible that mail receivers will use the SPF check as part of


   a larger set of tests on incoming mail.  The results of other tests
 
   a larger set of tests on incoming mail.  The results of other tests







   may influence whether or not a particular SPF check is performed.
 
   might influence whether or not a particular SPF check is performed.


   For example, finding the sending host's IP address on a local white
 
   For example, finding the sending host's IP address on a local white







   list may cause all other tests to be skipped and all mail from that
 
   list might cause all other tests to be skipped and all mail from that


   host to be accepted.
 
   host to be accepted.



 



   When a mail receiver decides to perform an SPF check, it MUST use a
 
   When a mail receiver decides to perform an SPF check, it MUST use a


   correctly-implemented check_host() function (Section 4) evaluated
 
   correctly-implemented check_host() function (Section 4) evaluated


   with the correct parameters.  Although the test as a whole is
 
   with the correct parameters.  Although the test as a whole is







   optional, once it has been decided to perform a test it must be
 
   optional, once it has been decided to perform a test it has to be


   performed as specified so that the correct semantics are preserved
 
   performed as specified so that the correct semantics are preserved


   between publisher and receiver.
 
   between publisher and receiver.



 



   To make the test, the mail receiver MUST evaluate the check_host()
 
   To make the test, the mail receiver MUST evaluate the check_host()


   function with the arguments set as follows:
 
   function with the arguments set as follows:



 



   <ip>     - the IP address of the SMTP client that is emitting the
 
   <ip>     - the IP address of the SMTP client that is emitting the


              mail, either IPv4 or IPv6.
 
              mail, either IPv4 or IPv6.



 



   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
 
   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.



 



   <sender> - the "MAIL FROM" or "HELO" identity.
 
   <sender> - the "MAIL FROM" or "HELO" identity.



 








   Note that the <domain> argument may not be a well-formed domain name.
 
   Note that the <domain> argument might not be a well-formed domain


   For example, if the reverse-path was null, then the EHLO/HELO domain
 
   name.  For example, if the reverse-path was null, then the EHLO/HELO


   is used, with its associated problems (see Section 2.1).  In these
 
   domain is used, with its associated problems (see Section 2.1).  In


   cases, check_host() is defined in Section 4.3 to return a "None"
 
   these cases, check_host() is defined in Section 4.3 to return a


   result.
 
   "none" result.



 



   Although invalid, malformed, or non-existent domains cause SPF checks
 
   Although invalid, malformed, or non-existent domains cause SPF checks







   to return "None" because no SPF record can be found, it has long been
 
   to return "none" because no SPF record can be found, it has long been


   the policy of many MTAs to reject E-Mail from such domains,
 
   the policy of many MTAs to reject email from such domains, especially


   especially in the case of invalid "MAIL FROM".  In order to prevent
 
   in the case of invalid "MAIL FROM".  Rejecting email will prevent one


   the circumvention of SPF records, rejecting E-Mail from invalid
 
   method of circumventing of SPF records.


   domains should be considered.
 




 








   Implementations must take care to correctly extract the <domain> from
 
   Implementations have to take care to correctly extract the <domain>


   the data given with the SMTP MAIL FROM command as many MTAs will
 
   from the data given with the SMTP MAIL FROM command as many MTAs will


   still accept such things as source routes (see [RFC2821], Appendix
 
   still accept such things as source routes (see [RFC5321], Appendix


   C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
 
   C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).


   These archaic features have been maliciously used to bypass security
 
   These archaic features have been maliciously used to bypass security


   systems.
 
   systems.



 



2.5.  Interpreting the Result
 
2.5.  Interpreting the Result



 



   This section describes how software that performs the authorization
 
   This section describes how software that performs the authorization







   should interpret the results of the check_host() function.  The
 
   interprets the results of the check_host() function.  The


   authorization check SHOULD be performed during the processing of the
 
   authorization check SHOULD be performed during the processing of the


   SMTP transaction that sends the mail.  This allows errors to be
 
   SMTP transaction that sends the mail.  This allows errors to be


   returned directly to the sending MTA by way of SMTP replies.
 
   returned directly to the sending MTA by way of SMTP replies.



 








   Performing the authorization after the SMTP transaction has finished
 
   Performing the authorization other than using the return-path and


   may cause problems, such as the following: (1) It may be difficult to
 
   client address at the time of the MAIL command during the SMTP


   accurately extract the required information from potentially
 
   transaction can cause problems, such as the following: (1) It might


   deceptive headers; (2) legitimate E-Mail may fail because the
 
   be difficult to accurately extract the required information from


   sender's policy may have since changed.
 
   potentially deceptive headers; (2) legitimate email might fail



 
   because the sender's policy had since changed.



 



   Generating non-delivery notifications to forged identities that have
 
   Generating non-delivery notifications to forged identities that have







   failed the authorization check is generally abusive and against the
 
   failed the authorization check is a source of backscatter and SHOULD


   explicit wishes of the identity owner.
 
   be avoided.  [RFC3834] section 2 describes backscatter and the



 
   problems it causes.



 



2.5.1.  None
 
2.5.1.  None



 








   A result of "None" means that no records were published by the domain
 
   A result of "none" means either (a) no syntactically valid DNS domain


   or that no checkable sender domain could be determined from the given
 
   name was extracted from the SMTP session that could be used as the


   identity.  The checking software cannot ascertain whether or not the
 
   one to be authorized, or (b) no TXT records were retrieved from the


   client host is authorized.
 
   DNS that appeared to be intended for use by SPF verifiers.



 



2.5.2.  Neutral
 
2.5.2.  Neutral



 








   The domain owner has explicitly stated that he cannot or does not
 
   The domain owner has explicitly stated that they cannot or do not


   want to assert whether or not the IP address is authorized.  A
 
   want to assert whether the IP address is authorized or not.  A


   "Neutral" result MUST be treated exactly like the "None" result; the
 
   "neutral" result MUST be treated exactly like the "none" result; the


   distinction exists only for informational purposes.  Treating
 
   distinction exists only for informational purposes.  Treating







   "Neutral" more harshly than "None" would discourage domain owners
 
   "neutral" more harshly than "none" would discourage domain owners


   from testing the use of SPF records (see Section 9.1).
 
   from testing the use of SPF records (see Section 9.1).



 



2.5.3.  Pass
 
2.5.3.  Pass



 








   A "Pass" result means that the client is authorized to inject mail
 
   A "pass" result means that the client is authorized to inject mail


   with the given identity.  The domain can now, in the sense of
 
   with the given identity.  The domain can now, in the sense of


   reputation, be considered responsible for sending the message.
 
   reputation, be considered responsible for sending the message.


   Further policy checks can now proceed with confidence in the
 
   Further policy checks can now proceed with confidence in the







   legitimate use of the identity.
 
   legitimate use of the identity.  This is further discussed in



 
   Section 9.3.1.



 



2.5.4.  Fail
 
2.5.4.  Fail



 








   A "Fail" result is an explicit statement that the client is not
 
   A "fail" result is an explicit statement that the client is not


   authorized to use the domain in the given identity.  The checking
 
   authorized to use the domain in the given identity.  Disposition of


   software can choose to mark the mail based on this or to reject the
 
   SPF fail messages is a matter of local policy.  See Section 9.3.2 for


   mail outright.
 
   considerations on developing local policy.



 



   If the checking software chooses to reject the mail during the SMTP
 
   If the checking software chooses to reject the mail during the SMTP


   transaction, then it SHOULD use an SMTP reply code of 550 (see
 
   transaction, then it SHOULD use an SMTP reply code of 550 (see







   [RFC2821]) and, if supported, the 5.7.1 Delivery Status Notification
 
   [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see


   (DSN) code (see [RFC3464]), in addition to an appropriate reply text.
 
   [RFC3463]), in addition to an appropriate reply text.  The


   The check_host() function may return either a default explanation
 
   check_host() function will return either a default explanation string


   string or one from the domain that published the SPF records (see
 
   or one from the domain that published the SPF records (see


   Section 6.2).  If the information does not originate with the
 
   Section 6.2).  If the information does not originate with the







   checking software, it should be made clear that the text is provided
 
   checking software, it is good to make it clear that the text is


   by the sender's domain.  For example:
 
   provided by the sender's domain.  For example:



 



       550-5.7.1 SPF MAIL FROM check failed:
 
       550-5.7.1 SPF MAIL FROM check failed:


       550-5.7.1 The domain example.com explains:
 
       550-5.7.1 The domain example.com explains:


       550 5.7.1 Please see http://www.example.com/mailpolicy.html
 
       550 5.7.1 Please see http://www.example.com/mailpolicy.html



 








2.5.5.  SoftFail
 
   If the checking software chooses not to reject the mail during the



 
   SMTP transaction, then it SHOULD add a Received-SPF or



 
   Authentication-Results header field (see Section 7) to communicate



 
   this result to downstream message processors.  While this is true for



 
   all SPF results, it is of particular importance for "fail" results



 
   since the message is explicitly not authorized by the domain owner.



 








   A "SoftFail" result should be treated as somewhere between a "Fail"
 
2.5.5.  Softfail


   and a "Neutral".  The domain believes the host is not authorized but
 
                                                                         


   is not willing to make that strong of a statement.  Receiving
 
   A "softfail" result ought to be treated as somewhere between "fail"


   software SHOULD NOT reject the message based solely on this result,
 
   and "neutral"/"none".  The domain owner believes the host is not


   but MAY subject the message to closer scrutiny than normal.
 
   authorized but is not willing to make a strong policy statement.



 
   Receiving software SHOULD NOT reject the message based solely on this



 
   result, but MAY subject the message to closer scrutiny than normal.



 



   The domain owner wants to discourage the use of this host and thus
 
   The domain owner wants to discourage the use of this host and thus







   desires limited feedback when a "SoftFail" result occurs.  For
 
   desires limited feedback when a "softfail" result occurs.  For


   example, the recipient's Mail User Agent (MUA) could highlight the
 
   example, the recipient's Mail User Agent (MUA) could highlight the







   "SoftFail" status, or the receiving MTA could give the sender a
 
   "softfail" status, or the receiving MTA could give the sender a


   message using a technique called "greylisting" whereby the MTA can
 
   message using greylisting, [RFC6647], with a note the first time the


   issue an SMTP reply code of 451 (4.3.0 DSN code) with a note the
 
   message is received, but accept it on a later attempt based on


   first time the message is received, but accept it the second time.
 
   receiver policy.



 








2.5.6.  TempError
 
2.5.6.  Temperror



 








   A "TempError" result means that the SPF client encountered a
 
   A "temperror" result means the SPF verifier encountered a transient


   transient error while performing the check.  Checking software can
 
   (generally DNS) error while performing the check.  Checking software


   choose to accept or temporarily reject the message.  If the message
 
   can choose to accept or temporarily reject the message.  If the


   is rejected during the SMTP transaction for this reason, the software
 
   message is rejected during the SMTP transaction for this reason, the


   SHOULD use an SMTP reply code of 451 and, if supported, the 4.4.3 DSN
 
   software SHOULD use an SMTP reply code of 451 and, if supported, the


   code.
 
   4.4.3 enhanced status code.  These errors can be caused by problems



 
   in either the sender's or receiver's DNS software.



 








2.5.7.  PermError
 
2.5.7.  Permerror



 








   A "PermError" result means that the domain's published records could
 
   A "permerror" result means the domain's published records could not


   not be correctly interpreted.  This signals an error condition that
 
   be correctly interpreted.  This signals an error condition that


   requires manual intervention to be resolved, as opposed to the
 
   definitely requires manual intervention to be resolved.  If the


   TempError result.  Be aware that if the domain owner uses macros
 
   message is rejected during the SMTP transaction for this reason, the


   (Section 8), it is possible that this result is due to the checked
 
   software SHOULD use an SMTP reply code of 550 and, if supported, the


   identities having an unexpected format.
 
   5.5.2 enhanced status code.  Be aware that if the domain owner uses



 
   macros (Section 8), it is possible that this result is due to the



 
   checked identities having an unexpected format.  It is also possible



 
   that this result is generated by certain SPF clients due to the input



 
   arguments having an unexpected format; see Section 4.8.



 



3.  SPF Records
 
3.  SPF Records



 








   An SPF record is a DNS Resource Record (RR) that declares which hosts
 
   An SPF record is a DNS record that declares which hosts are, and are


   are, and are not, authorized to use a domain name for the "HELO" and
 
   not, authorized to use a domain name for the "HELO" and "MAIL FROM"


   "MAIL FROM" identities.  Loosely, the record partitions all hosts
 
   identities.  Loosely, the record partitions all hosts into permitted


   into permitted and not-permitted sets (though some hosts might fall
 
   and not-permitted sets (though some hosts might fall into neither


   into neither category).
 
   category).



 








   The SPF record is a single string of text.  An example record is the
 
   The SPF record is a single string of text.  The record format is


   following:
 
   described below in Section 4.  An example record is the following:



 



      v=spf1 +mx a:colo.example.com/28 -all
 
      v=spf1 +mx a:colo.example.com/28 -all



 



   This record has a version of "spf1" and three directives: "+mx",
 
   This record has a version of "spf1" and three directives: "+mx",


   "a:colo.example.com/28" (the + is implied), and "-all".
 
   "a:colo.example.com/28" (the + is implied), and "-all".



 








3.1.  Publishing
 
   Each SPF record is placed in the DNS tree at the host name it



 



   Domain owners wishing to be SPF compliant must publish SPF records
 



   for the hosts that are used in the "MAIL FROM" and "HELO" identities.
 



   The SPF records are placed in the DNS tree at the host name it
 



   pertains to, not a subdomain under it, such as is done with SRV
 
   pertains to, not a subdomain under it, such as is done with SRV







   records.  This is the same whether the TXT or SPF RR type (see
 
   records [RFC2782].


   Section 3.1.1) is used.
 




 








   The example above in Section 3 might be published via these lines in
 
   The example in this section might be published via these lines in a


   a domain zone file:
 
   domain zone file:



 



      example.com.          TXT "v=spf1 +mx a:colo.example.com/28 -all"
 
      example.com.          TXT "v=spf1 +mx a:colo.example.com/28 -all"


      smtp-out.example.com. TXT "v=spf1 a -all"
 
      smtp-out.example.com. TXT "v=spf1 a -all"



 








   When publishing via TXT records, beware of other TXT records
 
   Since TXT records have multiple uses, beware of other TXT records


   published there for other purposes.  They may cause problems with
 
   published there for other purposes.  They might cause problems with


   size limits (see Section 3.1.4).
 
   size limits (see Section 3.4) and care has to be taken to ensure only



 
   SPF records are used for SPF processing.


3.1.1.  DNS Resource Record Types
 




 



   This document defines a new DNS RR of type SPF, code 99.  The format
 



   of this type is identical to the TXT RR [RFC1035].  For either type,
 



   the character content of the record is encoded as [US-ASCII].
 




 



   It is recognized that the current practice (using a TXT record) is
 



   not optimal, but it is necessary because there are a number of DNS
 



   server and resolver implementations in common use that cannot handle
 



   the new RR type.  The two-record-type scheme provides a forward path
 



   to the better solution of using an RR type reserved for this purpose.
 




 








   An SPF-compliant domain name SHOULD have SPF records of both RR
 
   ADMDs publishing SPF records SHOULD try to keep the number of


   types.  A compliant domain name MUST have a record of at least one
 
   "include" mechanisms and chained "redirect" modifiers to a minimum.


   type.  If a domain has records of both types, they MUST have
 
   ADMDs SHOULD also try to minimize the amount of other DNS information


   identical content.  For example, instead of publishing just one
 
   needed to evaluate a record.  Section 4.6.4 and Section 9.1.1 provide


   record as in Section 3.1 above, it is better to publish:
 
   some suggestions on how to achieve this.



 








      example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
 
3.1.  DNS Resource Records


      example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"
 




 








   Example RRs in this document are shown with the TXT record type;
 
   SPF records MUST be published as a DNS TXT (type 16) Resource Record


   however, they could be published with the SPF type or with both
 
   (RR) [RFC1035] only.  The character content of the record is encoded


   types.
 
   as [US-ASCII].  Use of alternate DNS RR types was supported in SPF's



 
   experimental phase, but has been discontinued.  See Appendix A of



 
   [RFC6686] for further information.



 








3.1.2.  Multiple DNS Records
 
3.2.  Multiple DNS Records



 



   A domain name MUST NOT have multiple records that would cause an
 
   A domain name MUST NOT have multiple records that would cause an


   authorization check to select more than one record.  See Section 4.5
 
   authorization check to select more than one record.  See Section 4.5


   for the selection rules.
 
   for the selection rules.



 








3.1.3.  Multiple Strings in a Single DNS record
 
3.3.  Multiple Strings in a Single DNS record



 



   As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS
 
   As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS







   record (either TXT or SPF RR types) can be composed of more than one
 
   record can be composed of more than one string.  If a published


   string.  If a published record contains multiple strings, then the
 
   record contains multiple character-strings, then the record MUST be


   record MUST be treated as if those strings are concatenated together
 
   treated as if those strings are concatenated together without adding


   without adding spaces.  For example:
 
   spaces.  For example:



 



      IN TXT "v=spf1 .... first" "second string..."
 
      IN TXT "v=spf1 .... first" "second string..."



 



   MUST be treated as equivalent to
 
   MUST be treated as equivalent to



 



      IN TXT "v=spf1 .... firstsecond string..."
 
      IN TXT "v=spf1 .... firstsecond string..."



 








   SPF or TXT records containing multiple strings are useful in
 
   TXT records containing multiple strings are useful in constructing


   constructing records that would exceed the 255-byte maximum length of
 
   records that would exceed the 255-byte maximum length of a character-


   a string within a single TXT or SPF RR record.
 
   string within a single TXT record.



 








3.1.4.  Record Size
 
3.4.  Record Size



 



   The published SPF record for a given domain name SHOULD remain small
 
   The published SPF record for a given domain name SHOULD remain small


   enough that the results of a query for it will fit within 512 octets.
 
   enough that the results of a query for it will fit within 512 octets.







   This will keep even older DNS implementations from falling over to
 
   This UDP limit is defined in [RFC1035] section 2.3.4.  This will keep


   TCP.  Since the answer size is dependent on many things outside the
 
   even older DNS implementations from falling over to TCP.  Since the


   scope of this document, it is only possible to give this guideline:
 
   answer size is dependent on many things outside the scope of this


   If the combined length of the DNS name and the text of all the
 
   document, it is only possible to give this guideline: If the combined


   records of a given type (TXT or SPF) is under 450 characters, then
 
   length of the DNS name and the text of all the records of a given


   DNS answers should fit in UDP packets.  Note that when computing the
 
   type is under 450 characters, then DNS answers ought to fit in UDP


   sizes for queries of the TXT format, one must take into account any
 
   packets.  Note that when computing the sizes for queries of the TXT


   other TXT records published at the domain name.  Records that are too
 
   format, one has to take into account any other TXT records published


   long to fit in a single UDP packet MAY be silently ignored by SPF
 
   at the domain name.  Records that are too long to fit in a single UDP


   clients.
 
   packet could be silently ignored by SPF verifiers due to firewall and



 
   other issues that cause DNS over TCP to be less reliable than DNS


3.1.5.  Wildcard Records
 
   over UDP.



 








   Use of wildcard records for publishing is not recommended.  Care must
 
3.5.  Wildcard Records


   be taken if wildcard records are used.  If a domain publishes
 



   wildcard MX records, it may want to publish wildcard declarations,
 



   subject to the same requirements and problems.  In particular, the
 



   declaration must be repeated for any host that has any RR records at
 



   all, and for subdomains thereof.  For example, the example given in
 



   [RFC1034], Section 4.3.3, could be extended with the following:
 




 








       X.COM.          MX      10      A.X.COM
 
   Use of wildcard records for publishing is discouraged and care has to


       X.COM.          TXT     "v=spf1 a:A.X.COM -all"
 
   be taken if they are used.  If a zone includes wildcard MX records,



 
   it might want to publish wildcard declarations, subject to the same



 
   requirements and problems.  In particular, the declaration MUST be



 
   repeated for any host that has any RR records at all, and for



 
   subdomains thereof.  Consider the example in [RFC1034], Section



 
   4.3.3.  Based on that, we can do the following:



 








       *.X.COM.        MX      10      A.X.COM
 
       EXAMPLE.COM.          MX      10      A.EXAMPLE.COM


       *.X.COM.        TXT     "v=spf1 a:A.X.COM -all"
 
       EXAMPLE.COM.          TXT     "v=spf1 a:A.EXAMPLE.COM -all"



 








       A.X.COM.        A       1.2.3.4
 
       *.EXAMPLE.COM.        MX      10      A.EXAMPLE.COM


       A.X.COM.        MX      10      A.X.COM
 
       *.EXAMPLE.COM.        TXT     "v=spf1 a:A.EXAMPLE.COM -all"


       A.X.COM.        TXT     "v=spf1 a:A.X.COM -all"
 




 








       *.A.X.COM.      MX      10      A.X.COM
 
       A.EXAMPLE.COM.        A       203.0.113.1


       *.A.X.COM.      TXT     "v=spf1 a:A.X.COM -all"
 
       A.EXAMPLE.COM.        MX      10      A.EXAMPLE.COM



 
       A.EXAMPLE.COM.        TXT     "v=spf1 a:A.EXAMPLE.COM -all"



 








   Notice that SPF records must be repeated twice for every name within
 
       *.A.EXAMPLE.COM.      MX      10      A.EXAMPLE.COM


   the domain: once for the name, and once with a wildcard to cover the
 
       *.A.EXAMPLE.COM.      TXT     "v=spf1 a:A.EXAMPLE.COM -all"


   tree under the name.
 




 








   Use of wildcards is discouraged in general as they cause every name
 
   SPF records have to be listed twice for every name within the zone:


   under the domain to exist and queries against arbitrary names will
 
   once for the name, and once with a wildcard to cover the tree under


   never return RCODE 3 (Name Error).
 
   the name, in order to cover all domains in use in outgoing mail.



 



4.  The check_host() Function
 
4.  The check_host() Function



 









 
   This description is not an API (Application Program Interface)



 
   definition, but rather a function description used to illustrate the



 
   algorithm.  A compliant SPF implementation MUST do something



 
   semantically equivalent to this description.



 
                                                                         


   The check_host() function fetches SPF records, parses them, and
 
   The check_host() function fetches SPF records, parses them, and







   interprets them to determine whether a particular host is or is not
 
   evaluates them to determine whether a particular host is or is not


   permitted to send mail with a given identity.  Mail receivers that
 
   permitted to send mail with a given identity.  Mail receivers that


   perform this check MUST correctly evaluate the check_host() function
 
   perform this check MUST correctly evaluate the check_host() function


   as described here.
 
   as described here.



 



   Implementations MAY use a different algorithm than the canonical
 
   Implementations MAY use a different algorithm than the canonical


   algorithm defined here, so long as the results are the same in all
 
   algorithm defined here, so long as the results are the same in all


   cases.
 
   cases.



 



4.1.  Arguments
 
4.1.  Arguments



 




 



skipping to change at page 13, line 12

skipping to change at page 17, line 40


              information; initially, the domain portion of the "MAIL
 
              information; initially, the domain portion of the "MAIL


              FROM" or "HELO" identity.
 
              FROM" or "HELO" identity.



 



   <sender> - the "MAIL FROM" or "HELO" identity.
 
   <sender> - the "MAIL FROM" or "HELO" identity.



 



   The domain portion of <sender> will usually be the same as the
 
   The domain portion of <sender> will usually be the same as the


   <domain> argument when check_host() is initially evaluated.  However,
 
   <domain> argument when check_host() is initially evaluated.  However,


   this will generally not be true for recursive evaluations (see
 
   this will generally not be true for recursive evaluations (see


   Section 5.2 below).
 
   Section 5.2 below).



 








   Actual implementations of the check_host() function may need
 



   additional arguments.
 



                                                                         
 



4.2.  Results
 
4.2.  Results



 



   The function check_host() can return one of several results described
 
   The function check_host() can return one of several results described


   in Section 2.5.  Based on the result, the action to be taken is
 
   in Section 2.5.  Based on the result, the action to be taken is


   determined by the local policies of the receiver.
 
   determined by the local policies of the receiver.



 



4.3.  Initial Processing
 
4.3.  Initial Processing



 








   If the <domain> is malformed (label longer than 63 characters, zero-
 
   If the <domain> is malformed (e.g. label longer than 63 characters,


   length label not at the end, etc.) or is not a fully qualified domain
 
   zero-length label not at the end, etc.) or is not a fully qualified


   name, or if the DNS lookup returns "domain does not exist" (RCODE 3),
 
   domain name, or if the DNS lookup returns "domain does not exist"


   check_host() immediately returns the result "None".
 
   (RCODE 3), check_host() immediately returns the result "none".



 
   Properly formed domains are fully qualified email domains as



 
   described in [RFC5321] Section 2.3.5.  Internationalized domain names



 
   MUST be encoded as A-labels, as described in Section 2.3 of



 
   [RFC5890].on 2.3 of [RFC5890].



 








   If the <sender> has no localpart, substitute the string "postmaster"
 
   If the <sender> has no local-part, substitute the string "postmaster"


   for the localpart.
 
   for the local-part.



 



4.4.  Record Lookup
 
4.4.  Record Lookup



 








   In accordance with how the records are published (see Section 3.1
 
   In accordance with how the records are published (see Section 3


   above), a DNS query needs to be made for the <domain> name, querying
 
   above), a DNS query needs to be made for the <domain> name, querying







   for either RR type TXT, SPF, or both.  If both SPF and TXT RRs are
 
   for type TXT only.


   looked up, the queries MAY be done in parallel.
 




 



   If all DNS lookups that are made return a server failure (RCODE 2),
 
   If all DNS lookups that are made return a server failure (RCODE 2),


   or other error (RCODE other than 0 or 3), or time out, then
 
   or other error (RCODE other than 0 or 3), or time out, then







   check_host() exits immediately with the result "TempError".
 
   check_host() terminates immediately with the result "temperror".



 
   Alternatively, for a server failure (RCODE 2) result, check_host()



 
   MAY track failures and treat multiple failures within 24 hours for



 
   the same domain as "permerror".



 




 
   This alternative is intended to shorten the queue time of messages



 
   that cannot be accepted, by returning a permanent negative completion



 
   reply code to the client, instead of a transient one.  [RFC2308]



 
   suggests on an algorithm for doing such tracking and handling of



 
   server failure codes.



 



4.5.  Selecting Records
 
4.5.  Selecting Records



 



   Records begin with a version section:
 
   Records begin with a version section:



 



   record           = version terms *SP
 
   record           = version terms *SP


   version          = "v=spf1"
 
   version          = "v=spf1"



 



   Starting with the set of records that were returned by the lookup,
 
   Starting with the set of records that were returned by the lookup,







   record selection proceeds in two steps:
 
   discard records that do not begin with a version section of exactly



 
   "v=spf1".  Note that the version section is terminated either by an


   1. Records that do not begin with a version section of exactly
 
   SP character or the end of the record.  A record with a version


      "v=spf1" are discarded.  Note that the version section is
 
   section of "v=spf10" does not match and MUST be discarded.


      terminated either by an SP character or the end of the record.  A
 



      record with a version section of "v=spf10" does not match and must
 



      be discarded.
 



                                                                         
 



   2. If any records of type SPF are in the set, then all records of
 



      type TXT are discarded.
 




 



   After the above steps, there should be exactly one record remaining
 



   and evaluation can proceed.  If there are two or more records
 



   remaining, then check_host() exits immediately with the result of
 



   "PermError".
 




 








   If no matching records are returned, an SPF client MUST assume that
 
   If the resultant record set includes no records, check_host()


   the domain makes no SPF declarations.  SPF processing MUST stop and
 
   produces the "none" result.  If the resultant record set includes


   return "None".
 
   more than one record, check_host() produces the "permerror" result.



 



4.6.  Record Evaluation
 
4.6.  Record Evaluation



 








   After one SPF record has been selected, the check_host() function
 
   The check_host() function parses and interprets the SPF record to


   parses and interprets it to find a result for the current test.  If
 
   find a result for the current test.  If there are any syntax errors,


   there are any syntax errors, check_host() returns immediately with
 
   check_host() returns immediately with the result "permerror".


   the result "PermError".
 




 



   Implementations MAY choose to parse the entire record first and
 
   Implementations MAY choose to parse the entire record first and







   return "PermError" if the record is not syntactically well formed.
 
   return "permerror" if the record is not syntactically well formed.


   However, in all cases, any syntax errors anywhere in the record MUST
 
   However, in all cases, any syntax errors anywhere in the record MUST


   be detected.
 
   be detected.



 



4.6.1.  Term Evaluation
 
4.6.1.  Term Evaluation



 



   There are two types of terms: mechanisms and modifiers.  A record
 
   There are two types of terms: mechanisms and modifiers.  A record


   contains an ordered list of these as specified in the following
 
   contains an ordered list of these as specified in the following


   Augmented Backus-Naur Form (ABNF).
 
   Augmented Backus-Naur Form (ABNF).



 



   terms            = *( 1*SP ( directive / modifier ) )
 
   terms            = *( 1*SP ( directive / modifier ) )



 



   directive        = [ qualifier ] mechanism
 
   directive        = [ qualifier ] mechanism


   qualifier        = "+" / "-" / "?" / "~"
 
   qualifier        = "+" / "-" / "?" / "~"


   mechanism        = ( all / include
 
   mechanism        = ( all / include


                      / A / MX / PTR / IP4 / IP6 / exists )
 
                      / A / MX / PTR / IP4 / IP6 / exists )


   modifier         = redirect / explanation / unknown-modifier
 
   modifier         = redirect / explanation / unknown-modifier


   unknown-modifier = name "=" macro-string
 
   unknown-modifier = name "=" macro-string








 
                      ; where name is not any known modifier



 



   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 
   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )



 



   Most mechanisms allow a ":" or "/" character after the name.
 
   Most mechanisms allow a ":" or "/" character after the name.



 



   Modifiers always contain an equals ('=') character immediately after
 
   Modifiers always contain an equals ('=') character immediately after







   the name, and before any ":" or "/" characters that may be part of
 
   the name, and before any ":" or "/" characters that might be part of


   the macro-string.
 
   the macro-string.



 



   Terms that do not contain any of "=", ":", or "/" are mechanisms, as
 
   Terms that do not contain any of "=", ":", or "/" are mechanisms, as


   defined in Section 5.
 
   defined in Section 5.



 








   As per the definition of the ABNF notation in [RFC4234], mechanism
 
   As per the definition of the ABNF notation in [RFC5234], mechanism


   and modifier names are case-insensitive.
 
   and modifier names are case-insensitive.



 



4.6.2.  Mechanisms
 
4.6.2.  Mechanisms



 



   Each mechanism is considered in turn from left to right.  If there
 
   Each mechanism is considered in turn from left to right.  If there


   are no more mechanisms, the result is specified in Section 4.7.
 
   are no more mechanisms, the result is specified in Section 4.7.



 



   When a mechanism is evaluated, one of three things can happen: it can
 
   When a mechanism is evaluated, one of three things can happen: it can







   match, not match, or throw an exception.
 
   match, not match, or return an exception.



 



   If it matches, processing ends and the qualifier value is returned as
 
   If it matches, processing ends and the qualifier value is returned as


   the result of that record.  If it does not match, processing
 
   the result of that record.  If it does not match, processing







   continues with the next mechanism.  If it throws an exception,
 
   continues with the next mechanism.  If it returns an exception,


   mechanism processing ends and the exception value is returned.
 
   mechanism processing ends and the exception value is returned.



 








   The possible qualifiers, and the results they return are as follows:
 
   The possible qualifiers, and the results they cause check_host() to



 
   return are as follows:



 








      "+" Pass
 
      "+" pass


      "-" Fail
 
      "-" fail


      "~" SoftFail
 
      "~" softfail


      "?" Neutral
 
      "?" neutral



 



   The qualifier is optional and defaults to "+".
 
   The qualifier is optional and defaults to "+".



 








   When a mechanism matches and the qualifier is "-", then a "Fail"
 
   When a mechanism matches and the qualifier is "-", then a "fail"


   result is returned and the explanation string is computed as
 
   result is returned and the explanation string is computed as


   described in Section 6.2.
 
   described in Section 6.2.



 



   The specific mechanisms are described in Section 5.
 
   The specific mechanisms are described in Section 5.



 



4.6.3.  Modifiers
 
4.6.3.  Modifiers



 








   Modifiers are not mechanisms: they do not return match or not-match.
 
   Modifiers are not mechanisms.  They do not return match or not-match.


   Instead they provide additional information.  Although modifiers do
 
   Instead, they provide additional information.  Although modifiers do


   not directly affect the evaluation of the record, the "redirect"
 
   not directly affect the evaluation of the record, the "redirect"


   modifier has an effect after all the mechanisms have been evaluated.
 
   modifier has an effect after all the mechanisms have been evaluated.



 









 
4.6.4.  DNS Lookup Limits



 




 
   SPF implementations MUST limit the number of mechanisms and modifiers



 
   ("terms") that cause any DNS query to at most 10 during SPF



 
   evaluation.  Specifically, the "include", "a", "mx", "ptr", and



 
   "exists" mechanisms as well as the "redirect" modifier count against



 
   this limit.  The "all", "ip4", and "ip6" mechanisms do not count



 
   against this limit.  If this number is exceeded during a check, a



 
   permerror MUST be returned.  The "exp" modifier does not count



 
   against this limit because the DNS lookup to fetch the explanation



 
   string occurs after the SPF record evaluation has been completed.



 




 
   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,



 
   there MUST be a limit of no more than 10 MX or PTR RRs looked up and



 
   checked.  If more than 10 "mx" or "ptr" records are returned for this



 
   further lookup, a permerror MUST be returned.  This limit is per



 
   mechanism or macro in the record and in addition to the lookup limits



 
   above.



 




 
   MTAs or other processors SHOULD impose a limit on the maximum amount



 
   of elapsed time to evaluate check_host().  Such a limit SHOULD allow



 
   at least 20 seconds.  If such a limit is exceeded, the result of



 
   authorization SHOULD be "temperror".



 
                                                                         


4.7.  Default Result
 
4.7.  Default Result



 



   If none of the mechanisms match and there is no "redirect" modifier,
 
   If none of the mechanisms match and there is no "redirect" modifier,







   then the check_host() returns a result of "Neutral", just as if
 
   then the check_host() returns a result of "neutral", just as if


   "?all" were specified as the last directive.  If there is a
 
   "?all" were specified as the last directive.  If there is a


   "redirect" modifier, check_host() proceeds as defined in Section 6.1.
 
   "redirect" modifier, check_host() proceeds as defined in Section 6.1.



 



   Note that records SHOULD always use either a "redirect" modifier or
 
   Note that records SHOULD always use either a "redirect" modifier or







   an "all" mechanism to explicitly terminate processing.
 
   an "all" mechanism to explicitly terminate processing.  Although the



 
   latter has default (specifically "?all"), it aids debugging efforts



 
   if it is explicitly included.



 



   For example:
 
   For example:



 



      v=spf1 +mx -all
 
      v=spf1 +mx -all


   or
 
   or


      v=spf1 +mx redirect=_spf.example.com
 
      v=spf1 +mx redirect=_spf.example.com



 



4.8.  Domain Specification
 
4.8.  Domain Specification



 








   Several of these mechanisms and modifiers have a <domain-spec>
 
   Several of these mechanisms and modifiers have a domain-spec section.


   section.  The <domain-spec> string is macro expanded (see Section 8).
 
   The domain-spec string is subject to macro expansion (see Section 8).


   The resulting string is the common presentation form of a fully-
 
   The resulting string is the common presentation form of a fully-


   qualified DNS name: a series of labels separated by periods.  This
 
   qualified DNS name: a series of labels separated by periods.  This


   domain is called the <target-name> in the rest of this document.
 
   domain is called the <target-name> in the rest of this document.



 



   Note: The result of the macro expansion is not subject to any further
 
   Note: The result of the macro expansion is not subject to any further


   escaping.  Hence, this facility cannot produce all characters that
 
   escaping.  Hence, this facility cannot produce all characters that


   are legal in a DNS label (e.g., the control characters).  However,
 
   are legal in a DNS label (e.g., the control characters).  However,


   this facility is powerful enough to express legal host names and
 
   this facility is powerful enough to express legal host names and


   common utility labels (such as "_spf") that are used in DNS.
 
   common utility labels (such as "_spf") that are used in DNS.



 



   For several mechanisms, the <domain-spec> is optional.  If it is not
 
   For several mechanisms, the <domain-spec> is optional.  If it is not







   provided, the <domain> is used as the <target-name>.
 
   provided, the <domain> is used as the <target-name>.  Domain and



 
   domain-spec are syntactically identical after macro expansion.



 
   Domain is an input value for check_host() while domain-spec is



 
   computed by check_host().



 




 
   Note: Historically, this document has made no provisions for how to



 
   handle domain-specs, or macro-expansions thereof, that are



 
   syntactically invalid per [RFC1035], such as names with empty labels



 
   (e.g., "foo..example.com") or overlong labels (more than 63



 
   characters).  Some implementations choose to treat as a no-match



 
   mechanisms, and ignore modifiers, with such names, whereas others



 
   return a "permerror" exception.  The outcome for an unexpected



 
   domain-spec without macros might even differ from that for an



 
   unexpected target-name after macro expansion.



 



5.  Mechanism Definitions
 
5.  Mechanism Definitions



 



   This section defines two types of mechanisms.
 
   This section defines two types of mechanisms.



 



   Basic mechanisms contribute to the language framework.  They do not
 
   Basic mechanisms contribute to the language framework.  They do not


   specify a particular type of authorization scheme.
 
   specify a particular type of authorization scheme.



 



      all
 
      all


      include
 
      include



 



   Designated sender mechanisms are used to designate a set of <ip>
 
   Designated sender mechanisms are used to designate a set of <ip>


   addresses as being permitted or not permitted to use the <domain> for
 
   addresses as being permitted or not permitted to use the <domain> for


   sending mail.
 
   sending mail.



 



      a
 
      a


      mx
 
      mx







      ptr
 
      ptr (deprecated)


      ip4
 
      ip4


      ip6
 
      ip6


      exists
 
      exists



 



   The following conventions apply to all mechanisms that perform a
 
   The following conventions apply to all mechanisms that perform a


   comparison between <ip> and an IP address at any point:
 
   comparison between <ip> and an IP address at any point:



 








   If no CIDR-length is given in the directive, then <ip> and the IP
 
   If no CIDR prefix length is given in the directive, then <ip> and the


   address are compared for equality. (Here, CIDR is Classless Inter-
 
   IP address are compared for equality.  (Here, CIDR is Classless


   Domain Routing.)
 
   Inter-Domain Routing, described in [RFC4632].)



 








   If a CIDR-length is specified, then only the specified number of
 
   If a CIDR prefix length is specified, then only the specified number


   high-order bits of <ip> and the IP address are compared for equality.
 
   of high-order bits of <ip> and the IP address are compared for



 
   equality.



 



   When any mechanism fetches host addresses to compare with <ip>, when
 
   When any mechanism fetches host addresses to compare with <ip>, when







   <ip> is an IPv4 address, A records are fetched, when <ip> is an IPv6
 
   <ip> is an IPv4 address, A records are fetched; when <ip> is an IPv6


   address, AAAA records are fetched.  Even if the SMTP connection is
 
   address, AAAA records are fetched.  Even if the SMTP connection uses


   via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513], Section
 
   IPv6, an IPv4-mapped IPv6 IP address (see [RFC4291], Section 2.5.5)


   2.5.5) MUST still be considered an IPv4 address.
 
   MUST still be considered an IPv4 address and MUST be evaluated using



 
   IPv4 mechanisms (i.e. "ip4" and "a").



 








   Several mechanisms rely on information fetched from DNS.  For these
 
   Several mechanisms rely on information fetched from the DNS.  For


   DNS queries, except where noted, if the DNS server returns an error
 
   these DNS queries, except where noted, if the DNS server returns an


   (RCODE other than 0 or 3) or the query times out, the mechanism
 
   error (RCODE other than 0 or 3) or the query times out, the mechanism


   throws the exception "TempError".  If the server returns "domain does
 
   stops and the topmost check_host() returns "temperror".  If the


   not exist" (RCODE 3), then evaluation of the mechanism continues as
 
   server returns "domain does not exist" (RCODE 3), then evaluation of


   if the server returned no error (RCODE 0) and zero answer records.
 
   the mechanism continues as if the server returned no error (RCODE 0)



 
   and zero answer records.



 



5.1.  "all"
 
5.1.  "all"



 



   all              = "all"
 
   all              = "all"



 



   The "all" mechanism is a test that always matches.  It is used as the
 
   The "all" mechanism is a test that always matches.  It is used as the


   rightmost mechanism in a record to provide an explicit default.
 
   rightmost mechanism in a record to provide an explicit default.



 



   For example:
 
   For example:



 



      v=spf1 a mx -all
 
      v=spf1 a mx -all



 








   Mechanisms after "all" will never be tested.  Any "redirect" modifier
 
   Mechanisms after "all" will never be tested.  Mechanisms listed after


   (Section 6.1) has no effect when there is an "all" mechanism.
 
   "all" MUST be ignored.  Any "redirect" modifier (Section 6.1) MUST be



 
   ignored when there is an "all" mechanism in the record.



 



5.2.  "include"
 
5.2.  "include"



 








      include          = "include"  ":" domain-spec
 
   include          = "include"  ":" domain-spec



 



   The "include" mechanism triggers a recursive evaluation of
 
   The "include" mechanism triggers a recursive evaluation of







   check_host().  The domain-spec is expanded as per Section 8.  Then
 
   check_host().


   check_host() is evaluated with the resulting string as the <domain>.
 
                                                                         


   The <ip> and <sender> arguments remain the same as in the current
 
   1.  The domain-spec is expanded as per Section 8.


   evaluation of check_host().
 
                                                                         



 
   2.  Check_host() is evaluated with the resulting string as the



 
       <domain>.  The <ip> and <sender> arguments remain the same as in



 
       the current evaluation of check_host().



 
                                                                         



 
   3.  The recursive evaluation returns either match, not match, or an



 
       error.  If it matches, then the appropriate result for the



 
       include: mechanism is used (e.g. include or +include gives a



 
       "pass" result and -include gives "fail).



 




 
   4.  If there is no match, the parent check_host() resumes processing



 
       as per the table below, with the previous value of <domain>



 
       restored.



 



   In hindsight, the name "include" was poorly chosen.  Only the
 
   In hindsight, the name "include" was poorly chosen.  Only the


   evaluated result of the referenced SPF record is used, rather than
 
   evaluated result of the referenced SPF record is used, rather than


   acting as if the referenced SPF record was literally included in the
 
   acting as if the referenced SPF record was literally included in the


   first.  For example, evaluating a "-all" directive in the referenced
 
   first.  For example, evaluating a "-all" directive in the referenced


   record does not terminate the overall processing and does not
 
   record does not terminate the overall processing and does not







   necessarily result in an overall "Fail".  (Better names for this
 
   necessarily result in an overall "fail".  (Better names for this


   mechanism would have been "if-pass", "on-pass", etc.)
 
   mechanism would have been "if-match", "on-match", etc.)



 



   The "include" mechanism makes it possible for one domain to designate
 
   The "include" mechanism makes it possible for one domain to designate


   multiple administratively-independent domains.  For example, a vanity
 
   multiple administratively-independent domains.  For example, a vanity


   domain "example.net" might send mail using the servers of
 
   domain "example.net" might send mail using the servers of


   administratively-independent domains example.com and example.org.
 
   administratively-independent domains example.com and example.org.



 



   Example.net could say
 
   Example.net could say



 



      IN TXT "v=spf1 include:example.com include:example.org -all"
 
      IN TXT "v=spf1 include:example.com include:example.org -all"



 



   This would direct check_host() to, in effect, check the records of
 
   This would direct check_host() to, in effect, check the records of







   example.com and example.org for a "Pass" result.  Only if the host
 
   example.com and example.org for a "pass" result.  Only if the host


   were not permitted for either of those domains would the result be
 
   were not permitted for either of those domains would the result be







   "Fail".
 
   "fail".



