Re: [Syslog] Document shepherd report of AD concerns

[<[e-mail]>]

To add to David's email, and Tom's concern that "[we] should only
change what the IESG wants us to to; and that I do not see":

All the changes in version -12 were done in response to my (and
earlier Sam's) AD review comments. However, based on the recent
discussions, it's clear that the text isn't final yet, and needs 
some improvement.

The intent of 4.2.1/4.2.2 was to specify a minimum level needed for
interoperability, not to prevent deployments for making their own
policy decisions. That minimum level can be pretty small -- and
there's some room for discussing what exactly it should be -- but it
must provide reasonable security for many typical syslog uses (and in
this particular case, having a full-fledged PKI as the only approach
isn't really a credible solution).

Perhaps we could rephrase the text to make this more clear, and 
also explain the motivation for these requirements.  Here's a first 
shot at that text -- feedback is welcome.

   To prevent disclosure of syslog message contents, the transport
   sender (TLS client) has to verify that it is sending the messages
   to an authorized transport receiver (TLS server).

   Typically, a client application using TLS achieves this by
   performing certification path validation (using previously
   configured trust anchors), and comparing the certificate subject
   name against the expected server name (or a list of authorized
   server names).
 
   Syslog transport senders MUST support this approach. The minimum
   requirements for certification path validation and subject name
   comparison are specified later in this section.

   However, syslog is frequently used in environments where assuming
   PKI deployment is not realistic. Thus, an alternative that provides
   a reasonable level of security, while being simple to deploy, is
   desired. This document requires that syslog transport senders MUST
   support a mechanism where the authorized transport receivers are
   identified by certificate fingerprints. The minimum requirements
   for this mechanism are specified in Section 4.2.3.

   However, these requirements are intended to specify only a minimum
   level of compliance required for interoperability.  Implementations
   MAY support additional mechanisms for expressing more complex
   policies, validating certificates, and determining whether a
   certificate represents an authorized transport receiver.

   There can be extreme situations where it is not feasible to verify
   that the transport sender is communicating with an authorized
   transport receiver. A syslog implementation MAY provide an option
   to skip this verification (i.e., accept any server certificate).
   This option MUST NOT be enabled by default.

