Re: Macro Expansion (was: Last Call: <draft-ietf-spfbis-4408bis-19.txt> (Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1) to Proposed Standard)

[Douglas Otis <[e-mail]>]

Dear SM,

See comments inline.

On Sep 16, 2013, at 9:00 AM, S Moonesamy <[e-mail]> wrote:

> Hi Doug,
> At 21:55 11-09-2013, Douglas Otis wrote:
>> Add to:
>> 11.5.3.  Macro Expansion
>> ,---
>> It is not within SPF's purview whether IPv6 or DNSSEC is being used.  IPv6 (RFC2460) increased the minimum MTU size to 1280 octets.  DNSSEC is deployed with EDNS0 (RFC6891) to avoid TCP fallback.  EDNS0 suggests an MTU increase between 1280 and 1410 octets offers a reasonable result starting from a request of 4096 octets.  A 1410 MTU offers a 2.4 times payload increase over the assumed MTU of 576 octets and is widely supported by Customer Premise Equipment.  With increased MTUs being used with DNS over UDP, network amplification concerns increase accordingly.
>> 
>> SPF macros can utilize SPF parameters derived from email messages that can modulate the names being queried in several ways without publishing additional DNS resources.  The SPF macro feature permits malefactors a means to covertly orchestrate directed DDoS attacks from an array of compromised systems while expending little of their own resources.
>> 
>> Since SPF does not make use of a dedicated resource record type or naming convention, this leaves few solutions available to DNS operations in offering a means to mitigate possible abuse.  This type of abuse becomes rather pernicious when used in conjunction with synthetic domains now popular for tracking users without using web cookies.
>> 
>> However, email providers can mitigate this type of abuse by ignoring SPF records containing macros.  Very few domains make use of macros, and ignoring these records result in neutral handling.  Some large providers have admitted they make use of this strategy without experiencing any notable problem.  AOL began their support of SPF by saying they would use SPF to construct whitelists prior to receipt of email.  Clearly, such whitelisting practices tends to preclude benefits derived from macro use.
>> '---
> 
> As background information I read draft-otis-spfbis-macros-nixed-01.  I read the messages where EDNS0 was mentioned [1].  I read the messages on the thread starting with msg-id: [e-mail].  I have followed the discussions about macros ever since the SPFBIS WG was chartered.
> 
> The above suggestion is to add text in the Security Considerations section of the draft.  The problem being pointed out is, in simple terms, DNS amplification.  The first (quoted) paragraph argues that there can be an acute problem because of EDNS0 as specified in the Internet Standard.
> 
> The second paragraph starts with SPF macros can utilize SPF parameters derived from email messages".  I do not understand that.  From what I understand the rest of the second (quoted) paragraph argues that the SPF macro feature permits evildoers to use it as an attack vector.

Since this was not understood, I'll attempt to clarify.  An effort to keep these conversations fairly concise seems to lead to a level of confusion with those not familiar with DNS.

DNS UDP traffic lacks congestion avoidance when used to covertly direct attacks.  Residential systems represent a large component of compromised systems involved with email although data centers measured by overall traffic is increasing.  Network amplification is measured by gains beyond exchanges initiating a higher volume of exchanges.  DNS caching tends to reduce subsequent exchanges.  SPFbis macros inhibit normal caching protections by imposing mechanisms not directly supported by DNS and having targets constructed from email message components.  SPFbis mechanism names can be misleading since they are given a related manipulated DNS resource name.  One SPFbis mechanism can represent more than 100 subsequent DNS transactions where normally resolving the resource would represent a single transaction.  Publishing new targets within DNS resources to circumvent caching would normally be expensive and unlikely to provide remarkable gain.  SPFbis macros change this equation significantly.  SPFbis offers macros to translate code points, restructure host labels, build labels from the client IP address, make use of the local-part of the message return path or some label in the EHLO hostname, etc.

In other words, SPFbis macros permit malefactors a means to modulate the target of their queries while still leveraging their own cached DNS records.  This means a malefactors' DNS resources can be highly leveraged as a result of recipient SPFbis macro processing.  Secondly, SPFbis also ignores the overall size of the resources being queried in many cases.   The most egregious is perhaps that of the unlimited PTR RRsets which then results in a series of address RRset resolutions cascading down the hostname labels that happens for a maximum of 10 PTRs that might be offered on either a random or round robin basis.  It would be extremely difficult to determine the number of transactions and overall traffic volume any single PTR mechanism might impose, for example. 

> The argument in the third (quoted) paragraph is that it is not possible to mitigate possible (DNS) abuse due to the SPF as it does not have a dedicated resource record type.

Or a naming convention that might support mitigation efforts.

> The fourth (quoted) paragraph argues that macros should be ignored.  That paragraph also mentions that some large providers admitted to using that strategy.  I am not aware of any public reports about that.

As was said, AOL made their use of prefetching of SPF public at the beginning which precluded use of macros.  Others have also adopted this practice but have not made their use public.

> I read draft-otis-spfbis-macros-nixed-01 again to try and understand the problem.  It seems to be the:
> 
>  '{%l}._spf.{%d} or exists:{%i}_spf.{%d} can  be used in "specialized"
>   DNS servers able to understand encrypted local-parts'

There is nothing in SPFbis that limits the structured use of DNS resources.  In this example, it shows the expansion of the return path local-part, which represents a non-suspicious variable not derived from DNS, that can increase the leverage obtained in DNS related attacks.  A similar attack might manipulate HELO hostname or  MAIL FROM domain labels as well. 

> which is discussed in Appendix E of draft-ietf-spfbis-4408bis-20.
> 
> Arthur Thisell commented about the "specialized DNS server".  He mentioned that at the time that text was written two people came forward to say that they were doing that.  During the SPFBIS discussions nobody stated that he or she has implemented or is using a "specialized" DNS server.
> 
> I'll ask the person editing draft-ietf-spfbis-4408bis or the SPFBIS WG to provide some publicly verifiable cases where these examples are used.
> 
> I assume that the SPFBIS WG and the Responsible Area Director have understood the mathematics relating to EDNS0 and DNS amplification.  Anyone who has not understood that part is welcome to raise the issue on the SPFBIS mailing list.
> 
> The discussion about the "dedicated resource record type" has led to agreement.  I'll describe the agreement as something people can live with.  In my opinion it is better not to start another discussion about that.
> 
> I hope that what I wrote above clearly explains what I have understood and what I have not understood.
> 
> Regards,
> S. Moonesamy (as document shepherd)

Regards,
Douglas Otis