 








   Whether this mechanism matches, does not match, or throws an
 
   Whether this mechanism matches, does not match, or returns an


   exception depends on the result of the recursive evaluation of
 
   exception depends on the result of the recursive evaluation of


   check_host():
 
   check_host():



 



   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+


   | A recursive check_host() result | Causes the "include" mechanism  |
 
   | A recursive check_host() result | Causes the "include" mechanism  |


   | of:                             | to:                             |
 
   | of:                             | to:                             |


   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+







   | Pass                            | match                           |
 
   | pass                            | match                           |


   |                                 |                                 |
 
   |                                 |                                 |







   | Fail                            | not match                       |
 
   | fail                            | not match                       |


   |                                 |                                 |
 
   |                                 |                                 |







   | SoftFail                        | not match                       |
 
   | softfail                        | not match                       |


   |                                 |                                 |
 
   |                                 |                                 |







   | Neutral                         | not match                       |
 
   | neutral                         | not match                       |


   |                                 |                                 |
 
   |                                 |                                 |







   | TempError                       | throw TempError                 |
 
   | temperror                       | return temperror                |


   |                                 |                                 |
 
   |                                 |                                 |







   | PermError                       | throw PermError                 |
 
   | permerror                       | return permerror                |


   |                                 |                                 |
 
   |                                 |                                 |







   | None                            | throw PermError                 |
 
   | none                            | return permerror                |


   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+



 



   The "include" mechanism is intended for crossing administrative
 
   The "include" mechanism is intended for crossing administrative







   boundaries.  Although it is possible to use includes to consolidate
 
   boundaries.  For example, if example.com and example.org were managed


   multiple domains that share the same set of designated hosts, domains
 
   by the same entity, and if the permitted set of hosts for both


   are encouraged to use redirects where possible, and to minimize the
 
   domains was


   number of includes within a single administrative domain.  For
 



   example, if example.com and example.org were managed by the same
 



   entity, and if the permitted set of hosts for both domains was
 



   "mx:example.com", it would be possible for example.org to specify
 
   "mx:example.com", it would be possible for example.org to specify


   "include:example.com", but it would be preferable to specify
 
   "include:example.com", but it would be preferable to specify


   "redirect=example.com" or even "mx:example.com".
 
   "redirect=example.com" or even "mx:example.com".



 









 
   With the "include" mechanism an administratively external set of



 
   hosts can be authorized, but determination of sender policy is still



 
   a function of the original domain's SPF record (as determined by the



 
   "all" mechanism in that record).  The redirect modifier is more



 
   suitable for consolidating both authorizations and policy into a



 
   common set to be shared within an ADMD.  Redirect is much more like a



 
   common code element to be shared among records in a single ADMD.  It



 
   is possible to control both authorized hosts and policy for an



 
   arbitrary number of domains from a single record.



 
                                                                         


5.3.  "a"
 
5.3.  "a"



 



   This mechanism matches if <ip> is one of the <target-name>'s IP
 
   This mechanism matches if <ip> is one of the <target-name>'s IP


   addresses.
 
   addresses.



 








   A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
 
   a                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]



 



   An address lookup is done on the <target-name>.  The <ip> is compared
 
   An address lookup is done on the <target-name>.  The <ip> is compared


   to the returned address(es).  If any address matches, the mechanism
 
   to the returned address(es).  If any address matches, the mechanism


   matches.
 
   matches.



 



5.4.  "mx"
 
5.4.  "mx"



 



   This mechanism matches if <ip> is one of the MX hosts for a domain
 
   This mechanism matches if <ip> is one of the MX hosts for a domain


   name.
 
   name.



 








   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 
   mx               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]



 



   check_host() first performs an MX lookup on the <target-name>.  Then
 
   check_host() first performs an MX lookup on the <target-name>.  Then


   it performs an address lookup on each MX name returned.  The <ip> is
 
   it performs an address lookup on each MX name returned.  The <ip> is


   compared to each returned IP address.  To prevent Denial of Service
 
   compared to each returned IP address.  To prevent Denial of Service


   (DoS) attacks, more than 10 MX names MUST NOT be looked up during the
 
   (DoS) attacks, more than 10 MX names MUST NOT be looked up during the







   evaluation of an "mx" mechanism (see Section 10).  If any address
 
   evaluation of an "mx" mechanism.  If there are more than 10 MX names


   matches, the mechanism matches.
 
   then permerror is returned and the evaluation terminated (see



 
   Section 4.6.4).  If any address matches, the mechanism matches.



 



   Note regarding implicit MXs: If the <target-name> has no MX records,
 
   Note regarding implicit MXs: If the <target-name> has no MX records,


   check_host() MUST NOT pretend the target is its single MX, and MUST
 
   check_host() MUST NOT pretend the target is its single MX, and MUST







   NOT default to an A lookup on the <target-name> directly.  This
 
   NOT default to an A or AAAA lookup on the <target-name> directly.


   behavior breaks with the legacy "implicit MX" rule.  See [RFC2821],
 
   This behavior diverges from the legacy "implicit MX" rule, (See


   Section 5.  If such behavior is desired, the publisher should specify
 
   [RFC5321], Section 5.  If such behavior is desired, the publisher


   an "a" directive.
 
   will have to specify an "a" directive).



 








5.5.  "ptr"
 
5.5.  "ptr" (deprecated)



 



   This mechanism tests whether the DNS reverse-mapping for <ip> exists
 
   This mechanism tests whether the DNS reverse-mapping for <ip> exists


   and correctly points to a domain name within a particular domain.
 
   and correctly points to a domain name within a particular domain.








 
   This mechanism is deprecated and SHOULD NOT be used.



 








   PTR              = "ptr"    [ ":" domain-spec ]
 
   ptr              = "ptr"    [ ":" domain-spec ]



 








   First, the <ip>'s name is looked up using this procedure: perform a
 
   The <ip>'s name is looked up using this procedure:


   DNS reverse-mapping for <ip>, looking up the corresponding PTR record
 



   in "in-addr.arpa." if the address is an IPv4 one and in "ip6.arpa."
 



   if it is an IPv6 address.  For each record returned, validate the
 



   domain name by looking up its IP address.  To prevent DoS attacks,
 



   more than 10 PTR names MUST NOT be looked up during the evaluation of
 



   a "ptr" mechanism (see Section 10).  If <ip> is among the returned IP
 



   addresses, then that domain name is validated.  In pseudocode:
 




 








   sending-domain_names := ptr_lookup(sending-host_IP); if more than 10
 
   1.  Perform a DNS reverse-mapping for <ip>: Look up the corresponding


   sending-domain_names are found, use at most 10.  for each name in
 
       PTR record in "in-addr.arpa." if the address is an IPv4 one and


   (sending-domain_names) {
 
       in "ip6.arpa." if it is an IPv6 address.


     IP_addresses := a_lookup(name);
 



     if the sending-domain_IP is one of the IP_addresses {
 



       validated-sending-domain_names += name;
 



     } }
 




 








   Check all validated domain names to see if they end in the
 
   2.  For each record returned, validate the domain name by looking up


   <target-name> domain.  If any do, this mechanism matches.  If no
 
       its IP addresses.  To prevent DoS attacks, more than 10 PTR names


   validated domain name can be found, or if none of the validated
 
       MUST NOT be looked up during the evaluation of a "ptr" mechanism


   domain names end in the <target-name>, this mechanism fails to match.
 
       (see Section 4.6.4).


   If a DNS error occurs while doing the PTR RR lookup, then this
 



   mechanism fails to match.  If a DNS error occurs while doing an A RR
 
   3.  If <ip> is among the returned IP addresses, then that domain name


   lookup, then that domain name is skipped and the search continues.
 
       is validated.



 
                                                                         



 
   Check all validated domain names to see if they either match the



 
   <target-name> domain or are a subdomain of the <target-name> domain.



 
   If any do, this mechanism matches.  If no validated domain name can



 
   be found, or if none of the validated domain names match or are a



 
   subdomain of the <target-name>, this mechanism fails to match.  If a



 
   DNS error occurs while doing the PTR RR lookup, then this mechanism



 
   fails to match.  If a DNS error occurs while doing an A RR lookup,



 
   then that domain name is skipped and the search continues.



 



   Pseudocode:
 
   Pseudocode:



 









 
   sending-domain_names := ptr_lookup(sending-host_IP);



 
   if more than 10 sending-domain_names are found, use at most 10.



 
   for each name in (sending-domain_names) {



 
     IP_addresses := a_lookup(name);



 
     if the sending-domain_IP is one of the IP_addresses {



 
       validated-sending-domain_names += name;



 
     }



 
   }



 
                                                                         


   for each name in (validated-sending-domain_names) {
 
   for each name in (validated-sending-domain_names) {


     if name ends in <domain-spec>, return match.
 
     if name ends in <domain-spec>, return match.


     if name is <domain-spec>, return match.
 
     if name is <domain-spec>, return match.


   }
 
   }


   return no-match.
 
   return no-match.



 








   This mechanism matches if the <target-name> is either an ancestor of
 
   This mechanism matches if the <target-name> is either a subdomain of


   a validated domain name or if the <target-name> and a validated
 
   a validated domain name or if the <target-name> and a validated


   domain name are the same.  For example: "mail.example.com" is within
 
   domain name are the same.  For example: "mail.example.com" is within


   the domain "example.com", but "mail.bad-example.com" is not.
 
   the domain "example.com", but "mail.bad-example.com" is not.



 








   Note: Use of this mechanism is discouraged because it is slow, it is
 
   Note: This mechanism has been deprecated because it is slow, it is


   not as reliable as other mechanisms in cases of DNS errors, and it
 
   not as reliable as other mechanisms in cases of DNS errors, and it







   places a large burden on the arpa name servers.  If used, proper PTR
 
   places a large burden on the .arpa name servers.  If used, proper PTR


   records must be in place for the domain's hosts and the "ptr"
 
   records MUST be in place for the domain's hosts and the "ptr"


   mechanism should be one of the last mechanisms checked.
 
   mechanism SHOULD be one of the last mechanisms checked.  After many



 
   years of SPF deployment experience it has been concluded it is



 
   unnecessary and more reliable alternatives used instead.  It is,



 
   however, still in use and part of the SPF protocol, so compliant



 
   check_host() implementations MUST support it.



 



5.6.  "ip4" and "ip6"
 
5.6.  "ip4" and "ip6"



 



   These mechanisms test whether <ip> is contained within a given IP
 
   These mechanisms test whether <ip> is contained within a given IP


   network.
 
   network.



 








   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 
   ip4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]


   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 
   ip6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]



 



   ip4-cidr-length  = "/" 1*DIGIT
 
   ip4-cidr-length  = "/" 1*DIGIT


   ip6-cidr-length  = "/" 1*DIGIT
 
   ip6-cidr-length  = "/" 1*DIGIT


   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
 
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]



 



   ip4-network      = qnum "." qnum "." qnum "." qnum
 
   ip4-network      = qnum "." qnum "." qnum "." qnum


   qnum             = DIGIT                 ; 0-9
 
   qnum             = DIGIT                 ; 0-9


                      / %x31-39 DIGIT       ; 10-99
 
                      / %x31-39 DIGIT       ; 10-99


                      / "1" 2DIGIT          ; 100-199
 
                      / "1" 2DIGIT          ; 100-199


                      / "2" %x30-34 DIGIT   ; 200-249
 
                      / "2" %x30-34 DIGIT   ; 200-249


                      / "25" %x30-35        ; 250-255
 
                      / "25" %x30-35        ; 250-255


            ; as per conventional dotted quad notation.  e.g., 192.0.2.0
 
            ; as per conventional dotted quad notation.  e.g., 192.0.2.0







   ip6-network      = <as per [RFC 3513], section 2.2>
 
   ip6-network      = <as per [RFC 4291], section 2.2>


            ; e.g., 2001:DB8::CD30
 
            ; e.g., 2001:DB8::CD30



 








   The <ip> is compared to the given network.  If CIDR-length high-order
 
   The <ip> is compared to the given network.  If CIDR prefix length


   bits match, the mechanism matches.
 
   high-order bits match, the mechanism matches.



 



   If ip4-cidr-length is omitted, it is taken to be "/32".  If
 
   If ip4-cidr-length is omitted, it is taken to be "/32".  If


   ip6-cidr-length is omitted, it is taken to be "/128".  It is not
 
   ip6-cidr-length is omitted, it is taken to be "/128".  It is not


   permitted to omit parts of the IP address instead of using CIDR
 
   permitted to omit parts of the IP address instead of using CIDR


   notations.  That is, use 192.0.2.0/24 instead of 192.0.2.
 
   notations.  That is, use 192.0.2.0/24 instead of 192.0.2.



 



5.7.  "exists"
 
5.7.  "exists"



 



   This mechanism is used to construct an arbitrary domain name that is
 
   This mechanism is used to construct an arbitrary domain name that is


   used for a DNS A record query.  It allows for complicated schemes
 
   used for a DNS A record query.  It allows for complicated schemes



 



skipping to change at page 22, line 34

skipping to change at page 28, line 15


   Domains can use this mechanism to specify arbitrarily complex
 
   Domains can use this mechanism to specify arbitrarily complex


   queries.  For example, suppose example.com publishes the record:
 
   queries.  For example, suppose example.com publishes the record:



 



      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
 
      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all



 



   The <target-name> might expand to
 
   The <target-name> might expand to


   "1.2.0.192.someuser._spf.example.com".  This makes fine-grained
 
   "1.2.0.192.someuser._spf.example.com".  This makes fine-grained


   decisions possible at the level of the user and client IP address.
 
   decisions possible at the level of the user and client IP address.



 



   This mechanism enables queries that mimic the style of tests that
 
   This mechanism enables queries that mimic the style of tests that







   existing anti-spam DNS blacklists (DNSBL) use.
 
   existing DNS white/black lists (DNSxLs) use, as described in



 
   [RFC5782].  The query will either return NXDOMAIN (no match), any



 
   valid answer (match), or an error.



 



6.  Modifier Definitions
 
6.  Modifier Definitions



 



   Modifiers are name/value pairs that provide additional information.
 
   Modifiers are name/value pairs that provide additional information.


   Modifiers always have an "=" separating the name and the value.
 
   Modifiers always have an "=" separating the name and the value.



 



   The modifiers defined in this document ("redirect" and "exp") MAY
 
   The modifiers defined in this document ("redirect" and "exp") MAY


   appear anywhere in the record, but SHOULD appear at the end, after
 
   appear anywhere in the record, but SHOULD appear at the end, after


   all mechanisms.  Ordering of these two modifiers does not matter.
 
   all mechanisms.  Ordering of these two modifiers does not matter.


   These two modifiers MUST NOT appear in a record more than once each.
 
   These two modifiers MUST NOT appear in a record more than once each.







   If they do, then check_host() exits with a result of "PermError".
 
   If they do, then check_host() exits with a result of "permerror".



 



   Unrecognized modifiers MUST be ignored no matter where in a record,
 
   Unrecognized modifiers MUST be ignored no matter where in a record,


   or how often.  This allows implementations of this document to
 
   or how often.  This allows implementations of this document to


   gracefully handle records with modifiers that are defined in other
 
   gracefully handle records with modifiers that are defined in other


   specifications.
 
   specifications.



 



6.1.  redirect: Redirected Query
 
6.1.  redirect: Redirected Query



 








   If all mechanisms fail to match, and a "redirect" modifier is
 
   The redirect modifier is intended for consolidating both


   present, then processing proceeds as follows:
 
   authorizations and policy into a common set to be shared within a



 
   single ADMD.  Redirect is like a common code element to be shared



 
   among records in a single ADMD.  It is possible to control both



 
   authorized hosts and policy for an arbitrary number of domains from a



 
   single record.



 



   redirect         = "redirect" "=" domain-spec
 
   redirect         = "redirect" "=" domain-spec



 









 
   If all mechanisms fail to match, and a "redirect" modifier is



 
   present, then processing proceeds as follows:



 
                                                                         


   The domain-spec portion of the redirect section is expanded as per
 
   The domain-spec portion of the redirect section is expanded as per


   the macro rules in Section 8.  Then check_host() is evaluated with
 
   the macro rules in Section 8.  Then check_host() is evaluated with


   the resulting string as the <domain>.  The <ip> and <sender>
 
   the resulting string as the <domain>.  The <ip> and <sender>







   arguments remain the same as current evaluation of check_host().
 
   arguments remain the same as in the current evaluation of



 
   check_host().



 



   The result of this new evaluation of check_host() is then considered
 
   The result of this new evaluation of check_host() is then considered


   the result of the current evaluation with the exception that if no
 
   the result of the current evaluation with the exception that if no


   SPF record is found, or if the target-name is malformed, the result
 
   SPF record is found, or if the target-name is malformed, the result







   is a "PermError" rather than "None".
 
   is a "permerror" rather than "none".



 








   Note that the newly-queried domain may itself specify redirect
 
   Note that the newly-queried domain can itself specify redirect


   processing.
 
   processing.



 



   This facility is intended for use by organizations that wish to apply
 
   This facility is intended for use by organizations that wish to apply


   the same record to multiple domains.  For example:
 
   the same record to multiple domains.  For example:



 



     la.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     la.example.com. TXT "v=spf1 redirect=_spf.example.com"


     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"


     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"


   _spf.example.com. TXT "v=spf1 mx:example.com -all"
 
   _spf.example.com. TXT "v=spf1 mx:example.com -all"



 



   In this example, mail from any of the three domains is described by
 
   In this example, mail from any of the three domains is described by


   the same record.  This can be an administrative advantage.
 
   the same record.  This can be an administrative advantage.



 



   Note: In general, the domain "A" cannot reliably use a redirect to
 
   Note: In general, the domain "A" cannot reliably use a redirect to


   another domain "B" not under the same administrative control.  Since
 
   another domain "B" not under the same administrative control.  Since


   the <sender> stays the same, there is no guarantee that the record at
 
   the <sender> stays the same, there is no guarantee that the record at


   domain "B" will correctly work for mailboxes in domain "A",
 
   domain "B" will correctly work for mailboxes in domain "A",







   especially if domain "B" uses mechanisms involving localparts.  An
 
   especially if domain "B" uses mechanisms involving local-parts.  An


   "include" directive may be more appropriate.
 
   "include" directive is generally be more appropriate.



 



   For clarity, it is RECOMMENDED that any "redirect" modifier appear as
 
   For clarity, it is RECOMMENDED that any "redirect" modifier appear as


   the very last term in a record.
 
   the very last term in a record.



 



6.2.  exp: Explanation
 
6.2.  exp: Explanation



 



   explanation      = "exp" "=" domain-spec
 
   explanation      = "exp" "=" domain-spec



 








   If check_host() results in a "Fail" due to a mechanism match (such as
 
   If check_host() results in a "fail" due to a mechanism match (such as


   "-all"), and the "exp" modifier is present, then the explanation
 
   "-all"), and the "exp" modifier is present, then the explanation


   string returned is computed as described below.  If no "exp" modifier
 
   string returned is computed as described below.  If no "exp" modifier


   is present, then either a default explanation string or an empty
 
   is present, then either a default explanation string or an empty







   explanation string may be returned.
 
   explanation string MUST be returned.



 








   The <domain-spec> is macro expanded (see Section 8) and becomes the
 
   The domain-spec is macro expanded (see Section 8) and becomes the


   <target-name>.  The DNS TXT record for the <target-name> is fetched.
 
   <target-name>.  The DNS TXT record for the <target-name> is fetched.



 








   If <domain-spec> is empty, or there are any DNS processing errors
 
   If there are any DNS processing errors (any RCODE other than 0), or


   (any RCODE other than 0), or if no records are returned, or if more
 
   if no records are returned, or if more than one record is returned,


   than one record is returned, or if there are syntax errors in the
 
   or if there are syntax errors in the explanation string, then proceed


   explanation string, then proceed as if no exp modifier was given.
 
   as if no exp modifier was given.



 



   The fetched TXT record's strings are concatenated with no spaces, and
 
   The fetched TXT record's strings are concatenated with no spaces, and







   then treated as an <explain-string>, which is macro-expanded.  This
 
   then treated as an explain-string, which is macro-expanded.  This


   final result is the explanation string.  Implementations MAY limit
 
   final result is the explanation string.  Implementations MAY limit


   the length of the resulting explanation string to allow for other
 
   the length of the resulting explanation string to allow for other


   protocol constraints and/or reasonable processing limits.  Since the
 
   protocol constraints and/or reasonable processing limits.  Since the







   explanation string is intended for an SMTP response and [RFC2821]
 
   explanation string is intended for an SMTP response and [RFC5321]


   Section 2.4 says that responses are in [US-ASCII], the explanation
 
   Section 2.4 says that responses are in [US-ASCII], the explanation







   string is also limited to US-ASCII.
 
   string MUST be limited to US-ASCII.



 



   Software evaluating check_host() can use this string to communicate
 
   Software evaluating check_host() can use this string to communicate


   information from the publishing domain in the form of a short message
 
   information from the publishing domain in the form of a short message


   or URL.  Software SHOULD make it clear that the explanation string
 
   or URL.  Software SHOULD make it clear that the explanation string


   comes from a third party.  For example, it can prepend the macro
 
   comes from a third party.  For example, it can prepend the macro







   string "%{o} explains: " to the explanation, such as shown in Section
 
   string "%{o} explains: " to the explanation, such as shown in


   2.5.4.
 
   Section 2.5.4.



 



   Suppose example.com has this record:
 
   Suppose example.com has this record:



 



      v=spf1 mx -all exp=explain._spf.%{d}
 
      v=spf1 mx -all exp=explain._spf.%{d}



 



   Here are some examples of possible explanation TXT records at
 
   Here are some examples of possible explanation TXT records at


   explain._spf.example.com:
 
   explain._spf.example.com:



 



      "Mail from example.com should only be sent by its own servers."
 
      "Mail from example.com should only be sent by its own servers."







         -- a simple, constant message
 
         --  a simple, constant message



 



      "%{i} is not one of %{d}'s designated mail servers."
 
      "%{i} is not one of %{d}'s designated mail servers."







         -- a message with a little more information, including the IP
 
         --  a message with a little more information, including the IP


            address that failed the check
 
             address that failed the check



 



      "See http://%{d}/why.html?s=%{S}&i=%{I}"
 
      "See http://%{d}/why.html?s=%{S}&i=%{I}"







         -- a complicated example that constructs a URL with the
 
         --  a complicated example that constructs a URL with the


            arguments to check_host() so that a web page can be
 
             arguments to check_host() so that a web page can be


            generated with detailed, custom instructions
 
             generated with detailed, custom instructions



 



   Note: During recursion into an "include" mechanism, an exp= modifier
 
   Note: During recursion into an "include" mechanism, an exp= modifier


   from the <target-name> MUST NOT be used.  In contrast, when executing
 
   from the <target-name> MUST NOT be used.  In contrast, when executing


   a "redirect" modifier, an exp= modifier from the original domain MUST
 
   a "redirect" modifier, an exp= modifier from the original domain MUST


   NOT be used.
 
   NOT be used.



 








7.  The Received-SPF Header Field
 
7.  Recording The Result



 








   It is RECOMMENDED that SMTP receivers record the result of SPF
 
   To provide downstream agents, such as MUAs, with the information they


   processing in the message header.  If an SMTP receiver chooses to do
 
   might need in terms of evaluating or representing the apparent safety


   so, it SHOULD use the "Received-SPF" header field defined here for
 
   of the message content, it is RECOMMENDED that SMTP receivers record


   each identity that was checked.  This information is intended for the
 
   the result of SPF processing in the message header.  For operators


   recipient.  (Information intended for the sender is described in
 
   that choose to record SPF results in the header of the message for


   Section 6.2, Explanation.)
 
   processing by internal filters or MUAs, two methods are presented.



 
   Section 7.1 defines the Received-SPF field, which is the results



 
   field originally defined for SPF use.  Section 7.2 discusses



 
   Authentication-Results [RFC5451] which was specified more recently



 
   and is designed for use by SPF and other authentication methods.



 








   The Received-SPF header field is a trace field (see [RFC2822] Section
 
   Both are in common use, and hence both are included here.  However,



 
   it is important to note that they were designed to serve slightly



 
   different purposes.  Received-SPF is intended to include enough



 
   forensic information to enable reconstruction of the SPF evaluation



 
   of the message, while Authentication-Results is designed only to



 
   relay the result itself and related output details of likely use to



 
   end users (e.g., what property of the message was actually



 
   authenticated and what it contained), leaving forensic work to the



 
   purview of system logs and the Received field contents.  Also,



 
   Received-SPF relies on compliance of agents within the receiving ADMD



 
   to adhere to the header field ordering rules of [RFC5321] and



 
   [RFC5322], while Authentication-Results includes some provisions to



 
   protect against non-compliant implementations.



 




 
   An operator could choose to use both to serve different downstream



 
   agents.  In such cases, care needs to be taken to ensure both fields



 
   are conveying the same details, or unexpected results can occur.



 




 
7.1.  The Received-SPF Header Field



 
                                                                         



 
   The Received-SPF header field is a trace field (see [RFC5322] Section


   3.6.7) and SHOULD be prepended to the existing header, above the
 
   3.6.7) and SHOULD be prepended to the existing header, above the


   Received: field that is generated by the SMTP receiver.  It MUST
 
   Received: field that is generated by the SMTP receiver.  It MUST


   appear above all other Received-SPF fields in the message.  The
 
   appear above all other Received-SPF fields in the message.  The


   header field has the following format:
 
   header field has the following format:



 



   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
 
   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]


                      [ key-value-list ] CRLF
 
                      [ key-value-list ] CRLF



 








   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
 
   result           = "pass" / "fail" / "softfail" / "neutral" /


                      "None" / "TempError" / "PermError"
 
                      "none" / "temperror" / "permerror"



 



   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
 
   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )


                      [";"]
 
                      [";"]



 



   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
 
   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )



 



   key              = "client-ip" / "envelope-from" / "helo" /
 
   key              = "client-ip" / "envelope-from" / "helo" /


                      "problem" / "receiver" / "identity" /
 
                      "problem" / "receiver" / "identity" /







                       mechanism / "x-" name / name
 
                       mechanism / name



 



   identity         = "mailfrom"   ; for the "MAIL FROM" identity
 
   identity         = "mailfrom"   ; for the "MAIL FROM" identity


                      / "helo"     ; for the "HELO" identity
 
                      / "helo"     ; for the "HELO" identity


                      / name       ; other identities
 
                      / name       ; other identities



 








   dot-atom         = <unquoted word as per [RFC2822]>
 
   dot-atom         = <unquoted word as per [RFC5322]>


   quoted-string    = <quoted string as per [RFC2822]>
 
   quoted-string    = <quoted string as per [RFC5322]>


   comment          = <comment string as per [RFC2822]>
 
   comment          = <comment string as per [RFC5322]>


   CFWS             = <comment or folding white space as per [RFC2822]>
 
   CFWS             = <comment or folding white space as per [RFC5322]>


   FWS              = <folding white space as per [RFC2822]>
 
   FWS              = <folding white space as per [RFC5322]>


   CRLF             = <standard end-of-line token as per [RFC2822]>
 
   CRLF             = <standard end-of-line token as per [RFC2532]>



 








   The header field SHOULD include a "(...)" style <comment> after the
 
   The header field SHOULD include a "(...)" style comment after the


   result, conveying supporting information for the result, such as
 
   result, conveying supporting information for the result, such as


   <ip>, <sender>, and <domain>.
 
   <ip>, <sender>, and <domain>.



 



   The following key-value pairs are designed for later machine parsing.
 
   The following key-value pairs are designed for later machine parsing.







   SPF clients SHOULD give enough information so that the SPF results
 
   SPF verifiers SHOULD give enough information so that the SPF results


   can be verified.  That is, at least "client-ip", "helo", and, if the
 
   can be verified.  That is, at least "client-ip", "helo", and, if the


   "MAIL FROM" identity was checked, "envelope-from".
 
   "MAIL FROM" identity was checked, "envelope-from".



 



   client-ip      the IP address of the SMTP client
 
   client-ip      the IP address of the SMTP client



 



   envelope-from  the envelope sender mailbox
 
   envelope-from  the envelope sender mailbox



 



   helo           the host name given in the HELO or EHLO command
 
   helo           the host name given in the HELO or EHLO command



 



   mechanism      the mechanism that matched (if no mechanisms matched,
 
   mechanism      the mechanism that matched (if no mechanisms matched,


                  substitute the word "default")
 
                  substitute the word "default")



 



   problem        if an error was returned, details about the error
 
   problem        if an error was returned, details about the error







                                                                         
 
   receiver       the host name of the SPF verifier


   receiver       the host name of the SPF client
 




 



   identity       the identity that was checked; see the <identity> ABNF
 
   identity       the identity that was checked; see the <identity> ABNF


                  rule
 
                  rule



 








   Other keys may be defined by SPF clients.  Until a new key name
 
   Other keys MAY be defined by SPF verifiers.


   becomes widely accepted, new key names should start with "x-".
 




 








   SPF clients MUST make sure that the Received-SPF header field does
 
   SPF verifiers MUST make sure that the Received-SPF header field does


   not contain invalid characters, is not excessively long, and does not
 
   not contain invalid characters, is not excessively long (See


   contain malicious data that has been provided by the sender.
 
   [RFC5322] Section 2.1.1), and does not contain malicious data that



 
   has been provided by the sender.



 








   Examples of various header styles that could be generated are the
 
   Examples of various header field styles that could be generated are


   following:
 
   the following:



 








   Received-SPF: Pass (mybox.example.org: domain of
 
   Received-SPF: pass (mybox.example.org: domain of


    [e-mail] designates 192.0.2.1 as permitted sender)
 
    [e-mail] designates 192.0.2.1 as permitted sender)


       receiver=mybox.example.org; client-ip=192.0.2.1;
 
       receiver=mybox.example.org; client-ip=192.0.2.1;







       envelope-from=<[e-mail]>; helo=foo.example.com;
 
       envelope-from="[e-mail]"; helo=foo.example.com;



 








   Received-SPF: Fail (mybox.example.org: domain of
 
   Received-SPF: fail (mybox.example.org: domain of


                     [e-mail] does not designate
 
                     [e-mail] does not designate


                     192.0.2.1 as permitted sender)
 
                     192.0.2.1 as permitted sender)


                     identity=mailfrom; client-ip=192.0.2.1;
 
                     identity=mailfrom; client-ip=192.0.2.1;







                     envelope-from=<[e-mail]>;
 
                     envelope-from="[e-mail]";



 




 
7.2.  SPF Results in the Authentication-Results Header Field



 




 
   As mentioned in Section 7, the Authentication-Results header field is



 
   designed to communicate lists of tests a border MTA did and their



 
   results.  The specified elements of the field provide less



 
   information than the SPF-Received field:



 




 
   Authentication-Results: myhost.example.org; spf=pass



 
     smtp.mailfrom=example.net



 




 
   Received-SPF: pass (myhost.example.org: domain of



 
    [e-mail] designates 192.0.2.1 as permitted sender)



 
       receiver=mybox.example.org; client-ip=192.0.2.1;



 
       envelope-from="[e-mail]"; helo=foo.example.com;



 




 
   It is, however, possible to add CFWS in the "reason" part of an



 
   Authentication-Results header field and provide the equivalent



 
   information, if desired.



 




 
   As an example, an expanded Authentication-Results header field might



 
   look like (for a "MAIL FROM" check in this example):



 




 
   Authentication-Results: myhost.example.org; spf=pass



 
     reason="client-ip=192.0.2.1; smtp.helo=foo.example.com"



 
     smtp.mailfrom=[e-mail]



 



8.  Macros
 
8.  Macros



 



8.1.  Macro Definitions
 
8.1.  Macro Definitions



 








   Many mechanisms and modifiers perform macro expansion on part of the
 
   Many mechanisms and modifiers perform macro expansion on a term.


   term.
 




 



   domain-spec      = macro-string domain-end
 
   domain-spec      = macro-string domain-end


   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
 
   domain-end       = ( "." toplabel [ "." ] ) / macro-expand



 



   toplabel         = ( *alphanum ALPHA *alphanum ) /
 
   toplabel         = ( *alphanum ALPHA *alphanum ) /


                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
 
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )


                      ; LDH rule plus additional TLD restrictions
 
                      ; LDH rule plus additional TLD restrictions







                      ; (see [RFC3696], Section 2)
 
                      ; (see [RFC3696], Section 2 for background)


   alphanum         = ALPHA / DIGIT
 
   alphanum         = ALPHA / DIGIT



 



   explain-string   = *( macro-string / SP )
 
   explain-string   = *( macro-string / SP )



 



   macro-string     = *( macro-expand / macro-literal )
 
   macro-string     = *( macro-expand / macro-literal )


   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
 
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )


                      / "%%" / "%_" / "%-"
 
                      / "%%" / "%_" / "%-"


   macro-literal    = %x21-24 / %x26-7E
 
   macro-literal    = %x21-24 / %x26-7E


                      ; visible characters except "%"
 
                      ; visible characters except "%"


   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
 
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /







                      "c" / "r" / "t"
 
                      "c" / "r" / "t" / "v"


   transformers     = *DIGIT [ "r" ]
 
   transformers     = *DIGIT [ "r" ]


   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="



 



   A literal "%" is expressed by "%%".
 
   A literal "%" is expressed by "%%".



 



      "%_" expands to a single " " space.
 
      "%_" expands to a single " " space.


      "%-" expands to a URL-encoded space, viz., "%20".
 
      "%-" expands to a URL-encoded space, viz., "%20".



 



   The following macro letters are expanded in term arguments:
 
   The following macro letters are expanded in term arguments:



 



      s = <sender>
 
      s = <sender>


      l = local-part of <sender>
 
      l = local-part of <sender>


      o = domain of <sender>
 
      o = domain of <sender>


      d = <domain>
 
      d = <domain>


      i = <ip>
 
      i = <ip>







      p = the validated domain name of <ip>
 
      p = the validated domain name of <ip> (deprecated)


      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
 
      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6


      h = HELO/EHLO domain
 
      h = HELO/EHLO domain



 



   The following macro letters are allowed only in "exp" text:
 
   The following macro letters are allowed only in "exp" text:



 



      c = SMTP client IP (easily readable format)
 
      c = SMTP client IP (easily readable format)


      r = domain name of host performing the check
 
      r = domain name of host performing the check


      t = current timestamp
 
      t = current timestamp



 



   A '%' character not followed by a '{', '%', '-', or '_' character is
 
   A '%' character not followed by a '{', '%', '-', or '_' character is



 



skipping to change at page 28, line 13

skipping to change at page 37, line 7


      h = HELO/EHLO domain
 
      h = HELO/EHLO domain



 



   The following macro letters are allowed only in "exp" text:
 
   The following macro letters are allowed only in "exp" text:



 



      c = SMTP client IP (easily readable format)
 
      c = SMTP client IP (easily readable format)


      r = domain name of host performing the check
 
      r = domain name of host performing the check


      t = current timestamp
 
      t = current timestamp



 



   A '%' character not followed by a '{', '%', '-', or '_' character is
 
   A '%' character not followed by a '{', '%', '-', or '_' character is


   a syntax error.  So
 
   a syntax error.  So







                                                                         
 



      -exists:%(ir).sbl.spamhaus.example.org
 
      -exists:%(ir).sbl.spamhaus.example.org







                                                                         
 
   is incorrect and will cause check_host() to yield a "permerror".


   is incorrect and will cause check_host() to return a "PermError".
 



   Instead, say
 
   Instead, say







                                                                         
 



      -exists:%{ir}.sbl.spamhaus.example.org
 
      -exists:%{ir}.sbl.spamhaus.example.org



 



   Optional transformers are the following:
 
   Optional transformers are the following:



 



      *DIGIT = zero or more digits
 
      *DIGIT = zero or more digits


      'r'    = reverse value, splitting on dots by default
 
      'r'    = reverse value, splitting on dots by default



 



   If transformers or delimiters are provided, the replacement value for
 
   If transformers or delimiters are provided, the replacement value for


   a macro letter is split into parts.  After performing any reversal
 
   a macro letter is split into parts.  After performing any reversal


   operation and/or removal of left-hand parts, the parts are rejoined
 
   operation and/or removal of left-hand parts, the parts are rejoined



 



skipping to change at page 28, line 32

skipping to change at page 37, line 23



 



      *DIGIT = zero or more digits
 
      *DIGIT = zero or more digits


      'r'    = reverse value, splitting on dots by default
 
      'r'    = reverse value, splitting on dots by default



 



   If transformers or delimiters are provided, the replacement value for
 
   If transformers or delimiters are provided, the replacement value for


   a macro letter is split into parts.  After performing any reversal
 
   a macro letter is split into parts.  After performing any reversal


   operation and/or removal of left-hand parts, the parts are rejoined
 
   operation and/or removal of left-hand parts, the parts are rejoined


   using "." and not the original splitting characters.
 
   using "." and not the original splitting characters.



 



   By default, strings are split on "." (dots).  Note that no special
 
   By default, strings are split on "." (dots).  Note that no special







   treatment is given to leading, trailing, or consecutive delimiters,
 
   treatment is given to leading, trailing, or consecutive delimiters in


   and so the list of parts may contain empty strings.  Older
 
   input strings, and so the list of parts might contain empty strings.


   implementations of SPF prohibit trailing dots in domain names, so
 
   Some older implementations of SPF prohibit trailing dots in domain


   trailing dots should not be published by domain owners, although they
 
   names, so trailing dots SHOULD NOT be published by domain owners,


   must be accepted by implementations conforming to this document.
 
   although they MUST be accepted by implementations conforming to this


   Macros may specify delimiter characters that are used instead of ".".
 
   document.  Macros can specify delimiter characters that are used



 
   instead of ".".



 



   The 'r' transformer indicates a reversal operation: if the client IP
 
   The 'r' transformer indicates a reversal operation: if the client IP


   address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
 
   address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"


   and the macro %{ir} would expand to "1.2.0.192".
 
   and the macro %{ir} would expand to "1.2.0.192".



 



   The DIGIT transformer indicates the number of right-hand parts to
 
   The DIGIT transformer indicates the number of right-hand parts to


   use, after optional reversal.  If a DIGIT is specified, the value
 
   use, after optional reversal.  If a DIGIT is specified, the value


   MUST be nonzero.  If no DIGITs are specified, or if the value
 
   MUST be nonzero.  If no DIGITs are specified, or if the value


   specifies more parts than are available, all the available parts are
 
   specifies more parts than are available, all the available parts are


   used.  If the DIGIT was 5, and only 3 parts were available, the macro
 
   used.  If the DIGIT was 5, and only 3 parts were available, the macro


   interpreter would pretend the DIGIT was 3.  Implementations MUST
 
   interpreter would pretend the DIGIT was 3.  Implementations MUST


   support at least a value of 128, as that is the maximum number of
 
   support at least a value of 128, as that is the maximum number of


   labels in a domain name.
 
   labels in a domain name.



 








   The "s" macro expands to the <sender> argument.  It is an E-Mail
 
   The "s" macro expands to the <sender> argument.  It is an email


   address with a localpart, an "@" character, and a domain.  The "l"
 
   address with a local-part, an "@" character, and a domain.  The "l"


   macro expands to just the localpart.  The "o" macro expands to just
 
   macro expands to just the local-part.  The "o" macro expands to just


   the domain part.  Note that these values remain the same during
 
   the domain part.  Note that these values remain the same during


   recursive and chained evaluations due to "include" and/or "redirect".
 
   recursive and chained evaluations due to "include" and/or "redirect".







   Note also that if the original <sender> had no localpart, the
 
   Note also that if the original <sender> had no local-part, the local-


   localpart was set to "postmaster" in initial processing (see Section
 
   part was set to "postmaster" in initial processing (see Section 4.3).


   4.3).
 




 



   For IPv4 addresses, both the "i" and "c" macros expand to the
 
   For IPv4 addresses, both the "i" and "c" macros expand to the


   standard dotted-quad format.
 
   standard dotted-quad format.



 



   For IPv6 addresses, the "i" macro expands to a dot-format address; it
 
   For IPv6 addresses, the "i" macro expands to a dot-format address; it







   is intended for use in %{ir}.  The "c" macro may expand to any of the
 
   is intended for use in %{ir}.  The "c" macro can expand to any of the


   hexadecimal colon-format addresses specified in [RFC3513], Section
 
   hexadecimal colon-format addresses specified in [RFC4291], Section


   2.2.  It is intended for humans to read.
 
   2.2.  It is intended for humans to read.



 



   The "p" macro expands to the validated domain name of <ip>.  The
 
   The "p" macro expands to the validated domain name of <ip>.  The







   procedure for finding the validated domain name is defined in Section
 
   procedure for finding the validated domain name is defined in


   5.5.  If the <domain> is present in the list of validated domains, it
 
   Section 5.5.  If the <domain> is present in the list of validated


   SHOULD be used.  Otherwise, if a subdomain of the <domain> is
 
   domains, it SHOULD be used.  Otherwise, if a subdomain of the


   present, it SHOULD be used.  Otherwise, any name from the list may be
 
   <domain> is present, it SHOULD be used.  Otherwise, any name from the


   used.  If there are no validated domain names or if a DNS error
 
   list can be used.  If there are no validated domain names or if a DNS


   occurs, the string "unknown" is used.
 
   error occurs, the string "unknown" is used.  This macro is deprecated



 
   and SHOULD NOT be used.



 



   The "r" macro expands to the name of the receiving MTA.  This SHOULD
 
   The "r" macro expands to the name of the receiving MTA.  This SHOULD


   be a fully qualified domain name, but if one does not exist (as when
 
   be a fully qualified domain name, but if one does not exist (as when


   the checking is done by a MUA) or if policy restrictions dictate
 
   the checking is done by a MUA) or if policy restrictions dictate


   otherwise, the word "unknown" SHOULD be substituted.  The domain name
 
   otherwise, the word "unknown" SHOULD be substituted.  The domain name







   may be different from the name found in the MX record that the client
 
   can be different from the name found in the MX record that the client


   MTA used to locate the receiving MTA.
 
   MTA used to locate the receiving MTA.



 



   The "t" macro expands to the decimal representation of the
 
   The "t" macro expands to the decimal representation of the


   approximate number of seconds since the Epoch (Midnight, January 1,
 
   approximate number of seconds since the Epoch (Midnight, January 1,







   1970, UTC).  This is the same value as is returned by the POSIX
 
   1970, UTC) at the time of the evaluation.  This is the same value as


   time() function in most standards-compliant libraries.
 
   is returned by the POSIX time() function in most standards-compliant



 
   libraries.



 



   When the result of macro expansion is used in a domain name query, if
 
   When the result of macro expansion is used in a domain name query, if


   the expanded domain name exceeds 253 characters (the maximum length
 
   the expanded domain name exceeds 253 characters (the maximum length


   of a domain name), the left side is truncated to fit, by removing
 
   of a domain name), the left side is truncated to fit, by removing







   successive domain labels until the total length does not exceed 253
 
   successive domain labels (and their following dots) until the total


   characters.
 
   length does not exceed 253 characters.



 



   Uppercased macros expand exactly as their lowercased equivalents, and
 
   Uppercased macros expand exactly as their lowercased equivalents, and







   are then URL escaped.  URL escaping must be performed for characters
 
   are then URL escaped.  URL escaping MUST be performed for characters


   not in the "uric" set, which is defined in [RFC3986].
 
   not in the "unreserved" set, which is defined in [RFC3986].



 








   Note: Care must be taken so that macro expansion for legitimate
 
   Note: Care has to be taken so that macro expansion for legitimate


   E-Mail does not exceed the 63-character limit on DNS labels.  The
 
   email does not exceed the 63-character limit on DNS labels.  The


   localpart of E-Mail addresses, in particular, can have more than 63
 
   local-part of email addresses, in particular, can have more than 63


   characters between dots.
 
   characters between dots.



 








   Note: Domains should avoid using the "s", "l", "o", or "h" macros in
 
   Note: Domains SHOULD avoid using the "s", "l", "o", or "h" macros in


   conjunction with any mechanism directive.  Although these macros are
 
   conjunction with any mechanism directive.  Although these macros are


   powerful and allow per-user records to be published, they severely
 
   powerful and allow per-user records to be published, they severely


   limit the ability of implementations to cache results of check_host()
 
   limit the ability of implementations to cache results of check_host()


   and they reduce the effectiveness of DNS caches.
 
   and they reduce the effectiveness of DNS caches.



 








   Implementations should be aware that if no directive processed during
 
   Note: If no directive processed during the evaluation of check_host()


   the evaluation of check_host() contains an "s", "l", "o", or "h"
 
   contains an "s", "l", "o", or "h" macro, then the results of the


   macro, then the results of the evaluation can be cached on the basis
 
   evaluation can be cached on the basis of <domain> and <ip> alone for


   of <domain> and <ip> alone for as long as the shortest Time To Live
 
   as long as the shortest Time To Live (TTL) of all the DNS records


   (TTL) of all the DNS records involved.
 
   involved.