Best regards,
Pasi

> -----Original Message-----
> From: David Harrington
> Sent: 15 May, 2008 21:02
> To: [e-mail]
> Subject: [Syslog] Document shepherd report of AD concerns
> 
> Hi,
> 
> Tom asked for a review of the IESG concerns.
> As document shepherd, let me address that question.
> 
> This email shows the AD comments as the draft progressed since -11-. 
> Some WG input was also taken into consideration during the document
> revisions.
> 
> Following the publication of -11-, Sam Hartman said:
> "Dave, the syslog tls document is much clearer and I think it does a
> good job of specifying what implementations need to do.
> I think  this is a huge improvement.
> 
> However  I have two major problems.
> 
> 1) The default/recommended configuration requires managed trust
>    anchors, a working DNS deployment and certificates provisioned to
>    syslog servers.  I do not find it plausible that operators will
>    actually do this and so I do not find the specification credible.
> 
> 2) The spec talks about caching of certificates in a manner similar to
>     ssh, but does not discuss operational concerns such as key
>     rollover.  Jeff, any chance you could help the syslog working
>     group understand the tradeoffs around leap of faith?
> "
> 
> Jeff Hutzelman provided expert advice on behalf of Sam:
> "I can try.  It may be worth noting that I don't think ssh talks much
> about 
> server key rollover, and it certainly isn't done much in practice.
> What 
> does happen is server keys changing because of a machine being
> reinstalled 
> or some similar event.  The approaches open to SSH in that case
> (prompt the 
> user, or just notify them and fail so they can fix it and try again)
> don't 
> work so well with syslog, where the originator of a message likely
> can't 
> fix the problem, and the way to notify the person who can is to send a
> 
> syslog message!"
> 
> Then the AD token passed from Sam to Pasi, who suggested:
> "Here's one idea how to handle certificates for TLS in order to make 
> it reasonably secure (in most cases), yet also deployable:
> 
> - When a tranport sender configures the address of transport receiver,
> and enables TLS, allow the following choices:
> 
>   1) configure set of trust anchors, and a name to be matched against
>      the cert. If the transport receiver address is configured as DNS
>      name, this name would usually be the same one. If the transport
>      receiver address is configured as IP address, this one could
>      be IP address (but could be also a domain name).
> 
>      (Note: in no situation, the result of A/AAAA or PTR lookup
>      would be used when checking the certificate. This part of the
>      current spec isn't correct.)
> 
>   2) configure the "certificate fingerprint" of the peer (typically 
>      a 40-digit hex string). Cut-and-pasting that from one SSH window 
>      to another isn't the world's friendliest UI, but would be doable
>      by admins, and is couple of orders of magnitude less work than 
>      setting up a real PKI.
> 
>   3) configure "anything goes, don't verify the cert" (obviously,  
>      not very secure; SHOULD NOT be done)
> 
> - A transport receiver would have similar choices:
> 
>   1) configure set of trust anchors, and a list of names (or 
>      wildcards matching names) to be matched against the
>      certificates; everyone listed here would be OK.
> 
>   2) configure list of certificate fingerprints that are OK.
> 
>   3) configure list of transport (IP) addresses/netmasks that are 
>      allowed to send messages regardless of the certificate
>      (obviously, not very secure; SHOULD NOT be done)
> 
> - I've intentionally omitted "SSH style leap of faith" (called "MAY
>   cache the certificate presented by its peer so it can compare the
>   certificate with what is presented in subsequent connections" in
>   current spec). .
> 
>   It could be added (but has some operational issues); or we could
>   specify slightly similar (but not quite) functionality where instead
>   of typing in the 40-digit fingerprint, the management interface
>   would fetch the fingerprint from the peer, show it to the
> administrator,
>   and the admin would then compare it against something (e.g., what
>   he/she sees in some other SSH window), and confirm that they match.
>   There are some issues with this, though.... (user studies made with 
>   similar approaches show that most users click "yes, they match" even
>   when shown two totally different values).
> 
> - We probably need to require that everyone must be able to generate a
>   keypair and self-signed certificate, and calculate/display the
>   fingerprint of the certificate (in some management interface).
> 
> Has something like this (in particular, using the fingerprint in 
> cases where deploying PKI isn't feasible) suggested before?  Do 
> you think it would be operationally workable?
> 
> If you think this might make sense, I can post something on the
> mailing list, too. But this is just one preliminary idea -- if 
> there are other ideas that would solve the issues, I'm in no way
> saying it must be done this way."
> 
> Joe did a revision which Paso reviewed, and commented:
> "Joe: thanks for working on this; I think we're making good
> progress, and I'm optimistic that we could get this draft
> done soon.
> 
> However, some of the TLS/certificate related text in version -11 
> was *really*...err... unclear, so I think we need to rewrite
> it to some extent before taking it to the WG list (fortunately, 
> the whole text is only two pages, so this shouldn't be
> too much effort). We also have an option (fingerprints)
> that wasn't present before, so we need to recheck the WG 
> consensus anyway.
> 
> Some proposed text fragments from me (Joe, please feel free 
> to edit as you like):
> 
> 4.2
> 
>    TLS typically uses certificates to authenticate
>    peers. Authentication and authorization of the TLS server
>    (transport receiver) is described in Section 4.2.1; authentication
>    and authorization of the TLS client (transport sender) is described
>    in Section 4.2.2.
> 
> 4.2.1 Server authentication/authorization
> 
>    The transport sender (TLS client) has three different options for
>    authenticating and authorizing the transport receiver (TLS server):
> 
>    (Pasi's note: it could be sufficient is one of the following
>    two is a MUST, and the other one is a SHOULD.)
> 
>    o  Certification path validation and subject name verification: the
> 
>       client is configured with one or more trust anchors, and for 
>       each transport receiver, the name to be matched against the  
>       certificate. This option MUST be supported.
> 
>    o  Certificate fingerprints: For each transport receiver, the
> client 
>       is configured with a fingerprint of the server's certificate
>       (which can be self-signed). This option MUST be supported.
> 
>    o  No server authentication/authorization: The client is configured
> 
>       to accept any certificate from the server. This option
>       MAY be supported, but MUST NOT be enabled by default.
> 
>    For certification path validation, client implementations MUST
>    provide a mechanism for configuring one or more trust anchors, and
>    MUST perform certification path validation as specified in
>    [RFC3280bis].
>  
>    For subject name verification, client implementations MUST support
>    configuring, for each transport receiver, the name to be matched
>    against the certificate.  This name may be the host name or IP
>    address used when opening the TCP connection, or a distinct host
>    name (...details of name matching...MUST support DNS names,
>    probably others can be MAY...)
> 
>    To support certificate fingerprints, client implementations MUST
>    support configuring, for each transport receiver, a fingerprint of
>    the server certificate. See Section 4.2.3 for details.
> 
>    If server authentication/authorization fails, the client SHOULD log
>    the error in some form or another (...more text...)
> 
> 4.2.2 Client authentication/authorization
>   
>    The transport receiver (TLS server) has three different options for
>    authenticating and authorizing the transport sender (TLS client).
> 
>    (Pasi's note: it could be sufficient is one of the following
>    two is a MUST, and the other one is a SHOULD.)
> 
>    o  Certification path validation and subject name verification:
>       the server is configured with one or more trust anchors,
>       and a set of names (or wildcard patterns) to be matched 
>       against the certificates.  This option MUST be supported.
> 
>    o  Certificate fingerprints: The server is configured with the 
>       fingerprints of client certificates. This option MUST be
>       supported.
> 
>    o  No client authentication/authorization (or authorization
>       based only on connection source IP address): This option MAY 
>       be supported, but MUST NOT be enabled by default.
> 
>    Certification path validation and subject name verification work
>    as described in Section 4.2.1 above, with following addition: 
>    (...locally configured client name can be a wildcard; different 
>    from wildcard-in-cert case...)
> 
>    To support certificate fingerprints, server implementations MUST
>    support configuring with a list of fingerprints of authorized
>    certificates. See Section 4.2.3 cor details.
> 
> 4.2.3 Certificate fingerprints
> 
>    Both client and server implementations MUST make the certificate
>    fingerprint available through a management interface. If no other
>    certificate is configured, both client and server implementations
>    MUST support generating a key pair and self-signed certificate.
> 
>    <describe details of certificate fingerprint calculation here. 
>    this should be compatible with the way e.g. current web browsers
>    calculate fingerprints> "
> 
> and after Joe did some additioon revising, Pasi said:
> "I'd be interested in getting the WG's opinion on this (in
> particular, the fingerprint stuff) as soon as possible."
> 
> At this point, Joe revised and published the draft and we brought it
> to the WG for comment.
> 
> Hope this helps
> 
> 
> David Harrington
> [e-mail]
> [e-mail]
> [e-mail]
> 
> _______________________________________________
> Syslog mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/syslog
> 
_______________________________________________
Syslog mailing list
[e-mail]
https://www.ietf.org/mailman/listinfo/syslog