 



8.2.  Expansion Examples
 
8.2.  Expansion Examples



 



      The <sender> is [e-mail].
 
      The <sender> is [e-mail].


      The IPv4 SMTP client IP is 192.0.2.3.
 
      The IPv4 SMTP client IP is 192.0.2.3.


      The IPv6 SMTP client IP is 2001:DB8::CB01.
 
      The IPv6 SMTP client IP is 2001:DB8::CB01.


      The PTR domain name of the client IP is mx.example.org.
 
      The PTR domain name of the client IP is mx.example.org.



 



   macro                       expansion
 
   macro                       expansion


   -------  ----------------------------
 
   -------  ----------------------------



 



skipping to change at page 31, line 20

skipping to change at page 40, line 4


                       bad.strong.lp.3.2.0.192.in-addr._spf.example.com
 
                       bad.strong.lp.3.2.0.192.in-addr._spf.example.com



 



   %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
 
   %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}


                           3.2.0.192.in-addr.strong.lp._spf.example.com
 
                           3.2.0.192.in-addr.strong.lp._spf.example.com



 



   %{d2}.trusted-domains.example.net
 
   %{d2}.trusted-domains.example.net


                                example.com.trusted-domains.example.net
 
                                example.com.trusted-domains.example.net



 



   IPv6:
 
   IPv6:


   %{ir}.%{v}._spf.%{d2}                               1.0.B.C.0.0.0.0.
 
   %{ir}.%{v}._spf.%{d2}                               1.0.B.C.0.0.0.0.








 
                                                                         


   0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com
 
   0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com



 



9.  Implications
 
9.  Implications



 



   This section outlines the major implications that adoption of this
 
   This section outlines the major implications that adoption of this







   document will have on various entities involved in Internet E-Mail.
 
   document will have on various entities involved in Internet email.


   It is intended to make clear to the reader where this document
 
   It is intended to make clear to the reader where this document


   knowingly affects the operation of such entities.  This section is
 
   knowingly affects the operation of such entities.  This section is


   not a "how-to" manual, or a "best practices" document, and it is not
 
   not a "how-to" manual, or a "best practices" document, and it is not







   a comprehensive list of what such entities should do in light of this
 
   a comprehensive list of what such entities SHOULD do in light of this


   document.
 
   document.



 








   This section is non-normative.
 
   This section is non-normative.  [RFC5598] describes the Internet



 
   email architecture.  This section is organized based on the different



 
   segments of the architecture.



 



9.1.  Sending Domains
 
9.1.  Sending Domains



 








   Domains that wish to be compliant with this specification will need
 
   Originating ADMDs (ADministrative Management Domains - [RFC5598]


   to determine the list of hosts that they allow to use their domain
 
   Section 2.2.1 and Section 2.3) that wish to be compliant with this


   name in the "HELO" and "MAIL FROM" identities.  It is recognized that
 
   specification will need to determine the list of relays ([RFC5598]


   forming such a list is not just a simple technical exercise, but
 
   Section 2.2.2) that they allow to use their domain name in the "HELO"


   involves policy decisions with both technical and administrative
 
   and "MAIL FROM" identities when relaying to other ADMDs.  It is


   considerations.
 
   recognized that forming such a list is not just a simple technical



 
   exercise, but involves policy decisions with both technical and



 
   administrative considerations.



 








   It can be helpful to publish records that include a "tracking
 
9.1.1.  DNS Resource Considerations



 




 
   Minimizing the DNS resources required for SPF lookups can be done by



 
   choosing directives that require less DNS information and by placing



 
   lower-cost mechanisms earlier in the SPF record.



 




 
             +----------+--------+-----------------+



 
             | term     | cost   | limit           |



 
             +----------+--------+-----------------+



 
             | ip4/ip6  | 0      | -               |



 
             | a        | 1      | 10              |



 
             | include  | 1      | 10              |



 
             | redirect | 1      | 10              |



 
             | exists   | 1      | 10              |



 
             | mx       | 1 + N* | 10 and N* <= 10 |



 
             | ptr/%{p} | 1 + N* | 10 and N* <= 10 |



 
             | all      | 0      | -               |



 
             +----------+--------+-----------------+



 
              * N is the number of RRs found during each term evaluation



 




 
   Section 4.6.4 specifies the limits receivers have to use.  It is



 
   essential to publish records that do not exceed these requirements.



 
   It is also required to carefully weight the cost and the



 
   maintainability of licit solutions.



 




 
   For example, consider a domain set up as follows:



 




 
      example.com.     IN MX   10 mx.example.com.



 
                       IN MX   20 mx2.example.com.



 
      mx.example.com.  IN A    192.0.2.1



 
      mx2.example.com. IN A    192.0.2.129



 




 
   Assume the administrative point is to authorize (pass) mx and mx2



 
   while failing every other host.  Compare the following solutions:



 




 
   Best record:



 
      example.com.   IN TXT  "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all"



 




 
   Good record:



 
      $ORIGIN example.com.



 
      @              IN TXT  "v=spf1 a:authorized-spf.example.com -all"



 
      authorized-spf IN A    192.0.2.1



 
                     IN A    192.0.2.129



 




 
   Expensive record:



 
      example.com.   IN TXT  "v=spf1 mx:example.com -all"



 




 
   Wasteful, bad record:



 
      example.com.   IN TXT  "v=spf1 ip4:192.0.2.0/24 mx -all"



 




 
9.1.2.  Administrator's Considerations



 




 
   There might be administrative considerations: using "a" over "ip4" or



 
   "ip6" allows hosts to be renumbered easily.  Using "mx" over "a"



 
   allows the set of mail hosts to be changed easily.  Unless such



 
   changes are common, it is better to use the less resource intensive



 
   mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".



 




 
   In some specific cases, standard advice on record content is



 
   appropriate.  Publishing SPF records for domains that send no mail is



 
   a well established best practice.  The record for a domain that sends



 
   no mail is:



 




 
      www.example.com.   IN TXT  "v=spf1 -all"



 




 
   Publishing SPF records for individual hosts is also best practice.



 
   The hostname is generally the identity used in the 5321.HELO/.EHLO



 
   command.  In the case of messages with a null 5321.MailFrom, this is



 
   used as the domain for 5321.MailFrom SPF checks, in addition to being



 
   used in 5321.HELO/.EHLO based SPF checks.  The standard SPF record



 
   for an individual host that is involved in mail processing is:



 




 
      relay.example.com.   IN TXT  "v=spf1 a -all"



 




 
   Validating correct deployment is difficult.  [RFC6652] describes one



 
   mechanism for soliciting feedback on SPF failures.  Another approach



 
   that can be helpful to publish records that include a "tracking


   exists:" mechanism.  By looking at the name server logs, a rough list
 
   exists:" mechanism.  By looking at the name server logs, a rough list







   may then be generated.  For example:
 
   can then be generated.  For example:



 



      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
 
      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all



 








9.2.  Mailing Lists
 
   Regardless of the method used, understanding the ADMD's outbound mail



 
   architecture is essential to effective deployment.



 








   Mailing lists must be aware of how they re-inject mail that is sent
 
9.1.3.  Bounces


   to the list.  Mailing lists MUST comply with the requirements in
 



   [RFC2821], Section 3.10, and [RFC1123], Section 5.3.6, that say that
 
   As explained in Section 1.3.3, [RFC5321] allows the reverse-path to



 
   be null, which is typical of some Delivery Status Notification



 
   [RFC3464], commonly called email bounces.  In this case the only



 
   entity available for performing an SPF check is the "HELO" identity



 
   defined in Section 1.3.4.  SPF functionality is enhanced by



 
   administrators ensuring this identity is set correctly and has an



 
   appropriate SPF record.  It is normal to have the HELO identity set



 
   to hostname instead of domain.  Zone file generation for significant



 
   numbers of hosts can be consolidated using the redirect modifier and



 
   scripted for initial deployment.  Specific deployment advice is given



 
   above in Section 9.1.2.



 




 
9.2.  Mediators



 




 
   Broadly speaking, there are two types of mediating ADMDs that can



 
   affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]



 
   Section 5.3) and ReSenders ([RFC5598] Section 5.2).



 




 
9.2.1.  Mailing Lists



 
                                                                         



 
   Mailing lists have to be aware of how they re-inject mail that is



 
   sent to the list.  Mailing lists MUST comply with the requirements in



 
   [RFC5321], Section 3.10, and [RFC1123], Section 5.3.6, that say that


   the reverse-path MUST be changed to be the mailbox of a person or
 
   the reverse-path MUST be changed to be the mailbox of a person or


   other entity who administers the list.  Whereas the reasons for
 
   other entity who administers the list.  Whereas the reasons for


   changing the reverse-path are many and long-standing, SPF adds
 
   changing the reverse-path are many and long-standing, SPF adds


   enforcement to this requirement.
 
   enforcement to this requirement.



 



   In practice, almost all mailing list software in use already complies
 
   In practice, almost all mailing list software in use already complies







   with this requirement.  Mailing lists that do not comply may or may
 
   with this requirement.  Mailing lists that do not comply might


   not encounter problems depending on how access to the list is
 
   encounter problems depending on how access to the list is restricted.


   restricted.  Such lists that are entirely internal to a domain (only
 
   Such lists that are entirely internal to a domain (only people in the


   people in the domain can send to or receive from the list) are not
 
   domain can send to or receive from the list) are not affected.


   affected.
 




 








9.3.  Forwarding Services and Aliases
 
9.2.2.  Forwarding Services and Aliases



 



   Forwarding services take mail that is received at a mailbox and
 
   Forwarding services take mail that is received at a mailbox and


   direct it to some external mailbox.  At the time of this writing, the
 
   direct it to some external mailbox.  At the time of this writing, the


   near-universal practice of such services is to use the original "MAIL
 
   near-universal practice of such services is to use the original "MAIL


   FROM" of a message when re-injecting it for delivery to the external
 
   FROM" of a message when re-injecting it for delivery to the external







   mailbox.  [RFC1123] and [RFC2821] describe this action as an "alias"
 
   mailbox.  [RFC1123] and [RFC5321] describe this action as an "alias"


   rather than a "mail list".  This means that the external mailbox's
 
   rather than a "mail list".  This means the external mailbox's MTA


   MTA sees all such mail in a connection from a host of the forwarding
 
   sees all such mail in a connection from a host of the forwarding


   service, and so the "MAIL FROM" identity will not, in general, pass
 
   service, and so the "MAIL FROM" identity will not, in general, pass


   authorization.
 
   authorization.



 



   There are three places that techniques can be used to ameliorate this
 
   There are three places that techniques can be used to ameliorate this


   problem.
 
   problem.



 








   1. The beginning, when E-Mail is first sent.
 
   1.  The beginning, when email is first sent (Originating ADMDs).



 








       1. "Neutral" results could be given for IP addresses that may be
 
       1.  "Neutral" results could be given for IP addresses that might


          forwarders, instead of "Fail" results.  For example:
 
           be forwarders, instead of "fail" results based on a list of



 
           known reliable forwarders.  For example:



 








             "v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all"
 
              "v=spf1 mx ?exists:%{ir}.whitlist.example.org -all"



 








          This would cause a lookup on an anti-spam DNS blacklist
 
           This would cause a lookup on an DNS white list (DNSWL) and


          (DNSBL) and cause a result of "Fail" only for E-Mail coming
 
           cause a result of "fail" only for email not either coming


          from listed sources.  All other E-Mail, including E-Mail sent
 
           from the domain's mx host(s) (SPF pass) or white listed


          through forwarders, would receive a "Neutral" result.  By
 
           sources (SPF neutral).  This, in effect, outsources an


          checking the DNSBL after the known good sources, problems with
 
           element of sender policy to the maintainer of the whitelist.


          incorrect listing on the DNSBL are greatly reduced.
 




 








       2. The "MAIL FROM" identity could have additional information in
 
       2.  The "MAIL FROM" identity could have additional information in


          the localpart that cryptographically identifies the mail as
 
           the local-part that cryptographically identifies the mail as


          coming from an authorized source.  In this case, such an SPF
 
           coming from an authorized source.  In this case, such an SPF


          record could be used:
 
           record could be used:



 








             "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"
 
              "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"



 








          Then, a specialized DNS server can be set up to serve the
 
           Then, a specialized DNS server can be set up to serve the


          _spf_verify subdomain that validates the localpart.  Although
 
           _spf_verify subdomain that validates the local-part.


          this requires an extra DNS lookup, this happens only when the
 
           Although this requires an extra DNS lookup, this happens only


          E-Mail would otherwise be rejected as not coming from a known
 
           when the email would otherwise be rejected as not coming from


          good source.
 
           a known good source.



 
           Note that due to the 63-character limit for domain labels,



 
           this approach only works reliably if the local-part signature



 
           scheme is guaranteed either to only produce local-parts with



 
           a maximum of 63 characters or to gracefully handle truncated



 
           local-parts.



 








          Note that due to the 63-character limit for domain labels,
 
       3.  Similarly, a specialized DNS server could be set up that will


          this approach only works reliably if the localpart signature
 
           rate-limit the email coming from unexpected IP addresses.


          scheme is guaranteed either to only produce localparts with a
 



          maximum of 63 characters or to gracefully handle truncated
 



          localparts.
 




 








       3. Similarly, a specialized DNS server could be set up that will
 
              "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"


          rate-limit the E-Mail coming from unexpected IP addresses.
 




 








             "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"
 
       4.  SPF allows the creation of per-user policies for special



 
           cases.  For example, the following SPF record and appropriate



 
           wildcard DNS records can be used:



 








       4. SPF allows the creation of per-user policies for special
 
              "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"


          cases.  For example, the following SPF record and appropriate
 



          wildcard DNS records can be used:
 




 








                 "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
 
   2.  The middle, when email is forwarded (Mediating ADMDs).



 








   2.  The middle, when E-Mail is forwarded.
 
       1.  Forwarding services can solve the problem by rewriting the



 
           "MAIL FROM" to be in their own domain.  This means mail



 
           rejected from the external mailbox will have to be forwarded



 
           back to the original sender by the forwarding service.



 
           Various schemes to do this exist though they vary widely in



 
           complexity and resource requirements on the part of the



 
           forwarding service.



 








       1. Forwarding services can solve the problem by rewriting the
 
       2.  Several popular MTAs can be forced from "alias" semantics to


          "MAIL FROM" to be in their own domain.  This means that mail
 
           "mailing list" semantics by configuring an additional alias


          bounced from the external mailbox will have to be re-bounced
 
           with "owner-" prepended to the original alias name (e.g., an


          by the forwarding service.  Various schemes to do this exist
 
           alias of "friends: [e-mail], [e-mail]"


          though they vary widely in complexity and resource
 
           would need another alias of the form "owner-friends:


          requirements on the part of the forwarding service.
 
           localowner").



 








       2. Several popular MTAs can be forced from "alias" semantics to
 
       3.  Forwarding servers could reject mail that would "fail" SPF if


          "mailing list" semantics by configuring an additional alias
 
           forwarded using an SMTP reply code of 551, User not local,


          with "owner-" prepended to the original alias name (e.g., an
 
           (see [RFC5321] section 3.4) to communicate the correct target


          alias of "friends: [e-mail], [e-mail]" would
 
           address to resend the mail to.


          need another alias of the form "owner-friends:  localowner").
 




 








   3. The end, when E-Mail is received.
 
   3.  The end, when email is received (Receiving ADMDs).



 








       1. If the owner of the external mailbox wishes to trust the
 
       1.  If the owner of the external mailbox wishes to trust the


          forwarding service, he can direct the external mailbox's MTA
 
           forwarding service, he can direct the external mailbox's MTA


          to skip SPF tests when the client host belongs to the
 
           to skip SPF tests when the client host belongs to the


          forwarding service.
 
           forwarding service.



 








       2. Tests against other identities, such as the "HELO" identity,
 
       2.  Tests against other identities, such as the "HELO" identity,


          may be used to override a failed test against the "MAIL FROM"
 
           MAY be used to override a failed test against the "MAIL FROM"


          identity.
 
           identity.



 








       3. For larger domains, it may not be possible to have a complete
 
       3.  For larger domains, it might not be possible to have a


          or accurate list of forwarding services used by the owners of
 
           complete or accurate list of forwarding services used by the


          the domain's mailboxes.  In such cases, whitelists of
 
           owners of the domain's mailboxes.  In such cases, whitelists


          generally-recognized forwarding services could be employed.
 
           of generally-recognized forwarding services could be



 
           employed.



 








9.4.  Mail Services
 
9.2.3.  Mail Services



 








   Service providers that offer mail services to third-party domains,
 
   MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail


   such as sending of bulk mail, may want to adjust their setup in light
 
   services to third-party domains, such as sending of bulk mail, might


   of the authorization check described in this document.  If the "MAIL
 
   want to adjust their configurations in light of the authorization


   FROM" identity used for such E-Mail uses the domain of the service
 
   check described in this document.  If the domain part of the "MAIL


   provider, then the provider needs only to ensure that its sending
 
   FROM" identity used for such email uses the domain of one of the MSPs


   host is authorized by its own SPF record, if any.
 
   domain, then the provider needs only to ensure that its sending host



 
   is authorized by its own SPF record, if any.



 








   If the "MAIL FROM" identity does not use the mail service provider's
 
   If the "MAIL FROM" identity does not use the MSP's domain, then extra


   domain, then extra care must be taken.  The SPF record format has
 
   care has to be taken.  The SPF record format has several options for


   several options for the third-party domain to authorize the service
 
   the third-party domain to authorize the service provider's MTAs to


   provider's MTAs to send mail on its behalf.  For mail service
 
   send mail on its behalf.  For MSPs, such as ISPs, that have a wide


   providers, such as ISPs, that have a wide variety of customers using
 
   variety of customers using the same MTA, steps are required to


   the same MTA, steps should be taken to prevent cross-customer forgery
 
   mitiate the risk of cross-customer forgery (see Section 10.4).


   (see Section 10.4).
 




 








9.5.  MTA Relays
 
9.2.4.  MTA Relays



 








   The authorization check generally precludes the use of arbitrary MTA
 
   Relays are described in [RFC5598] Section 2.2.2.  The authorization


   relays between sender and receiver of an E-Mail message.
 
   check generally precludes the use of arbitrary MTA relays between



 
   sender and receiver of an email message.



 



   Within an organization, MTA relays can be effectively deployed.
 
   Within an organization, MTA relays can be effectively deployed.


   However, for purposes of this document, such relays are effectively
 
   However, for purposes of this document, such relays are effectively


   transparent.  The SPF authorization check is a check between border
 
   transparent.  The SPF authorization check is a check between border







   MTAs of different domains.
 
   MTAs of different ADMDs.



 








   For mail senders, this means that published SPF records must
 
   For mail senders, this means that published SPF records have to


   authorize any MTAs that actually send across the Internet.  Usually,
 
   authorize any MTAs that actually send across the Internet.  Usually,


   these are just the border MTAs as internal MTAs simply forward mail
 
   these are just the border MTAs as internal MTAs simply forward mail







   to these MTAs for delivery.
 
   to these MTAs for relaying.



 








   Mail receivers will generally want to perform the authorization check
 
   The receiving ADMD will generally want to perform the authorization


   at the border MTAs, specifically including all secondary MXs.  This
 
   check at the boundary MTAs, including all secondary MXs.  Internal


   allows mail that fails to be rejected during the SMTP session rather
 
   MTAs (including MTAs that might serve both as boundary MTAs and


   than bounced.  Internal MTAs then do not perform the authorization
 
   internal relays from secondary MXs when they are processing the


   test.  To perform the authorization test other than at the border,
 
   relayed mail stream) then do not perform the authorization test.  To


   the host that first transferred the message to the organization must
 
   perform the authorization test other than at the boundary, the host


   be determined, which can be difficult to extract from the message
 
   that first transferred the message to the receiving ADMD have to be


   header.  Testing other than at the border is not recommended.
 
   determined, which can be difficult to extract from the message header



 
   because (a) header fields can be forged or malformed, and (b) there's



 
   no standard way to encode that information such that it can be



 
   reliably extracted.  Testing other than at the boundary is likely to



 
   produce unreliable results.



 




 
9.3.  Receivers



 




 
   SPF results can be used in combination with other methods to



 
   determine the final local disposition (either positive or negative of



 
   a message.  It can also be considered dispositive on its own.



 




 
9.3.1.  Policy For SPF Pass



 




 
   SPF pass results can be used in combination with "white lists" of



 
   known "good" domains to bypass some or all additional pre-delivery



 
   email checks.  Exactly which checks and how to determine appropriate



 
   white list entries has to be based on local conditions and



 
   requirements.



 




 
9.3.2.  Policy For SPF Fail



 




 
   SPF fail results can be used to reject messages during the SMTP



 
   transaction based on either "MAIL FROM" or "HELO" identity results.



 
   This reduces resource requirements for various content filtering



 
   methods and conserves bandwidth since rejection can be done before



 
   the SMTP content is transferred.  It also gives immediate feedback to



 
   the sender who might then be able to resolve the issue.  Due to some



 
   of the issues described above in this section (Section 9), SPF based



 
   rejection does present some risk of rejecting legitimate email when



 
   rejecting based on "MAIL FROM" results.



 




 
   SPF fail results can alternately be used as one input into a larger



 
   set of evaluations which might, based on a combination with other



 
   evaluation techniques, result in the email being marked negatively in



 
   some way (this might be via delivery to a special spam folder,



 
   modifying subject lines, or other locally determined means).



 
   Developing the details of such an approach have to be based on local



 
   conditions and requirements.  Using SPF results in this way does not



 
   have the advantages of resource conservation and immediate feedback



 
   to the sender associated with SMTP rejection, but could produce fewer



 
   undesirable rejections in a well designed system.  Such an approach



 
   might result in email that was not authorized by the sending ADMD



 
   being unknowingly delivered to end users.



 




 
   Either general approach can be used as they both leave a clear



 
   disposition of emails.  They are either delivered in some manner or



 
   the sender is notified of the failure.  Other dispositions such as



 
   "dropping" or deleting email after acceptance are inappropriate



 
   because they leave uncertainty and reduce the overall reliabilility



 
   and utility of email across the Internet.



 




 
9.3.3.  Policy For SPF Permerror



 




 
   The "permerror" result (see Section 2.5.7) indicates the SPF



 
   processing module at the receiver determined that the retrieved SPF



 
   policy record could not be interpreted.  This gives no true



 
   indication about the authorized use of the data found in the



 
   envelope.



 




 
   As with all results, implementers have a choice to make regarding



 
   what to do with a message that yields this result.  SMTP allows only



 
   a few basic options.



 




 
   Rejection of the message is an option, in that it is the one thing a



 
   receiver can do to draw attention to the difficulty encountered while



 
   protecting itself from messages that do not have a definite SPF



 
   result of some kind.  However, if the SPF implementation is defective



 
   and returns spurious "permerror" results, only the sender is actively



 
   notified of the defect (in the form of rejected mail), and not the



 
   receiver making use of SPF.



 




 
   The less intrusive handling choice is to deliver the message, perhaps



 
   with some kind of annotation of the difficulty encountered and/or



 
   logging of a similar nature.  However, this will not be desirable to



 
   operators that wish to implement SPF checking as strictly as



 
   possible, nor is this sort of passive problem reporting typically



 
   effective.



 




 
   There is of course the option placing this choice in the hands of the



 
   operator rather than the implementer since this kind of choice is



 
   often a matter of local policy rather than a condition with a



 
   universal solution, but this adds one more piece of complexity to an



 
   already non-trivial environment.



 




 
   Both implementers and operators need to be cautious of all choices



 
   and outcomes when handling SPF results.



 



10.  Security Considerations
 
10.  Security Considerations



 



10.1.  Processing Limits
 
10.1.  Processing Limits



 








   As with most aspects of E-Mail, there are a number of ways that
 
   As with most aspects of email, there are a number of ways that


   malicious parties could use the protocol as an avenue for a
 
   malicious parties could use the protocol as an avenue for a







   Denial-of-Service (DoS) attack.  The processing limits outlined here
 
   Denial-of-Service (DoS) attack.  The processing limits outlined in


   are designed to prevent attacks such as the following:
 
   Section 4.6.4 are designed to prevent attacks such as the following:



 



   o  A malicious party could create an SPF record with many references
 
   o  A malicious party could create an SPF record with many references







      to a victim's domain and send many E-Mails to different SPF
 
      to a victim's domain and send many emails to different SPF


      clients; those SPF clients would then create a DoS attack.  In
 
      verifiers; those SPF verifiers would then create a DoS attack.  In


      effect, the SPF clients are being used to amplify the attacker's
 
      effect, the SPF verifiers are being used to amplify the attacker's


      bandwidth by using fewer bytes in the SMTP session than are used
 
      bandwidth by using fewer bytes in the SMTP session than are used


      by the DNS queries.  Using SPF clients also allows the attacker to
 
      by the DNS queries.  Using SPF clients also allows the attacker to


      hide the true source of the attack.
 
      hide the true source of the attack.



 



   o  Whereas implementations of check_host() are supposed to limit the
 
   o  Whereas implementations of check_host() are supposed to limit the


      number of DNS lookups, malicious domains could publish records
 
      number of DNS lookups, malicious domains could publish records


      that exceed these limits in an attempt to waste computation effort
 
      that exceed these limits in an attempt to waste computation effort


      at their targets when they send them mail.  Malicious domains
 
      at their targets when they send them mail.  Malicious domains


      could also design SPF records that cause particular
 
      could also design SPF records that cause particular


      implementations to use excessive memory or CPU usage, or to
 
      implementations to use excessive memory or CPU usage, or to


      trigger bugs.
 
      trigger bugs.



 



   o  Malicious parties could send a large volume of mail purporting to
 
   o  Malicious parties could send a large volume of mail purporting to


      come from the intended target to a wide variety of legitimate mail
 
      come from the intended target to a wide variety of legitimate mail


      hosts.  These legitimate machines would then present a DNS load on
 
      hosts.  These legitimate machines would then present a DNS load on


      the target as they fetched the relevant records.
 
      the target as they fetched the relevant records.



 



   Of these, the case of a third party referenced in the SPF record is
 
   Of these, the case of a third party referenced in the SPF record is


   the easiest for a DoS attack to effectively exploit.  As a result,
 
   the easiest for a DoS attack to effectively exploit.  As a result,







   limits that may seem reasonable for an individual mail server can
 
   limits that might seem reasonable for an individual mail server can


   still allow an unreasonable amount of bandwidth amplification.
 
   still allow an unreasonable amount of bandwidth amplification.


   Therefore, the processing limits need to be quite low.
 
   Therefore, the processing limits need to be quite low.



 








   SPF implementations MUST limit the number of mechanisms and modifiers
 
10.2.  SPF-Authorized Email May Contain Other False Identities


   that do DNS lookups to at most 10 per SPF check, including any
 



   lookups caused by the use of the "include" mechanism or the
 



   "redirect" modifier.  If this number is exceeded during a check, a
 



   PermError MUST be returned.  The "include", "a", "mx", "ptr", and
 



   "exists" mechanisms as well as the "redirect" modifier do count
 



   against this limit.  The "all", "ip4", and "ip6" mechanisms do not
 



   require DNS lookups and therefore do not count against this limit.
 



   The "exp" modifier does not count against this limit because the DNS
 



   lookup to fetch the explanation string occurs after the SPF record
 



   has been evaluated.
 




 



   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
 



   there MUST be a limit of no more than 10 MX or PTR RRs looked up and
 



   checked.
 




 



   SPF implementations SHOULD limit the total amount of data obtained
 



   from the DNS queries.  For example, when DNS over TCP or EDNS0 are
 



   available, there may need to be an explicit limit to how much data
 



   will be accepted to prevent excessive bandwidth usage or memory usage
 



   and DoS attacks.
 




 



   MTAs or other processors MAY also impose a limit on the maximum
 



   amount of elapsed time to evaluate check_host().  Such a limit SHOULD
 



   allow at least 20 seconds.  If such a limit is exceeded, the result
 



   of authorization SHOULD be "TempError".
 




 



   Domains publishing records SHOULD try to keep the number of "include"
 



   mechanisms and chained "redirect" modifiers to a minimum.  Domains
 



   SHOULD also try to minimize the amount of other DNS information
 



   needed to evaluate a record.  This can be done by choosing directives
 



   that require less DNS information and placing lower-cost mechanisms
 



   earlier in the SPF record.
 




 



   For example, consider a domain set up as follows:
 




 



   example.com.      IN MX   10 mx.example.com.
 



   mx.example.com.   IN A    192.0.2.1
 



   a.example.com.    IN TXT  "v=spf1 mx:example.com -all"
 



   b.example.com.    IN TXT  "v=spf1 a:mx.example.com -all"
 



   c.example.com.    IN TXT  "v=spf1 ip4:192.0.2.1 -all"
 




 



   Evaluating check_host() for the domain "a.example.com" requires the
 



   MX records for "example.com", and then the A records for the listed
 



   hosts.  Evaluating for "b.example.com" requires only the A records.
 



   Evaluating for "c.example.com" requires none.
 




 



   However, there may be administrative considerations: using "a" over
 



   "ip4" allows hosts to be renumbered easily.  Using "mx" over "a"
 



   allows the set of mail hosts to be changed easily.
 



                                                                         
 



10.2.  SPF-Authorized E-Mail May Contain Other False Identities
 




 








   The "MAIL FROM" and "HELO" identity authorizations must not be
 
   Do not construe the "MAIL FROM" and "HELO" identity authorizations to


   construed to provide more assurance than they do.  It is entirely
 
   provide more assurance than they do.  It is entirely possible for a


   possible for a malicious sender to inject a message using his own
 
   malicious sender to inject a message using his own domain in the


   domain in the identities used by SPF, to have that domain's SPF
 
   identities used by SPF, to have that domain's SPF record authorize


   record authorize the sending host, and yet the message can easily
 
   the sending host, and yet the message can easily list other


   list other identities in its header.  Unless the user or the MUA
 
   identities in its header.  Unless the user or the MUA takes care to


   takes care to note that the authorized identity does not match the
 
   note that the authorized identity does not match the other more


   other more commonly-presented identities (such as the From:  header
 
   commonly-presented identities (such as the From: header field), the


   field), the user may be lulled into a false sense of security.
 
   user might be lulled into a false sense of security.



 



10.3.  Spoofed DNS and IP Data
 
10.3.  Spoofed DNS and IP Data



 



   There are two aspects of this protocol that malicious parties could
 
   There are two aspects of this protocol that malicious parties could


   exploit to undermine the validity of the check_host() function:
 
   exploit to undermine the validity of the check_host() function:



 



   o  The evaluation of check_host() relies heavily on DNS.  A malicious
 
   o  The evaluation of check_host() relies heavily on DNS.  A malicious


      attacker could attack the DNS infrastructure and cause
 
      attacker could attack the DNS infrastructure and cause


      check_host() to see spoofed DNS data, and then return incorrect
 
      check_host() to see spoofed DNS data, and then return incorrect







      results.  This could include returning "Pass" for an <ip> value
 
      results.  This could include returning "pass" for an <ip> value


      where the actual domain's record would evaluate to "Fail".  See
 
      where the actual domain's record would evaluate to "fail".  See


      [RFC3833] for a description of DNS weaknesses.
 
      [RFC3833] for a description of DNS weaknesses.



 








   o  The client IP address, <ip>, is assumed to be correct.  A
 
   o  The client IP address, <ip>, is assumed to be correct.  In a


      malicious attacker could spoof TCP sequence numbers to make mail
 
      modern, correctly configured system the risk of this not being


      appear to come from a permitted host for a domain that the
 
      true is nil.


      attacker is impersonating.
 




 



10.4.  Cross-User Forgery
 
10.4.  Cross-User Forgery



 



   By definition, SPF policies just map domain names to sets of
 
   By definition, SPF policies just map domain names to sets of







   authorized MTAs, not whole E-Mail addresses to sets of authorized
 
   authorized MTAs, not whole email addresses to sets of authorized


   users.  Although the "l" macro (Section 8) provides a limited way to
 
   users.  Although the "l" macro (Section 8) provides a limited way to







   define individual sets of authorized MTAs for specific E-Mail
 
   define individual sets of authorized MTAs for specific email


   addresses, it is generally impossible to verify, through SPF, the use
 
   addresses, it is generally impossible to verify, through SPF, the use







   of specific E-Mail addresses by individual users of the same MTA.
 
   of specific email addresses by individual users of the same MTA.



 



   It is up to mail services and their MTAs to directly prevent
 
   It is up to mail services and their MTAs to directly prevent







   cross-user forgery: based on SMTP AUTH ([RFC2554]), users should be
 
   cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be


   restricted to using only those E-Mail addresses that are actually
 
   restricted to using only those email addresses that are actually


   under their control (see [RFC4409], Section 6.1).  Another means to
 
   under their control (see [RFC6409], Section 6.1).  Another means to


   verify the identity of individual users is message cryptography such
 
   verify the identity of individual users is message cryptography such







   as PGP ([RFC2440]) or S/MIME ([RFC3851]).
 
   as PGP ([RFC4880]) or S/MIME ([RFC5751]).



 



10.5.  Untrusted Information Sources
 
10.5.  Untrusted Information Sources



 








   SPF uses information supplied by third parties, such as the "HELO"
 
   An SPF compliant receiver gathers information from the SMTP commands


   domain name, the "MAIL FROM" address, and SPF records.  This
 
   it receives and from the published DNS records of the sending domain


   information is then passed to the receiver in the Received-SPF: trace
 
   holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the


   fields and possibly returned to the client MTA in the form of an SMTP
 
   envelope, and SPF DNS records published by the domain holder).


   rejection message.  This information must be checked for invalid
 



   characters and excessively long lines.
 




 








   When the authorization check fails, an explanation string may be
 
10.5.1.  Recorded Results



 




 
   This information, passed to the receiver in the Received-SPF: or



 
   Authentication-Results: trace fields, may be returned to the client



 
   MTA as an SMTP rejection message.  If such an SMTP rejection message



 
   is generated, the information from the trace fields has to be checked



 
   for such problems as invalid characters and excessively long lines.



 




 
10.5.2.  External Explanations



 
                                                                         



 
   When the authorization check fails, an explanation string could be


   included in the reject response.  Both the sender and the rejecting
 
   included in the reject response.  Both the sender and the rejecting


   receiver need to be aware that the explanation was determined by the
 
   receiver need to be aware that the explanation was determined by the


   publisher of the SPF record checked and, in general, not the
 
   publisher of the SPF record checked and, in general, not the







   receiver.  The explanation may contain malicious URLs, or it may be
 
   receiver.  The explanation can contain malicious URLs, or it might be


   offensive or misleading.
 
   offensive or misleading.



 








   This is probably less of a concern than it may initially seem since
 
   Explanations returned to sender domains due to "exp" modifiers,


   such messages are returned to the sender, and the explanation strings
 
   (Section 6.2), were generated by the sender policy published by the


   come from the sender policy published by the domain in the identity
 
   domain holders themselves.  As long as messages are only returned


   claimed by that very sender.  As long as the DSN is not redirected to
 
   with non-delivery notification ([RFC3464]) to domains publishing the


   someone other than the actual sender, the only people who see
 
   explanation strings from their own DNS SPF records, the only affected


   malicious explanation strings are people whose messages claim to be
 
   parties are the original publishers of the domain's SPF records.


   from domains that publish such strings in their SPF records.  In
 
                                                                         


   practice, DSNs can be misdirected, such as when an MTA accepts an
 
   In practice, such non-delivery notifications can be misdirected, such


   E-Mail and then later generates a DSN to a forged address, or when an
 
   as when an MTA accepts an email and only later generates the


   E-Mail forwarder does not direct the DSN back to the original sender.
 
   notification to a forged address, or when an email forwarder does not



 
   direct the bounce back to the original sender.



 
                                                                         



 
10.5.3.  Macro Expansion



 




 
   Macros (Section 8) allow senders to inject arbitrary text (any non-



 
   null [US-ASCII] character) into receiver DNS queries.  It is necesary



 
   to be prepared for hostile or unexpected content.



 



10.6.  Privacy Exposure
 
10.6.  Privacy Exposure



 



   Checking SPF records causes DNS queries to be sent to the domain
 
   Checking SPF records causes DNS queries to be sent to the domain


   owner.  These DNS queries, especially if they are caused by the
 
   owner.  These DNS queries, especially if they are caused by the


   "exists" mechanism, can contain information about who is sending
 
   "exists" mechanism, can contain information about who is sending







   E-Mail and likely to which MTA the E-Mail is being sent.  This can
 
   email and likely to which MTA the email is being sent.  This can


   introduce some privacy concerns, which may be more or less of an
 
   introduce some privacy concerns, which are more or less of an issue


   issue depending on local laws and the relationship between the domain
 
   depending on local laws and the relationship between the domain owner


   owner and the person sending the E-Mail.
 
   and the person sending the email.



 



11.  Contributors and Acknowledgements
 
11.  Contributors and Acknowledgements



 








   This document is largely based on the work of Meng Weng Wong and Mark
 
   This document is largely based on the work of Meng Weng Wong, Mark


   Lentczner.  Although, as this section acknowledges, many people have
 
   Lentczner, and Wayne Schlitt.  Although, as this section


   contributed to this document, a very large portion of the writing and
 
   acknowledges, many people have contributed to this document, a very


   editing are due to Meng and Mark.
 
   large portion of the writing and editing are due to Meng, Mark, and



 
   Wayne.



 



   This design owes a debt of parentage to [RMX] by Hadmut Danisch and
 
   This design owes a debt of parentage to [RMX] by Hadmut Danisch and


   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
 
   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check







   the legitimacy of an E-Mail address traces its ancestry further back
 
   the legitimacy of an email address traces its ancestry further back


   through messages on the namedroppers mailing list by Paul Vixie
 
   through messages on the namedroppers mailing list by Paul Vixie







                                                                         
 



   [Vixie] (based on suggestion by Jim Miller) and by David Green
 
   [Vixie] (based on suggestion by Jim Miller) and by David Green


   [Green].
 
   [Green].



 



   Philip Gladstone contributed the concept of macros to the
 
   Philip Gladstone contributed the concept of macros to the


   specification, multiplying the expressiveness of the language and
 
   specification, multiplying the expressiveness of the language and


   making per-user and per-IP lookups possible.
 
   making per-user and per-IP lookups possible.



 








   The authors would also like to thank the literally hundreds of
 
   The authors of both this document and [RFC4408] would also like to


   individuals who have participated in the development of this design.
 
   thank the literally hundreds of individuals who have participated in


   They are far too numerous to name, but they include the following:
 
   the development of this design.  They are far too numerous to name,



 
   but they include the following:



 









 
      The participants in the SPFbis working group.


      The folks on the spf-discuss mailing list.
 
      The folks on the spf-discuss mailing list.


      The folks on the SPAM-L mailing list.
 
      The folks on the SPAM-L mailing list.


      The folks on the IRTF ASRG mailing list.
 
      The folks on the IRTF ASRG mailing list.


      The folks on the IETF MARID mailing list.
 
      The folks on the IETF MARID mailing list.


      The folks on #perl.
 
      The folks on #perl.



 



12.  IANA Considerations
 
12.  IANA Considerations



 



12.1.  The SPF DNS Record Type
 
12.1.  The SPF DNS Record Type



 








   The IANA has assigned a new Resource Record Type and Qtype from the
 
   Per [RFC4408], the IANA assigned the Resource Record Type and Qtype


   DNS Parameters Registry for the SPF RR type with code 99.
 
   from the DNS Parameters Registry for the SPF RR type with code 99.



 
   The format of this type is identical to the TXT RR [RFC1035].  The



 
   character content of the record is encoded as [US-ASCII].  Use of



 
   this record type is obsolete for SPF Version 1.



 




 
   IANA is requested to add an annotation to the SPF RRTYPE saying



 
   "(OBSOLETE - use TXT)" in the DNS Parameters registry.



 




 
   [NOTE TO RFC EDITOR: (to be changed to " ... has added ..." upon



 
   publication)]



 



12.2.  The Received-SPF Mail Header Field
 
12.2.  The Received-SPF Mail Header Field



 



   Per [RFC3864], the "Received-SPF:" header field is added to the IANA
 
   Per [RFC3864], the "Received-SPF:" header field is added to the IANA


   Permanent Message Header Field Registry.  The following is the
 
   Permanent Message Header Field Registry.  The following is the


   registration template:
 
   registration template:



 



      Header field name: Received-SPF
 
      Header field name: Received-SPF







      Applicable protocol: mail ([RFC2822])
 
      Applicable protocol: mail ([RFC5322])


      Status: Experimental
 
      Status: Standards Track


      Author/Change controller: IETF
 
      Author/Change controller: IETF







      Specification document(s): RFC 4408
 
      Specification document(s): RFC XXXX


      Related information:
 
      [NOTE TO RFC EDITOR: (this document)]


      Requesting SPF Council review of any proposed changes and
 



      additions to this field are recommended.  For information about
 
12.3.  SPF Modifier Registration


      the SPF Council see http://www.openspf.org/Council
 




 
   [RFC6652] created a new SPF Modifier Registration.  IANA is requested



 
   to change the reference for the exp and redirect modifiers from



 
   [RFC4408] to this document.  Their status should not be changed.



 



13.  References
 
13.  References



 



13.1.  Normative References
 
13.1.  Normative References



 



   [RFC1035]  Mockapetris, P., "Domain names - implementation and
 
   [RFC1035]  Mockapetris, P., "Domain names - implementation and


              specification", STD 13, RFC 1035, November 1987.
 
              specification", STD 13, RFC 1035, November 1987.



 



   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
 
   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application


              and Support", STD 3, RFC 1123, October 1989.
 
              and Support", STD 3, RFC 1123, October 1989.



 



   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
 
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate


              Requirement Levels", BCP 14, RFC 2119, March 1997.
 
              Requirement Levels", BCP 14, RFC 2119, March 1997.



 








   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
 
   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",


              April 2001.
 
              RFC 3463, January 2003.



 



   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
 



              2001.
 




 



   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
 



              for Delivery Status Notifications", RFC 3464, January
 



              2003.
 




 



   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
 



              (IPv6) Addressing Architecture", RFC 3513, April 2003.
 




 



   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
 
   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration


              Procedures for Message Header Fields", BCP 90, RFC 3864,
 
              Procedures for Message Header Fields", BCP 90, RFC 3864,


              September 2004.
 
              September 2004.



 



   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
 
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform







              Resource Identifier (URI): Generic Syntax", STD 66, RFC
 
              Resource Identifier (URI): Generic Syntax", STD 66,


              3986, January 2005.
 
              RFC 3986, January 2005.



 








   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
 
   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing


              Specifications: ABNF", RFC 4234, October 2005.
 
              Architecture", RFC 4291, February 2006.



 








   [US-ASCII] American National Standards Institute (formerly United
 
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax



 
              Specifications: ABNF", STD 68, RFC 5234, January 2008.



 




 
   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,



 
              October 2008.



 




 
   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,



 
              October 2008.



 




 
   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating



 
              Message Authentication Status", RFC 5451, April 2009.



 




 
   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,



 
              July 2009.



 




 
   [RFC5890]  Klensin, J., "Internationalized Domain Names for



 
              Applications (IDNA): Definitions and Document Framework",



 
              RFC 5890, August 2010.



 
                                                                         



 
   [US-ASCII]



 
              American National Standards Institute (formerly United


              States of America Standards Institute), "USA Code for
 
              States of America Standards Institute), "USA Code for


              Information Interchange, X3.4", 1968.
 
              Information Interchange, X3.4", 1968.



 








   ANSI X3.4-1968 has been replaced by newer versions with slight
 
              ANSI X3.4-1968 has been replaced by newer versions with


              modifications, but the 1968 version remains definitive for
 
              slight modifications, but the 1968 version remains


              the Internet.
 
              definitive for the Internet.



 








13.2  Informative References
 
13.2.  Informative References



 
                                                                         



 
   [DMP]      Fecyk, G., "Designated Mailers Protocol".



 




 
              Work In Progress



 




 
   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.



 



   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
 
   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",


              STD 13, RFC 1034, November 1987.
 
              STD 13, RFC 1034, November 1987.



 








   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983, August
 
   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983,


              1996.
 
              August 1996.



 








   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
 
   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS


              "OpenPGP Message Format", RFC 2440, November 1998.
 
              NCACHE)", RFC 2308, March 1998.



 








   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
 
   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for


              RFC 2554, March 1999.
 
              specifying the location of services (DNS SRV)", RFC 2782,



 
              February 2000.



 




 
   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format



 
              for Delivery Status Notifications", RFC 3464,



 
              January 2003.



 



   [RFC3696]  Klensin, J., "Application Techniques for Checking and
 
   [RFC3696]  Klensin, J., "Application Techniques for Checking and


              Transformation of Names", RFC 3696, February 2004.
 
              Transformation of Names", RFC 3696, February 2004.



 



   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
 
   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain


              Name System (DNS)", RFC 3833, August 2004.
 
              Name System (DNS)", RFC 3833, August 2004.



 








   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
 
   [RFC3834]  Moore, K., "Recommendations for Automatic Responses to


              Extensions (S/MIME) Version 3.1 Message Specification",
 
              Electronic Mail", RFC 3834, August 2004.


              RFC 3851, July 2004.
 




 








   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
 
   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)


              RFC 4409, April 2006.
 
              for Authorizing Use of Domains in E-Mail, Version 1",



 
              RFC 4408, April 2006.



 








   [RMX]      Danish, H., "The RMX DNS RR Type for light weight sender
 
   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing


              authentication", Work In Progress
 
              (CIDR): The Internet Address Assignment and Aggregation



 
              Plan", BCP 122, RFC 4632, August 2006.



 








   [DMP]      Fecyk, G., "Designated Mailers Protocol", Work In Progress
 
   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.



 
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.



 








   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.
 
   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension



 
              for Authentication", RFC 4954, July 2007.



 








   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.
 
   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet



 
              Mail Extensions (S/MIME) Version 3.2 Message



 
              Specification", RFC 5751, January 2010.



 




 
   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,



 
              February 2010.



 




 
   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",



 
              STD 72, RFC 6409, November 2011.



 




 
   [RFC6647]  Kucherawy, M. and D. Crocker, "Email Greylisting: An



 
              Applicability Statement for SMTP", RFC 6647, June 2012.



 




 
   [RFC6652]  Kitterman, S., "Sender Policy Framework (SPF)



 
              Authentication Failure Reporting Using the Abuse Reporting



 
              Format", RFC 6652, June 2012.



 




 
   [RFC6686]  Kucherawy, M., "Resolution of the Sender Policy Framework



 
              (SPF) and Sender ID Experiments", RFC 6686, July 2012.



 




 
   [RMX]      Danisch, H., "The RMX DNS RR Type for light weight sender



 
              authentication".



 




 
              Work In Progress



 




 
   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.



 



Appendix A.  Collected ABNF
 
Appendix A.  Collected ABNF



 



   This section is normative and any discrepancies with the ABNF
 
   This section is normative and any discrepancies with the ABNF


   fragments in the preceding text are to be resolved in favor of this
 
   fragments in the preceding text are to be resolved in favor of this


   grammar.
 
   grammar.



 








   See [RFC4234] for ABNF notation.  Please note that as per this ABNF
 
   See [RFC5234] for ABNF notation.  Please note that as per this ABNF


   definition, literal text strings (those in quotes) are case-
 
   definition, literal text strings (those in quotes) are case-


   insensitive.  Hence, "mx" matches "mx", "MX", "mX", and "Mx".
 
   insensitive.  Hence, "mx" matches "mx", "MX", "mX", and "Mx".



 



   record           = version terms *SP
 
   record           = version terms *SP


   version          = "v=spf1"
 
   version          = "v=spf1"



 



   terms            = *( 1*SP ( directive / modifier ) )
 
   terms            = *( 1*SP ( directive / modifier ) )



 



   directive        = [ qualifier ] mechanism
 
   directive        = [ qualifier ] mechanism


   qualifier        = "+" / "-" / "?" / "~"
 
   qualifier        = "+" / "-" / "?" / "~"



 



skipping to change at page 42, line 38

skipping to change at page 57, line 38


   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 
   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]


   PTR              = "ptr"    [ ":" domain-spec ]
 
   PTR              = "ptr"    [ ":" domain-spec ]


   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 
   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]


   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 
   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]


   exists           = "exists"   ":" domain-spec
 
   exists           = "exists"   ":" domain-spec



 



   modifier         = redirect / explanation / unknown-modifier
 
   modifier         = redirect / explanation / unknown-modifier


   redirect         = "redirect" "=" domain-spec
 
   redirect         = "redirect" "=" domain-spec


   explanation      = "exp" "=" domain-spec
 
   explanation      = "exp" "=" domain-spec


   unknown-modifier = name "=" macro-string
 
   unknown-modifier = name "=" macro-string








 
                      ; where name is not any known modifier



 



   ip4-cidr-length  = "/" 1*DIGIT
 
   ip4-cidr-length  = "/" 1*DIGIT


   ip6-cidr-length  = "/" 1*DIGIT
 
   ip6-cidr-length  = "/" 1*DIGIT


   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
 
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]



 



   ip4-network      = qnum "." qnum "." qnum "." qnum
 
   ip4-network      = qnum "." qnum "." qnum "." qnum


   qnum             = DIGIT                 ; 0-9
 
   qnum             = DIGIT                 ; 0-9


                      / %x31-39 DIGIT       ; 10-99
 
                      / %x31-39 DIGIT       ; 10-99


                      / "1" 2DIGIT          ; 100-199
 
                      / "1" 2DIGIT          ; 100-199


                      / "2" %x30-34 DIGIT   ; 200-249
 
                      / "2" %x30-34 DIGIT   ; 200-249


                      / "25" %x30-35        ; 250-255
 
                      / "25" %x30-35        ; 250-255







             ; conventional dotted quad notation.  e.g., 192.0.2.0
 
            ; conventional dotted quad notation.  e.g., 192.0.2.0


   ip6-network      = <as per [RFC 3513], section 2.2>
 
   ip6-network      = <as per [RFC 4291], section 2.2>


             ; e.g., 2001:DB8::CD30
 
            ; e.g., 2001:DB8::CD30



 



   domain-spec      = macro-string domain-end
 
   domain-spec      = macro-string domain-end


   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
 
   domain-end       = ( "." toplabel [ "." ] ) / macro-expand








 
                                                                         


   toplabel         = ( *alphanum ALPHA *alphanum ) /
 
   toplabel         = ( *alphanum ALPHA *alphanum ) /


                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
 
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )


                      ; LDH rule plus additional TLD restrictions
 
                      ; LDH rule plus additional TLD restrictions







                      ; (see [RFC3696], Section 2)
 
                      ; (see [RFC3696], Section 2 for background)


                                                                         
 



   alphanum         = ALPHA / DIGIT
 
   alphanum         = ALPHA / DIGIT



 



   explain-string   = *( macro-string / SP )
 
   explain-string   = *( macro-string / SP )



 



   macro-string     = *( macro-expand / macro-literal )
 
   macro-string     = *( macro-expand / macro-literal )


   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
 
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )


                      / "%%" / "%_" / "%-"
 
                      / "%%" / "%_" / "%-"


   macro-literal    = %x21-24 / %x26-7E
 
   macro-literal    = %x21-24 / %x26-7E


                      ; visible characters except "%"
 
                      ; visible characters except "%"


   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
 
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /







                      "c" / "r" / "t"
 
                      "c" / "r" / "t" / "v"


   transformers     = *DIGIT [ "r" ]
 
   transformers     = *DIGIT [ "r" ]


   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="



 



   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 
   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )



 



   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
 
   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]


                      [ key-value-list ] CRLF
 
                      [ key-value-list ] CRLF



 








   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
 
   result           = "pass" / "fail" / "softfail" / "neutral" /


                      "None" / "TempError" / "PermError"
 
                      "none" / "temperror" / "permerror"



 



   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
 
   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )


                      [";"]
 
                      [";"]



 



   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
 
   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )



 



   key              = "client-ip" / "envelope-from" / "helo" /
 
   key              = "client-ip" / "envelope-from" / "helo" /







                      "problem" / "receiver" / "identity" /
 
                      "problem" / "receiver" / identity /


                       mechanism / "x-" name / name
 
                       mechanism / name



 



   identity         = "mailfrom"   ; for the "MAIL FROM" identity
 
   identity         = "mailfrom"   ; for the "MAIL FROM" identity


                      / "helo"     ; for the "HELO" identity
 
                      / "helo"     ; for the "HELO" identity


                      / name       ; other identities
 
                      / name       ; other identities



 








   dot-atom         = <unquoted word as per [RFC2822]>
 
   ALPHA            = <A-Z / a-z as per [RFC5234]>


   quoted-string    = <quoted string as per [RFC2822]>
 
   DIGIT            = <0-9 as per [RFC5234]>


   comment          = <comment string as per [RFC2822]>
 
   SP               = <space character as per [RFC5234]>


   CFWS             = <comment or folding white space as per [RFC2822]>
 
   domain           = <fully qualified domain as per [RFC5321]>


   FWS              = <folding white space as per [RFC2822]>
 
   dot-atom         = <unquoted word as per [RFC5322]>


   CRLF             = <standard end-of-line token as per [RFC2822]>
 
   quoted-string    = <quoted string as per [RFC5322]>



 
   comment          = <comment string as per [RFC5322]>



 
   CFWS             = <comment or folding white space as per [RFC5322]>



 
   FWS              = <folding white space as per [RFC5322]>



 
   CRLF             = <standard end-of-line token as per [RFC5322]>



 
   authserv-id      = <authserv-id per [RFC5451]>



 
   reasonspec       = <reason per [RFC5451]>



 



Appendix B.  Extended Examples
 
Appendix B.  Extended Examples



 



   These examples are based on the following DNS setup:
 
   These examples are based on the following DNS setup:



 



   ; A domain with two mail servers, two hosts
 
   ; A domain with two mail servers, two hosts


   ; and two servers at the domain name
 
   ; and two servers at the domain name


   $ORIGIN example.com.
 
   $ORIGIN example.com.


   @           MX  10 mail-a
 
   @           MX  10 mail-a


               MX  20 mail-b
 
               MX  20 mail-b



 



skipping to change at page 44, line 46

skipping to change at page 60, line 46



 



   ; A rogue reverse IP domain that claims to be
 
   ; A rogue reverse IP domain that claims to be


   ; something it's not
 
   ; something it's not


   $ORIGIN 0.0.10.in-addr.arpa.
 
   $ORIGIN 0.0.10.in-addr.arpa.


   4           PTR bob.example.com.
 
   4           PTR bob.example.com.



 



B.1.  Simple Examples
 
B.1.  Simple Examples



 



   These examples show various possible published records for
 
   These examples show various possible published records for


   example.com and which values if <ip> would cause check_host() to
 
   example.com and which values if <ip> would cause check_host() to







   return "Pass".  Note that <domain> is "example.com".
 
   return "pass".  Note that <domain> is "example.com".



 



   v=spf1 +all
 
   v=spf1 +all







      -- any <ip> passes
 
      --  any <ip> passes



 



   v=spf1 a -all
 
   v=spf1 a -all







      -- hosts 192.0.2.10 and 192.0.2.11 pass
 
      --  hosts 192.0.2.10 and 192.0.2.11 pass



 



   v=spf1 a:example.org -all
 
   v=spf1 a:example.org -all







      -- no sending hosts pass since example.org has no A records
 
      --  no sending hosts pass since example.org has no A records



 



   v=spf1 mx -all
 
   v=spf1 mx -all







      -- sending hosts 192.0.2.129 and 192.0.2.130 pass
 
      --  sending hosts 192.0.2.129 and 192.0.2.130 pass



 



   v=spf1 mx:example.org -all
 
   v=spf1 mx:example.org -all







      -- sending host 192.0.2.140 passes
 
      --  sending host 192.0.2.140 passes



 



   v=spf1 mx mx:example.org -all
 
   v=spf1 mx mx:example.org -all







      -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass
 
      --  sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass



 



   v=spf1 mx/30 mx:example.org/30 -all
 
   v=spf1 mx/30 mx:example.org/30 -all







      -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
 
      --  any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes



 



   v=spf1 ptr -all
 
   v=spf1 ptr -all







      -- sending host 192.0.2.65 passes (reverse DNS is valid and is in
 
      --  sending host 192.0.2.65 passes (reverse DNS is valid and is in


         example.com)
 
          example.com)


      -- sending host 192.0.2.140 fails (reverse DNS is valid, but not
 
      --  sending host 192.0.2.140 fails (reverse DNS is valid, but not


         in example.com)
 
          in example.com)


      -- sending host 10.0.0.4 fails (reverse IP is not valid)
 
      --  sending host 10.0.0.4 fails (reverse IP is not valid)



 



   v=spf1 ip4:192.0.2.128/28 -all
 
   v=spf1 ip4:192.0.2.128/28 -all







      -- sending host 192.0.2.65 fails
 
      --  sending host 192.0.2.65 fails


      -- sending host 192.0.2.129 passes
 
      --  sending host 192.0.2.129 passes



 



B.2.  Multiple Domain Example
 
B.2.  Multiple Domain Example



 



   These examples show the effect of related records:
 
   These examples show the effect of related records:



 



      example.org: "v=spf1 include:example.com include:example.net -all"
 
      example.org: "v=spf1 include:example.com include:example.net -all"



 



   This record would be used if mail from example.org actually came
 
   This record would be used if mail from example.org actually came


   through servers at example.com and example.net.  Example.org's
 
   through servers at example.com and example.net.  Example.org's


   designated servers are the union of example.com's and example.net's
 
   designated servers are the union of example.com's and example.net's



 



skipping to change at page 46, line 10

skipping to change at page 62, line 12


   These records allow a set of domains that all use the same mail
 
   These records allow a set of domains that all use the same mail


   system to make use of that mail system's record.  In this way, only
 
   system to make use of that mail system's record.  In this way, only


   the mail system's record needs to be updated when the mail setup
 
   the mail system's record needs to be updated when the mail setup


   changes.  These domains' records never have to change.
 
   changes.  These domains' records never have to change.



 



B.3.  DNSBL Style Example
 
B.3.  DNSBL Style Example



 



   Imagine that, in addition to the domain records listed above, there
 
   Imagine that, in addition to the domain records listed above, there


   are these:
 
   are these:



 








   $ORIGIN _spf.example.com.  mary.mobile-users                   A
 
   $ORIGIN _spf.example.com.


   127.0.0.2 fred.mobile-users                   A 127.0.0.2
 
   mary.mobile-users                   A 127.0.0.2



 
   fred.mobile-users                   A 127.0.0.2


   15.15.168.192.joel.remote-users     A 127.0.0.2
 
   15.15.168.192.joel.remote-users     A 127.0.0.2


   16.15.168.192.joel.remote-users     A 127.0.0.2
 
   16.15.168.192.joel.remote-users     A 127.0.0.2



 



   The following records describe users at example.com who mail from
 
   The following records describe users at example.com who mail from


   arbitrary servers, or who mail from personal servers.
 
   arbitrary servers, or who mail from personal servers.



 



   example.com:
 
   example.com:



 



   v=spf1 mx
 
   v=spf1 mx


          include:mobile-users._spf.%{d}
 
          include:mobile-users._spf.%{d}



 



skipping to change at page 47, line 5

skipping to change at page 63, line 5


                                 "-include:ip4._spf.%{d} "
 
                                 "-include:ip4._spf.%{d} "


                                 "-include:ptr._spf.%{d} "
 
                                 "-include:ptr._spf.%{d} "


                                 "+all" )
 
                                 "+all" )


   ip4._spf.example.com.  SPF  "v=spf1 -ip4:192.0.2.0/24 +all"
 
   ip4._spf.example.com.  SPF  "v=spf1 -ip4:192.0.2.0/24 +all"


   ptr._spf.example.com.  SPF  "v=spf1 -ptr +all"
 
   ptr._spf.example.com.  SPF  "v=spf1 -ptr +all"



 



   This example shows how the "-include" mechanism can be useful, how an
 
   This example shows how the "-include" mechanism can be useful, how an


   SPF record that ends in "+all" can be very restrictive, and the use
 
   SPF record that ends in "+all" can be very restrictive, and the use


   of De Morgan's Law.
 
   of De Morgan's Law.



 








Authors' Addresses
 
Appendix C.  Change History



 








   Meng Weng Wong
 
   Changes since RFC 4408 (to be removed prior to publication)


   Singapore
 




 








   EMail: [e-mail]
 
      Moved to standards track



 








   Wayne Schlitt
 
      Authors updated


   4615 Meredeth #9
 



   Lincoln Nebraska, NE  68506
 



   United States of America
 




 








   EMail: [e-mail]
 
      IESG Note regarding experimental use replaced with discussion of


   URI:   http://www.schlitt.net/spf/
 
      results



 








Full Copyright Statement
 
      Process errata:



 








   Copyright (C) The Internet Society (2006).
 
      Resolved Section 2.5.7 PermError on invalid domains after macro



 
      expansion errata in favor of documenting that different clients



 
      produce different results.



 








   This document is subject to the rights, licenses and restrictions
 
      Add %v macro to ABNF grammar


   contained in BCP 78, and except as set forth therein, the authors
 



   retain all their rights.
 




 








   This document and the information contained herein are provided on an
 
      Replace "uric" by "unreserved"


   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 



   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 



   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 



   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 



   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 



   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 




 








Intellectual Property
 
      Recommend an SMTP reply code for optional permerror rejections



 








   The IETF takes no position regarding the validity or scope of any
 
      Correct syntax in Received-SPF examples


   Intellectual Property Rights or other rights that might be claimed to
 



   pertain to the implementation or use of the technology described in
 



   this document or the extent to which any license under such rights
 



   might or might not be available; nor does it represent that it has
 



   made any independent effort to identify any such rights.  Information
 



   on the procedures with respect to rights in RFC documents can be
 



   found in BCP 78 and BCP 79.
 




 








   Copies of IPR disclosures made to the IETF Secretariat and any
 
      Fix unknown-modifier clause is too greedy in ABNF


   assurances of licenses to be made available, or the result of an
 



   attempt made to obtain a general license or permission for the use of
 



   such proprietary rights by implementers or users of this
 



   specification can be obtained from the IETF on-line IPR repository at
 



   http://www.ietf.org/ipr.
 




 








   The IETF invites any interested party to bring to its attention any
 
      Correct use of empty domain-spec on exp modifier


   copyrights, patents or patent applications, or other proprietary
 



   rights that may cover technology that may be required to implement
 



   this standard.  Please address the information to the IETF at
 



   [e-mail].
 




 








Acknowledgement
 
      Fix minor typo errata



 








   Funding for the RFC Editor function is provided by the IETF
 
      Convert to spfbis working group draft,


   Administrative Support Activity (IASA).
 
      draft-ietf-spfbis-4408bis-00



 




 
      Addressed Ticket #1, RFC 4408 Section 2.5.6 - Temporary errors by



 
      giving the option to turn repeated SERVFAIL into permerror and



 
      adding RFC 2308 reference.



 




 
      Clarified text about IPv4 mapped addresses to resolve test suite



 
      ambiguity



 




 
      Clarified ambiguity about result when more than 10 "mx" or "ptr"



 
      records are returned for lookup to specify permerror.  This



 
      resolves one of the test suite ambiguities



 




 
      Made all references to result codes lower case per issue #7



 
      Adjusted section 2.2 Requirement to check mail from per issue #15



 




 
      Added missing "v" element in macro-letter in the collected ABNF



 
      per issue #16 - section 8.1 was already fixed in the pre-WG draft



 




 
      Marked ptr and "p" macro deprecated/SHOULD NOT use per issue #27



 




 
      Expunged lower case may from the draft per issue #8



 




 
      Expunged "x-" name as an obsolete concept



 




 
      Updated obslete references: RFC2821 to RFC5321, RFC2822 to



 
      RFC5322, and RFC4234 to RFC5234



 




 
      Refer to RFC6647 to describe greylisting instead of trying to



 
      describe it directly.



 




 
      Updated informative references to the current versions.



 




 
      Added definition for deprecated since there are questions.



 




 
      Start to rework section 9 with some RFC5598 terms.



 




 
      Added mention of RFC 6552 feedback reports in section 9.



 




 
      Added draft-ietf-spfbis-experiment as an informational reference.



 




 
      Drop Type SPF.



 




 
      Try and clarify informational nature of RFC3696



 




 
      Fix ABNF nits and add missing definitions per Bill's ABNF checker.



 




 
      Make DNS lookup time limit SHOULD instead of MAY.



 




 
      Reorganize and clarify processing limits.  Move hard limits to new



 
      section 4.6.4, Evaluation Limits.  Move advice to non-normative



 
      section 9.



 




 
      Removed paragraph in section 10.1 about limiting total data



 
      volumes as it is unused (and removable per the charter) and serves



 
      no purpose (it isn't something that actually can be implemented in



 
      any reasonable way).



 




 
      Added text and figures from Alessandro Vesely in section 9.1 to



 
      better explain DNS resource limits.



 




 
      Multiple editorial fixes from Murray Kucherawy's review.



 




 
      Also based on Murray's review, reworked SMTP identity definitions



 
      and made RFC 5598 a normative reference instead of informative.



 
      This is a downref that will have to be mentioned in the last call.



 




 
      Added RFC 3834 as an informative reference about backscatter.



 




 
      Added IDN requirements and normative reference to RFC 5890 to deal



 
      with the question "like DKIM did it.:



 




 
      Added informative reference to RFC 4632 for CIDR and use CIDR



 
      prefix length instead of CIDR-length to match its terminology.



 




 
      Added RFC 5782 informative reference on DNSxLs to support



 
      improving the exists description.



 




 
      Added text on creating a Authentication-Results header field that



 
      matches the Received-SPF header field information and added a



 
      normative reference to RFC 5451.



 




 
      Added informative reference to RFC 2782 due to SRV mention.



 




 
      Added informative reference to RFC 3464 due to DSN mention.



 




 
      Added informative reference to RFC 5617 for it's DNS wildcard use.



 




 
      Added informative reference to RFC 5782 to enhance the explanation



 
      of how the exists mechanism works.  Clarified the intended match/



 
      no-match method.



 




 
      Added new sections on Receiver policy for SPF pass, fail, and



 
      permerror.



 




 
      Added new section 9 discussion on treatment of bounces and the



 
      significance of HELO records.



 




 
      Added request to IANA to update the SPF modifier registry.



 




 
Author's Address



 




 
   Scott Kitterman



 
   Kitterman Technical Services



 
   3611 Scheel Dr



 
   Ellicott City, MD  21042



 
   United States of America



 




 
   Email: [e-mail]



 


 End of changes. 318 change blocks. 

943 lines changed or deleted

1353 lines changed or added


This html diff was produced by rfcdiff 1.41. The latest version is available 
from http://tools.ietf.org/tools/rfcdiff/ 
        
   

 rfc4408.txt 
 
 draft-ietf-spfbis-4408bis-08.txt 
       


 

      
      





Network Working Group                                            M. Wong
 
Network Working Group                                       S. Kitterman
      

Request for Comments: 4408                                    W. Schlitt
 
Internet-Draft                              Kitterman Technical Services
      

Category: Experimental                                        April 2006
 
Obsoletes: 4408 (if approved)                           October 22, 2012
      


 
Intended status: Standards Track
      

                   Sender Policy Framework (SPF) for
 
Expires: April 25, 2013
      

            Authorizing Use of Domains in E-Mail, Version 1
 

      


 

      
      





Status of This Memo
 
 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,
      


 
                               Version 1
      


 
                    draft-ietf-spfbis-4408bis-08.txt
      


 

      
      





   This memo defines an Experimental Protocol for the Internet
 
Abstract
      

   community.  It does not specify an Internet standard of any kind.
 

      

   Discussion and suggestions for improvement are requested.
 

      

   Distribution of this memo is unlimited.
 

      


 

      
      





Copyright Notice
 
   Email on the Internet can be forged in a number of ways.  In
      


 
   particular, existing protocols place no restriction on what a sending
      


 
   host can use as the "MAIL FROM" of a message or the domain given on
      


 
   the SMTP HELO/EHLO commands.  This document describes version 1 of
      


 
   the Sender Policy Framework (SPF) protocol, whereby an ADMD can
      


 
   explicitly authorize the hosts that are allowed to use its domain
      


 
   names, and a receiving host can check such authorization.
      


 

      
      





   Copyright (C) The Internet Society (2006).
 
   This document obsoletes RFC4408.
      


 

      
      





IESG Note
 
Status of this Memo
      


 

      
      





   The following documents  (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
 
   This Internet-Draft is submitted in full conformance with the
      

   are published simultaneously as Experimental RFCs, although there is
 
   provisions of BCP 78 and BCP 79.
      

   no general technical consensus and efforts to reconcile the two
 

      

   approaches have failed.  As such, these documents have not received
 

      

   full IETF review and are published "AS-IS" to document the different
 

      

   approaches as they were considered in the MARID working group.
 

      


 

      
      





   The IESG takes no position about which approach is to be preferred
 
   Internet-Drafts are working documents of the Internet Engineering
      

   and cautions the reader that there are serious open issues for each
 
   Task Force (IETF).  Note that other groups may also distribute
      

   approach and concerns about using them in tandem.  The IESG believes
 
   working documents as Internet-Drafts.  The list of current Internet-
      

   that documenting the different approaches does less harm than not
 
   Drafts is at http://datatracker.ietf.org/drafts/current/.
      

   documenting them.
 

      


 

      
      





   Note that the Sender ID experiment may use DNS records that may have
 
   Internet-Drafts are draft documents valid for a maximum of six months
      

   been created for the current SPF experiment or earlier versions in
 
   and may be updated, replaced, or obsoleted by other documents at any
      

   this set of experiments.  Depending on the content of the record,
 
   time.  It is inappropriate to use Internet-Drafts as reference
      

   this may mean that Sender-ID heuristics would be applied incorrectly
 
   material or to cite them other than as "work in progress."
      

   to a message.  Depending on the actions associated by the recipient
 

      

   with those heuristics, the message may not be delivered or may be
 

      

   discarded on receipt.
 

      


 

      
      





   Participants relying on Sender ID experiment DNS records are warned
 
   This Internet-Draft will expire on April 25, 2013.
      

   that they may lose valid messages in this set of circumstances.
 

      

   aParticipants publishing SPF experiment DNS records should consider
 

      

   the advice given in section 3.4 of RFC 4406 and may wish to publish
 

      

   both v=spf1 and spf2.0 records to avoid the conflict.
 

      


 

      
      





   Participants in the Sender-ID experiment need to be aware that the
 
Copyright Notice
      

   way Resent-* header fields are used will result in failure to receive
 

      

   legitimate email when interacting with standards-compliant systems
 

      

   (specifically automatic forwarders which comply with the standards by
 

      

   not adding Resent-* headers, and systems which comply with RFC 822
 

      

   but have not yet implemented RFC 2822 Resent-* semantics).  It would
 

      

   be inappropriate to advance Sender-ID on the standards track without
 

      

   resolving this interoperability problem.
 

      


 

      
      





   The community is invited to observe the success or failure of the two
 
   Copyright (c) 2012 IETF Trust and the persons identified as the
      

   approaches during the two years following publication, in order that
 
   document authors.  All rights reserved.
      

   a community consensus can be reached in the future.
 

      


 

      
      





Abstract
 
   This document is subject to BCP 78 and the IETF Trust's Legal
      


 
   Provisions Relating to IETF Documents
      


 
   (http://trustee.ietf.org/license-info) in effect on the date of
      


 
   publication of this document.  Please review these documents
      


 
   carefully, as they describe your rights and restrictions with respect
      


 
   to this document.  Code Components extracted from this document must
      


 
   include Simplified BSD License text as described in Section 4.e of
      


 
   the Trust Legal Provisions and are provided without warranty as
      


 
   described in the Simplified BSD License.
      


 

      
      





   E-mail on the Internet can be forged in a number of ways.  In
 
   This document may contain material from IETF Documents or IETF
      

   particular, existing protocols place no restriction on what a sending
 
   Contributions published or made publicly available before November
      

   host can use as the reverse-path of a message or the domain given on
 
   10, 2008.  The person(s) controlling the copyright in some of this
      

   the SMTP HELO/EHLO commands.  This document describes version 1 of
 
   material may not have granted the IETF Trust the right to allow
      

   the Sender Policy Framework (SPF) protocol, whereby a domain may
 
   modifications of such material outside the IETF Standards Process.
      

   explicitly authorize the hosts that are allowed to use its domain
 
   Without obtaining an adequate license from the person(s) controlling
      

   name, and a receiving host may check such authorization.
 
   the copyright in such materials, this document may not be modified
      


 
   outside the IETF Standards Process, and derivative works of it may
      


 
   not be created outside the IETF Standards Process, except to format
      


 
   it for publication as an RFC or to translate it into languages other
      


 
   than English.
      


 

      

Table of Contents
 
Table of Contents
      


 

      
      





   1. Introduction ....................................................4
 
   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
      

      1.1. Protocol Status ............................................4
 
     1.1.  Protocol Status  . . . . . . . . . . . . . . . . . . . . .  6
      

      1.2. Terminology ................................................5
 
     1.2.  Experimental History . . . . . . . . . . . . . . . . . . .  7
      

   2. Operation .......................................................5
 
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
      

      2.1. The HELO Identity ..........................................5
 
       1.3.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . .  7
      

      2.2. The MAIL FROM Identity .....................................5
 
       1.3.2.  Imported Definitions . . . . . . . . . . . . . . . . .  7
      

      2.3. Publishing Authorization ...................................6
 
       1.3.3.  Mail From Definition . . . . . . . . . . . . . . . . .  7
      

      2.4. Checking Authorization .....................................6
 
       1.3.4.  HELO Definition  . . . . . . . . . . . . . . . . . . .  8
      

      2.5. Interpreting the Result ....................................7
 
       1.3.5.  Deprecated . . . . . . . . . . . . . . . . . . . . . .  8
      

           2.5.1. None ................................................8
 
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
      

           2.5.2. Neutral .............................................8
 
     2.1.  The "HELO" Identity  . . . . . . . . . . . . . . . . . . .  9
      

           2.5.3. Pass ................................................8
 
     2.2.  The "MAIL FROM" Identity . . . . . . . . . . . . . . . . .  9
      

           2.5.4. Fail ................................................8
 
     2.3.  Publishing Authorization . . . . . . . . . . . . . . . . .  9
      

           2.5.5. SoftFail ............................................9
 
     2.4.  Checking Authorization . . . . . . . . . . . . . . . . . . 10
      

           2.5.6. TempError ...........................................9
 
     2.5.  Interpreting the Result  . . . . . . . . . . . . . . . . . 11
      

           2.5.7. PermError ...........................................9
 
       2.5.1.  None . . . . . . . . . . . . . . . . . . . . . . . . . 12
      

   3. SPF Records .....................................................9
 
       2.5.2.  Neutral  . . . . . . . . . . . . . . . . . . . . . . . 12
      

      3.1. Publishing ................................................10
 
       2.5.3.  Pass . . . . . . . . . . . . . . . . . . . . . . . . . 12
      

           3.1.1. DNS Resource Record Types ..........................10
 
       2.5.4.  Fail . . . . . . . . . . . . . . . . . . . . . . . . . 12
      

           3.1.2. Multiple DNS Records ...............................11
 
       2.5.5.  Softfail . . . . . . . . . . . . . . . . . . . . . . . 13
      

           3.1.3. Multiple Strings in a Single DNS record ............11
 
       2.5.6.  Temperror  . . . . . . . . . . . . . . . . . . . . . . 13
      

           3.1.4. Record Size ........................................11
 
       2.5.7.  Permerror  . . . . . . . . . . . . . . . . . . . . . . 13
      

           3.1.5. Wildcard Records ...................................11
 
   3.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . . 14
      

                                                                         
 
     3.1.  DNS Resource Records . . . . . . . . . . . . . . . . . . . 14
      

   4. The check_host() Function ......................................12
 
     3.2.  Multiple DNS Records . . . . . . . . . . . . . . . . . . . 15
      

      4.1. Arguments .................................................12
 
     3.3.  Multiple Strings in a Single DNS record  . . . . . . . . . 15
      

      4.2. Results ...................................................13
 
     3.4.  Record Size  . . . . . . . . . . . . . . . . . . . . . . . 15
      

      4.3. Initial Processing ........................................13
 
     3.5.  Wildcard Records . . . . . . . . . . . . . . . . . . . . . 15
      

      4.4. Record Lookup .............................................13
 
   4.  The check_host() Function  . . . . . . . . . . . . . . . . . . 17
      

      4.5. Selecting Records .........................................13
 
     4.1.  Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 17
      

      4.6. Record Evaluation .........................................14
 
     4.2.  Results  . . . . . . . . . . . . . . . . . . . . . . . . . 17
      

           4.6.1. Term Evaluation ....................................14
 
     4.3.  Initial Processing . . . . . . . . . . . . . . . . . . . . 17
      

           4.6.2. Mechanisms .........................................15
 
     4.4.  Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 18
      

           4.6.3. Modifiers ..........................................15
 
     4.5.  Selecting Records  . . . . . . . . . . . . . . . . . . . . 18
      

      4.7. Default Result ............................................16
 
     4.6.  Record Evaluation  . . . . . . . . . . . . . . . . . . . . 18
      

      4.8. Domain Specification ......................................16
 
       4.6.1.  Term Evaluation  . . . . . . . . . . . . . . . . . . . 19
      

   5. Mechanism Definitions ..........................................16
 
       4.6.2.  Mechanisms . . . . . . . . . . . . . . . . . . . . . . 19
      

      5.1. "all" .....................................................17
 
       4.6.3.  Modifiers  . . . . . . . . . . . . . . . . . . . . . . 20
      

      5.2. "include" .................................................18
 
       4.6.4.  DNS Lookup Limits  . . . . . . . . . . . . . . . . . . 20
      

      5.3. "a" .......................................................19
 
     4.7.  Default Result . . . . . . . . . . . . . . . . . . . . . . 21
      

      5.4. "mx" ......................................................20
 
     4.8.  Domain Specification . . . . . . . . . . . . . . . . . . . 21
      

      5.5. "ptr" .....................................................20
 
   5.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 22
      

      5.6. "ip4" and "ip6" ...........................................21
 
     5.1.  "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 23
      

      5.7. "exists" ..................................................22
 
     5.2.  "include"  . . . . . . . . . . . . . . . . . . . . . . . . 23
      

   6. Modifier Definitions ...........................................22
 
     5.3.  "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
      

      6.1. redirect: Redirected Query ................................23
 
     5.4.  "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
      

      6.2. exp: Explanation ..........................................23
 
     5.5.  "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 25
      

   7. The Received-SPF Header Field ..................................25
 
     5.6.  "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 27
      

   8. Macros .........................................................27
 
     5.7.  "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 27
      

      8.1. Macro Definitions .........................................27
 
   6.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 29
      

      8.2. Expansion Examples ........................................30
 
     6.1.  redirect: Redirected Query . . . . . . . . . . . . . . . . 29
      

   9. Implications ...................................................31
 
     6.2.  exp: Explanation . . . . . . . . . . . . . . . . . . . . . 30
      

      9.1. Sending Domains ...........................................31
 
   7.  Recording The Result . . . . . . . . . . . . . . . . . . . . . 32
      

      9.2. Mailing Lists .............................................32
 
     7.1.  The Received-SPF Header Field  . . . . . . . . . . . . . . 32
      

      9.3. Forwarding Services and Aliases ...........................32
 
     7.2.  SPF Results in the Authentication-Results Header Field . . 34
      

      9.4. Mail Services .............................................34
 
   8.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
      

      9.5. MTA Relays ................................................34
 
     8.1.  Macro Definitions  . . . . . . . . . . . . . . . . . . . . 36
      

   10. Security Considerations .......................................35
 
     8.2.  Expansion Examples . . . . . . . . . . . . . . . . . . . . 39
      

      10.1. Processing Limits ........................................35
 
   9.  Implications . . . . . . . . . . . . . . . . . . . . . . . . . 41
      

      10.2. SPF-Authorized E-Mail May Contain Other False
 
     9.1.  Sending Domains  . . . . . . . . . . . . . . . . . . . . . 41
      

            Identities ...............................................37
 
       9.1.1.  DNS Resource Considerations  . . . . . . . . . . . . . 41
      

      10.3. Spoofed DNS and IP Data ..................................37
 
       9.1.2.  Administrator's Considerations . . . . . . . . . . . . 42
      

      10.4. Cross-User Forgery .......................................37
 
       9.1.3.  Bounces  . . . . . . . . . . . . . . . . . . . . . . . 43
      

      10.5. Untrusted Information Sources ............................38
 
     9.2.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . 43
      

      10.6. Privacy Exposure .........................................38
 
       9.2.1.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . 43
      

   11. Contributors and Acknowledgements .............................38
 
       9.2.2.  Forwarding Services and Aliases  . . . . . . . . . . . 44
      

   12. IANA Considerations ...........................................39
 
       9.2.3.  Mail Services  . . . . . . . . . . . . . . . . . . . . 46
      

      12.1. The SPF DNS Record Type ..................................39
 
       9.2.4.  MTA Relays . . . . . . . . . . . . . . . . . . . . . . 46
      

      12.2. The Received-SPF Mail Header Field .......................39
 
     9.3.  Receivers  . . . . . . . . . . . . . . . . . . . . . . . . 47
      

   13. References ....................................................39
 
       9.3.1.  Policy For SPF Pass  . . . . . . . . . . . . . . . . . 47
      

      13.1. Normative References .....................................39
 
       9.3.2.  Policy For SPF Fail  . . . . . . . . . . . . . . . . . 47
      

      13.2. Informative References ...................................40
 
       9.3.3.  Policy For SPF Permerror . . . . . . . . . . . . . . . 48
      

                                                                         
 
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 49
      

   Appendix A.  Collected ABNF .......................................42
 
     10.1. Processing Limits  . . . . . . . . . . . . . . . . . . . . 49
      

   Appendix B.  Extended Examples ....................................44
 
     10.2. SPF-Authorized Email May Contain Other False Identities  . 49
      

      B.1.  Simple Examples ..........................................44
 
     10.3. Spoofed DNS and IP Data  . . . . . . . . . . . . . . . . . 50
      

      B.2.  Multiple Domain Example ..................................45
 
     10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 50
      

      B.3.  DNSBL Style Example ......................................46
 
     10.5. Untrusted Information Sources  . . . . . . . . . . . . . . 50
      

      B.4.  Multiple Requirements Example ............................46
 
       10.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 50
      


 
       10.5.2. External Explanations  . . . . . . . . . . . . . . . . 51
      


 
       10.5.3. Macro Expansion  . . . . . . . . . . . . . . . . . . . 51
      


 
     10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 51
      


 
   11. Contributors and Acknowledgements  . . . . . . . . . . . . . . 52
      


 
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 53
      


 
     12.1. The SPF DNS Record Type  . . . . . . . . . . . . . . . . . 53
      


 
     12.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 53
      


 
     12.3. SPF Modifier Registration  . . . . . . . . . . . . . . . . 53
      


 
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54
      


 
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 54
      


 
     13.2. Informative References . . . . . . . . . . . . . . . . . . 55
      


 
   Appendix A.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 57
      


 
   Appendix B.  Extended Examples . . . . . . . . . . . . . . . . . . 60
      


 
     B.1.  Simple Examples  . . . . . . . . . . . . . . . . . . . . . 60
      


 
     B.2.  Multiple Domain Example  . . . . . . . . . . . . . . . . . 61
      


 
     B.3.  DNSBL Style Example  . . . . . . . . . . . . . . . . . . . 62
      


 
     B.4.  Multiple Requirements Example  . . . . . . . . . . . . . . 62
      


 
   Appendix C.  Change History  . . . . . . . . . . . . . . . . . . . 63
      


 
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 66
      


 

      

1.  Introduction
 
1.  Introduction
      


 

      
      





   The current E-Mail infrastructure has the property that any host
 
   The current email infrastructure has the property that any host
      

   injecting mail into the mail system can identify itself as any domain
 
   injecting mail into the system can use any DNS domain name it wants
      

   name it wants.  Hosts can do this at a variety of levels: in
 
   in each of the various identifiers specified by [RFC5321] and
      

   particular, the session, the envelope, and the mail headers.
 
   [RFC5322].  Although this feature is desirable in some circumstances,
      

   Although this feature is desirable in some circumstances, it is a
 
   it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka
      

   major obstacle to reducing Unsolicited Bulk E-Mail (UBE, aka spam).
 
   spam).  Furthermore, many domain owning ADMDs (ADministrative
      

   Furthermore, many domain name holders are understandably concerned
 
   Management Domains, see [RFC5598]) are understandably concerned about
      

   about the ease with which other entities may make use of their domain
 
   the ease with which other entities can make use of their domain
      

   names, often with malicious intent.
 
   names, often with malicious intent.
      


 

      
      





   This document defines a protocol by which domain owners may authorize
 
   This document defines a protocol by which ADMDs can authorize hosts
      

   hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
 
   to use their domain names in the "MAIL FROM" or "HELO" identities.
      

   Compliant domain holders publish Sender Policy Framework (SPF)
 
   Compliant ADMDs publish Sender Policy Framework (SPF) records in the
      

   records specifying which hosts are permitted to use their names, and
 
   DNS specifying which hosts are permitted to use their names, and
      

   compliant mail receivers use the published SPF records to test the
 
   compliant mail receivers use the published SPF records to test the
      

   authorization of sending Mail Transfer Agents (MTAs) using a given
 
   authorization of sending Mail Transfer Agents (MTAs) using a given
      

   "HELO" or "MAIL FROM" identity during a mail transaction.
 
   "HELO" or "MAIL FROM" identity during a mail transaction.
      


 

      

   An additional benefit to mail receivers is that after the use of an
 
   An additional benefit to mail receivers is that after the use of an
      

   identity is verified, local policy decisions about the mail can be
 
   identity is verified, local policy decisions about the mail can be
      

   made based on the sender's domain, rather than the host's IP address.
 
   made based on the sender's domain, rather than the host's IP address.
      

   This is advantageous because reputation of domain names is likely to
 
   This is advantageous because reputation of domain names is likely to
      

   be more accurate than reputation of host IP addresses.  Furthermore,
 
   be more accurate than reputation of host IP addresses.  Furthermore,
      

   if a claimed identity fails verification, local policy can take
 
   if a claimed identity fails verification, local policy can take
      
      





   stronger action against such E-Mail, such as rejecting it.
 
   stronger action against such email, such as rejecting it.
      


 

      

1.1.  Protocol Status
 
1.1.  Protocol Status
      


 

      

   SPF has been in development since the summer of 2003 and has seen
 
   SPF has been in development since the summer of 2003 and has seen
      

   deployment beyond the developers beginning in December 2003.  The
 
   deployment beyond the developers beginning in December 2003.  The
      

   design of SPF slowly evolved until the spring of 2004 and has since
 
   design of SPF slowly evolved until the spring of 2004 and has since
      

   stabilized.  There have been quite a number of forms of SPF, some
 
   stabilized.  There have been quite a number of forms of SPF, some
      

   written up as documents, some submitted as Internet Drafts, and many
 
   written up as documents, some submitted as Internet Drafts, and many
      
      





   discussed and debated in development forums.
 
   discussed and debated in development forums.  The protocol was
      


 
   originally defined in [RFC4408], which this document replaces.
      


 

      
      





   The goal of this document is to clearly document the protocol defined
 
   [RFC4408] was designed to clearly document the protocol defined by
      

   by earlier draft specifications of SPF as used in existing
 
   earlier draft specifications of SPF as used in existing
      

   implementations.  This conception of SPF is sometimes called "SPF
 
   implementations.  This updated specification is intended to clarify
      

   Classic".  It is understood that particular implementations and
 
   identified ambiguities in [RFC4408], resolve techincal issues
      

   deployments may differ from, and build upon, this work.  It is hoped
 
   identified in post-RFC 4408 deplyment experience, and document widely
      

   that we have nonetheless captured the common understanding of SPF
 
   deployed extensions to SPF that have been developed since [RFC4408]
      

   version 1.
 
   was published.
      


 

      
      





1.2.  Terminology
 
1.2.  Experimental History
      


 

      


 
   This document updates and replaces RFC 4408 that was part of a group
      


 
   of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,
      


 
   RFC 4407, and RFC 4408) in 2006.  At that time the IESG requested the
      


 
   community observe the success or failure of the two approaches
      


 
   documented in these RFCs during the two years following publication,
      


 
   in order that a community consensus could be reached in the future.
      


 

      


 
   SPF is widely deployed by large and small email providers alike.
      


 
   There are multiple, interoperable implementations.
      


 

      


 
   For SPF (as documented in RFC 4408) a careful effort was made to
      


 
   collect and document lessons learned and errata during the two year
      


 
   period.  The errata list has been stable (no new submissions) and
      


 
   only minor protocol lessons learned were identified.  Resolution of
      


 
   the IESG's experiment is documented in [RFC6686].
      


 

      


 
1.3.  Terminology
      


 
                                                                         
      


 
1.3.1.  Keywords
      


 

      

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      
      





   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
      

   document are to be interpreted as described in [RFC2119].
 
   "OPTIONAL" in this document are to be interpreted as described in
      


 
   [RFC2119].
      


 
                                                                         
      


 
1.3.2.  Imported Definitions
      


 

      


 
   The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].
      


 

      


 
   The token "local-part" is defined in [RFC5321].
      


 

      


 
   "dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are
      


 
   defined in [RFC5322].
      


 

      


 
1.3.3.  Mail From Definition
      


 

      

   This document is concerned with the portion of a mail message
 
   This document is concerned with the portion of a mail message
      

   commonly called "envelope sender", "return path", "reverse path",
 
   commonly called "envelope sender", "return path", "reverse path",
      
      





   "bounce address", "2821 FROM", or "MAIL FROM".  Since these terms are
 
   "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom.
      

   either not well defined or often used casually, this document defines
 
   Since these terms are either not well defined or often used casually,
      

   the "MAIL FROM" identity in Section 2.2.  Note that other terms that
 
   this document uses "MAIL FROM" for consistency.  This means the
      

   may superficially look like the common terms, such as "reverse-path",
 
   RFC5321.MailFrom as defined in [RFC5598].  Note that other terms that
      

   are used only with the defined meanings from normative documents.
 
   might superficially look like the common terms, such as "reverse-
      


 
   path", are used only with the defined meanings from normative
      


 
   documents.
      


 

      
      





2.  Operation
 
1.3.4.  HELO Definition
      


 

      
      





2.1.  The HELO Identity
 
   This document also makes use of the HELO/EHLO identity.  The "HELO"
      


 
   identity derives from either the SMTP HELO or EHLO command (see
      


 
   [RFC5321]).  Since HELO and EHLO can, in many cases, be used
      


 
   interchangeably, they are identified commonly as "HELO" in this
      


 
   document.  This means RFC5321.HELO/.EHLO as defined in [RFC5598].
      


 
   These commands supply the identity of the SMTP client (sending host)
      


 
   for the SMTP session.
      


 

      
      





   The "HELO" identity derives from either the SMTP HELO or EHLO command
 
1.3.5.  Deprecated
      

   (see [RFC2821]).  These commands supply the SMTP client (sending
 

      

   host) for the SMTP session.  Note that requirements for the domain
 

      

   presented in the EHLO or HELO command are not always clear to the
 

      

   sending party, and SPF clients must be prepared for the "HELO"
 

      

   identity to be malformed or an IP address literal.  At the time of
 

      

   this writing, many legitimate E-Mails are delivered with invalid HELO
 

      

   domains.
 

      


 

      
      





   It is RECOMMENDED that SPF clients not only check the "MAIL FROM"
 
   There are [RFC4408] features that are marked "deprecated".  In the
      


 
   context of this document, deprecated means that senders SHOULD NOT
      


 
   publish SPF records that make use of such features because they might
      


 
   be removed entirely in future updates to the protocol.  Such features
      


 
   do, however, remain part of the SPF protocol and receiving systems
      


 
   MUST support them unless this document explicitly says otherwise.
      


 

      


 
2.  Operation
      


 

      


 
2.1.  The "HELO" Identity
      


 
                                                                         
      


 
   It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"
      

   identity, but also separately check the "HELO" identity by applying
 
   identity, but also separately check the "HELO" identity by applying
      

   the check_host() function (Section 4) to the "HELO" identity as the
 
   the check_host() function (Section 4) to the "HELO" identity as the
      
      





   <sender>.
 
   <sender>.  Checking "HELO" promotes consistency of results and can
      


 
   reduce DNS resource usage.  Additionally, since SPF records published
      


 
   for "HELO" identities refer to a single host, when available, they
      


 
   are a very reliable source of host authorization status.
      


 

      
      





2.2.  The MAIL FROM Identity
 
   Note that requirements for the domain presented in the EHLO or HELO
      


 
   command are not always clear to the sending party, and SPF verifiers
      


 
   MUST be prepared for the "HELO" identity to be malformed or an IP
      


 
   address literal.  This SPF check can only be performed when the
      


 
   "HELO" string is a valid fully qualified domain.
      


 

      
      





   The "MAIL FROM" identity derives from the SMTP MAIL command (see
 
2.2.  The "MAIL FROM" Identity
      

   [RFC2821]).  This command supplies the "reverse-path" for a message,
 

      

   which generally consists of the sender mailbox, and is the mailbox to
 

      

   which notification messages are to be sent if there are problems
 

      

   delivering the message.
 

      


 

      
      





   [RFC2821] allows the reverse-path to be null (see Section 4.5.5 in
 
   SPF verifiers MUST check the ""MAIL FROM" identity if a completed
      

   RFC 2821).  In this case, there is no explicit sender mailbox, and
 
   "HELO" check has not reached a definitive policy result by applying
      


 
   the check_host() function to the "MAIL FROM" identity as the
      


 
   <sender>.
      


 

      


 
   [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in
      


 
   [RFC5321]).  In this case, there is no explicit sender mailbox, and
      

   such a message can be assumed to be a notification message from the
 
   such a message can be assumed to be a notification message from the
      

   mail system itself.  When the reverse-path is null, this document
 
   mail system itself.  When the reverse-path is null, this document
      

   defines the "MAIL FROM" identity to be the mailbox composed of the
 
   defines the "MAIL FROM" identity to be the mailbox composed of the
      
      





   localpart "postmaster" and the "HELO" identity (which may or may not
 
   local-part "postmaster" and the "HELO" identity (which might or might
      

   have been checked separately before).
 
   not have been checked separately before).
      

                                                                         
 

      

   SPF clients MUST check the "MAIL FROM" identity.  SPF clients check
 

      

   the "MAIL FROM" identity by applying the check_host() function to the
 

      

   "MAIL FROM" identity as the <sender>.
 

      


 

      

2.3.  Publishing Authorization
 
2.3.  Publishing Authorization
      


 

      
      





   An SPF-compliant domain MUST publish a valid SPF record as described
 
   An SPF-compliant domain MUST have valid SPF records as described in
      

   in Section 3.  This record authorizes the use of the domain name in
 
   Section 3.  These records authorize the use of the relevant domain
      

   the "HELO" and "MAIL FROM" identities by the MTAs it specifies.
 
   names in the "HELO" and "MAIL FROM" identities by the MTAs specified
      


 
   therein.
      


 

      
      





   If domain owners choose to publish SPF records, it is RECOMMENDED
 
   SPF results can be used to make both positive (source is authorized)
      

   that they end in "-all", or redirect to other records that do, so
 
   and negative (source is not authorized) determinations.  If domain
      

   that a definitive determination of authorization can be made.
 
   owners choose to publish SPF records and want to support receivers
      


 
   making negative authorization determinations, then they MUST publish
      


 
   records that end in "-all", or redirect to other records that do,
      


 
   otherwise, no definitive determination of authorization can be made.
      


 
   Potential issues and mitigations associated with negative
      


 
   determinations are discussed in Section 9.
      


 

      
      





   Domain holders may publish SPF records that explicitly authorize no
 
   ADMDs can publish SPF records that explicitly authorize no hosts for
      

   hosts if mail should never originate using that domain.
 
   domain names that are neither used in the domain part of email
      


 
   addresses nor expected to originate mail.
      


 

      
      





   When changing SPF records, care must be taken to ensure that there is
 
   When changing SPF records, care has to be taken to ensure that there
      

   a transition period so that the old policy remains valid until all
 
   is a transition period so that the old policy remains valid until all
      

   legitimate E-Mail has been checked.
 
   legitimate email can reasonably expect to have been checked.  This
      


 
   can be as much as 30 days.
      


 

      

2.4.  Checking Authorization
 
2.4.  Checking Authorization
      


 

      

   A mail receiver can perform a set of SPF checks for each mail message
 
   A mail receiver can perform a set of SPF checks for each mail message
      

   it receives.  An SPF check tests the authorization of a client host
 
   it receives.  An SPF check tests the authorization of a client host
      

   to emit mail with a given identity.  Typically, such checks are done
 
   to emit mail with a given identity.  Typically, such checks are done
      

   by a receiving MTA, but can be performed elsewhere in the mail
 
   by a receiving MTA, but can be performed elsewhere in the mail
      

   processing chain so long as the required information is available and
 
   processing chain so long as the required information is available and
      

   reliable.  At least the "MAIL FROM" identity MUST be checked, but it
 
   reliable.  At least the "MAIL FROM" identity MUST be checked, but it
      

   is RECOMMENDED that the "HELO" identity also be checked beforehand.
 
   is RECOMMENDED that the "HELO" identity also be checked beforehand.
      


 

      

   Without explicit approval of the domain owner, checking other
 
   Without explicit approval of the domain owner, checking other
      

   identities against SPF version 1 records is NOT RECOMMENDED because
 
   identities against SPF version 1 records is NOT RECOMMENDED because
      

   there are cases that are known to give incorrect results.  For
 
   there are cases that are known to give incorrect results.  For
      

   example, almost all mailing lists rewrite the "MAIL FROM" identity
 
   example, almost all mailing lists rewrite the "MAIL FROM" identity
      
      





   (see Section 9.2), but some do not change any other identities in the
 
   (see Section 9.2.1), but some do not change any other identities in
      

   message.  The scenario described in Section 9.3, sub-section 1.2, is
 
   the message.  The scenario described in Section 9.2.2, sub-section
      

   another example.  Documents that define other identities should
 
   1.2, is another example.  Documents that define other identities will
      

   define the method for explicit approval.
 
   have to define the method for explicit approval.
      


 

      

   It is possible that mail receivers will use the SPF check as part of
 
   It is possible that mail receivers will use the SPF check as part of
      

   a larger set of tests on incoming mail.  The results of other tests
 
   a larger set of tests on incoming mail.  The results of other tests
      
      





   may influence whether or not a particular SPF check is performed.
 
   might influence whether or not a particular SPF check is performed.
      

   For example, finding the sending host's IP address on a local white
 
   For example, finding the sending host's IP address on a local white
      
      





   list may cause all other tests to be skipped and all mail from that
 
   list might cause all other tests to be skipped and all mail from that
      

   host to be accepted.
 
   host to be accepted.
      


 

      

   When a mail receiver decides to perform an SPF check, it MUST use a
 
   When a mail receiver decides to perform an SPF check, it MUST use a
      

   correctly-implemented check_host() function (Section 4) evaluated
 
   correctly-implemented check_host() function (Section 4) evaluated
      

   with the correct parameters.  Although the test as a whole is
 
   with the correct parameters.  Although the test as a whole is
      
      





   optional, once it has been decided to perform a test it must be
 
   optional, once it has been decided to perform a test it has to be
      

   performed as specified so that the correct semantics are preserved
 
   performed as specified so that the correct semantics are preserved
      

   between publisher and receiver.
 
   between publisher and receiver.
      


 

      

   To make the test, the mail receiver MUST evaluate the check_host()
 
   To make the test, the mail receiver MUST evaluate the check_host()
      

   function with the arguments set as follows:
 
   function with the arguments set as follows:
      


 

      

   <ip>     - the IP address of the SMTP client that is emitting the
 
   <ip>     - the IP address of the SMTP client that is emitting the
      

              mail, either IPv4 or IPv6.
 
              mail, either IPv4 or IPv6.
      


 

      

   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
 
   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
      


 

      

   <sender> - the "MAIL FROM" or "HELO" identity.
 
   <sender> - the "MAIL FROM" or "HELO" identity.
      


 

      
      





   Note that the <domain> argument may not be a well-formed domain name.
 
   Note that the <domain> argument might not be a well-formed domain
      

   For example, if the reverse-path was null, then the EHLO/HELO domain
 
   name.  For example, if the reverse-path was null, then the EHLO/HELO
      

   is used, with its associated problems (see Section 2.1).  In these
 
   domain is used, with its associated problems (see Section 2.1).  In
      

   cases, check_host() is defined in Section 4.3 to return a "None"
 
   these cases, check_host() is defined in Section 4.3 to return a
      

   result.
 
   "none" result.
      


 

      

   Although invalid, malformed, or non-existent domains cause SPF checks
 
   Although invalid, malformed, or non-existent domains cause SPF checks
      
      





   to return "None" because no SPF record can be found, it has long been
 
   to return "none" because no SPF record can be found, it has long been
      

   the policy of many MTAs to reject E-Mail from such domains,
 
   the policy of many MTAs to reject email from such domains, especially
      

   especially in the case of invalid "MAIL FROM".  In order to prevent
 
   in the case of invalid "MAIL FROM".  Rejecting email will prevent one
      

   the circumvention of SPF records, rejecting E-Mail from invalid
 
   method of circumventing of SPF records.
      

   domains should be considered.
 

      


 

      
      





   Implementations must take care to correctly extract the <domain> from
 
   Implementations have to take care to correctly extract the <domain>
      

   the data given with the SMTP MAIL FROM command as many MTAs will
 
   from the data given with the SMTP MAIL FROM command as many MTAs will
      

   still accept such things as source routes (see [RFC2821], Appendix
 
   still accept such things as source routes (see [RFC5321], Appendix
      

   C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
 
   C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
      

   These archaic features have been maliciously used to bypass security
 
   These archaic features have been maliciously used to bypass security
      

   systems.
 
   systems.
      


 

      

2.5.  Interpreting the Result
 
2.5.  Interpreting the Result
      


 

      

   This section describes how software that performs the authorization
 
   This section describes how software that performs the authorization
      
      





   should interpret the results of the check_host() function.  The
 
   interprets the results of the check_host() function.  The
      

   authorization check SHOULD be performed during the processing of the
 
   authorization check SHOULD be performed during the processing of the
      

   SMTP transaction that sends the mail.  This allows errors to be
 
   SMTP transaction that sends the mail.  This allows errors to be
      

   returned directly to the sending MTA by way of SMTP replies.
 
   returned directly to the sending MTA by way of SMTP replies.
      


 

      
      





   Performing the authorization after the SMTP transaction has finished
 
   Performing the authorization other than using the return-path and
      

   may cause problems, such as the following: (1) It may be difficult to
 
   client address at the time of the MAIL command during the SMTP
      

   accurately extract the required information from potentially
 
   transaction can cause problems, such as the following: (1) It might
      

   deceptive headers; (2) legitimate E-Mail may fail because the
 
   be difficult to accurately extract the required information from
      

   sender's policy may have since changed.
 
   potentially deceptive headers; (2) legitimate email might fail
      


 
   because the sender's policy had since changed.
      


 

      

   Generating non-delivery notifications to forged identities that have
 
   Generating non-delivery notifications to forged identities that have
      
      





   failed the authorization check is generally abusive and against the
 
   failed the authorization check is a source of backscatter and SHOULD
      

   explicit wishes of the identity owner.
 
   be avoided.  [RFC3834] section 2 describes backscatter and the
      


 
   problems it causes.
      


 

      

2.5.1.  None
 
2.5.1.  None
      


 

      
      





   A result of "None" means that no records were published by the domain
 
   A result of "none" means either (a) no syntactically valid DNS domain
      

   or that no checkable sender domain could be determined from the given
 
   name was extracted from the SMTP session that could be used as the
      

   identity.  The checking software cannot ascertain whether or not the
 
   one to be authorized, or (b) no TXT records were retrieved from the
      

   client host is authorized.
 
   DNS that appeared to be intended for use by SPF verifiers.
      


 

      

2.5.2.  Neutral
 
2.5.2.  Neutral
      


 

      
      





   The domain owner has explicitly stated that he cannot or does not
 
   The domain owner has explicitly stated that they cannot or do not
      

   want to assert whether or not the IP address is authorized.  A
 
   want to assert whether the IP address is authorized or not.  A
      

   "Neutral" result MUST be treated exactly like the "None" result; the
 
   "neutral" result MUST be treated exactly like the "none" result; the
      

   distinction exists only for informational purposes.  Treating
 
   distinction exists only for informational purposes.  Treating
      
      





   "Neutral" more harshly than "None" would discourage domain owners
 
   "neutral" more harshly than "none" would discourage domain owners
      

   from testing the use of SPF records (see Section 9.1).
 
   from testing the use of SPF records (see Section 9.1).
      


 

      

2.5.3.  Pass
 
2.5.3.  Pass
      


 

      
      





   A "Pass" result means that the client is authorized to inject mail
 
   A "pass" result means that the client is authorized to inject mail
      

   with the given identity.  The domain can now, in the sense of
 
   with the given identity.  The domain can now, in the sense of
      

   reputation, be considered responsible for sending the message.
 
   reputation, be considered responsible for sending the message.
      

   Further policy checks can now proceed with confidence in the
 
   Further policy checks can now proceed with confidence in the
      
      





   legitimate use of the identity.
 
   legitimate use of the identity.  This is further discussed in
      


 
   Section 9.3.1.
      


 

      

2.5.4.  Fail
 
2.5.4.  Fail
      


 

      
      





   A "Fail" result is an explicit statement that the client is not
 
   A "fail" result is an explicit statement that the client is not
      

   authorized to use the domain in the given identity.  The checking
 
   authorized to use the domain in the given identity.  Disposition of
      

   software can choose to mark the mail based on this or to reject the
 
   SPF fail messages is a matter of local policy.  See Section 9.3.2 for
      

   mail outright.
 
   considerations on developing local policy.
      


 

      

   If the checking software chooses to reject the mail during the SMTP
 
   If the checking software chooses to reject the mail during the SMTP
      

   transaction, then it SHOULD use an SMTP reply code of 550 (see
 
   transaction, then it SHOULD use an SMTP reply code of 550 (see
      
      





   [RFC2821]) and, if supported, the 5.7.1 Delivery Status Notification
 
   [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see
      

   (DSN) code (see [RFC3464]), in addition to an appropriate reply text.
 
   [RFC3463]), in addition to an appropriate reply text.  The
      

   The check_host() function may return either a default explanation
 
   check_host() function will return either a default explanation string
      

   string or one from the domain that published the SPF records (see
 
   or one from the domain that published the SPF records (see
      

   Section 6.2).  If the information does not originate with the
 
   Section 6.2).  If the information does not originate with the
      
      





   checking software, it should be made clear that the text is provided
 
   checking software, it is good to make it clear that the text is
      

   by the sender's domain.  For example:
 
   provided by the sender's domain.  For example:
      


 

      

       550-5.7.1 SPF MAIL FROM check failed:
 
       550-5.7.1 SPF MAIL FROM check failed:
      

       550-5.7.1 The domain example.com explains:
 
       550-5.7.1 The domain example.com explains:
      

       550 5.7.1 Please see http://www.example.com/mailpolicy.html
 
       550 5.7.1 Please see http://www.example.com/mailpolicy.html
      


 

      
      





2.5.5.  SoftFail
 
   If the checking software chooses not to reject the mail during the
      


 
   SMTP transaction, then it SHOULD add a Received-SPF or
      


 
   Authentication-Results header field (see Section 7) to communicate
      


 
   this result to downstream message processors.  While this is true for
      


 
   all SPF results, it is of particular importance for "fail" results
      


 
   since the message is explicitly not authorized by the domain owner.
      


 

      
      





   A "SoftFail" result should be treated as somewhere between a "Fail"
 
2.5.5.  Softfail
      

   and a "Neutral".  The domain believes the host is not authorized but
 
                                                                         
      

   is not willing to make that strong of a statement.  Receiving
 
   A "softfail" result ought to be treated as somewhere between "fail"
      

   software SHOULD NOT reject the message based solely on this result,
 
   and "neutral"/"none".  The domain owner believes the host is not
      

   but MAY subject the message to closer scrutiny than normal.
 
   authorized but is not willing to make a strong policy statement.
      


 
   Receiving software SHOULD NOT reject the message based solely on this
      


 
   result, but MAY subject the message to closer scrutiny than normal.
      


 

      

   The domain owner wants to discourage the use of this host and thus
 
   The domain owner wants to discourage the use of this host and thus
      
      





   desires limited feedback when a "SoftFail" result occurs.  For
 
   desires limited feedback when a "softfail" result occurs.  For
      

   example, the recipient's Mail User Agent (MUA) could highlight the
 
   example, the recipient's Mail User Agent (MUA) could highlight the
      
      





   "SoftFail" status, or the receiving MTA could give the sender a
 
   "softfail" status, or the receiving MTA could give the sender a
      

   message using a technique called "greylisting" whereby the MTA can
 
   message using greylisting, [RFC6647], with a note the first time the
      

   issue an SMTP reply code of 451 (4.3.0 DSN code) with a note the
 
   message is received, but accept it on a later attempt based on
      

   first time the message is received, but accept it the second time.
 
   receiver policy.
      


 

      
      





2.5.6.  TempError
 
2.5.6.  Temperror
      


 

      
      





   A "TempError" result means that the SPF client encountered a
 
   A "temperror" result means the SPF verifier encountered a transient
      

   transient error while performing the check.  Checking software can
 
   (generally DNS) error while performing the check.  Checking software
      

   choose to accept or temporarily reject the message.  If the message
 
   can choose to accept or temporarily reject the message.  If the
      

   is rejected during the SMTP transaction for this reason, the software
 
   message is rejected during the SMTP transaction for this reason, the
      

   SHOULD use an SMTP reply code of 451 and, if supported, the 4.4.3 DSN
 
   software SHOULD use an SMTP reply code of 451 and, if supported, the
      

   code.
 
   4.4.3 enhanced status code.  These errors can be caused by problems
      


 
   in either the sender's or receiver's DNS software.
      


 

      
      





2.5.7.  PermError
 
2.5.7.  Permerror
      


 

      
      





   A "PermError" result means that the domain's published records could
 
   A "permerror" result means the domain's published records could not
      

   not be correctly interpreted.  This signals an error condition that
 
   be correctly interpreted.  This signals an error condition that
      

   requires manual intervention to be resolved, as opposed to the
 
   definitely requires manual intervention to be resolved.  If the
      

   TempError result.  Be aware that if the domain owner uses macros
 
   message is rejected during the SMTP transaction for this reason, the
      

   (Section 8), it is possible that this result is due to the checked
 
   software SHOULD use an SMTP reply code of 550 and, if supported, the
      

   identities having an unexpected format.
 
   5.5.2 enhanced status code.  Be aware that if the domain owner uses
      


 
   macros (Section 8), it is possible that this result is due to the
      


 
   checked identities having an unexpected format.  It is also possible
      


 
   that this result is generated by certain SPF clients due to the input
      


 
   arguments having an unexpected format; see Section 4.8.
      


 

      

3.  SPF Records
 
3.  SPF Records
      


 

      
      





   An SPF record is a DNS Resource Record (RR) that declares which hosts
 
   An SPF record is a DNS record that declares which hosts are, and are
      

   are, and are not, authorized to use a domain name for the "HELO" and
 
   not, authorized to use a domain name for the "HELO" and "MAIL FROM"
      

   "MAIL FROM" identities.  Loosely, the record partitions all hosts
 
   identities.  Loosely, the record partitions all hosts into permitted
      

   into permitted and not-permitted sets (though some hosts might fall
 
   and not-permitted sets (though some hosts might fall into neither
      

   into neither category).
 
   category).
      


 

      
      





   The SPF record is a single string of text.  An example record is the
 
   The SPF record is a single string of text.  The record format is
      

   following:
 
   described below in Section 4.  An example record is the following:
      


 

      

      v=spf1 +mx a:colo.example.com/28 -all
 
      v=spf1 +mx a:colo.example.com/28 -all
      


 

      

   This record has a version of "spf1" and three directives: "+mx",
 
   This record has a version of "spf1" and three directives: "+mx",
      

   "a:colo.example.com/28" (the + is implied), and "-all".
 
   "a:colo.example.com/28" (the + is implied), and "-all".
      


 

      
      





3.1.  Publishing
 
   Each SPF record is placed in the DNS tree at the host name it
      


 

      

   Domain owners wishing to be SPF compliant must publish SPF records
 

      

   for the hosts that are used in the "MAIL FROM" and "HELO" identities.
 

      

   The SPF records are placed in the DNS tree at the host name it
 

      

   pertains to, not a subdomain under it, such as is done with SRV
 
   pertains to, not a subdomain under it, such as is done with SRV
      
      





   records.  This is the same whether the TXT or SPF RR type (see
 
   records [RFC2782].
      

   Section 3.1.1) is used.
 

      


 

      
      





   The example above in Section 3 might be published via these lines in
 
   The example in this section might be published via these lines in a
      

   a domain zone file:
 
   domain zone file:
      


 

      

      example.com.          TXT "v=spf1 +mx a:colo.example.com/28 -all"
 
      example.com.          TXT "v=spf1 +mx a:colo.example.com/28 -all"
      

      smtp-out.example.com. TXT "v=spf1 a -all"
 
      smtp-out.example.com. TXT "v=spf1 a -all"
      


 

      
      





   When publishing via TXT records, beware of other TXT records
 
   Since TXT records have multiple uses, beware of other TXT records
      

   published there for other purposes.  They may cause problems with
 
   published there for other purposes.  They might cause problems with
      

   size limits (see Section 3.1.4).
 
   size limits (see Section 3.4) and care has to be taken to ensure only
      


 
   SPF records are used for SPF processing.
      

3.1.1.  DNS Resource Record Types
 

      


 

      

   This document defines a new DNS RR of type SPF, code 99.  The format
 

      

   of this type is identical to the TXT RR [RFC1035].  For either type,
 

      

   the character content of the record is encoded as [US-ASCII].
 

      


 

      

   It is recognized that the current practice (using a TXT record) is
 

      

   not optimal, but it is necessary because there are a number of DNS
 

      

   server and resolver implementations in common use that cannot handle
 

      

   the new RR type.  The two-record-type scheme provides a forward path
 

      

   to the better solution of using an RR type reserved for this purpose.
 

      


 

      
      





   An SPF-compliant domain name SHOULD have SPF records of both RR
 
   ADMDs publishing SPF records SHOULD try to keep the number of
      

   types.  A compliant domain name MUST have a record of at least one
 
   "include" mechanisms and chained "redirect" modifiers to a minimum.
      

   type.  If a domain has records of both types, they MUST have
 
   ADMDs SHOULD also try to minimize the amount of other DNS information
      

   identical content.  For example, instead of publishing just one
 
   needed to evaluate a record.  Section 4.6.4 and Section 9.1.1 provide
      

   record as in Section 3.1 above, it is better to publish:
 
   some suggestions on how to achieve this.
      


 

      
      





      example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
 
3.1.  DNS Resource Records
      

      example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"
 

      


 

      
      





   Example RRs in this document are shown with the TXT record type;
 
   SPF records MUST be published as a DNS TXT (type 16) Resource Record
      

   however, they could be published with the SPF type or with both
 
   (RR) [RFC1035] only.  The character content of the record is encoded
      

   types.
 
   as [US-ASCII].  Use of alternate DNS RR types was supported in SPF's
      


 
   experimental phase, but has been discontinued.  See Appendix A of
      


 
   [RFC6686] for further information.
      


 

      
      





3.1.2.  Multiple DNS Records
 
3.2.  Multiple DNS Records
      


 

      

   A domain name MUST NOT have multiple records that would cause an
 
   A domain name MUST NOT have multiple records that would cause an
      

   authorization check to select more than one record.  See Section 4.5
 
   authorization check to select more than one record.  See Section 4.5
      

   for the selection rules.
 
   for the selection rules.
      


 

      
      





3.1.3.  Multiple Strings in a Single DNS record
 
3.3.  Multiple Strings in a Single DNS record
      


 

      

   As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS
 
   As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS
      
      





   record (either TXT or SPF RR types) can be composed of more than one
 
   record can be composed of more than one string.  If a published
      

   string.  If a published record contains multiple strings, then the
 
   record contains multiple character-strings, then the record MUST be
      

   record MUST be treated as if those strings are concatenated together
 
   treated as if those strings are concatenated together without adding
      

   without adding spaces.  For example:
 
   spaces.  For example:
      


 

      

      IN TXT "v=spf1 .... first" "second string..."
 
      IN TXT "v=spf1 .... first" "second string..."
      


 

      

   MUST be treated as equivalent to
 
   MUST be treated as equivalent to
      


 

      

      IN TXT "v=spf1 .... firstsecond string..."
 
      IN TXT "v=spf1 .... firstsecond string..."
      


 

      
      





   SPF or TXT records containing multiple strings are useful in
 
   TXT records containing multiple strings are useful in constructing
      

   constructing records that would exceed the 255-byte maximum length of
 
   records that would exceed the 255-byte maximum length of a character-
      

   a string within a single TXT or SPF RR record.
 
   string within a single TXT record.
      


 

      
      





3.1.4.  Record Size
 
3.4.  Record Size
      


 

      

   The published SPF record for a given domain name SHOULD remain small
 
   The published SPF record for a given domain name SHOULD remain small
      

   enough that the results of a query for it will fit within 512 octets.
 
   enough that the results of a query for it will fit within 512 octets.
      
      





   This will keep even older DNS implementations from falling over to
 
   This UDP limit is defined in [RFC1035] section 2.3.4.  This will keep
      

   TCP.  Since the answer size is dependent on many things outside the
 
   even older DNS implementations from falling over to TCP.  Since the
      

   scope of this document, it is only possible to give this guideline:
 
   answer size is dependent on many things outside the scope of this
      

   If the combined length of the DNS name and the text of all the
 
   document, it is only possible to give this guideline: If the combined
      

   records of a given type (TXT or SPF) is under 450 characters, then
 
   length of the DNS name and the text of all the records of a given
      

   DNS answers should fit in UDP packets.  Note that when computing the
 
   type is under 450 characters, then DNS answers ought to fit in UDP
      

   sizes for queries of the TXT format, one must take into account any
 
   packets.  Note that when computing the sizes for queries of the TXT
      

   other TXT records published at the domain name.  Records that are too
 
   format, one has to take into account any other TXT records published
      

   long to fit in a single UDP packet MAY be silently ignored by SPF
 
   at the domain name.  Records that are too long to fit in a single UDP
      

   clients.
 
   packet could be silently ignored by SPF verifiers due to firewall and
      


 
   other issues that cause DNS over TCP to be less reliable than DNS
      

3.1.5.  Wildcard Records
 
   over UDP.
      


 

      
      





   Use of wildcard records for publishing is not recommended.  Care must
 
3.5.  Wildcard Records
      

   be taken if wildcard records are used.  If a domain publishes
 

      

   wildcard MX records, it may want to publish wildcard declarations,
 

      

   subject to the same requirements and problems.  In particular, the
 

      

   declaration must be repeated for any host that has any RR records at
 

      

   all, and for subdomains thereof.  For example, the example given in
 

      

   [RFC1034], Section 4.3.3, could be extended with the following:
 

      


 

      
      





       X.COM.          MX      10      A.X.COM
 
   Use of wildcard records for publishing is discouraged and care has to
      

       X.COM.          TXT     "v=spf1 a:A.X.COM -all"
 
   be taken if they are used.  If a zone includes wildcard MX records,
      


 
   it might want to publish wildcard declarations, subject to the same
      


 
   requirements and problems.  In particular, the declaration MUST be
      


 
   repeated for any host that has any RR records at all, and for
      


 
   subdomains thereof.  Consider the example in [RFC1034], Section
      


 
   4.3.3.  Based on that, we can do the following:
      


 

      
      





       *.X.COM.        MX      10      A.X.COM
 
       EXAMPLE.COM.          MX      10      A.EXAMPLE.COM
      

       *.X.COM.        TXT     "v=spf1 a:A.X.COM -all"
 
       EXAMPLE.COM.          TXT     "v=spf1 a:A.EXAMPLE.COM -all"
      


 

      
      





       A.X.COM.        A       1.2.3.4
 
       *.EXAMPLE.COM.        MX      10      A.EXAMPLE.COM
      

       A.X.COM.        MX      10      A.X.COM
 
       *.EXAMPLE.COM.        TXT     "v=spf1 a:A.EXAMPLE.COM -all"
      

       A.X.COM.        TXT     "v=spf1 a:A.X.COM -all"
 

      


 

      
      





       *.A.X.COM.      MX      10      A.X.COM
 
       A.EXAMPLE.COM.        A       203.0.113.1
      

       *.A.X.COM.      TXT     "v=spf1 a:A.X.COM -all"
 
       A.EXAMPLE.COM.        MX      10      A.EXAMPLE.COM
      


 
       A.EXAMPLE.COM.        TXT     "v=spf1 a:A.EXAMPLE.COM -all"
      


 

      
      





   Notice that SPF records must be repeated twice for every name within
 
       *.A.EXAMPLE.COM.      MX      10      A.EXAMPLE.COM
      

   the domain: once for the name, and once with a wildcard to cover the
 
       *.A.EXAMPLE.COM.      TXT     "v=spf1 a:A.EXAMPLE.COM -all"
      

   tree under the name.
 

      


 

      
      





   Use of wildcards is discouraged in general as they cause every name
 
   SPF records have to be listed twice for every name within the zone:
      

   under the domain to exist and queries against arbitrary names will
 
   once for the name, and once with a wildcard to cover the tree under
      

   never return RCODE 3 (Name Error).
 
   the name, in order to cover all domains in use in outgoing mail.
      


 

      

4.  The check_host() Function
 
4.  The check_host() Function
      


 

      
      






 
   This description is not an API (Application Program Interface)
      


 
   definition, but rather a function description used to illustrate the
      


 
   algorithm.  A compliant SPF implementation MUST do something
      


 
   semantically equivalent to this description.
      


 
                                                                         
      

   The check_host() function fetches SPF records, parses them, and
 
   The check_host() function fetches SPF records, parses them, and
      
      





   interprets them to determine whether a particular host is or is not
 
   evaluates them to determine whether a particular host is or is not
      

   permitted to send mail with a given identity.  Mail receivers that
 
   permitted to send mail with a given identity.  Mail receivers that
      

   perform this check MUST correctly evaluate the check_host() function
 
   perform this check MUST correctly evaluate the check_host() function
      

   as described here.
 
   as described here.
      


 

      

   Implementations MAY use a different algorithm than the canonical
 
   Implementations MAY use a different algorithm than the canonical
      

   algorithm defined here, so long as the results are the same in all
 
   algorithm defined here, so long as the results are the same in all
      

   cases.
 
   cases.
      


 

      

4.1.  Arguments
 
4.1.  Arguments
      


 

      


 

      

skipping to change at page 13, line 12
 
skipping to change at page 17, line 40
      

              information; initially, the domain portion of the "MAIL
 
              information; initially, the domain portion of the "MAIL
      

              FROM" or "HELO" identity.
 
              FROM" or "HELO" identity.
      


 

      

   <sender> - the "MAIL FROM" or "HELO" identity.
 
   <sender> - the "MAIL FROM" or "HELO" identity.
      


 

      

   The domain portion of <sender> will usually be the same as the
 
   The domain portion of <sender> will usually be the same as the
      

   <domain> argument when check_host() is initially evaluated.  However,
 
   <domain> argument when check_host() is initially evaluated.  However,
      

   this will generally not be true for recursive evaluations (see
 
   this will generally not be true for recursive evaluations (see
      

   Section 5.2 below).
 
   Section 5.2 below).
      


 

      
      





   Actual implementations of the check_host() function may need
 

      

   additional arguments.
 

      

                                                                         
 

      

4.2.  Results
 
4.2.  Results
      


 

      

   The function check_host() can return one of several results described
 
   The function check_host() can return one of several results described
      

   in Section 2.5.  Based on the result, the action to be taken is
 
   in Section 2.5.  Based on the result, the action to be taken is
      

   determined by the local policies of the receiver.
 
   determined by the local policies of the receiver.
      


 

      

4.3.  Initial Processing
 
4.3.  Initial Processing
      


 

      
      





   If the <domain> is malformed (label longer than 63 characters, zero-
 
   If the <domain> is malformed (e.g. label longer than 63 characters,
      

   length label not at the end, etc.) or is not a fully qualified domain
 
   zero-length label not at the end, etc.) or is not a fully qualified
      

   name, or if the DNS lookup returns "domain does not exist" (RCODE 3),
 
   domain name, or if the DNS lookup returns "domain does not exist"
      

   check_host() immediately returns the result "None".
 
   (RCODE 3), check_host() immediately returns the result "none".
      


 
   Properly formed domains are fully qualified email domains as
      


 
   described in [RFC5321] Section 2.3.5.  Internationalized domain names
      


 
   MUST be encoded as A-labels, as described in Section 2.3 of
      


 
   [RFC5890].on 2.3 of [RFC5890].
      


 

      
      





   If the <sender> has no localpart, substitute the string "postmaster"
 
   If the <sender> has no local-part, substitute the string "postmaster"
      

   for the localpart.
 
   for the local-part.
      


 

      

4.4.  Record Lookup
 
4.4.  Record Lookup
      


 

      
      





   In accordance with how the records are published (see Section 3.1
 
   In accordance with how the records are published (see Section 3
      

   above), a DNS query needs to be made for the <domain> name, querying
 
   above), a DNS query needs to be made for the <domain> name, querying
      
      





   for either RR type TXT, SPF, or both.  If both SPF and TXT RRs are
 
   for type TXT only.
      

   looked up, the queries MAY be done in parallel.
 

      


 

      

   If all DNS lookups that are made return a server failure (RCODE 2),
 
   If all DNS lookups that are made return a server failure (RCODE 2),
      

   or other error (RCODE other than 0 or 3), or time out, then
 
   or other error (RCODE other than 0 or 3), or time out, then
      
      





   check_host() exits immediately with the result "TempError".
 
   check_host() terminates immediately with the result "temperror".
      


 
   Alternatively, for a server failure (RCODE 2) result, check_host()
      


 
   MAY track failures and treat multiple failures within 24 hours for
      


 
   the same domain as "permerror".
      


 

      


 
   This alternative is intended to shorten the queue time of messages
      


 
   that cannot be accepted, by returning a permanent negative completion
      


 
   reply code to the client, instead of a transient one.  [RFC2308]
      


 
   suggests on an algorithm for doing such tracking and handling of
      


 
   server failure codes.
      


 

      

4.5.  Selecting Records
 
4.5.  Selecting Records
      


 

      

   Records begin with a version section:
 
   Records begin with a version section:
      


 

      

   record           = version terms *SP
 
   record           = version terms *SP
      

   version          = "v=spf1"
 
   version          = "v=spf1"
      


 

      

   Starting with the set of records that were returned by the lookup,
 
   Starting with the set of records that were returned by the lookup,
      
      





   record selection proceeds in two steps:
 
   discard records that do not begin with a version section of exactly
      


 
   "v=spf1".  Note that the version section is terminated either by an
      

   1. Records that do not begin with a version section of exactly
 
   SP character or the end of the record.  A record with a version
      

      "v=spf1" are discarded.  Note that the version section is
 
   section of "v=spf10" does not match and MUST be discarded.
      

      terminated either by an SP character or the end of the record.  A
 

      

      record with a version section of "v=spf10" does not match and must
 

      

      be discarded.
 

      

                                                                         
 

      

   2. If any records of type SPF are in the set, then all records of
 

      

      type TXT are discarded.
 

      


 

      

   After the above steps, there should be exactly one record remaining
 

      

   and evaluation can proceed.  If there are two or more records
 

      

   remaining, then check_host() exits immediately with the result of
 

      

   "PermError".
 

      


 

      
      





   If no matching records are returned, an SPF client MUST assume that
 
   If the resultant record set includes no records, check_host()
      

   the domain makes no SPF declarations.  SPF processing MUST stop and
 
   produces the "none" result.  If the resultant record set includes
      

   return "None".
 
   more than one record, check_host() produces the "permerror" result.
      


 

      

4.6.  Record Evaluation
 
4.6.  Record Evaluation
      


 

      
      





   After one SPF record has been selected, the check_host() function
 
   The check_host() function parses and interprets the SPF record to
      

   parses and interprets it to find a result for the current test.  If
 
   find a result for the current test.  If there are any syntax errors,
      

   there are any syntax errors, check_host() returns immediately with
 
   check_host() returns immediately with the result "permerror".
      

   the result "PermError".
 

      


 

      

   Implementations MAY choose to parse the entire record first and
 
   Implementations MAY choose to parse the entire record first and
      
      





   return "PermError" if the record is not syntactically well formed.
 
   return "permerror" if the record is not syntactically well formed.
      

   However, in all cases, any syntax errors anywhere in the record MUST
 
   However, in all cases, any syntax errors anywhere in the record MUST
      

   be detected.
 
   be detected.
      


 

      

4.6.1.  Term Evaluation
 
4.6.1.  Term Evaluation
      


 

      

   There are two types of terms: mechanisms and modifiers.  A record
 
   There are two types of terms: mechanisms and modifiers.  A record
      

   contains an ordered list of these as specified in the following
 
   contains an ordered list of these as specified in the following
      

   Augmented Backus-Naur Form (ABNF).
 
   Augmented Backus-Naur Form (ABNF).
      


 

      

   terms            = *( 1*SP ( directive / modifier ) )
 
   terms            = *( 1*SP ( directive / modifier ) )
      


 

      

   directive        = [ qualifier ] mechanism
 
   directive        = [ qualifier ] mechanism
      

   qualifier        = "+" / "-" / "?" / "~"
 
   qualifier        = "+" / "-" / "?" / "~"
      

   mechanism        = ( all / include
 
   mechanism        = ( all / include
      

                      / A / MX / PTR / IP4 / IP6 / exists )
 
                      / A / MX / PTR / IP4 / IP6 / exists )
      

   modifier         = redirect / explanation / unknown-modifier
 
   modifier         = redirect / explanation / unknown-modifier
      

   unknown-modifier = name "=" macro-string
 
   unknown-modifier = name "=" macro-string
      
      






 
                      ; where name is not any known modifier
      


 

      

   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 
   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
      


 

      

   Most mechanisms allow a ":" or "/" character after the name.
 
   Most mechanisms allow a ":" or "/" character after the name.
      


 

      

   Modifiers always contain an equals ('=') character immediately after
 
   Modifiers always contain an equals ('=') character immediately after
      
      





   the name, and before any ":" or "/" characters that may be part of
 
   the name, and before any ":" or "/" characters that might be part of
      

   the macro-string.
 
   the macro-string.
      


 

      

   Terms that do not contain any of "=", ":", or "/" are mechanisms, as
 
   Terms that do not contain any of "=", ":", or "/" are mechanisms, as
      

   defined in Section 5.
 
   defined in Section 5.
      


 

      
      





   As per the definition of the ABNF notation in [RFC4234], mechanism
 
   As per the definition of the ABNF notation in [RFC5234], mechanism
      

   and modifier names are case-insensitive.
 
   and modifier names are case-insensitive.
      


 

      

4.6.2.  Mechanisms
 
4.6.2.  Mechanisms
      


 

      

   Each mechanism is considered in turn from left to right.  If there
 
   Each mechanism is considered in turn from left to right.  If there
      

   are no more mechanisms, the result is specified in Section 4.7.
 
   are no more mechanisms, the result is specified in Section 4.7.
      


 

      

   When a mechanism is evaluated, one of three things can happen: it can
 
   When a mechanism is evaluated, one of three things can happen: it can
      
      





   match, not match, or throw an exception.
 
   match, not match, or return an exception.
      


 

      

   If it matches, processing ends and the qualifier value is returned as
 
   If it matches, processing ends and the qualifier value is returned as
      

   the result of that record.  If it does not match, processing
 
   the result of that record.  If it does not match, processing
      
      





   continues with the next mechanism.  If it throws an exception,
 
   continues with the next mechanism.  If it returns an exception,
      

   mechanism processing ends and the exception value is returned.
 
   mechanism processing ends and the exception value is returned.
      


 

      
      





   The possible qualifiers, and the results they return are as follows:
 
   The possible qualifiers, and the results they cause check_host() to
      


 
   return are as follows:
      


 

      
      





      "+" Pass
 
      "+" pass
      

      "-" Fail
 
      "-" fail
      

      "~" SoftFail
 
      "~" softfail
      

      "?" Neutral
 
      "?" neutral
      


 

      

   The qualifier is optional and defaults to "+".
 
   The qualifier is optional and defaults to "+".
      


 

      
      





   When a mechanism matches and the qualifier is "-", then a "Fail"
 
   When a mechanism matches and the qualifier is "-", then a "fail"
      

   result is returned and the explanation string is computed as
 
   result is returned and the explanation string is computed as
      

   described in Section 6.2.
 
   described in Section 6.2.
      


 

      

   The specific mechanisms are described in Section 5.
 
   The specific mechanisms are described in Section 5.
      


 

      

4.6.3.  Modifiers
 
4.6.3.  Modifiers
      


 

      
      





   Modifiers are not mechanisms: they do not return match or not-match.
 
   Modifiers are not mechanisms.  They do not return match or not-match.
      

   Instead they provide additional information.  Although modifiers do
 
   Instead, they provide additional information.  Although modifiers do
      

   not directly affect the evaluation of the record, the "redirect"
 
   not directly affect the evaluation of the record, the "redirect"
      

   modifier has an effect after all the mechanisms have been evaluated.
 
   modifier has an effect after all the mechanisms have been evaluated.
      


 

      
      






 
4.6.4.  DNS Lookup Limits
      


 

      


 
   SPF implementations MUST limit the number of mechanisms and modifiers
      


 
   ("terms") that cause any DNS query to at most 10 during SPF
      


 
   evaluation.  Specifically, the "include", "a", "mx", "ptr", and
      


 
   "exists" mechanisms as well as the "redirect" modifier count against
      


 
   this limit.  The "all", "ip4", and "ip6" mechanisms do not count
      


 
   against this limit.  If this number is exceeded during a check, a
      


 
   permerror MUST be returned.  The "exp" modifier does not count
      


 
   against this limit because the DNS lookup to fetch the explanation
      


 
   string occurs after the SPF record evaluation has been completed.
      


 

      


 
   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
      


 
   there MUST be a limit of no more than 10 MX or PTR RRs looked up and
      


 
   checked.  If more than 10 "mx" or "ptr" records are returned for this
      


 
   further lookup, a permerror MUST be returned.  This limit is per
      


 
   mechanism or macro in the record and in addition to the lookup limits
      


 
   above.
      


 

      


 
   MTAs or other processors SHOULD impose a limit on the maximum amount
      


 
   of elapsed time to evaluate check_host().  Such a limit SHOULD allow
      


 
   at least 20 seconds.  If such a limit is exceeded, the result of
      


 
   authorization SHOULD be "temperror".
      


 
                                                                         
      

4.7.  Default Result
 
4.7.  Default Result
      


 

      

   If none of the mechanisms match and there is no "redirect" modifier,
 
   If none of the mechanisms match and there is no "redirect" modifier,
      
      





   then the check_host() returns a result of "Neutral", just as if
 
   then the check_host() returns a result of "neutral", just as if
      

   "?all" were specified as the last directive.  If there is a
 
   "?all" were specified as the last directive.  If there is a
      

   "redirect" modifier, check_host() proceeds as defined in Section 6.1.
 
   "redirect" modifier, check_host() proceeds as defined in Section 6.1.
      


 

      

   Note that records SHOULD always use either a "redirect" modifier or
 
   Note that records SHOULD always use either a "redirect" modifier or
      
      





   an "all" mechanism to explicitly terminate processing.
 
   an "all" mechanism to explicitly terminate processing.  Although the
      


 
   latter has default (specifically "?all"), it aids debugging efforts
      


 
   if it is explicitly included.
      


 

      

   For example:
 
   For example:
      


 

      

      v=spf1 +mx -all
 
      v=spf1 +mx -all
      

   or
 
   or
      

      v=spf1 +mx redirect=_spf.example.com
 
      v=spf1 +mx redirect=_spf.example.com
      


 

      

4.8.  Domain Specification
 
4.8.  Domain Specification
      


 

      
      





   Several of these mechanisms and modifiers have a <domain-spec>
 
   Several of these mechanisms and modifiers have a domain-spec section.
      

   section.  The <domain-spec> string is macro expanded (see Section 8).
 
   The domain-spec string is subject to macro expansion (see Section 8).
      

   The resulting string is the common presentation form of a fully-
 
   The resulting string is the common presentation form of a fully-
      

   qualified DNS name: a series of labels separated by periods.  This
 
   qualified DNS name: a series of labels separated by periods.  This
      

   domain is called the <target-name> in the rest of this document.
 
   domain is called the <target-name> in the rest of this document.
      


 

      

   Note: The result of the macro expansion is not subject to any further
 
   Note: The result of the macro expansion is not subject to any further
      

   escaping.  Hence, this facility cannot produce all characters that
 
   escaping.  Hence, this facility cannot produce all characters that
      

   are legal in a DNS label (e.g., the control characters).  However,
 
   are legal in a DNS label (e.g., the control characters).  However,
      

   this facility is powerful enough to express legal host names and
 
   this facility is powerful enough to express legal host names and
      

   common utility labels (such as "_spf") that are used in DNS.
 
   common utility labels (such as "_spf") that are used in DNS.
      


 

      

   For several mechanisms, the <domain-spec> is optional.  If it is not
 
   For several mechanisms, the <domain-spec> is optional.  If it is not
      
      





   provided, the <domain> is used as the <target-name>.
 
   provided, the <domain> is used as the <target-name>.  Domain and
      


 
   domain-spec are syntactically identical after macro expansion.
      


 
   Domain is an input value for check_host() while domain-spec is
      


 
   computed by check_host().
      


 

      


 
   Note: Historically, this document has made no provisions for how to
      


 
   handle domain-specs, or macro-expansions thereof, that are
      


 
   syntactically invalid per [RFC1035], such as names with empty labels
      


 
   (e.g., "foo..example.com") or overlong labels (more than 63
      


 
   characters).  Some implementations choose to treat as a no-match
      


 
   mechanisms, and ignore modifiers, with such names, whereas others
      


 
   return a "permerror" exception.  The outcome for an unexpected
      


 
   domain-spec without macros might even differ from that for an
      


 
   unexpected target-name after macro expansion.
      


 

      

5.  Mechanism Definitions
 
5.  Mechanism Definitions
      


 

      

   This section defines two types of mechanisms.
 
   This section defines two types of mechanisms.
      


 

      

   Basic mechanisms contribute to the language framework.  They do not
 
   Basic mechanisms contribute to the language framework.  They do not
      

   specify a particular type of authorization scheme.
 
   specify a particular type of authorization scheme.
      


 

      

      all
 
      all
      

      include
 
      include
      


 

      

   Designated sender mechanisms are used to designate a set of <ip>
 
   Designated sender mechanisms are used to designate a set of <ip>
      

   addresses as being permitted or not permitted to use the <domain> for
 
   addresses as being permitted or not permitted to use the <domain> for
      

   sending mail.
 
   sending mail.
      


 

      

      a
 
      a
      

      mx
 
      mx
      
      





      ptr
 
      ptr (deprecated)
      

      ip4
 
      ip4
      

      ip6
 
      ip6
      

      exists
 
      exists
      


 

      

   The following conventions apply to all mechanisms that perform a
 
   The following conventions apply to all mechanisms that perform a
      

   comparison between <ip> and an IP address at any point:
 
   comparison between <ip> and an IP address at any point:
      


 

      
      





   If no CIDR-length is given in the directive, then <ip> and the IP
 
   If no CIDR prefix length is given in the directive, then <ip> and the
      

   address are compared for equality. (Here, CIDR is Classless Inter-
 
   IP address are compared for equality.  (Here, CIDR is Classless
      

   Domain Routing.)
 
   Inter-Domain Routing, described in [RFC4632].)
      


 

      
      





   If a CIDR-length is specified, then only the specified number of
 
   If a CIDR prefix length is specified, then only the specified number
      

   high-order bits of <ip> and the IP address are compared for equality.
 
   of high-order bits of <ip> and the IP address are compared for
      


 
   equality.
      


 

      

   When any mechanism fetches host addresses to compare with <ip>, when
 
   When any mechanism fetches host addresses to compare with <ip>, when
      
      





   <ip> is an IPv4 address, A records are fetched, when <ip> is an IPv6
 
   <ip> is an IPv4 address, A records are fetched; when <ip> is an IPv6
      

   address, AAAA records are fetched.  Even if the SMTP connection is
 
   address, AAAA records are fetched.  Even if the SMTP connection uses
      

   via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513], Section
 
   IPv6, an IPv4-mapped IPv6 IP address (see [RFC4291], Section 2.5.5)
      

   2.5.5) MUST still be considered an IPv4 address.
 
   MUST still be considered an IPv4 address and MUST be evaluated using
      


 
   IPv4 mechanisms (i.e. "ip4" and "a").
      


 

      
      





   Several mechanisms rely on information fetched from DNS.  For these
 
   Several mechanisms rely on information fetched from the DNS.  For
      

   DNS queries, except where noted, if the DNS server returns an error
 
   these DNS queries, except where noted, if the DNS server returns an
      

   (RCODE other than 0 or 3) or the query times out, the mechanism
 
   error (RCODE other than 0 or 3) or the query times out, the mechanism
      

   throws the exception "TempError".  If the server returns "domain does
 
   stops and the topmost check_host() returns "temperror".  If the
      

   not exist" (RCODE 3), then evaluation of the mechanism continues as
 
   server returns "domain does not exist" (RCODE 3), then evaluation of
      

   if the server returned no error (RCODE 0) and zero answer records.
 
   the mechanism continues as if the server returned no error (RCODE 0)
      


 
   and zero answer records.
      


 

      

5.1.  "all"
 
5.1.  "all"
      


 

      

   all              = "all"
 
   all              = "all"
      


 

      

   The "all" mechanism is a test that always matches.  It is used as the
 
   The "all" mechanism is a test that always matches.  It is used as the
      

   rightmost mechanism in a record to provide an explicit default.
 
   rightmost mechanism in a record to provide an explicit default.
      


 

      

   For example:
 
   For example:
      


 

      

      v=spf1 a mx -all
 
      v=spf1 a mx -all
      


 

      
      





   Mechanisms after "all" will never be tested.  Any "redirect" modifier
 
   Mechanisms after "all" will never be tested.  Mechanisms listed after
      

   (Section 6.1) has no effect when there is an "all" mechanism.
 
   "all" MUST be ignored.  Any "redirect" modifier (Section 6.1) MUST be
      


 
   ignored when there is an "all" mechanism in the record.
      


 

      

5.2.  "include"
 
5.2.  "include"
      


 

      
      





      include          = "include"  ":" domain-spec
 
   include          = "include"  ":" domain-spec
      


 

      

   The "include" mechanism triggers a recursive evaluation of
 
   The "include" mechanism triggers a recursive evaluation of
      
      





   check_host().  The domain-spec is expanded as per Section 8.  Then
 
   check_host().
      

   check_host() is evaluated with the resulting string as the <domain>.
 
                                                                         
      

   The <ip> and <sender> arguments remain the same as in the current
 
   1.  The domain-spec is expanded as per Section 8.
      

   evaluation of check_host().
 
                                                                         
      


 
   2.  Check_host() is evaluated with the resulting string as the
      


 
       <domain>.  The <ip> and <sender> arguments remain the same as in
      


 
       the current evaluation of check_host().
      


 
                                                                         
      


 
   3.  The recursive evaluation returns either match, not match, or an
      


 
       error.  If it matches, then the appropriate result for the
      


 
       include: mechanism is used (e.g. include or +include gives a
      


 
       "pass" result and -include gives "fail).
      


 

      


 
   4.  If there is no match, the parent check_host() resumes processing
      


 
       as per the table below, with the previous value of <domain>
      


 
       restored.
      


 

      

   In hindsight, the name "include" was poorly chosen.  Only the
 
   In hindsight, the name "include" was poorly chosen.  Only the
      

   evaluated result of the referenced SPF record is used, rather than
 
   evaluated result of the referenced SPF record is used, rather than
      

   acting as if the referenced SPF record was literally included in the
 
   acting as if the referenced SPF record was literally included in the
      

   first.  For example, evaluating a "-all" directive in the referenced
 
   first.  For example, evaluating a "-all" directive in the referenced
      

   record does not terminate the overall processing and does not
 
   record does not terminate the overall processing and does not
      
      





   necessarily result in an overall "Fail".  (Better names for this
 
   necessarily result in an overall "fail".  (Better names for this
      

   mechanism would have been "if-pass", "on-pass", etc.)
 
   mechanism would have been "if-match", "on-match", etc.)
      


 

      

   The "include" mechanism makes it possible for one domain to designate
 
   The "include" mechanism makes it possible for one domain to designate
      

   multiple administratively-independent domains.  For example, a vanity
 
   multiple administratively-independent domains.  For example, a vanity
      

   domain "example.net" might send mail using the servers of
 
   domain "example.net" might send mail using the servers of
      

   administratively-independent domains example.com and example.org.
 
   administratively-independent domains example.com and example.org.
      


 

      

   Example.net could say
 
   Example.net could say
      


 

      

      IN TXT "v=spf1 include:example.com include:example.org -all"
 
      IN TXT "v=spf1 include:example.com include:example.org -all"
      


 

      

   This would direct check_host() to, in effect, check the records of
 
   This would direct check_host() to, in effect, check the records of
      
      





   example.com and example.org for a "Pass" result.  Only if the host
 
   example.com and example.org for a "pass" result.  Only if the host
      

   were not permitted for either of those domains would the result be
 
   were not permitted for either of those domains would the result be
      
      





   "Fail".
 
   "fail".
      


 

      
      





   Whether this mechanism matches, does not match, or throws an
 
   Whether this mechanism matches, does not match, or returns an
      

   exception depends on the result of the recursive evaluation of
 
   exception depends on the result of the recursive evaluation of
      

   check_host():
 
   check_host():
      


 

      

   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+
      

   | A recursive check_host() result | Causes the "include" mechanism  |
 
   | A recursive check_host() result | Causes the "include" mechanism  |
      

   | of:                             | to:                             |
 
   | of:                             | to:                             |
      

   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+
      
      





   | Pass                            | match                           |
 
   | pass                            | match                           |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | Fail                            | not match                       |
 
   | fail                            | not match                       |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | SoftFail                        | not match                       |
 
   | softfail                        | not match                       |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | Neutral                         | not match                       |
 
   | neutral                         | not match                       |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | TempError                       | throw TempError                 |
 
   | temperror                       | return temperror                |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | PermError                       | throw PermError                 |
 
   | permerror                       | return permerror                |
      

   |                                 |                                 |
 
   |                                 |                                 |
      
      





   | None                            | throw PermError                 |
 
   | none                            | return permerror                |
      

   +---------------------------------+---------------------------------+
 
   +---------------------------------+---------------------------------+
      


 

      

   The "include" mechanism is intended for crossing administrative
 
   The "include" mechanism is intended for crossing administrative
      
      





   boundaries.  Although it is possible to use includes to consolidate
 
   boundaries.  For example, if example.com and example.org were managed
      

   multiple domains that share the same set of designated hosts, domains
 
   by the same entity, and if the permitted set of hosts for both
      

   are encouraged to use redirects where possible, and to minimize the
 
   domains was
      

   number of includes within a single administrative domain.  For
 

      

   example, if example.com and example.org were managed by the same
 

      

   entity, and if the permitted set of hosts for both domains was
 

      

   "mx:example.com", it would be possible for example.org to specify
 
   "mx:example.com", it would be possible for example.org to specify
      

   "include:example.com", but it would be preferable to specify
 
   "include:example.com", but it would be preferable to specify
      

   "redirect=example.com" or even "mx:example.com".
 
   "redirect=example.com" or even "mx:example.com".
      


 

      
      






 
   With the "include" mechanism an administratively external set of
      


 
   hosts can be authorized, but determination of sender policy is still
      


 
   a function of the original domain's SPF record (as determined by the
      


 
   "all" mechanism in that record).  The redirect modifier is more
      


 
   suitable for consolidating both authorizations and policy into a
      


 
   common set to be shared within an ADMD.  Redirect is much more like a
      


 
   common code element to be shared among records in a single ADMD.  It
      


 
   is possible to control both authorized hosts and policy for an
      


 
   arbitrary number of domains from a single record.
      


 
                                                                         
      

5.3.  "a"
 
5.3.  "a"
      


 

      

   This mechanism matches if <ip> is one of the <target-name>'s IP
 
   This mechanism matches if <ip> is one of the <target-name>'s IP
      

   addresses.
 
   addresses.
      


 

      
      





   A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
 
   a                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
      


 

      

   An address lookup is done on the <target-name>.  The <ip> is compared
 
   An address lookup is done on the <target-name>.  The <ip> is compared
      

   to the returned address(es).  If any address matches, the mechanism
 
   to the returned address(es).  If any address matches, the mechanism
      

   matches.
 
   matches.
      


 

      

5.4.  "mx"
 
5.4.  "mx"
      


 

      

   This mechanism matches if <ip> is one of the MX hosts for a domain
 
   This mechanism matches if <ip> is one of the MX hosts for a domain
      

   name.
 
   name.
      


 

      
      





   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 
   mx               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
      


 

      

   check_host() first performs an MX lookup on the <target-name>.  Then
 
   check_host() first performs an MX lookup on the <target-name>.  Then
      

   it performs an address lookup on each MX name returned.  The <ip> is
 
   it performs an address lookup on each MX name returned.  The <ip> is
      

   compared to each returned IP address.  To prevent Denial of Service
 
   compared to each returned IP address.  To prevent Denial of Service
      

   (DoS) attacks, more than 10 MX names MUST NOT be looked up during the
 
   (DoS) attacks, more than 10 MX names MUST NOT be looked up during the
      
      





   evaluation of an "mx" mechanism (see Section 10).  If any address
 
   evaluation of an "mx" mechanism.  If there are more than 10 MX names
      

   matches, the mechanism matches.
 
   then permerror is returned and the evaluation terminated (see
      


 
   Section 4.6.4).  If any address matches, the mechanism matches.
      


 

      

   Note regarding implicit MXs: If the <target-name> has no MX records,
 
   Note regarding implicit MXs: If the <target-name> has no MX records,
      

   check_host() MUST NOT pretend the target is its single MX, and MUST
 
   check_host() MUST NOT pretend the target is its single MX, and MUST
      
      





   NOT default to an A lookup on the <target-name> directly.  This
 
   NOT default to an A or AAAA lookup on the <target-name> directly.
      

   behavior breaks with the legacy "implicit MX" rule.  See [RFC2821],
 
   This behavior diverges from the legacy "implicit MX" rule, (See
      

   Section 5.  If such behavior is desired, the publisher should specify
 
   [RFC5321], Section 5.  If such behavior is desired, the publisher
      

   an "a" directive.
 
   will have to specify an "a" directive).
      


 

      
      





5.5.  "ptr"
 
5.5.  "ptr" (deprecated)
      


 

      

   This mechanism tests whether the DNS reverse-mapping for <ip> exists
 
   This mechanism tests whether the DNS reverse-mapping for <ip> exists
      

   and correctly points to a domain name within a particular domain.
 
   and correctly points to a domain name within a particular domain.
      
      






 
   This mechanism is deprecated and SHOULD NOT be used.
      


 

      
      





   PTR              = "ptr"    [ ":" domain-spec ]
 
   ptr              = "ptr"    [ ":" domain-spec ]
      


 

      
      





   First, the <ip>'s name is looked up using this procedure: perform a
 
   The <ip>'s name is looked up using this procedure:
      

   DNS reverse-mapping for <ip>, looking up the corresponding PTR record
 

      

   in "in-addr.arpa." if the address is an IPv4 one and in "ip6.arpa."
 

      

   if it is an IPv6 address.  For each record returned, validate the
 

      

   domain name by looking up its IP address.  To prevent DoS attacks,
 

      

   more than 10 PTR names MUST NOT be looked up during the evaluation of
 

      

   a "ptr" mechanism (see Section 10).  If <ip> is among the returned IP
 

      

   addresses, then that domain name is validated.  In pseudocode:
 

      


 

      
      





   sending-domain_names := ptr_lookup(sending-host_IP); if more than 10
 
   1.  Perform a DNS reverse-mapping for <ip>: Look up the corresponding
      

   sending-domain_names are found, use at most 10.  for each name in
 
       PTR record in "in-addr.arpa." if the address is an IPv4 one and
      

   (sending-domain_names) {
 
       in "ip6.arpa." if it is an IPv6 address.
      

     IP_addresses := a_lookup(name);
 

      

     if the sending-domain_IP is one of the IP_addresses {
 

      

       validated-sending-domain_names += name;
 

      

     } }
 

      


 

      
      





   Check all validated domain names to see if they end in the
 
   2.  For each record returned, validate the domain name by looking up
      

   <target-name> domain.  If any do, this mechanism matches.  If no
 
       its IP addresses.  To prevent DoS attacks, more than 10 PTR names
      

   validated domain name can be found, or if none of the validated
 
       MUST NOT be looked up during the evaluation of a "ptr" mechanism
      

   domain names end in the <target-name>, this mechanism fails to match.
 
       (see Section 4.6.4).
      

   If a DNS error occurs while doing the PTR RR lookup, then this
 

      

   mechanism fails to match.  If a DNS error occurs while doing an A RR
 
   3.  If <ip> is among the returned IP addresses, then that domain name
      

   lookup, then that domain name is skipped and the search continues.
 
       is validated.
      


 
                                                                         
      


 
   Check all validated domain names to see if they either match the
      


 
   <target-name> domain or are a subdomain of the <target-name> domain.
      


 
   If any do, this mechanism matches.  If no validated domain name can
      


 
   be found, or if none of the validated domain names match or are a
      


 
   subdomain of the <target-name>, this mechanism fails to match.  If a
      


 
   DNS error occurs while doing the PTR RR lookup, then this mechanism
      


 
   fails to match.  If a DNS error occurs while doing an A RR lookup,
      


 
   then that domain name is skipped and the search continues.
      


 

      

   Pseudocode:
 
   Pseudocode:
      


 

      
      






 
   sending-domain_names := ptr_lookup(sending-host_IP);
      


 
   if more than 10 sending-domain_names are found, use at most 10.
      


 
   for each name in (sending-domain_names) {
      


 
     IP_addresses := a_lookup(name);
      


 
     if the sending-domain_IP is one of the IP_addresses {
      


 
       validated-sending-domain_names += name;
      


 
     }
      


 
   }
      


 
                                                                         
      

   for each name in (validated-sending-domain_names) {
 
   for each name in (validated-sending-domain_names) {
      

     if name ends in <domain-spec>, return match.
 
     if name ends in <domain-spec>, return match.
      

     if name is <domain-spec>, return match.
 
     if name is <domain-spec>, return match.
      

   }
 
   }
      

   return no-match.
 
   return no-match.
      


 

      
      





   This mechanism matches if the <target-name> is either an ancestor of
 
   This mechanism matches if the <target-name> is either a subdomain of
      

   a validated domain name or if the <target-name> and a validated
 
   a validated domain name or if the <target-name> and a validated
      

   domain name are the same.  For example: "mail.example.com" is within
 
   domain name are the same.  For example: "mail.example.com" is within
      

   the domain "example.com", but "mail.bad-example.com" is not.
 
   the domain "example.com", but "mail.bad-example.com" is not.
      


 

      
      





   Note: Use of this mechanism is discouraged because it is slow, it is
 
   Note: This mechanism has been deprecated because it is slow, it is
      

   not as reliable as other mechanisms in cases of DNS errors, and it
 
   not as reliable as other mechanisms in cases of DNS errors, and it
      
      





   places a large burden on the arpa name servers.  If used, proper PTR
 
   places a large burden on the .arpa name servers.  If used, proper PTR
      

   records must be in place for the domain's hosts and the "ptr"
 
   records MUST be in place for the domain's hosts and the "ptr"
      

   mechanism should be one of the last mechanisms checked.
 
   mechanism SHOULD be one of the last mechanisms checked.  After many
      


 
   years of SPF deployment experience it has been concluded it is
      


 
   unnecessary and more reliable alternatives used instead.  It is,
      


 
   however, still in use and part of the SPF protocol, so compliant
      


 
   check_host() implementations MUST support it.
      


 

      

5.6.  "ip4" and "ip6"
 
5.6.  "ip4" and "ip6"
      


 

      

   These mechanisms test whether <ip> is contained within a given IP
 
   These mechanisms test whether <ip> is contained within a given IP
      

   network.
 
   network.
      


 

      
      





   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 
   ip4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
      

   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 
   ip6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
      


 

      

   ip4-cidr-length  = "/" 1*DIGIT
 
   ip4-cidr-length  = "/" 1*DIGIT
      

   ip6-cidr-length  = "/" 1*DIGIT
 
   ip6-cidr-length  = "/" 1*DIGIT
      

   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
 
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
      


 

      

   ip4-network      = qnum "." qnum "." qnum "." qnum
 
   ip4-network      = qnum "." qnum "." qnum "." qnum
      

   qnum             = DIGIT                 ; 0-9
 
   qnum             = DIGIT                 ; 0-9
      

                      / %x31-39 DIGIT       ; 10-99
 
                      / %x31-39 DIGIT       ; 10-99
      

                      / "1" 2DIGIT          ; 100-199
 
                      / "1" 2DIGIT          ; 100-199
      

                      / "2" %x30-34 DIGIT   ; 200-249
 
                      / "2" %x30-34 DIGIT   ; 200-249
      

                      / "25" %x30-35        ; 250-255
 
                      / "25" %x30-35        ; 250-255
      

            ; as per conventional dotted quad notation.  e.g., 192.0.2.0
 
            ; as per conventional dotted quad notation.  e.g., 192.0.2.0
      
      





   ip6-network      = <as per [RFC 3513], section 2.2>
 
   ip6-network      = <as per [RFC 4291], section 2.2>
      

            ; e.g., 2001:DB8::CD30
 
            ; e.g., 2001:DB8::CD30
      


 

      
      





   The <ip> is compared to the given network.  If CIDR-length high-order
 
   The <ip> is compared to the given network.  If CIDR prefix length
      

   bits match, the mechanism matches.
 
   high-order bits match, the mechanism matches.
      


 

      

   If ip4-cidr-length is omitted, it is taken to be "/32".  If
 
   If ip4-cidr-length is omitted, it is taken to be "/32".  If
      

   ip6-cidr-length is omitted, it is taken to be "/128".  It is not
 
   ip6-cidr-length is omitted, it is taken to be "/128".  It is not
      

   permitted to omit parts of the IP address instead of using CIDR
 
   permitted to omit parts of the IP address instead of using CIDR
      

   notations.  That is, use 192.0.2.0/24 instead of 192.0.2.
 
   notations.  That is, use 192.0.2.0/24 instead of 192.0.2.
      


 

      

5.7.  "exists"
 
5.7.  "exists"
      


 

      

   This mechanism is used to construct an arbitrary domain name that is
 
   This mechanism is used to construct an arbitrary domain name that is
      

   used for a DNS A record query.  It allows for complicated schemes
 
   used for a DNS A record query.  It allows for complicated schemes
      


 

      

skipping to change at page 22, line 34
 
skipping to change at page 28, line 15
      

   Domains can use this mechanism to specify arbitrarily complex
 
   Domains can use this mechanism to specify arbitrarily complex
      

   queries.  For example, suppose example.com publishes the record:
 
   queries.  For example, suppose example.com publishes the record:
      


 

      

      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
 
      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
      


 

      

   The <target-name> might expand to
 
   The <target-name> might expand to
      

   "1.2.0.192.someuser._spf.example.com".  This makes fine-grained
 
   "1.2.0.192.someuser._spf.example.com".  This makes fine-grained
      

   decisions possible at the level of the user and client IP address.
 
   decisions possible at the level of the user and client IP address.
      


 

      

   This mechanism enables queries that mimic the style of tests that
 
   This mechanism enables queries that mimic the style of tests that
      
      





   existing anti-spam DNS blacklists (DNSBL) use.
 
   existing DNS white/black lists (DNSxLs) use, as described in
      


 
   [RFC5782].  The query will either return NXDOMAIN (no match), any
      


 
   valid answer (match), or an error.
      


 

      

6.  Modifier Definitions
 
6.  Modifier Definitions
      


 

      

   Modifiers are name/value pairs that provide additional information.
 
   Modifiers are name/value pairs that provide additional information.
      

   Modifiers always have an "=" separating the name and the value.
 
   Modifiers always have an "=" separating the name and the value.
      


 

      

   The modifiers defined in this document ("redirect" and "exp") MAY
 
   The modifiers defined in this document ("redirect" and "exp") MAY
      

   appear anywhere in the record, but SHOULD appear at the end, after
 
   appear anywhere in the record, but SHOULD appear at the end, after
      

   all mechanisms.  Ordering of these two modifiers does not matter.
 
   all mechanisms.  Ordering of these two modifiers does not matter.
      

   These two modifiers MUST NOT appear in a record more than once each.
 
   These two modifiers MUST NOT appear in a record more than once each.
      
      





   If they do, then check_host() exits with a result of "PermError".
 
   If they do, then check_host() exits with a result of "permerror".
      


 

      

   Unrecognized modifiers MUST be ignored no matter where in a record,
 
   Unrecognized modifiers MUST be ignored no matter where in a record,
      

   or how often.  This allows implementations of this document to
 
   or how often.  This allows implementations of this document to
      

   gracefully handle records with modifiers that are defined in other
 
   gracefully handle records with modifiers that are defined in other
      

   specifications.
 
   specifications.
      


 

      

6.1.  redirect: Redirected Query
 
6.1.  redirect: Redirected Query
      


 

      
      





   If all mechanisms fail to match, and a "redirect" modifier is
 
   The redirect modifier is intended for consolidating both
      

   present, then processing proceeds as follows:
 
   authorizations and policy into a common set to be shared within a
      


 
   single ADMD.  Redirect is like a common code element to be shared
      


 
   among records in a single ADMD.  It is possible to control both
      


 
   authorized hosts and policy for an arbitrary number of domains from a
      


 
   single record.
      


 

      

   redirect         = "redirect" "=" domain-spec
 
   redirect         = "redirect" "=" domain-spec
      


 

      
      






 
   If all mechanisms fail to match, and a "redirect" modifier is
      


 
   present, then processing proceeds as follows:
      


 
                                                                         
      

   The domain-spec portion of the redirect section is expanded as per
 
   The domain-spec portion of the redirect section is expanded as per
      

   the macro rules in Section 8.  Then check_host() is evaluated with
 
   the macro rules in Section 8.  Then check_host() is evaluated with
      

   the resulting string as the <domain>.  The <ip> and <sender>
 
   the resulting string as the <domain>.  The <ip> and <sender>
      
      





   arguments remain the same as current evaluation of check_host().
 
   arguments remain the same as in the current evaluation of
      


 
   check_host().
      


 

      

   The result of this new evaluation of check_host() is then considered
 
   The result of this new evaluation of check_host() is then considered
      

   the result of the current evaluation with the exception that if no
 
   the result of the current evaluation with the exception that if no
      

   SPF record is found, or if the target-name is malformed, the result
 
   SPF record is found, or if the target-name is malformed, the result
      
      





   is a "PermError" rather than "None".
 
   is a "permerror" rather than "none".
      


 

      
      





   Note that the newly-queried domain may itself specify redirect
 
   Note that the newly-queried domain can itself specify redirect
      

   processing.
 
   processing.
      


 

      

   This facility is intended for use by organizations that wish to apply
 
   This facility is intended for use by organizations that wish to apply
      

   the same record to multiple domains.  For example:
 
   the same record to multiple domains.  For example:
      


 

      

     la.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     la.example.com. TXT "v=spf1 redirect=_spf.example.com"
      

     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
      

     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
 
     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
      

   _spf.example.com. TXT "v=spf1 mx:example.com -all"
 
   _spf.example.com. TXT "v=spf1 mx:example.com -all"
      


 

      

   In this example, mail from any of the three domains is described by
 
   In this example, mail from any of the three domains is described by
      

   the same record.  This can be an administrative advantage.
 
   the same record.  This can be an administrative advantage.
      


 

      

   Note: In general, the domain "A" cannot reliably use a redirect to
 
   Note: In general, the domain "A" cannot reliably use a redirect to
      

   another domain "B" not under the same administrative control.  Since
 
   another domain "B" not under the same administrative control.  Since
      

   the <sender> stays the same, there is no guarantee that the record at
 
   the <sender> stays the same, there is no guarantee that the record at
      

   domain "B" will correctly work for mailboxes in domain "A",
 
   domain "B" will correctly work for mailboxes in domain "A",
      
      





   especially if domain "B" uses mechanisms involving localparts.  An
 
   especially if domain "B" uses mechanisms involving local-parts.  An
      

   "include" directive may be more appropriate.
 
   "include" directive is generally be more appropriate.
      


 

      

   For clarity, it is RECOMMENDED that any "redirect" modifier appear as
 
   For clarity, it is RECOMMENDED that any "redirect" modifier appear as
      

   the very last term in a record.
 
   the very last term in a record.
      


 

      

6.2.  exp: Explanation
 
6.2.  exp: Explanation
      


 

      

   explanation      = "exp" "=" domain-spec
 
   explanation      = "exp" "=" domain-spec
      


 

      
      





   If check_host() results in a "Fail" due to a mechanism match (such as
 
   If check_host() results in a "fail" due to a mechanism match (such as
      

   "-all"), and the "exp" modifier is present, then the explanation
 
   "-all"), and the "exp" modifier is present, then the explanation
      

   string returned is computed as described below.  If no "exp" modifier
 
   string returned is computed as described below.  If no "exp" modifier
      

   is present, then either a default explanation string or an empty
 
   is present, then either a default explanation string or an empty
      
      





   explanation string may be returned.
 
   explanation string MUST be returned.
      


 

      
      





   The <domain-spec> is macro expanded (see Section 8) and becomes the
 
   The domain-spec is macro expanded (see Section 8) and becomes the
      

   <target-name>.  The DNS TXT record for the <target-name> is fetched.
 
   <target-name>.  The DNS TXT record for the <target-name> is fetched.
      


 

      
      





   If <domain-spec> is empty, or there are any DNS processing errors
 
   If there are any DNS processing errors (any RCODE other than 0), or
      

   (any RCODE other than 0), or if no records are returned, or if more
 
   if no records are returned, or if more than one record is returned,
      

   than one record is returned, or if there are syntax errors in the
 
   or if there are syntax errors in the explanation string, then proceed
      

   explanation string, then proceed as if no exp modifier was given.
 
   as if no exp modifier was given.
      


 

      

   The fetched TXT record's strings are concatenated with no spaces, and
 
   The fetched TXT record's strings are concatenated with no spaces, and
      
      





   then treated as an <explain-string>, which is macro-expanded.  This
 
   then treated as an explain-string, which is macro-expanded.  This
      

   final result is the explanation string.  Implementations MAY limit
 
   final result is the explanation string.  Implementations MAY limit
      

   the length of the resulting explanation string to allow for other
 
   the length of the resulting explanation string to allow for other
      

   protocol constraints and/or reasonable processing limits.  Since the
 
   protocol constraints and/or reasonable processing limits.  Since the
      
      





   explanation string is intended for an SMTP response and [RFC2821]
 
   explanation string is intended for an SMTP response and [RFC5321]
      

   Section 2.4 says that responses are in [US-ASCII], the explanation
 
   Section 2.4 says that responses are in [US-ASCII], the explanation
      
      





   string is also limited to US-ASCII.
 
   string MUST be limited to US-ASCII.
      


 

      

   Software evaluating check_host() can use this string to communicate
 
   Software evaluating check_host() can use this string to communicate
      

   information from the publishing domain in the form of a short message
 
   information from the publishing domain in the form of a short message
      

   or URL.  Software SHOULD make it clear that the explanation string
 
   or URL.  Software SHOULD make it clear that the explanation string
      

   comes from a third party.  For example, it can prepend the macro
 
   comes from a third party.  For example, it can prepend the macro
      
      





   string "%{o} explains: " to the explanation, such as shown in Section
 
   string "%{o} explains: " to the explanation, such as shown in
      

   2.5.4.
 
   Section 2.5.4.
      


 

      

   Suppose example.com has this record:
 
   Suppose example.com has this record:
      


 

      

      v=spf1 mx -all exp=explain._spf.%{d}
 
      v=spf1 mx -all exp=explain._spf.%{d}
      


 

      

   Here are some examples of possible explanation TXT records at
 
   Here are some examples of possible explanation TXT records at
      

   explain._spf.example.com:
 
   explain._spf.example.com:
      


 

      

      "Mail from example.com should only be sent by its own servers."
 
      "Mail from example.com should only be sent by its own servers."
      
      





         -- a simple, constant message
 
         --  a simple, constant message
      


 

      

      "%{i} is not one of %{d}'s designated mail servers."
 
      "%{i} is not one of %{d}'s designated mail servers."
      
      





         -- a message with a little more information, including the IP
 
         --  a message with a little more information, including the IP
      

            address that failed the check
 
             address that failed the check
      


 

      

      "See http://%{d}/why.html?s=%{S}&i=%{I}"
 
      "See http://%{d}/why.html?s=%{S}&i=%{I}"
      
      





         -- a complicated example that constructs a URL with the
 
         --  a complicated example that constructs a URL with the
      

            arguments to check_host() so that a web page can be
 
             arguments to check_host() so that a web page can be
      

            generated with detailed, custom instructions
 
             generated with detailed, custom instructions
      


 

      

   Note: During recursion into an "include" mechanism, an exp= modifier
 
   Note: During recursion into an "include" mechanism, an exp= modifier
      

   from the <target-name> MUST NOT be used.  In contrast, when executing
 
   from the <target-name> MUST NOT be used.  In contrast, when executing
      

   a "redirect" modifier, an exp= modifier from the original domain MUST
 
   a "redirect" modifier, an exp= modifier from the original domain MUST
      

   NOT be used.
 
   NOT be used.
      


 

      
      





7.  The Received-SPF Header Field
 
7.  Recording The Result
      


 

      
      





   It is RECOMMENDED that SMTP receivers record the result of SPF
 
   To provide downstream agents, such as MUAs, with the information they
      

   processing in the message header.  If an SMTP receiver chooses to do
 
   might need in terms of evaluating or representing the apparent safety
      

   so, it SHOULD use the "Received-SPF" header field defined here for
 
   of the message content, it is RECOMMENDED that SMTP receivers record
      

   each identity that was checked.  This information is intended for the
 
   the result of SPF processing in the message header.  For operators
      

   recipient.  (Information intended for the sender is described in
 
   that choose to record SPF results in the header of the message for
      

   Section 6.2, Explanation.)
 
   processing by internal filters or MUAs, two methods are presented.
      


 
   Section 7.1 defines the Received-SPF field, which is the results
      


 
   field originally defined for SPF use.  Section 7.2 discusses
      


 
   Authentication-Results [RFC5451] which was specified more recently
      


 
   and is designed for use by SPF and other authentication methods.
      


 

      
      





   The Received-SPF header field is a trace field (see [RFC2822] Section
 
   Both are in common use, and hence both are included here.  However,
      


 
   it is important to note that they were designed to serve slightly
      


 
   different purposes.  Received-SPF is intended to include enough
      


 
   forensic information to enable reconstruction of the SPF evaluation
      


 
   of the message, while Authentication-Results is designed only to
      


 
   relay the result itself and related output details of likely use to
      


 
   end users (e.g., what property of the message was actually
      


 
   authenticated and what it contained), leaving forensic work to the
      


 
   purview of system logs and the Received field contents.  Also,
      


 
   Received-SPF relies on compliance of agents within the receiving ADMD
      


 
   to adhere to the header field ordering rules of [RFC5321] and
      


 
   [RFC5322], while Authentication-Results includes some provisions to
      


 
   protect against non-compliant implementations.
      


 

      


 
   An operator could choose to use both to serve different downstream
      


 
   agents.  In such cases, care needs to be taken to ensure both fields
      


 
   are conveying the same details, or unexpected results can occur.
      


 

      


 
7.1.  The Received-SPF Header Field
      


 
                                                                         
      


 
   The Received-SPF header field is a trace field (see [RFC5322] Section
      

   3.6.7) and SHOULD be prepended to the existing header, above the
 
   3.6.7) and SHOULD be prepended to the existing header, above the
      

   Received: field that is generated by the SMTP receiver.  It MUST
 
   Received: field that is generated by the SMTP receiver.  It MUST
      

   appear above all other Received-SPF fields in the message.  The
 
   appear above all other Received-SPF fields in the message.  The
      

   header field has the following format:
 
   header field has the following format:
      


 

      

   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
 
   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
      

                      [ key-value-list ] CRLF
 
                      [ key-value-list ] CRLF
      


 

      
      





   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
 
   result           = "pass" / "fail" / "softfail" / "neutral" /
      

                      "None" / "TempError" / "PermError"
 
                      "none" / "temperror" / "permerror"
      


 

      

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
 
   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
      

                      [";"]
 
                      [";"]
      


 

      

   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
 
   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
      


 

      

   key              = "client-ip" / "envelope-from" / "helo" /
 
   key              = "client-ip" / "envelope-from" / "helo" /
      

                      "problem" / "receiver" / "identity" /
 
                      "problem" / "receiver" / "identity" /
      
      





                       mechanism / "x-" name / name
 
                       mechanism / name
      


 

      

   identity         = "mailfrom"   ; for the "MAIL FROM" identity
 
   identity         = "mailfrom"   ; for the "MAIL FROM" identity
      

                      / "helo"     ; for the "HELO" identity
 
                      / "helo"     ; for the "HELO" identity
      

                      / name       ; other identities
 
                      / name       ; other identities
      


 

      
      





   dot-atom         = <unquoted word as per [RFC2822]>
 
   dot-atom         = <unquoted word as per [RFC5322]>
      

   quoted-string    = <quoted string as per [RFC2822]>
 
   quoted-string    = <quoted string as per [RFC5322]>
      

   comment          = <comment string as per [RFC2822]>
 
   comment          = <comment string as per [RFC5322]>
      

   CFWS             = <comment or folding white space as per [RFC2822]>
 
   CFWS             = <comment or folding white space as per [RFC5322]>
      

   FWS              = <folding white space as per [RFC2822]>
 
   FWS              = <folding white space as per [RFC5322]>
      

   CRLF             = <standard end-of-line token as per [RFC2822]>
 
   CRLF             = <standard end-of-line token as per [RFC2532]>
      


 

      
      





   The header field SHOULD include a "(...)" style <comment> after the
 
   The header field SHOULD include a "(...)" style comment after the
      

   result, conveying supporting information for the result, such as
 
   result, conveying supporting information for the result, such as
      

   <ip>, <sender>, and <domain>.
 
   <ip>, <sender>, and <domain>.
      


 

      

   The following key-value pairs are designed for later machine parsing.
 
   The following key-value pairs are designed for later machine parsing.
      
      





   SPF clients SHOULD give enough information so that the SPF results
 
   SPF verifiers SHOULD give enough information so that the SPF results
      

   can be verified.  That is, at least "client-ip", "helo", and, if the
 
   can be verified.  That is, at least "client-ip", "helo", and, if the
      

   "MAIL FROM" identity was checked, "envelope-from".
 
   "MAIL FROM" identity was checked, "envelope-from".
      


 

      

   client-ip      the IP address of the SMTP client
 
   client-ip      the IP address of the SMTP client
      


 

      

   envelope-from  the envelope sender mailbox
 
   envelope-from  the envelope sender mailbox
      


 

      

   helo           the host name given in the HELO or EHLO command
 
   helo           the host name given in the HELO or EHLO command
      


 

      

   mechanism      the mechanism that matched (if no mechanisms matched,
 
   mechanism      the mechanism that matched (if no mechanisms matched,
      

                  substitute the word "default")
 
                  substitute the word "default")
      


 

      

   problem        if an error was returned, details about the error
 
   problem        if an error was returned, details about the error
      
      





                                                                         
 
   receiver       the host name of the SPF verifier
      

   receiver       the host name of the SPF client
 

      


 

      

   identity       the identity that was checked; see the <identity> ABNF
 
   identity       the identity that was checked; see the <identity> ABNF
      

                  rule
 
                  rule
      


 

      
      





   Other keys may be defined by SPF clients.  Until a new key name
 
   Other keys MAY be defined by SPF verifiers.
      

   becomes widely accepted, new key names should start with "x-".
 

      


 

      
      





   SPF clients MUST make sure that the Received-SPF header field does
 
   SPF verifiers MUST make sure that the Received-SPF header field does
      

   not contain invalid characters, is not excessively long, and does not
 
   not contain invalid characters, is not excessively long (See
      

   contain malicious data that has been provided by the sender.
 
   [RFC5322] Section 2.1.1), and does not contain malicious data that
      


 
   has been provided by the sender.
      


 

      
      





   Examples of various header styles that could be generated are the
 
   Examples of various header field styles that could be generated are
      

   following:
 
   the following:
      


 

      
      





   Received-SPF: Pass (mybox.example.org: domain of
 
   Received-SPF: pass (mybox.example.org: domain of
      

    [e-mail] designates 192.0.2.1 as permitted sender)
 
    [e-mail] designates 192.0.2.1 as permitted sender)
      

       receiver=mybox.example.org; client-ip=192.0.2.1;
 
       receiver=mybox.example.org; client-ip=192.0.2.1;
      
      





       envelope-from=<[e-mail]>; helo=foo.example.com;
 
       envelope-from="[e-mail]"; helo=foo.example.com;
      


 

      
      





   Received-SPF: Fail (mybox.example.org: domain of
 
   Received-SPF: fail (mybox.example.org: domain of
      

                     [e-mail] does not designate
 
                     [e-mail] does not designate
      

                     192.0.2.1 as permitted sender)
 
                     192.0.2.1 as permitted sender)
      

                     identity=mailfrom; client-ip=192.0.2.1;
 
                     identity=mailfrom; client-ip=192.0.2.1;
      
      





                     envelope-from=<[e-mail]>;
 
                     envelope-from="[e-mail]";
      


 

      


 
7.2.  SPF Results in the Authentication-Results Header Field
      


 

      


 
   As mentioned in Section 7, the Authentication-Results header field is
      


 
   designed to communicate lists of tests a border MTA did and their
      


 
   results.  The specified elements of the field provide less
      


 
   information than the SPF-Received field:
      


 

      


 
   Authentication-Results: myhost.example.org; spf=pass
      


 
     smtp.mailfrom=example.net
      


 

      


 
   Received-SPF: pass (myhost.example.org: domain of
      


 
    [e-mail] designates 192.0.2.1 as permitted sender)
      


 
       receiver=mybox.example.org; client-ip=192.0.2.1;
      


 
       envelope-from="[e-mail]"; helo=foo.example.com;
      


 

      


 
   It is, however, possible to add CFWS in the "reason" part of an
      


 
   Authentication-Results header field and provide the equivalent
      


 
   information, if desired.
      


 

      


 
   As an example, an expanded Authentication-Results header field might
      


 
   look like (for a "MAIL FROM" check in this example):
      


 

      


 
   Authentication-Results: myhost.example.org; spf=pass
      


 
     reason="client-ip=192.0.2.1; smtp.helo=foo.example.com"
      


 
     smtp.mailfrom=[e-mail]
      


 

      

8.  Macros
 
8.  Macros
      


 

      

8.1.  Macro Definitions
 
8.1.  Macro Definitions
      


 

      
      





   Many mechanisms and modifiers perform macro expansion on part of the
 
   Many mechanisms and modifiers perform macro expansion on a term.
      

   term.
 

      


 

      

   domain-spec      = macro-string domain-end
 
   domain-spec      = macro-string domain-end
      

   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
 
   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
      


 

      

   toplabel         = ( *alphanum ALPHA *alphanum ) /
 
   toplabel         = ( *alphanum ALPHA *alphanum ) /
      

                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
 
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
      

                      ; LDH rule plus additional TLD restrictions
 
                      ; LDH rule plus additional TLD restrictions
      
      





                      ; (see [RFC3696], Section 2)
 
                      ; (see [RFC3696], Section 2 for background)
      

   alphanum         = ALPHA / DIGIT
 
   alphanum         = ALPHA / DIGIT
      


 

      

   explain-string   = *( macro-string / SP )
 
   explain-string   = *( macro-string / SP )
      


 

      

   macro-string     = *( macro-expand / macro-literal )
 
   macro-string     = *( macro-expand / macro-literal )
      

   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
 
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
      

                      / "%%" / "%_" / "%-"
 
                      / "%%" / "%_" / "%-"
      

   macro-literal    = %x21-24 / %x26-7E
 
   macro-literal    = %x21-24 / %x26-7E
      

                      ; visible characters except "%"
 
                      ; visible characters except "%"
      

   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
 
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
      
      





                      "c" / "r" / "t"
 
                      "c" / "r" / "t" / "v"
      

   transformers     = *DIGIT [ "r" ]
 
   transformers     = *DIGIT [ "r" ]
      

   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
      


 

      

   A literal "%" is expressed by "%%".
 
   A literal "%" is expressed by "%%".
      


 

      

      "%_" expands to a single " " space.
 
      "%_" expands to a single " " space.
      

      "%-" expands to a URL-encoded space, viz., "%20".
 
      "%-" expands to a URL-encoded space, viz., "%20".
      


 

      

   The following macro letters are expanded in term arguments:
 
   The following macro letters are expanded in term arguments:
      


 

      

      s = <sender>
 
      s = <sender>
      

      l = local-part of <sender>
 
      l = local-part of <sender>
      

      o = domain of <sender>
 
      o = domain of <sender>
      

      d = <domain>
 
      d = <domain>
      

      i = <ip>
 
      i = <ip>
      
      





      p = the validated domain name of <ip>
 
      p = the validated domain name of <ip> (deprecated)
      

      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
 
      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
      

      h = HELO/EHLO domain
 
      h = HELO/EHLO domain
      


 

      

   The following macro letters are allowed only in "exp" text:
 
   The following macro letters are allowed only in "exp" text:
      


 

      

      c = SMTP client IP (easily readable format)
 
      c = SMTP client IP (easily readable format)
      

      r = domain name of host performing the check
 
      r = domain name of host performing the check
      

      t = current timestamp
 
      t = current timestamp
      


 

      

   A '%' character not followed by a '{', '%', '-', or '_' character is
 
   A '%' character not followed by a '{', '%', '-', or '_' character is
      


 

      

skipping to change at page 28, line 13
 
skipping to change at page 37, line 7
      

      h = HELO/EHLO domain
 
      h = HELO/EHLO domain
      


 

      

   The following macro letters are allowed only in "exp" text:
 
   The following macro letters are allowed only in "exp" text:
      


 

      

      c = SMTP client IP (easily readable format)
 
      c = SMTP client IP (easily readable format)
      

      r = domain name of host performing the check
 
      r = domain name of host performing the check
      

      t = current timestamp
 
      t = current timestamp
      


 

      

   A '%' character not followed by a '{', '%', '-', or '_' character is
 
   A '%' character not followed by a '{', '%', '-', or '_' character is
      

   a syntax error.  So
 
   a syntax error.  So
      
      





                                                                         
 

      

      -exists:%(ir).sbl.spamhaus.example.org
 
      -exists:%(ir).sbl.spamhaus.example.org
      
      





                                                                         
 
   is incorrect and will cause check_host() to yield a "permerror".
      

   is incorrect and will cause check_host() to return a "PermError".
 

      

   Instead, say
 
   Instead, say
      
      





                                                                         
 

      

      -exists:%{ir}.sbl.spamhaus.example.org
 
      -exists:%{ir}.sbl.spamhaus.example.org
      


 

      

   Optional transformers are the following:
 
   Optional transformers are the following:
      


 

      

      *DIGIT = zero or more digits
 
      *DIGIT = zero or more digits
      

      'r'    = reverse value, splitting on dots by default
 
      'r'    = reverse value, splitting on dots by default
      


 

      

   If transformers or delimiters are provided, the replacement value for
 
   If transformers or delimiters are provided, the replacement value for
      

   a macro letter is split into parts.  After performing any reversal
 
   a macro letter is split into parts.  After performing any reversal
      

   operation and/or removal of left-hand parts, the parts are rejoined
 
   operation and/or removal of left-hand parts, the parts are rejoined
      


 

      

skipping to change at page 28, line 32
 
skipping to change at page 37, line 23
      


 

      

      *DIGIT = zero or more digits
 
      *DIGIT = zero or more digits
      

      'r'    = reverse value, splitting on dots by default
 
      'r'    = reverse value, splitting on dots by default
      


 

      

   If transformers or delimiters are provided, the replacement value for
 
   If transformers or delimiters are provided, the replacement value for
      

   a macro letter is split into parts.  After performing any reversal
 
   a macro letter is split into parts.  After performing any reversal
      

   operation and/or removal of left-hand parts, the parts are rejoined
 
   operation and/or removal of left-hand parts, the parts are rejoined
      

   using "." and not the original splitting characters.
 
   using "." and not the original splitting characters.
      


 

      

   By default, strings are split on "." (dots).  Note that no special
 
   By default, strings are split on "." (dots).  Note that no special
      
      





   treatment is given to leading, trailing, or consecutive delimiters,
 
   treatment is given to leading, trailing, or consecutive delimiters in
      

   and so the list of parts may contain empty strings.  Older
 
   input strings, and so the list of parts might contain empty strings.
      

   implementations of SPF prohibit trailing dots in domain names, so
 
   Some older implementations of SPF prohibit trailing dots in domain
      

   trailing dots should not be published by domain owners, although they
 
   names, so trailing dots SHOULD NOT be published by domain owners,
      

   must be accepted by implementations conforming to this document.
 
   although they MUST be accepted by implementations conforming to this
      

   Macros may specify delimiter characters that are used instead of ".".
 
   document.  Macros can specify delimiter characters that are used
      


 
   instead of ".".
      


 

      

   The 'r' transformer indicates a reversal operation: if the client IP
 
   The 'r' transformer indicates a reversal operation: if the client IP
      

   address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
 
   address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
      

   and the macro %{ir} would expand to "1.2.0.192".
 
   and the macro %{ir} would expand to "1.2.0.192".
      


 

      

   The DIGIT transformer indicates the number of right-hand parts to
 
   The DIGIT transformer indicates the number of right-hand parts to
      

   use, after optional reversal.  If a DIGIT is specified, the value
 
   use, after optional reversal.  If a DIGIT is specified, the value
      

   MUST be nonzero.  If no DIGITs are specified, or if the value
 
   MUST be nonzero.  If no DIGITs are specified, or if the value
      

   specifies more parts than are available, all the available parts are
 
   specifies more parts than are available, all the available parts are
      

   used.  If the DIGIT was 5, and only 3 parts were available, the macro
 
   used.  If the DIGIT was 5, and only 3 parts were available, the macro
      

   interpreter would pretend the DIGIT was 3.  Implementations MUST
 
   interpreter would pretend the DIGIT was 3.  Implementations MUST
      

   support at least a value of 128, as that is the maximum number of
 
   support at least a value of 128, as that is the maximum number of
      

   labels in a domain name.
 
   labels in a domain name.
      


 

      
      





   The "s" macro expands to the <sender> argument.  It is an E-Mail
 
   The "s" macro expands to the <sender> argument.  It is an email
      

   address with a localpart, an "@" character, and a domain.  The "l"
 
   address with a local-part, an "@" character, and a domain.  The "l"
      

   macro expands to just the localpart.  The "o" macro expands to just
 
   macro expands to just the local-part.  The "o" macro expands to just
      

   the domain part.  Note that these values remain the same during
 
   the domain part.  Note that these values remain the same during
      

   recursive and chained evaluations due to "include" and/or "redirect".
 
   recursive and chained evaluations due to "include" and/or "redirect".
      
      





   Note also that if the original <sender> had no localpart, the
 
   Note also that if the original <sender> had no local-part, the local-
      

   localpart was set to "postmaster" in initial processing (see Section
 
   part was set to "postmaster" in initial processing (see Section 4.3).
      

   4.3).
 

      


 

      

   For IPv4 addresses, both the "i" and "c" macros expand to the
 
   For IPv4 addresses, both the "i" and "c" macros expand to the
      

   standard dotted-quad format.
 
   standard dotted-quad format.
      


 

      

   For IPv6 addresses, the "i" macro expands to a dot-format address; it
 
   For IPv6 addresses, the "i" macro expands to a dot-format address; it
      
      





   is intended for use in %{ir}.  The "c" macro may expand to any of the
 
   is intended for use in %{ir}.  The "c" macro can expand to any of the
      

   hexadecimal colon-format addresses specified in [RFC3513], Section
 
   hexadecimal colon-format addresses specified in [RFC4291], Section
      

   2.2.  It is intended for humans to read.
 
   2.2.  It is intended for humans to read.
      


 

      

   The "p" macro expands to the validated domain name of <ip>.  The
 
   The "p" macro expands to the validated domain name of <ip>.  The
      
      





   procedure for finding the validated domain name is defined in Section
 
   procedure for finding the validated domain name is defined in
      

   5.5.  If the <domain> is present in the list of validated domains, it
 
   Section 5.5.  If the <domain> is present in the list of validated
      

   SHOULD be used.  Otherwise, if a subdomain of the <domain> is
 
   domains, it SHOULD be used.  Otherwise, if a subdomain of the
      

   present, it SHOULD be used.  Otherwise, any name from the list may be
 
   <domain> is present, it SHOULD be used.  Otherwise, any name from the
      

   used.  If there are no validated domain names or if a DNS error
 
   list can be used.  If there are no validated domain names or if a DNS
      

   occurs, the string "unknown" is used.
 
   error occurs, the string "unknown" is used.  This macro is deprecated
      


 
   and SHOULD NOT be used.
      


 

      

   The "r" macro expands to the name of the receiving MTA.  This SHOULD
 
   The "r" macro expands to the name of the receiving MTA.  This SHOULD
      

   be a fully qualified domain name, but if one does not exist (as when
 
   be a fully qualified domain name, but if one does not exist (as when
      

   the checking is done by a MUA) or if policy restrictions dictate
 
   the checking is done by a MUA) or if policy restrictions dictate
      

   otherwise, the word "unknown" SHOULD be substituted.  The domain name
 
   otherwise, the word "unknown" SHOULD be substituted.  The domain name
      
      





   may be different from the name found in the MX record that the client
 
   can be different from the name found in the MX record that the client
      

   MTA used to locate the receiving MTA.
 
   MTA used to locate the receiving MTA.
      


 

      

   The "t" macro expands to the decimal representation of the
 
   The "t" macro expands to the decimal representation of the
      

   approximate number of seconds since the Epoch (Midnight, January 1,
 
   approximate number of seconds since the Epoch (Midnight, January 1,
      
      





   1970, UTC).  This is the same value as is returned by the POSIX
 
   1970, UTC) at the time of the evaluation.  This is the same value as
      

   time() function in most standards-compliant libraries.
 
   is returned by the POSIX time() function in most standards-compliant
      


 
   libraries.
      


 

      

   When the result of macro expansion is used in a domain name query, if
 
   When the result of macro expansion is used in a domain name query, if
      

   the expanded domain name exceeds 253 characters (the maximum length
 
   the expanded domain name exceeds 253 characters (the maximum length
      

   of a domain name), the left side is truncated to fit, by removing
 
   of a domain name), the left side is truncated to fit, by removing
      
      





   successive domain labels until the total length does not exceed 253
 
   successive domain labels (and their following dots) until the total
      

   characters.
 
   length does not exceed 253 characters.
      


 

      

   Uppercased macros expand exactly as their lowercased equivalents, and
 
   Uppercased macros expand exactly as their lowercased equivalents, and
      
      





   are then URL escaped.  URL escaping must be performed for characters
 
   are then URL escaped.  URL escaping MUST be performed for characters
      

   not in the "uric" set, which is defined in [RFC3986].
 
   not in the "unreserved" set, which is defined in [RFC3986].
      


 

      
      





   Note: Care must be taken so that macro expansion for legitimate
 
   Note: Care has to be taken so that macro expansion for legitimate
      

   E-Mail does not exceed the 63-character limit on DNS labels.  The
 
   email does not exceed the 63-character limit on DNS labels.  The
      

   localpart of E-Mail addresses, in particular, can have more than 63
 
   local-part of email addresses, in particular, can have more than 63
      

   characters between dots.
 
   characters between dots.
      


 

      
      





   Note: Domains should avoid using the "s", "l", "o", or "h" macros in
 
   Note: Domains SHOULD avoid using the "s", "l", "o", or "h" macros in
      

   conjunction with any mechanism directive.  Although these macros are
 
   conjunction with any mechanism directive.  Although these macros are
      

   powerful and allow per-user records to be published, they severely
 
   powerful and allow per-user records to be published, they severely
      

   limit the ability of implementations to cache results of check_host()
 
   limit the ability of implementations to cache results of check_host()
      

   and they reduce the effectiveness of DNS caches.
 
   and they reduce the effectiveness of DNS caches.
      


 

      
      





   Implementations should be aware that if no directive processed during
 
   Note: If no directive processed during the evaluation of check_host()
      

   the evaluation of check_host() contains an "s", "l", "o", or "h"
 
   contains an "s", "l", "o", or "h" macro, then the results of the
      

   macro, then the results of the evaluation can be cached on the basis
 
   evaluation can be cached on the basis of <domain> and <ip> alone for
      

   of <domain> and <ip> alone for as long as the shortest Time To Live
 
   as long as the shortest Time To Live (TTL) of all the DNS records
      

   (TTL) of all the DNS records involved.
 
   involved.
      


 

      

8.2.  Expansion Examples
 
8.2.  Expansion Examples
      


 

      

      The <sender> is [e-mail].
 
      The <sender> is [e-mail].
      

      The IPv4 SMTP client IP is 192.0.2.3.
 
      The IPv4 SMTP client IP is 192.0.2.3.
      

      The IPv6 SMTP client IP is 2001:DB8::CB01.
 
      The IPv6 SMTP client IP is 2001:DB8::CB01.
      

      The PTR domain name of the client IP is mx.example.org.
 
      The PTR domain name of the client IP is mx.example.org.
      


 

      

   macro                       expansion
 
   macro                       expansion
      

   -------  ----------------------------
 
   -------  ----------------------------
      


 

      

skipping to change at page 31, line 20
 
skipping to change at page 40, line 4
      

                       bad.strong.lp.3.2.0.192.in-addr._spf.example.com
 
                       bad.strong.lp.3.2.0.192.in-addr._spf.example.com
      


 

      

   %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
 
   %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
      

                           3.2.0.192.in-addr.strong.lp._spf.example.com
 
                           3.2.0.192.in-addr.strong.lp._spf.example.com
      


 

      

   %{d2}.trusted-domains.example.net
 
   %{d2}.trusted-domains.example.net
      

                                example.com.trusted-domains.example.net
 
                                example.com.trusted-domains.example.net
      


 

      

   IPv6:
 
   IPv6:
      

   %{ir}.%{v}._spf.%{d2}                               1.0.B.C.0.0.0.0.
 
   %{ir}.%{v}._spf.%{d2}                               1.0.B.C.0.0.0.0.
      
      






 
                                                                         
      

   0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com
 
   0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com
      


 

      

9.  Implications
 
9.  Implications
      


 

      

   This section outlines the major implications that adoption of this
 
   This section outlines the major implications that adoption of this
      
      





   document will have on various entities involved in Internet E-Mail.
 
   document will have on various entities involved in Internet email.
      

   It is intended to make clear to the reader where this document
 
   It is intended to make clear to the reader where this document
      

   knowingly affects the operation of such entities.  This section is
 
   knowingly affects the operation of such entities.  This section is
      

   not a "how-to" manual, or a "best practices" document, and it is not
 
   not a "how-to" manual, or a "best practices" document, and it is not
      
      





   a comprehensive list of what such entities should do in light of this
 
   a comprehensive list of what such entities SHOULD do in light of this
      

   document.
 
   document.
      


 

      
      





   This section is non-normative.
 
   This section is non-normative.  [RFC5598] describes the Internet
      


 
   email architecture.  This section is organized based on the different
      


 
   segments of the architecture.
      


 

      

9.1.  Sending Domains
 
9.1.  Sending Domains
      


 

      
      





   Domains that wish to be compliant with this specification will need
 
   Originating ADMDs (ADministrative Management Domains - [RFC5598]
      

   to determine the list of hosts that they allow to use their domain
 
   Section 2.2.1 and Section 2.3) that wish to be compliant with this
      

   name in the "HELO" and "MAIL FROM" identities.  It is recognized that
 
   specification will need to determine the list of relays ([RFC5598]
      

   forming such a list is not just a simple technical exercise, but
 
   Section 2.2.2) that they allow to use their domain name in the "HELO"
      

   involves policy decisions with both technical and administrative
 
   and "MAIL FROM" identities when relaying to other ADMDs.  It is
      

   considerations.
 
   recognized that forming such a list is not just a simple technical
      


 
   exercise, but involves policy decisions with both technical and
      


 
   administrative considerations.
      


 

      
      





   It can be helpful to publish records that include a "tracking
 
9.1.1.  DNS Resource Considerations
      


 

      


 
   Minimizing the DNS resources required for SPF lookups can be done by
      


 
   choosing directives that require less DNS information and by placing
      


 
   lower-cost mechanisms earlier in the SPF record.
      


 

      


 
             +----------+--------+-----------------+
      


 
             | term     | cost   | limit           |
      


 
             +----------+--------+-----------------+
      


 
             | ip4/ip6  | 0      | -               |
      


 
             | a        | 1      | 10              |
      


 
             | include  | 1      | 10              |
      


 
             | redirect | 1      | 10              |
      


 
             | exists   | 1      | 10              |
      


 
             | mx       | 1 + N* | 10 and N* <= 10 |
      


 
             | ptr/%{p} | 1 + N* | 10 and N* <= 10 |
      


 
             | all      | 0      | -               |
      


 
             +----------+--------+-----------------+
      


 
              * N is the number of RRs found during each term evaluation
      


 

      


 
   Section 4.6.4 specifies the limits receivers have to use.  It is
      


 
   essential to publish records that do not exceed these requirements.
      


 
   It is also required to carefully weight the cost and the
      


 
   maintainability of licit solutions.
      


 

      


 
   For example, consider a domain set up as follows:
      


 

      


 
      example.com.     IN MX   10 mx.example.com.
      


 
                       IN MX   20 mx2.example.com.
      


 
      mx.example.com.  IN A    192.0.2.1
      


 
      mx2.example.com. IN A    192.0.2.129
      


 

      


 
   Assume the administrative point is to authorize (pass) mx and mx2
      


 
   while failing every other host.  Compare the following solutions:
      


 

      


 
   Best record:
      


 
      example.com.   IN TXT  "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all"
      


 

      


 
   Good record:
      


 
      $ORIGIN example.com.
      


 
      @              IN TXT  "v=spf1 a:authorized-spf.example.com -all"
      


 
      authorized-spf IN A    192.0.2.1
      


 
                     IN A    192.0.2.129
      


 

      


 
   Expensive record:
      


 
      example.com.   IN TXT  "v=spf1 mx:example.com -all"
      


 

      


 
   Wasteful, bad record:
      


 
      example.com.   IN TXT  "v=spf1 ip4:192.0.2.0/24 mx -all"
      


 

      


 
9.1.2.  Administrator's Considerations
      


 

      


 
   There might be administrative considerations: using "a" over "ip4" or
      


 
   "ip6" allows hosts to be renumbered easily.  Using "mx" over "a"
      


 
   allows the set of mail hosts to be changed easily.  Unless such
      


 
   changes are common, it is better to use the less resource intensive
      


 
   mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".
      


 

      


 
   In some specific cases, standard advice on record content is
      


 
   appropriate.  Publishing SPF records for domains that send no mail is
      


 
   a well established best practice.  The record for a domain that sends
      


 
   no mail is:
      


 

      


 
      www.example.com.   IN TXT  "v=spf1 -all"
      


 

      


 
   Publishing SPF records for individual hosts is also best practice.
      


 
   The hostname is generally the identity used in the 5321.HELO/.EHLO
      


 
   command.  In the case of messages with a null 5321.MailFrom, this is
      


 
   used as the domain for 5321.MailFrom SPF checks, in addition to being
      


 
   used in 5321.HELO/.EHLO based SPF checks.  The standard SPF record
      


 
   for an individual host that is involved in mail processing is:
      


 

      


 
      relay.example.com.   IN TXT  "v=spf1 a -all"
      


 

      


 
   Validating correct deployment is difficult.  [RFC6652] describes one
      


 
   mechanism for soliciting feedback on SPF failures.  Another approach
      


 
   that can be helpful to publish records that include a "tracking
      

   exists:" mechanism.  By looking at the name server logs, a rough list
 
   exists:" mechanism.  By looking at the name server logs, a rough list
      
      





   may then be generated.  For example:
 
   can then be generated.  For example:
      


 

      

      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
 
      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
      


 

      
      





9.2.  Mailing Lists
 
   Regardless of the method used, understanding the ADMD's outbound mail
      


 
   architecture is essential to effective deployment.
      


 

      
      





   Mailing lists must be aware of how they re-inject mail that is sent
 
9.1.3.  Bounces
      

   to the list.  Mailing lists MUST comply with the requirements in
 

      

   [RFC2821], Section 3.10, and [RFC1123], Section 5.3.6, that say that
 
   As explained in Section 1.3.3, [RFC5321] allows the reverse-path to
      


 
   be null, which is typical of some Delivery Status Notification
      


 
   [RFC3464], commonly called email bounces.  In this case the only
      


 
   entity available for performing an SPF check is the "HELO" identity
      


 
   defined in Section 1.3.4.  SPF functionality is enhanced by
      


 
   administrators ensuring this identity is set correctly and has an
      


 
   appropriate SPF record.  It is normal to have the HELO identity set
      


 
   to hostname instead of domain.  Zone file generation for significant
      


 
   numbers of hosts can be consolidated using the redirect modifier and
      


 
   scripted for initial deployment.  Specific deployment advice is given
      


 
   above in Section 9.1.2.
      


 

      


 
9.2.  Mediators
      


 

      


 
   Broadly speaking, there are two types of mediating ADMDs that can
      


 
   affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]
      


 
   Section 5.3) and ReSenders ([RFC5598] Section 5.2).
      


 

      


 
9.2.1.  Mailing Lists
      


 
                                                                         
      


 
   Mailing lists have to be aware of how they re-inject mail that is
      


 
   sent to the list.  Mailing lists MUST comply with the requirements in
      


 
   [RFC5321], Section 3.10, and [RFC1123], Section 5.3.6, that say that
      

   the reverse-path MUST be changed to be the mailbox of a person or
 
   the reverse-path MUST be changed to be the mailbox of a person or
      

   other entity who administers the list.  Whereas the reasons for
 
   other entity who administers the list.  Whereas the reasons for
      

   changing the reverse-path are many and long-standing, SPF adds
 
   changing the reverse-path are many and long-standing, SPF adds
      

   enforcement to this requirement.
 
   enforcement to this requirement.
      


 

      

   In practice, almost all mailing list software in use already complies
 
   In practice, almost all mailing list software in use already complies
      
      





   with this requirement.  Mailing lists that do not comply may or may
 
   with this requirement.  Mailing lists that do not comply might
      

   not encounter problems depending on how access to the list is
 
   encounter problems depending on how access to the list is restricted.
      

   restricted.  Such lists that are entirely internal to a domain (only
 
   Such lists that are entirely internal to a domain (only people in the
      

   people in the domain can send to or receive from the list) are not
 
   domain can send to or receive from the list) are not affected.
      

   affected.
 

      


 

      
      





9.3.  Forwarding Services and Aliases
 
9.2.2.  Forwarding Services and Aliases
      


 

      

   Forwarding services take mail that is received at a mailbox and
 
   Forwarding services take mail that is received at a mailbox and
      

   direct it to some external mailbox.  At the time of this writing, the
 
   direct it to some external mailbox.  At the time of this writing, the
      

   near-universal practice of such services is to use the original "MAIL
 
   near-universal practice of such services is to use the original "MAIL
      

   FROM" of a message when re-injecting it for delivery to the external
 
   FROM" of a message when re-injecting it for delivery to the external
      
      





   mailbox.  [RFC1123] and [RFC2821] describe this action as an "alias"
 
   mailbox.  [RFC1123] and [RFC5321] describe this action as an "alias"
      

   rather than a "mail list".  This means that the external mailbox's
 
   rather than a "mail list".  This means the external mailbox's MTA
      

   MTA sees all such mail in a connection from a host of the forwarding
 
   sees all such mail in a connection from a host of the forwarding
      

   service, and so the "MAIL FROM" identity will not, in general, pass
 
   service, and so the "MAIL FROM" identity will not, in general, pass
      

   authorization.
 
   authorization.
      


 

      

   There are three places that techniques can be used to ameliorate this
 
   There are three places that techniques can be used to ameliorate this
      

   problem.
 
   problem.
      


 

      
      





   1. The beginning, when E-Mail is first sent.
 
   1.  The beginning, when email is first sent (Originating ADMDs).
      


 

      
      





       1. "Neutral" results could be given for IP addresses that may be
 
       1.  "Neutral" results could be given for IP addresses that might
      

          forwarders, instead of "Fail" results.  For example:
 
           be forwarders, instead of "fail" results based on a list of
      


 
           known reliable forwarders.  For example:
      


 

      
      





             "v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all"
 
              "v=spf1 mx ?exists:%{ir}.whitlist.example.org -all"
      


 

      
      





          This would cause a lookup on an anti-spam DNS blacklist
 
           This would cause a lookup on an DNS white list (DNSWL) and
      

          (DNSBL) and cause a result of "Fail" only for E-Mail coming
 
           cause a result of "fail" only for email not either coming
      

          from listed sources.  All other E-Mail, including E-Mail sent
 
           from the domain's mx host(s) (SPF pass) or white listed
      

          through forwarders, would receive a "Neutral" result.  By
 
           sources (SPF neutral).  This, in effect, outsources an
      

          checking the DNSBL after the known good sources, problems with
 
           element of sender policy to the maintainer of the whitelist.
      

          incorrect listing on the DNSBL are greatly reduced.
 

      


 

      
      





       2. The "MAIL FROM" identity could have additional information in
 
       2.  The "MAIL FROM" identity could have additional information in
      

          the localpart that cryptographically identifies the mail as
 
           the local-part that cryptographically identifies the mail as
      

          coming from an authorized source.  In this case, such an SPF
 
           coming from an authorized source.  In this case, such an SPF
      

          record could be used:
 
           record could be used:
      


 

      
      





             "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"
 
              "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"
      


 

      
      





          Then, a specialized DNS server can be set up to serve the
 
           Then, a specialized DNS server can be set up to serve the
      

          _spf_verify subdomain that validates the localpart.  Although
 
           _spf_verify subdomain that validates the local-part.
      

          this requires an extra DNS lookup, this happens only when the
 
           Although this requires an extra DNS lookup, this happens only
      

          E-Mail would otherwise be rejected as not coming from a known
 
           when the email would otherwise be rejected as not coming from
      

          good source.
 
           a known good source.
      


 
           Note that due to the 63-character limit for domain labels,
      


 
           this approach only works reliably if the local-part signature
      


 
           scheme is guaranteed either to only produce local-parts with
      


 
           a maximum of 63 characters or to gracefully handle truncated
      


 
           local-parts.
      


 

      
      





          Note that due to the 63-character limit for domain labels,
 
       3.  Similarly, a specialized DNS server could be set up that will
      

          this approach only works reliably if the localpart signature
 
           rate-limit the email coming from unexpected IP addresses.
      

          scheme is guaranteed either to only produce localparts with a
 

      

          maximum of 63 characters or to gracefully handle truncated
 

      

          localparts.
 

      


 

      
      





       3. Similarly, a specialized DNS server could be set up that will
 
              "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"
      

          rate-limit the E-Mail coming from unexpected IP addresses.
 

      


 

      
      





             "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"
 
       4.  SPF allows the creation of per-user policies for special
      


 
           cases.  For example, the following SPF record and appropriate
      


 
           wildcard DNS records can be used:
      


 

      
      





       4. SPF allows the creation of per-user policies for special
 
              "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
      

          cases.  For example, the following SPF record and appropriate
 

      

          wildcard DNS records can be used:
 

      


 

      
      





                 "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
 
   2.  The middle, when email is forwarded (Mediating ADMDs).
      


 

      
      





   2.  The middle, when E-Mail is forwarded.
 
       1.  Forwarding services can solve the problem by rewriting the
      


 
           "MAIL FROM" to be in their own domain.  This means mail
      


 
           rejected from the external mailbox will have to be forwarded
      


 
           back to the original sender by the forwarding service.
      


 
           Various schemes to do this exist though they vary widely in
      


 
           complexity and resource requirements on the part of the
      


 
           forwarding service.
      


 

      
      





       1. Forwarding services can solve the problem by rewriting the
 
       2.  Several popular MTAs can be forced from "alias" semantics to
      

          "MAIL FROM" to be in their own domain.  This means that mail
 
           "mailing list" semantics by configuring an additional alias
      

          bounced from the external mailbox will have to be re-bounced
 
           with "owner-" prepended to the original alias name (e.g., an
      

          by the forwarding service.  Various schemes to do this exist
 
           alias of "friends: [e-mail], [e-mail]"
      

          though they vary widely in complexity and resource
 
           would need another alias of the form "owner-friends:
      

          requirements on the part of the forwarding service.
 
           localowner").
      


 

      
      





       2. Several popular MTAs can be forced from "alias" semantics to
 
       3.  Forwarding servers could reject mail that would "fail" SPF if
      

          "mailing list" semantics by configuring an additional alias
 
           forwarded using an SMTP reply code of 551, User not local,
      

          with "owner-" prepended to the original alias name (e.g., an
 
           (see [RFC5321] section 3.4) to communicate the correct target
      

          alias of "friends: [e-mail], [e-mail]" would
 
           address to resend the mail to.
      

          need another alias of the form "owner-friends:  localowner").
 

      


 

      
      





   3. The end, when E-Mail is received.
 
   3.  The end, when email is received (Receiving ADMDs).
      


 

      
      





       1. If the owner of the external mailbox wishes to trust the
 
       1.  If the owner of the external mailbox wishes to trust the
      

          forwarding service, he can direct the external mailbox's MTA
 
           forwarding service, he can direct the external mailbox's MTA
      

          to skip SPF tests when the client host belongs to the
 
           to skip SPF tests when the client host belongs to the
      

          forwarding service.
 
           forwarding service.
      


 

      
      





       2. Tests against other identities, such as the "HELO" identity,
 
       2.  Tests against other identities, such as the "HELO" identity,
      

          may be used to override a failed test against the "MAIL FROM"
 
           MAY be used to override a failed test against the "MAIL FROM"
      

          identity.
 
           identity.
      


 

      
      





       3. For larger domains, it may not be possible to have a complete
 
       3.  For larger domains, it might not be possible to have a
      

          or accurate list of forwarding services used by the owners of
 
           complete or accurate list of forwarding services used by the
      

          the domain's mailboxes.  In such cases, whitelists of
 
           owners of the domain's mailboxes.  In such cases, whitelists
      

          generally-recognized forwarding services could be employed.
 
           of generally-recognized forwarding services could be
      


 
           employed.
      


 

      
      





9.4.  Mail Services
 
9.2.3.  Mail Services
      


 

      
      





   Service providers that offer mail services to third-party domains,
 
   MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail
      

   such as sending of bulk mail, may want to adjust their setup in light
 
   services to third-party domains, such as sending of bulk mail, might
      

   of the authorization check described in this document.  If the "MAIL
 
   want to adjust their configurations in light of the authorization
      

   FROM" identity used for such E-Mail uses the domain of the service
 
   check described in this document.  If the domain part of the "MAIL
      

   provider, then the provider needs only to ensure that its sending
 
   FROM" identity used for such email uses the domain of one of the MSPs
      

   host is authorized by its own SPF record, if any.
 
   domain, then the provider needs only to ensure that its sending host
      


 
   is authorized by its own SPF record, if any.
      


 

      
      





   If the "MAIL FROM" identity does not use the mail service provider's
 
   If the "MAIL FROM" identity does not use the MSP's domain, then extra
      

   domain, then extra care must be taken.  The SPF record format has
 
   care has to be taken.  The SPF record format has several options for
      

   several options for the third-party domain to authorize the service
 
   the third-party domain to authorize the service provider's MTAs to
      

   provider's MTAs to send mail on its behalf.  For mail service
 
   send mail on its behalf.  For MSPs, such as ISPs, that have a wide
      

   providers, such as ISPs, that have a wide variety of customers using
 
   variety of customers using the same MTA, steps are required to
      

   the same MTA, steps should be taken to prevent cross-customer forgery
 
   mitiate the risk of cross-customer forgery (see Section 10.4).
      

   (see Section 10.4).
 

      


 

      
      





9.5.  MTA Relays
 
9.2.4.  MTA Relays
      


 

      
      





   The authorization check generally precludes the use of arbitrary MTA
 
   Relays are described in [RFC5598] Section 2.2.2.  The authorization
      

   relays between sender and receiver of an E-Mail message.
 
   check generally precludes the use of arbitrary MTA relays between
      


 
   sender and receiver of an email message.
      


 

      

   Within an organization, MTA relays can be effectively deployed.
 
   Within an organization, MTA relays can be effectively deployed.
      

   However, for purposes of this document, such relays are effectively
 
   However, for purposes of this document, such relays are effectively
      

   transparent.  The SPF authorization check is a check between border
 
   transparent.  The SPF authorization check is a check between border
      
      





   MTAs of different domains.
 
   MTAs of different ADMDs.
      


 

      
      





   For mail senders, this means that published SPF records must
 
   For mail senders, this means that published SPF records have to
      

   authorize any MTAs that actually send across the Internet.  Usually,
 
   authorize any MTAs that actually send across the Internet.  Usually,
      

   these are just the border MTAs as internal MTAs simply forward mail
 
   these are just the border MTAs as internal MTAs simply forward mail
      
      





   to these MTAs for delivery.
 
   to these MTAs for relaying.
      


 

      
      





   Mail receivers will generally want to perform the authorization check
 
   The receiving ADMD will generally want to perform the authorization
      

   at the border MTAs, specifically including all secondary MXs.  This
 
   check at the boundary MTAs, including all secondary MXs.  Internal
      

   allows mail that fails to be rejected during the SMTP session rather
 
   MTAs (including MTAs that might serve both as boundary MTAs and
      

   than bounced.  Internal MTAs then do not perform the authorization
 
   internal relays from secondary MXs when they are processing the
      

   test.  To perform the authorization test other than at the border,
 
   relayed mail stream) then do not perform the authorization test.  To
      

   the host that first transferred the message to the organization must
 
   perform the authorization test other than at the boundary, the host
      

   be determined, which can be difficult to extract from the message
 
   that first transferred the message to the receiving ADMD have to be
      

   header.  Testing other than at the border is not recommended.
 
   determined, which can be difficult to extract from the message header
      


 
   because (a) header fields can be forged or malformed, and (b) there's
      


 
   no standard way to encode that information such that it can be
      


 
   reliably extracted.  Testing other than at the boundary is likely to
      


 
   produce unreliable results.
      


 

      


 
9.3.  Receivers
      


 

      


 
   SPF results can be used in combination with other methods to
      


 
   determine the final local disposition (either positive or negative of
      


 
   a message.  It can also be considered dispositive on its own.
      


 

      


 
9.3.1.  Policy For SPF Pass
      


 

      


 
   SPF pass results can be used in combination with "white lists" of
      


 
   known "good" domains to bypass some or all additional pre-delivery
      


 
   email checks.  Exactly which checks and how to determine appropriate
      


 
   white list entries has to be based on local conditions and
      


 
   requirements.
      


 

      


 
9.3.2.  Policy For SPF Fail
      


 

      


 
   SPF fail results can be used to reject messages during the SMTP
      


 
   transaction based on either "MAIL FROM" or "HELO" identity results.
      


 
   This reduces resource requirements for various content filtering
      


 
   methods and conserves bandwidth since rejection can be done before
      


 
   the SMTP content is transferred.  It also gives immediate feedback to
      


 
   the sender who might then be able to resolve the issue.  Due to some
      


 
   of the issues described above in this section (Section 9), SPF based
      


 
   rejection does present some risk of rejecting legitimate email when
      


 
   rejecting based on "MAIL FROM" results.
      


 

      


 
   SPF fail results can alternately be used as one input into a larger
      


 
   set of evaluations which might, based on a combination with other
      


 
   evaluation techniques, result in the email being marked negatively in
      


 
   some way (this might be via delivery to a special spam folder,
      


 
   modifying subject lines, or other locally determined means).
      


 
   Developing the details of such an approach have to be based on local
      


 
   conditions and requirements.  Using SPF results in this way does not
      


 
   have the advantages of resource conservation and immediate feedback
      


 
   to the sender associated with SMTP rejection, but could produce fewer
      


 
   undesirable rejections in a well designed system.  Such an approach
      


 
   might result in email that was not authorized by the sending ADMD
      


 
   being unknowingly delivered to end users.
      


 

      


 
   Either general approach can be used as they both leave a clear
      


 
   disposition of emails.  They are either delivered in some manner or
      


 
   the sender is notified of the failure.  Other dispositions such as
      


 
   "dropping" or deleting email after acceptance are inappropriate
      


 
   because they leave uncertainty and reduce the overall reliabilility
      


 
   and utility of email across the Internet.
      


 

      


 
9.3.3.  Policy For SPF Permerror
      


 

      


 
   The "permerror" result (see Section 2.5.7) indicates the SPF
      


 
   processing module at the receiver determined that the retrieved SPF
      


 
   policy record could not be interpreted.  This gives no true
      


 
   indication about the authorized use of the data found in the
      


 
   envelope.
      


 

      


 
   As with all results, implementers have a choice to make regarding
      


 
   what to do with a message that yields this result.  SMTP allows only
      


 
   a few basic options.
      


 

      


 
   Rejection of the message is an option, in that it is the one thing a
      


 
   receiver can do to draw attention to the difficulty encountered while
      


 
   protecting itself from messages that do not have a definite SPF
      


 
   result of some kind.  However, if the SPF implementation is defective
      


 
   and returns spurious "permerror" results, only the sender is actively
      


 
   notified of the defect (in the form of rejected mail), and not the
      


 
   receiver making use of SPF.
      


 

      


 
   The less intrusive handling choice is to deliver the message, perhaps
      


 
   with some kind of annotation of the difficulty encountered and/or
      


 
   logging of a similar nature.  However, this will not be desirable to
      


 
   operators that wish to implement SPF checking as strictly as
      


 
   possible, nor is this sort of passive problem reporting typically
      


 
   effective.
      


 

      


 
   There is of course the option placing this choice in the hands of the
      


 
   operator rather than the implementer since this kind of choice is
      


 
   often a matter of local policy rather than a condition with a
      


 
   universal solution, but this adds one more piece of complexity to an
      


 
   already non-trivial environment.
      


 

      


 
   Both implementers and operators need to be cautious of all choices
      


 
   and outcomes when handling SPF results.
      


 

      

10.  Security Considerations
 
10.  Security Considerations
      


 

      

10.1.  Processing Limits
 
10.1.  Processing Limits
      


 

      
      





   As with most aspects of E-Mail, there are a number of ways that
 
   As with most aspects of email, there are a number of ways that
      

   malicious parties could use the protocol as an avenue for a
 
   malicious parties could use the protocol as an avenue for a
      
      





   Denial-of-Service (DoS) attack.  The processing limits outlined here
 
   Denial-of-Service (DoS) attack.  The processing limits outlined in
      

   are designed to prevent attacks such as the following:
 
   Section 4.6.4 are designed to prevent attacks such as the following:
      


 

      

   o  A malicious party could create an SPF record with many references
 
   o  A malicious party could create an SPF record with many references
      
      





      to a victim's domain and send many E-Mails to different SPF
 
      to a victim's domain and send many emails to different SPF
      

      clients; those SPF clients would then create a DoS attack.  In
 
      verifiers; those SPF verifiers would then create a DoS attack.  In
      

      effect, the SPF clients are being used to amplify the attacker's
 
      effect, the SPF verifiers are being used to amplify the attacker's
      

      bandwidth by using fewer bytes in the SMTP session than are used
 
      bandwidth by using fewer bytes in the SMTP session than are used
      

      by the DNS queries.  Using SPF clients also allows the attacker to
 
      by the DNS queries.  Using SPF clients also allows the attacker to
      

      hide the true source of the attack.
 
      hide the true source of the attack.
      


 

      

   o  Whereas implementations of check_host() are supposed to limit the
 
   o  Whereas implementations of check_host() are supposed to limit the
      

      number of DNS lookups, malicious domains could publish records
 
      number of DNS lookups, malicious domains could publish records
      

      that exceed these limits in an attempt to waste computation effort
 
      that exceed these limits in an attempt to waste computation effort
      

      at their targets when they send them mail.  Malicious domains
 
      at their targets when they send them mail.  Malicious domains
      

      could also design SPF records that cause particular
 
      could also design SPF records that cause particular
      

      implementations to use excessive memory or CPU usage, or to
 
      implementations to use excessive memory or CPU usage, or to
      

      trigger bugs.
 
      trigger bugs.
      


 

      

   o  Malicious parties could send a large volume of mail purporting to
 
   o  Malicious parties could send a large volume of mail purporting to
      

      come from the intended target to a wide variety of legitimate mail
 
      come from the intended target to a wide variety of legitimate mail
      

      hosts.  These legitimate machines would then present a DNS load on
 
      hosts.  These legitimate machines would then present a DNS load on
      

      the target as they fetched the relevant records.
 
      the target as they fetched the relevant records.
      


 

      

   Of these, the case of a third party referenced in the SPF record is
 
   Of these, the case of a third party referenced in the SPF record is
      

   the easiest for a DoS attack to effectively exploit.  As a result,
 
   the easiest for a DoS attack to effectively exploit.  As a result,
      
      





   limits that may seem reasonable for an individual mail server can
 
   limits that might seem reasonable for an individual mail server can
      

   still allow an unreasonable amount of bandwidth amplification.
 
   still allow an unreasonable amount of bandwidth amplification.
      

   Therefore, the processing limits need to be quite low.
 
   Therefore, the processing limits need to be quite low.
      


 

      
      





   SPF implementations MUST limit the number of mechanisms and modifiers
 
10.2.  SPF-Authorized Email May Contain Other False Identities
      

   that do DNS lookups to at most 10 per SPF check, including any
 

      

   lookups caused by the use of the "include" mechanism or the
 

      

   "redirect" modifier.  If this number is exceeded during a check, a
 

      

   PermError MUST be returned.  The "include", "a", "mx", "ptr", and
 

      

   "exists" mechanisms as well as the "redirect" modifier do count
 

      

   against this limit.  The "all", "ip4", and "ip6" mechanisms do not
 

      

   require DNS lookups and therefore do not count against this limit.
 

      

   The "exp" modifier does not count against this limit because the DNS
 

      

   lookup to fetch the explanation string occurs after the SPF record
 

      

   has been evaluated.
 

      


 

      

   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
 

      

   there MUST be a limit of no more than 10 MX or PTR RRs looked up and
 

      

   checked.
 

      


 

      

   SPF implementations SHOULD limit the total amount of data obtained
 

      

   from the DNS queries.  For example, when DNS over TCP or EDNS0 are
 

      

   available, there may need to be an explicit limit to how much data
 

      

   will be accepted to prevent excessive bandwidth usage or memory usage
 

      

   and DoS attacks.
 

      


 

      

   MTAs or other processors MAY also impose a limit on the maximum
 

      

   amount of elapsed time to evaluate check_host().  Such a limit SHOULD
 

      

   allow at least 20 seconds.  If such a limit is exceeded, the result
 

      

   of authorization SHOULD be "TempError".
 

      


 

      

   Domains publishing records SHOULD try to keep the number of "include"
 

      

   mechanisms and chained "redirect" modifiers to a minimum.  Domains
 

      

   SHOULD also try to minimize the amount of other DNS information
 

      

   needed to evaluate a record.  This can be done by choosing directives
 

      

   that require less DNS information and placing lower-cost mechanisms
 

      

   earlier in the SPF record.
 

      


 

      

   For example, consider a domain set up as follows:
 

      


 

      

   example.com.      IN MX   10 mx.example.com.
 

      

   mx.example.com.   IN A    192.0.2.1
 

      

   a.example.com.    IN TXT  "v=spf1 mx:example.com -all"
 

      

   b.example.com.    IN TXT  "v=spf1 a:mx.example.com -all"
 

      

   c.example.com.    IN TXT  "v=spf1 ip4:192.0.2.1 -all"
 

      


 

      

   Evaluating check_host() for the domain "a.example.com" requires the
 

      

   MX records for "example.com", and then the A records for the listed
 

      

   hosts.  Evaluating for "b.example.com" requires only the A records.
 

      

   Evaluating for "c.example.com" requires none.
 

      


 

      

   However, there may be administrative considerations: using "a" over
 

      

   "ip4" allows hosts to be renumbered easily.  Using "mx" over "a"
 

      

   allows the set of mail hosts to be changed easily.
 

      

                                                                         
 

      

10.2.  SPF-Authorized E-Mail May Contain Other False Identities
 

      


 

      
      





   The "MAIL FROM" and "HELO" identity authorizations must not be
 
   Do not construe the "MAIL FROM" and "HELO" identity authorizations to
      

   construed to provide more assurance than they do.  It is entirely
 
   provide more assurance than they do.  It is entirely possible for a
      

   possible for a malicious sender to inject a message using his own
 
   malicious sender to inject a message using his own domain in the
      

   domain in the identities used by SPF, to have that domain's SPF
 
   identities used by SPF, to have that domain's SPF record authorize
      

   record authorize the sending host, and yet the message can easily
 
   the sending host, and yet the message can easily list other
      

   list other identities in its header.  Unless the user or the MUA
 
   identities in its header.  Unless the user or the MUA takes care to
      

   takes care to note that the authorized identity does not match the
 
   note that the authorized identity does not match the other more
      

   other more commonly-presented identities (such as the From:  header
 
   commonly-presented identities (such as the From: header field), the
      

   field), the user may be lulled into a false sense of security.
 
   user might be lulled into a false sense of security.
      


 

      

10.3.  Spoofed DNS and IP Data
 
10.3.  Spoofed DNS and IP Data
      


 

      

   There are two aspects of this protocol that malicious parties could
 
   There are two aspects of this protocol that malicious parties could
      

   exploit to undermine the validity of the check_host() function:
 
   exploit to undermine the validity of the check_host() function:
      


 

      

   o  The evaluation of check_host() relies heavily on DNS.  A malicious
 
   o  The evaluation of check_host() relies heavily on DNS.  A malicious
      

      attacker could attack the DNS infrastructure and cause
 
      attacker could attack the DNS infrastructure and cause
      

      check_host() to see spoofed DNS data, and then return incorrect
 
      check_host() to see spoofed DNS data, and then return incorrect
      
      





      results.  This could include returning "Pass" for an <ip> value
 
      results.  This could include returning "pass" for an <ip> value
      

      where the actual domain's record would evaluate to "Fail".  See
 
      where the actual domain's record would evaluate to "fail".  See
      

      [RFC3833] for a description of DNS weaknesses.
 
      [RFC3833] for a description of DNS weaknesses.
      


 

      
      





   o  The client IP address, <ip>, is assumed to be correct.  A
 
   o  The client IP address, <ip>, is assumed to be correct.  In a
      

      malicious attacker could spoof TCP sequence numbers to make mail
 
      modern, correctly configured system the risk of this not being
      

      appear to come from a permitted host for a domain that the
 
      true is nil.
      

      attacker is impersonating.
 

      


 

      

10.4.  Cross-User Forgery
 
10.4.  Cross-User Forgery
      


 

      

   By definition, SPF policies just map domain names to sets of
 
   By definition, SPF policies just map domain names to sets of
      
      





   authorized MTAs, not whole E-Mail addresses to sets of authorized
 
   authorized MTAs, not whole email addresses to sets of authorized
      

   users.  Although the "l" macro (Section 8) provides a limited way to
 
   users.  Although the "l" macro (Section 8) provides a limited way to
      
      





   define individual sets of authorized MTAs for specific E-Mail
 
   define individual sets of authorized MTAs for specific email
      

   addresses, it is generally impossible to verify, through SPF, the use
 
   addresses, it is generally impossible to verify, through SPF, the use
      
      





   of specific E-Mail addresses by individual users of the same MTA.
 
   of specific email addresses by individual users of the same MTA.
      


 

      

   It is up to mail services and their MTAs to directly prevent
 
   It is up to mail services and their MTAs to directly prevent
      
      





   cross-user forgery: based on SMTP AUTH ([RFC2554]), users should be
 
   cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be
      

   restricted to using only those E-Mail addresses that are actually
 
   restricted to using only those email addresses that are actually
      

   under their control (see [RFC4409], Section 6.1).  Another means to
 
   under their control (see [RFC6409], Section 6.1).  Another means to
      

   verify the identity of individual users is message cryptography such
 
   verify the identity of individual users is message cryptography such
      
      





   as PGP ([RFC2440]) or S/MIME ([RFC3851]).
 
   as PGP ([RFC4880]) or S/MIME ([RFC5751]).
      


 

      

10.5.  Untrusted Information Sources
 
10.5.  Untrusted Information Sources
      


 

      
      





   SPF uses information supplied by third parties, such as the "HELO"
 
   An SPF compliant receiver gathers information from the SMTP commands
      

   domain name, the "MAIL FROM" address, and SPF records.  This
 
   it receives and from the published DNS records of the sending domain
      

   information is then passed to the receiver in the Received-SPF: trace
 
   holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the
      

   fields and possibly returned to the client MTA in the form of an SMTP
 
   envelope, and SPF DNS records published by the domain holder).
      

   rejection message.  This information must be checked for invalid
 

      

   characters and excessively long lines.
 

      


 

      
      





   When the authorization check fails, an explanation string may be
 
10.5.1.  Recorded Results
      


 

      


 
   This information, passed to the receiver in the Received-SPF: or
      


 
   Authentication-Results: trace fields, may be returned to the client
      


 
   MTA as an SMTP rejection message.  If such an SMTP rejection message
      


 
   is generated, the information from the trace fields has to be checked
      


 
   for such problems as invalid characters and excessively long lines.
      


 

      


 
10.5.2.  External Explanations
      


 
                                                                         
      


 
   When the authorization check fails, an explanation string could be
      

   included in the reject response.  Both the sender and the rejecting
 
   included in the reject response.  Both the sender and the rejecting
      

   receiver need to be aware that the explanation was determined by the
 
   receiver need to be aware that the explanation was determined by the
      

   publisher of the SPF record checked and, in general, not the
 
   publisher of the SPF record checked and, in general, not the
      
      





   receiver.  The explanation may contain malicious URLs, or it may be
 
   receiver.  The explanation can contain malicious URLs, or it might be
      

   offensive or misleading.
 
   offensive or misleading.
      


 

      
      





   This is probably less of a concern than it may initially seem since
 
   Explanations returned to sender domains due to "exp" modifiers,
      

   such messages are returned to the sender, and the explanation strings
 
   (Section 6.2), were generated by the sender policy published by the
      

   come from the sender policy published by the domain in the identity
 
   domain holders themselves.  As long as messages are only returned
      

   claimed by that very sender.  As long as the DSN is not redirected to
 
   with non-delivery notification ([RFC3464]) to domains publishing the
      

   someone other than the actual sender, the only people who see
 
   explanation strings from their own DNS SPF records, the only affected
      

   malicious explanation strings are people whose messages claim to be
 
   parties are the original publishers of the domain's SPF records.
      

   from domains that publish such strings in their SPF records.  In
 
                                                                         
      

   practice, DSNs can be misdirected, such as when an MTA accepts an
 
   In practice, such non-delivery notifications can be misdirected, such
      

   E-Mail and then later generates a DSN to a forged address, or when an
 
   as when an MTA accepts an email and only later generates the
      

   E-Mail forwarder does not direct the DSN back to the original sender.
 
   notification to a forged address, or when an email forwarder does not
      


 
   direct the bounce back to the original sender.
      


 
                                                                         
      


 
10.5.3.  Macro Expansion
      


 

      


 
   Macros (Section 8) allow senders to inject arbitrary text (any non-
      


 
   null [US-ASCII] character) into receiver DNS queries.  It is necesary
      


 
   to be prepared for hostile or unexpected content.
      


 

      

10.6.  Privacy Exposure
 
10.6.  Privacy Exposure
      


 

      

   Checking SPF records causes DNS queries to be sent to the domain
 
   Checking SPF records causes DNS queries to be sent to the domain
      

   owner.  These DNS queries, especially if they are caused by the
 
   owner.  These DNS queries, especially if they are caused by the
      

   "exists" mechanism, can contain information about who is sending
 
   "exists" mechanism, can contain information about who is sending
      
      





   E-Mail and likely to which MTA the E-Mail is being sent.  This can
 
   email and likely to which MTA the email is being sent.  This can
      

   introduce some privacy concerns, which may be more or less of an
 
   introduce some privacy concerns, which are more or less of an issue
      

   issue depending on local laws and the relationship between the domain
 
   depending on local laws and the relationship between the domain owner
      

   owner and the person sending the E-Mail.
 
   and the person sending the email.
      


 

      

11.  Contributors and Acknowledgements
 
11.  Contributors and Acknowledgements
      


 

      
      





   This document is largely based on the work of Meng Weng Wong and Mark
 
   This document is largely based on the work of Meng Weng Wong, Mark
      

   Lentczner.  Although, as this section acknowledges, many people have
 
   Lentczner, and Wayne Schlitt.  Although, as this section
      

   contributed to this document, a very large portion of the writing and
 
   acknowledges, many people have contributed to this document, a very
      

   editing are due to Meng and Mark.
 
   large portion of the writing and editing are due to Meng, Mark, and
      


 
   Wayne.
      


 

      

   This design owes a debt of parentage to [RMX] by Hadmut Danisch and
 
   This design owes a debt of parentage to [RMX] by Hadmut Danisch and
      

   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
 
   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
      
      





   the legitimacy of an E-Mail address traces its ancestry further back
 
   the legitimacy of an email address traces its ancestry further back
      

   through messages on the namedroppers mailing list by Paul Vixie
 
   through messages on the namedroppers mailing list by Paul Vixie
      
      





                                                                         
 

      

   [Vixie] (based on suggestion by Jim Miller) and by David Green
 
   [Vixie] (based on suggestion by Jim Miller) and by David Green
      

   [Green].
 
   [Green].
      


 

      

   Philip Gladstone contributed the concept of macros to the
 
   Philip Gladstone contributed the concept of macros to the
      

   specification, multiplying the expressiveness of the language and
 
   specification, multiplying the expressiveness of the language and
      

   making per-user and per-IP lookups possible.
 
   making per-user and per-IP lookups possible.
      


 

      
      





   The authors would also like to thank the literally hundreds of
 
   The authors of both this document and [RFC4408] would also like to
      

   individuals who have participated in the development of this design.
 
   thank the literally hundreds of individuals who have participated in
      

   They are far too numerous to name, but they include the following:
 
   the development of this design.  They are far too numerous to name,
      


 
   but they include the following:
      


 

      
      






 
      The participants in the SPFbis working group.
      

      The folks on the spf-discuss mailing list.
 
      The folks on the spf-discuss mailing list.
      

      The folks on the SPAM-L mailing list.
 
      The folks on the SPAM-L mailing list.
      

      The folks on the IRTF ASRG mailing list.
 
      The folks on the IRTF ASRG mailing list.
      

      The folks on the IETF MARID mailing list.
 
      The folks on the IETF MARID mailing list.
      

      The folks on #perl.
 
      The folks on #perl.
      


 

      

12.  IANA Considerations
 
12.  IANA Considerations
      


 

      

12.1.  The SPF DNS Record Type
 
12.1.  The SPF DNS Record Type
      


 

      
      





   The IANA has assigned a new Resource Record Type and Qtype from the
 
   Per [RFC4408], the IANA assigned the Resource Record Type and Qtype
      

   DNS Parameters Registry for the SPF RR type with code 99.
 
   from the DNS Parameters Registry for the SPF RR type with code 99.
      


 
   The format of this type is identical to the TXT RR [RFC1035].  The
      


 
   character content of the record is encoded as [US-ASCII].  Use of
      


 
   this record type is obsolete for SPF Version 1.
      


 

      


 
   IANA is requested to add an annotation to the SPF RRTYPE saying
      


 
   "(OBSOLETE - use TXT)" in the DNS Parameters registry.
      


 

      


 
   [NOTE TO RFC EDITOR: (to be changed to " ... has added ..." upon
      


 
   publication)]
      


 

      

12.2.  The Received-SPF Mail Header Field
 
12.2.  The Received-SPF Mail Header Field
      


 

      

   Per [RFC3864], the "Received-SPF:" header field is added to the IANA
 
   Per [RFC3864], the "Received-SPF:" header field is added to the IANA
      

   Permanent Message Header Field Registry.  The following is the
 
   Permanent Message Header Field Registry.  The following is the
      

   registration template:
 
   registration template:
      


 

      

      Header field name: Received-SPF
 
      Header field name: Received-SPF
      
      





      Applicable protocol: mail ([RFC2822])
 
      Applicable protocol: mail ([RFC5322])
      

      Status: Experimental
 
      Status: Standards Track
      

      Author/Change controller: IETF
 
      Author/Change controller: IETF
      
      





      Specification document(s): RFC 4408
 
      Specification document(s): RFC XXXX
      

      Related information:
 
      [NOTE TO RFC EDITOR: (this document)]
      

      Requesting SPF Council review of any proposed changes and
 

      

      additions to this field are recommended.  For information about
 
12.3.  SPF Modifier Registration
      

      the SPF Council see http://www.openspf.org/Council
 

      


 
   [RFC6652] created a new SPF Modifier Registration.  IANA is requested
      


 
   to change the reference for the exp and redirect modifiers from
      


 
   [RFC4408] to this document.  Their status should not be changed.
      


 

      

13.  References
 
13.  References
      


 

      

13.1.  Normative References
 
13.1.  Normative References
      


 

      

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
 
   [RFC1035]  Mockapetris, P., "Domain names - implementation and
      

              specification", STD 13, RFC 1035, November 1987.
 
              specification", STD 13, RFC 1035, November 1987.
      


 

      

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
 
   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
      

              and Support", STD 3, RFC 1123, October 1989.
 
              and Support", STD 3, RFC 1123, October 1989.
      


 

      

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
 
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
      

              Requirement Levels", BCP 14, RFC 2119, March 1997.
 
              Requirement Levels", BCP 14, RFC 2119, March 1997.
      


 

      
      





   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
 
   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
      

              April 2001.
 
              RFC 3463, January 2003.
      


 

      

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
 

      

              2001.
 

      


 

      

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
 

      

              for Delivery Status Notifications", RFC 3464, January
 

      

              2003.
 

      


 

      

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
 

      

              (IPv6) Addressing Architecture", RFC 3513, April 2003.
 

      


 

      

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
 
   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
      

              Procedures for Message Header Fields", BCP 90, RFC 3864,
 
              Procedures for Message Header Fields", BCP 90, RFC 3864,
      

              September 2004.
 
              September 2004.
      


 

      

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
 
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
      
      





              Resource Identifier (URI): Generic Syntax", STD 66, RFC
 
              Resource Identifier (URI): Generic Syntax", STD 66,
      

              3986, January 2005.
 
              RFC 3986, January 2005.
      


 

      
      





   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
 
   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
      

              Specifications: ABNF", RFC 4234, October 2005.
 
              Architecture", RFC 4291, February 2006.
      


 

      
      





   [US-ASCII] American National Standards Institute (formerly United
 
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
      


 
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
      


 

      


 
   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
      


 
              October 2008.
      


 

      


 
   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
      


 
              October 2008.
      


 

      


 
   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
      


 
              Message Authentication Status", RFC 5451, April 2009.
      


 

      


 
   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
      


 
              July 2009.
      


 

      


 
   [RFC5890]  Klensin, J., "Internationalized Domain Names for
      


 
              Applications (IDNA): Definitions and Document Framework",
      


 
              RFC 5890, August 2010.
      


 
                                                                         
      


 
   [US-ASCII]
      


 
              American National Standards Institute (formerly United
      

              States of America Standards Institute), "USA Code for
 
              States of America Standards Institute), "USA Code for
      

              Information Interchange, X3.4", 1968.
 
              Information Interchange, X3.4", 1968.
      


 

      
      





   ANSI X3.4-1968 has been replaced by newer versions with slight
 
              ANSI X3.4-1968 has been replaced by newer versions with
      

              modifications, but the 1968 version remains definitive for
 
              slight modifications, but the 1968 version remains
      

              the Internet.
 
              definitive for the Internet.
      


 

      
      





13.2  Informative References
 
13.2.  Informative References
      


 
                                                                         
      


 
   [DMP]      Fecyk, G., "Designated Mailers Protocol".
      


 

      


 
              Work In Progress
      


 

      


 
   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.
      


 

      

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
 
   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
      

              STD 13, RFC 1034, November 1987.
 
              STD 13, RFC 1034, November 1987.
      


 

      
      





   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983, August
 
   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983,
      

              1996.
 
              August 1996.
      


 

      
      





   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
 
   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
      

              "OpenPGP Message Format", RFC 2440, November 1998.
 
              NCACHE)", RFC 2308, March 1998.
      


 

      
      





   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
 
   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
      

              RFC 2554, March 1999.
 
              specifying the location of services (DNS SRV)", RFC 2782,
      


 
              February 2000.
      


 

      


 
   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
      


 
              for Delivery Status Notifications", RFC 3464,
      


 
              January 2003.
      


 

      

   [RFC3696]  Klensin, J., "Application Techniques for Checking and
 
   [RFC3696]  Klensin, J., "Application Techniques for Checking and
      

              Transformation of Names", RFC 3696, February 2004.
 
              Transformation of Names", RFC 3696, February 2004.
      


 

      

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
 
   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
      

              Name System (DNS)", RFC 3833, August 2004.
 
              Name System (DNS)", RFC 3833, August 2004.
      


 

      
      





   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
 
   [RFC3834]  Moore, K., "Recommendations for Automatic Responses to
      

              Extensions (S/MIME) Version 3.1 Message Specification",
 
              Electronic Mail", RFC 3834, August 2004.
      

              RFC 3851, July 2004.
 

      


 

      
      





   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
 
   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
      

              RFC 4409, April 2006.
 
              for Authorizing Use of Domains in E-Mail, Version 1",
      


 
              RFC 4408, April 2006.
      


 

      
      





   [RMX]      Danish, H., "The RMX DNS RR Type for light weight sender
 
   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
      

              authentication", Work In Progress
 
              (CIDR): The Internet Address Assignment and Aggregation
      


 
              Plan", BCP 122, RFC 4632, August 2006.
      


 

      
      





   [DMP]      Fecyk, G., "Designated Mailers Protocol", Work In Progress
 
   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
      


 
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
      


 

      
      





   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.
 
   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
      


 
              for Authentication", RFC 4954, July 2007.
      


 

      
      





   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.
 
   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
      


 
              Mail Extensions (S/MIME) Version 3.2 Message
      


 
              Specification", RFC 5751, January 2010.
      


 

      


 
   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
      


 
              February 2010.
      


 

      


 
   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
      


 
              STD 72, RFC 6409, November 2011.
      


 

      


 
   [RFC6647]  Kucherawy, M. and D. Crocker, "Email Greylisting: An
      


 
              Applicability Statement for SMTP", RFC 6647, June 2012.
      


 

      


 
   [RFC6652]  Kitterman, S., "Sender Policy Framework (SPF)
      


 
              Authentication Failure Reporting Using the Abuse Reporting
      


 
              Format", RFC 6652, June 2012.
      


 

      


 
   [RFC6686]  Kucherawy, M., "Resolution of the Sender Policy Framework
      


 
              (SPF) and Sender ID Experiments", RFC 6686, July 2012.
      


 

      


 
   [RMX]      Danisch, H., "The RMX DNS RR Type for light weight sender
      


 
              authentication".
      


 

      


 
              Work In Progress
      


 

      


 
   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.
      


 

      

Appendix A.  Collected ABNF
 
Appendix A.  Collected ABNF
      


 

      

   This section is normative and any discrepancies with the ABNF
 
   This section is normative and any discrepancies with the ABNF
      

   fragments in the preceding text are to be resolved in favor of this
 
   fragments in the preceding text are to be resolved in favor of this
      

   grammar.
 
   grammar.
      


 

      
      





   See [RFC4234] for ABNF notation.  Please note that as per this ABNF
 
   See [RFC5234] for ABNF notation.  Please note that as per this ABNF
      

   definition, literal text strings (those in quotes) are case-
 
   definition, literal text strings (those in quotes) are case-
      

   insensitive.  Hence, "mx" matches "mx", "MX", "mX", and "Mx".
 
   insensitive.  Hence, "mx" matches "mx", "MX", "mX", and "Mx".
      


 

      

   record           = version terms *SP
 
   record           = version terms *SP
      

   version          = "v=spf1"
 
   version          = "v=spf1"
      


 

      

   terms            = *( 1*SP ( directive / modifier ) )
 
   terms            = *( 1*SP ( directive / modifier ) )
      


 

      

   directive        = [ qualifier ] mechanism
 
   directive        = [ qualifier ] mechanism
      

   qualifier        = "+" / "-" / "?" / "~"
 
   qualifier        = "+" / "-" / "?" / "~"
      


 

      

skipping to change at page 42, line 38
 
skipping to change at page 57, line 38
      

   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 
   MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
      

   PTR              = "ptr"    [ ":" domain-spec ]
 
   PTR              = "ptr"    [ ":" domain-spec ]
      

   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 
   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
      

   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 
   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
      

   exists           = "exists"   ":" domain-spec
 
   exists           = "exists"   ":" domain-spec
      


 

      

   modifier         = redirect / explanation / unknown-modifier
 
   modifier         = redirect / explanation / unknown-modifier
      

   redirect         = "redirect" "=" domain-spec
 
   redirect         = "redirect" "=" domain-spec
      

   explanation      = "exp" "=" domain-spec
 
   explanation      = "exp" "=" domain-spec
      

   unknown-modifier = name "=" macro-string
 
   unknown-modifier = name "=" macro-string
      
      






 
                      ; where name is not any known modifier
      


 

      

   ip4-cidr-length  = "/" 1*DIGIT
 
   ip4-cidr-length  = "/" 1*DIGIT
      

   ip6-cidr-length  = "/" 1*DIGIT
 
   ip6-cidr-length  = "/" 1*DIGIT
      

   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
 
   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
      


 

      

   ip4-network      = qnum "." qnum "." qnum "." qnum
 
   ip4-network      = qnum "." qnum "." qnum "." qnum
      

   qnum             = DIGIT                 ; 0-9
 
   qnum             = DIGIT                 ; 0-9
      

                      / %x31-39 DIGIT       ; 10-99
 
                      / %x31-39 DIGIT       ; 10-99
      

                      / "1" 2DIGIT          ; 100-199
 
                      / "1" 2DIGIT          ; 100-199
      

                      / "2" %x30-34 DIGIT   ; 200-249
 
                      / "2" %x30-34 DIGIT   ; 200-249
      

                      / "25" %x30-35        ; 250-255
 
                      / "25" %x30-35        ; 250-255
      
      





             ; conventional dotted quad notation.  e.g., 192.0.2.0
 
            ; conventional dotted quad notation.  e.g., 192.0.2.0
      

   ip6-network      = <as per [RFC 3513], section 2.2>
 
   ip6-network      = <as per [RFC 4291], section 2.2>
      

             ; e.g., 2001:DB8::CD30
 
            ; e.g., 2001:DB8::CD30
      


 

      

   domain-spec      = macro-string domain-end
 
   domain-spec      = macro-string domain-end
      

   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
 
   domain-end       = ( "." toplabel [ "." ] ) / macro-expand
      
      






 
                                                                         
      

   toplabel         = ( *alphanum ALPHA *alphanum ) /
 
   toplabel         = ( *alphanum ALPHA *alphanum ) /
      

                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
 
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
      

                      ; LDH rule plus additional TLD restrictions
 
                      ; LDH rule plus additional TLD restrictions
      
      





                      ; (see [RFC3696], Section 2)
 
                      ; (see [RFC3696], Section 2 for background)
      

                                                                         
 

      

   alphanum         = ALPHA / DIGIT
 
   alphanum         = ALPHA / DIGIT
      


 

      

   explain-string   = *( macro-string / SP )
 
   explain-string   = *( macro-string / SP )
      


 

      

   macro-string     = *( macro-expand / macro-literal )
 
   macro-string     = *( macro-expand / macro-literal )
      

   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
 
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
      

                      / "%%" / "%_" / "%-"
 
                      / "%%" / "%_" / "%-"
      

   macro-literal    = %x21-24 / %x26-7E
 
   macro-literal    = %x21-24 / %x26-7E
      

                      ; visible characters except "%"
 
                      ; visible characters except "%"
      

   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
 
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
      
      





                      "c" / "r" / "t"
 
                      "c" / "r" / "t" / "v"
      

   transformers     = *DIGIT [ "r" ]
 
   transformers     = *DIGIT [ "r" ]
      

   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
      


 

      

   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 
   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
      


 

      

   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
 
   header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
      

                      [ key-value-list ] CRLF
 
                      [ key-value-list ] CRLF
      


 

      
      





   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
 
   result           = "pass" / "fail" / "softfail" / "neutral" /
      

                      "None" / "TempError" / "PermError"
 
                      "none" / "temperror" / "permerror"
      


 

      

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
 
   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
      

                      [";"]
 
                      [";"]
      


 

      

   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
 
   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
      


 

      

   key              = "client-ip" / "envelope-from" / "helo" /
 
   key              = "client-ip" / "envelope-from" / "helo" /
      
      





                      "problem" / "receiver" / "identity" /
 
                      "problem" / "receiver" / identity /
      

                       mechanism / "x-" name / name
 
                       mechanism / name
      


 

      

   identity         = "mailfrom"   ; for the "MAIL FROM" identity
 
   identity         = "mailfrom"   ; for the "MAIL FROM" identity
      

                      / "helo"     ; for the "HELO" identity
 
                      / "helo"     ; for the "HELO" identity
      

                      / name       ; other identities
 
                      / name       ; other identities
      


 

      
      





   dot-atom         = <unquoted word as per [RFC2822]>
 
   ALPHA            = <A-Z / a-z as per [RFC5234]>
      

   quoted-string    = <quoted string as per [RFC2822]>
 
   DIGIT            = <0-9 as per [RFC5234]>
      

   comment          = <comment string as per [RFC2822]>
 
   SP               = <space character as per [RFC5234]>
      

   CFWS             = <comment or folding white space as per [RFC2822]>
 
   domain           = <fully qualified domain as per [RFC5321]>
      

   FWS              = <folding white space as per [RFC2822]>
 
   dot-atom         = <unquoted word as per [RFC5322]>
      

   CRLF             = <standard end-of-line token as per [RFC2822]>
 
   quoted-string    = <quoted string as per [RFC5322]>
      


 
   comment          = <comment string as per [RFC5322]>
      


 
   CFWS             = <comment or folding white space as per [RFC5322]>
      


 
   FWS              = <folding white space as per [RFC5322]>
      


 
   CRLF             = <standard end-of-line token as per [RFC5322]>
      


 
   authserv-id      = <authserv-id per [RFC5451]>
      


 
   reasonspec       = <reason per [RFC5451]>
      


 

      

Appendix B.  Extended Examples
 
Appendix B.  Extended Examples
      


 

      

   These examples are based on the following DNS setup:
 
   These examples are based on the following DNS setup:
      


 

      

   ; A domain with two mail servers, two hosts
 
   ; A domain with two mail servers, two hosts
      

   ; and two servers at the domain name
 
   ; and two servers at the domain name
      

   $ORIGIN example.com.
 
   $ORIGIN example.com.
      

   @           MX  10 mail-a
 
   @           MX  10 mail-a
      

               MX  20 mail-b
 
               MX  20 mail-b
      


 

      

skipping to change at page 44, line 46
 
skipping to change at page 60, line 46
      


 

      

   ; A rogue reverse IP domain that claims to be
 
   ; A rogue reverse IP domain that claims to be
      

   ; something it's not
 
   ; something it's not
      

   $ORIGIN 0.0.10.in-addr.arpa.
 
   $ORIGIN 0.0.10.in-addr.arpa.
      

   4           PTR bob.example.com.
 
   4           PTR bob.example.com.
      


 

      

B.1.  Simple Examples
 
B.1.  Simple Examples
      


 

      

   These examples show various possible published records for
 
   These examples show various possible published records for
      

   example.com and which values if <ip> would cause check_host() to
 
   example.com and which values if <ip> would cause check_host() to
      
      





   return "Pass".  Note that <domain> is "example.com".
 
   return "pass".  Note that <domain> is "example.com".
      


 

      

   v=spf1 +all
 
   v=spf1 +all
      
      





      -- any <ip> passes
 
      --  any <ip> passes
      


 

      

   v=spf1 a -all
 
   v=spf1 a -all
      
      





      -- hosts 192.0.2.10 and 192.0.2.11 pass
 
      --  hosts 192.0.2.10 and 192.0.2.11 pass
      


 

      

   v=spf1 a:example.org -all
 
   v=spf1 a:example.org -all
      
      





      -- no sending hosts pass since example.org has no A records
 
      --  no sending hosts pass since example.org has no A records
      


 

      

   v=spf1 mx -all
 
   v=spf1 mx -all
      
      





      -- sending hosts 192.0.2.129 and 192.0.2.130 pass
 
      --  sending hosts 192.0.2.129 and 192.0.2.130 pass
      


 

      

   v=spf1 mx:example.org -all
 
   v=spf1 mx:example.org -all
      
      





      -- sending host 192.0.2.140 passes
 
      --  sending host 192.0.2.140 passes
      


 

      

   v=spf1 mx mx:example.org -all
 
   v=spf1 mx mx:example.org -all
      
      





      -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass
 
      --  sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass
      


 

      

   v=spf1 mx/30 mx:example.org/30 -all
 
   v=spf1 mx/30 mx:example.org/30 -all
      
      





      -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
 
      --  any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
      


 

      

   v=spf1 ptr -all
 
   v=spf1 ptr -all
      
      





      -- sending host 192.0.2.65 passes (reverse DNS is valid and is in
 
      --  sending host 192.0.2.65 passes (reverse DNS is valid and is in
      

         example.com)
 
          example.com)
      

      -- sending host 192.0.2.140 fails (reverse DNS is valid, but not
 
      --  sending host 192.0.2.140 fails (reverse DNS is valid, but not
      

         in example.com)
 
          in example.com)
      

      -- sending host 10.0.0.4 fails (reverse IP is not valid)
 
      --  sending host 10.0.0.4 fails (reverse IP is not valid)
      


 

      

   v=spf1 ip4:192.0.2.128/28 -all
 
   v=spf1 ip4:192.0.2.128/28 -all
      
      





      -- sending host 192.0.2.65 fails
 
      --  sending host 192.0.2.65 fails
      

      -- sending host 192.0.2.129 passes
 
      --  sending host 192.0.2.129 passes
      


 

      

B.2.  Multiple Domain Example
 
B.2.  Multiple Domain Example
      


 

      

   These examples show the effect of related records:
 
   These examples show the effect of related records:
      


 

      

      example.org: "v=spf1 include:example.com include:example.net -all"
 
      example.org: "v=spf1 include:example.com include:example.net -all"
      


 

      

   This record would be used if mail from example.org actually came
 
   This record would be used if mail from example.org actually came
      

   through servers at example.com and example.net.  Example.org's
 
   through servers at example.com and example.net.  Example.org's
      

   designated servers are the union of example.com's and example.net's
 
   designated servers are the union of example.com's and example.net's
      


 

      

skipping to change at page 46, line 10
 
skipping to change at page 62, line 12
      

   These records allow a set of domains that all use the same mail
 
   These records allow a set of domains that all use the same mail
      

   system to make use of that mail system's record.  In this way, only
 
   system to make use of that mail system's record.  In this way, only
      

   the mail system's record needs to be updated when the mail setup
 
   the mail system's record needs to be updated when the mail setup
      

   changes.  These domains' records never have to change.
 
   changes.  These domains' records never have to change.
      


 

      

B.3.  DNSBL Style Example
 
B.3.  DNSBL Style Example
      


 

      

   Imagine that, in addition to the domain records listed above, there
 
   Imagine that, in addition to the domain records listed above, there
      

   are these:
 
   are these:
      


 

      
      





   $ORIGIN _spf.example.com.  mary.mobile-users                   A
 
   $ORIGIN _spf.example.com.
      

   127.0.0.2 fred.mobile-users                   A 127.0.0.2
 
   mary.mobile-users                   A 127.0.0.2
      


 
   fred.mobile-users                   A 127.0.0.2
      

   15.15.168.192.joel.remote-users     A 127.0.0.2
 
   15.15.168.192.joel.remote-users     A 127.0.0.2
      

   16.15.168.192.joel.remote-users     A 127.0.0.2
 
   16.15.168.192.joel.remote-users     A 127.0.0.2
      


 

      

   The following records describe users at example.com who mail from
 
   The following records describe users at example.com who mail from
      

   arbitrary servers, or who mail from personal servers.
 
   arbitrary servers, or who mail from personal servers.
      


 

      

   example.com:
 
   example.com:
      


 

      

   v=spf1 mx
 
   v=spf1 mx
      

          include:mobile-users._spf.%{d}
 
          include:mobile-users._spf.%{d}
      


 

      

skipping to change at page 47, line 5
 
skipping to change at page 63, line 5
      

                                 "-include:ip4._spf.%{d} "
 
                                 "-include:ip4._spf.%{d} "
      

                                 "-include:ptr._spf.%{d} "
 
                                 "-include:ptr._spf.%{d} "
      

                                 "+all" )
 
                                 "+all" )
      

   ip4._spf.example.com.  SPF  "v=spf1 -ip4:192.0.2.0/24 +all"
 
   ip4._spf.example.com.  SPF  "v=spf1 -ip4:192.0.2.0/24 +all"
      

   ptr._spf.example.com.  SPF  "v=spf1 -ptr +all"
 
   ptr._spf.example.com.  SPF  "v=spf1 -ptr +all"
      


 

      

   This example shows how the "-include" mechanism can be useful, how an
 
   This example shows how the "-include" mechanism can be useful, how an
      

   SPF record that ends in "+all" can be very restrictive, and the use
 
   SPF record that ends in "+all" can be very restrictive, and the use
      

   of De Morgan's Law.
 
   of De Morgan's Law.
      


 

      
      





Authors' Addresses
 
Appendix C.  Change History
      


 

      
      





   Meng Weng Wong
 
   Changes since RFC 4408 (to be removed prior to publication)
      

   Singapore
 

      


 

      
      





   EMail: [e-mail]
 
      Moved to standards track
      


 

      
      





   Wayne Schlitt
 
      Authors updated
      

   4615 Meredeth #9
 

      

   Lincoln Nebraska, NE  68506
 

      

   United States of America
 

      


 

      
      





   EMail: [e-mail]
 
      IESG Note regarding experimental use replaced with discussion of
      

   URI:   http://www.schlitt.net/spf/
 
      results
      


 

      
      





Full Copyright Statement
 
      Process errata:
      


 

      
      





   Copyright (C) The Internet Society (2006).
 
      Resolved Section 2.5.7 PermError on invalid domains after macro
      


 
      expansion errata in favor of documenting that different clients
      


 
      produce different results.
      


 

      
      





   This document is subject to the rights, licenses and restrictions
 
      Add %v macro to ABNF grammar
      

   contained in BCP 78, and except as set forth therein, the authors
 

      

   retain all their rights.
 

      


 

      
      





   This document and the information contained herein are provided on an
 
      Replace "uric" by "unreserved"
      

   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 

      

   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 

      

   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 

      

   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 

      

   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 

      

   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 

      


 

      
      





Intellectual Property
 
      Recommend an SMTP reply code for optional permerror rejections
      


 

      
      





   The IETF takes no position regarding the validity or scope of any
 
      Correct syntax in Received-SPF examples
      

   Intellectual Property Rights or other rights that might be claimed to
 

      

   pertain to the implementation or use of the technology described in
 

      

   this document or the extent to which any license under such rights
 

      

   might or might not be available; nor does it represent that it has
 

      

   made any independent effort to identify any such rights.  Information
 

      

   on the procedures with respect to rights in RFC documents can be
 

      

   found in BCP 78 and BCP 79.
 

      


 

      
      





   Copies of IPR disclosures made to the IETF Secretariat and any
 
      Fix unknown-modifier clause is too greedy in ABNF
      

   assurances of licenses to be made available, or the result of an
 

      

   attempt made to obtain a general license or permission for the use of
 

      

   such proprietary rights by implementers or users of this
 

      

   specification can be obtained from the IETF on-line IPR repository at
 

      

   http://www.ietf.org/ipr.
 

      


 

      
      





   The IETF invites any interested party to bring to its attention any
 
      Correct use of empty domain-spec on exp modifier
      

   copyrights, patents or patent applications, or other proprietary
 

      

   rights that may cover technology that may be required to implement
 

      

   this standard.  Please address the information to the IETF at
 

      

   [e-mail].
 

      


 

      
      





Acknowledgement
 
      Fix minor typo errata
      


 

      
      





   Funding for the RFC Editor function is provided by the IETF
 
      Convert to spfbis working group draft,
      

   Administrative Support Activity (IASA).
 
      draft-ietf-spfbis-4408bis-00
      


 

      


 
      Addressed Ticket #1, RFC 4408 Section 2.5.6 - Temporary errors by
      


 
      giving the option to turn repeated SERVFAIL into permerror and
      


 
      adding RFC 2308 reference.
      


 

      


 
      Clarified text about IPv4 mapped addresses to resolve test suite
      


 
      ambiguity
      


 

      


 
      Clarified ambiguity about result when more than 10 "mx" or "ptr"
      


 
      records are returned for lookup to specify permerror.  This
      


 
      resolves one of the test suite ambiguities
      


 

      


 
      Made all references to result codes lower case per issue #7
      


 
      Adjusted section 2.2 Requirement to check mail from per issue #15
      


 

      


 
      Added missing "v" element in macro-letter in the collected ABNF
      


 
      per issue #16 - section 8.1 was already fixed in the pre-WG draft
      


 

      


 
      Marked ptr and "p" macro deprecated/SHOULD NOT use per issue #27
      


 

      


 
      Expunged lower case may from the draft per issue #8
      


 

      


 
      Expunged "x-" name as an obsolete concept
      


 

      


 
      Updated obslete references: RFC2821 to RFC5321, RFC2822 to
      


 
      RFC5322, and RFC4234 to RFC5234
      


 

      


 
      Refer to RFC6647 to describe greylisting instead of trying to
      


 
      describe it directly.
      


 

      


 
      Updated informative references to the current versions.
      


 

      


 
      Added definition for deprecated since there are questions.
      


 

      


 
      Start to rework section 9 with some RFC5598 terms.
      


 

      


 
      Added mention of RFC 6552 feedback reports in section 9.
      


 

      


 
      Added draft-ietf-spfbis-experiment as an informational reference.
      


 

      


 
      Drop Type SPF.
      


 

      


 
      Try and clarify informational nature of RFC3696
      


 

      


 
      Fix ABNF nits and add missing definitions per Bill's ABNF checker.
      


 

      


 
      Make DNS lookup time limit SHOULD instead of MAY.
      


 

      


 
      Reorganize and clarify processing limits.  Move hard limits to new
      


 
      section 4.6.4, Evaluation Limits.  Move advice to non-normative
      


 
      section 9.
      


 

      


 
      Removed paragraph in section 10.1 about limiting total data
      


 
      volumes as it is unused (and removable per the charter) and serves
      


 
      no purpose (it isn't something that actually can be implemented in
      


 
      any reasonable way).
      


 

      


 
      Added text and figures from Alessandro Vesely in section 9.1 to
      


 
      better explain DNS resource limits.
      


 

      


 
      Multiple editorial fixes from Murray Kucherawy's review.
      


 

      


 
      Also based on Murray's review, reworked SMTP identity definitions
      


 
      and made RFC 5598 a normative reference instead of informative.
      


 
      This is a downref that will have to be mentioned in the last call.
      


 

      


 
      Added RFC 3834 as an informative reference about backscatter.
      


 

      


 
      Added IDN requirements and normative reference to RFC 5890 to deal
      


 
      with the question "like DKIM did it.:
      


 

      


 
      Added informative reference to RFC 4632 for CIDR and use CIDR
      


 
      prefix length instead of CIDR-length to match its terminology.
      


 

      


 
      Added RFC 5782 informative reference on DNSxLs to support
      


 
      improving the exists description.
      


 

      


 
      Added text on creating a Authentication-Results header field that
      


 
      matches the Received-SPF header field information and added a
      


 
      normative reference to RFC 5451.
      


 

      


 
      Added informative reference to RFC 2782 due to SRV mention.
      


 

      


 
      Added informative reference to RFC 3464 due to DSN mention.
      


 

      


 
      Added informative reference to RFC 5617 for it's DNS wildcard use.
      


 

      


 
      Added informative reference to RFC 5782 to enhance the explanation
      


 
      of how the exists mechanism works.  Clarified the intended match/
      


 
      no-match method.
      


 

      


 
      Added new sections on Receiver policy for SPF pass, fail, and
      


 
      permerror.
      


 

      


 
      Added new section 9 discussion on treatment of bounces and the
      


 
      significance of HELO records.
      


 

      


 
      Added request to IANA to update the SPF modifier registry.
      


 

      


 
Author's Address
      


 

      


 
   Scott Kitterman
      


 
   Kitterman Technical Services
      


 
   3611 Scheel Dr
      


 
   Ellicott City, MD  21042
      


 
   United States of America
      


 

      


 
   Email: [e-mail]
     


 

     
 End of changes. 318 change blocks.      

943 lines changed or deleted
 
1353 lines changed or added
     

This html diff was produced by rfcdiff 1.41. The latest version is available 
from http://tools.ietf.org/tools/rfcdiff/