Resolution of The SPF/Sender-ID Experiment
draft-ietf-spfbis-experiment-05

SPFBIS Working Group                                        M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Informational                            April 19, 2012
Expires: October 21, 2012

               Resolution of The SPF/Sender-ID Experiment
                    draft-ietf-spfbis-experiment-05

Abstract

   In 2006 the IETF published a suite of protocol documents comprising
   SPF and Sender-ID, two proposed email authentication protocols.
   Because of possible interoperability issues, particularly but not
   only those created by simultaneous use of the two protocols by a
   receiver, the IESG was unable to determine technical consensus and
   decided it was best to publish all of RFC4405, RFC4406, RFC4407 and
   RFC4408 as Experimental documents.  The IESG invited the community to
   observe their deployments for a period of time, and expressed hope
   for later convergence of opinion.

   After six years, sufficient experience and evidence have been
   collected that the experiment thus created can be considered
   concluded, and a single protocol can be advanced.  This document
   presents those findings.

Status of this Memo

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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Evidence of Deployment . . . . . . . . . . . . . . . . . . . .  4
     3.1.  DNS Resource Record Types  . . . . . . . . . . . . . . . .  4
     3.2.  Implementations  . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  The SUBMITTER SMTP Extension . . . . . . . . . . . . . . .  5
   4.  Evidence of Differences  . . . . . . . . . . . . . . . . . . .  6
   5.  Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   9.  Informative References . . . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Experiences Developing SPF  . . . . . . . . . . . . .  9
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   In April 2006, the IETF published the [SPF] and Sender-ID email
   authentication protocols, the latter consisting of three documents
   ([SUBMITTER], [SENDER-ID], and [PRA]).  Both of these protocols
   enable one to publish via the Domain Name System a policy declaring
   which mail servers were authorized to send email on behalf of a
   specific domain name.  The two protocols made use of this policy
   statement and some specific (but different) logic to evaluate whether
   the email client sending or relaying a message was authorized to do
   so.

   Due to the absence of consensus behind one or the other, and because
   Sender-ID supported use of the same policy statement defined by SPF,
   the IESG at the time was concerned that an implementation of
   Sender-ID might erroneously apply that statement to a message and,
   depending on selected recipient actions, could improperly interfere
   with message delivery.  As a result, the IESG required the
   publication of all of these documents as Experimental, and requested
   that the community observe deployment and operation of the protocols
   over a period of two years from the date of publication in order to
   determine a reasonable path forward.  (For further details about the
   IESG's concern, see the IESG Note prepended to all of those
   documents.)

   In line with the IESG's request to evaluate after a period of time,
   this document concludes the experiment by presenting evidence
   regarding both deployment and comparative effect of the two
   protocols.  At the end it presents conclusions based on the data
   collected.

   It is important to note that this document makes no direct technical
   comparison of the two protocols in terms of correctness, weaknesses,
   or use case coverage.  The email community at large has already done
   that.  Rather, the analysis presented here merely observes what has
   been deployed and supported in the time since the protocols were
   published, and draws conclusions based on those observations.

2.  Definitions

   The term "RRTYPE" is used to refer to a Domain Name System ([DNS])
   Resource Record (RR) type.  These are always expressed internally in
   software as numbers, assigned by IANA under Expert Review provisions.
   Assigned RRTYPEs also have names.  The two of interest in this work
   are the TXT RRTYPE (16) and the SPF RRTYPE (99).

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3.  Evidence of Deployment

   This section presents the collected research done to determine what
   parts of the two protocol suites are in general use, as well as
   related issues like [DNS] support.

3.1.  DNS Resource Record Types

   Two large-scale DNS surveys were run that looked for the two
   supported kinds of RRTYPEs that can contain SPF policy statements.
   Specifically, these surveys pulled a large number of domain names
   from recent activity logs and queried their nameservers for both
   RRTYPEs that can be used for SPF and/or Sender-ID.

   DNS Survey #1

     +------------------+-----------+-------+
     | Domains queried  | 1,000,000 |   -   |
     | TXT replies      |   397,511 | 39.8% |
     | SPF replies      |     6,627 | <0.1% |
     | SPF+TXT replies  |     6,603 | <0.1% |
     | spf2.0/* replies |     5,291 | <0.1% |
     +------------------+-----------+-------+

   The "spf2.0/*" replies are those replies whose payload started with
   the string "spf2.0/", which are express requests for Sender-ID
   processing.

   DNS Survey #2

     +------------------+-----------+-------+
     | Domains queried  |   259,918 |   -   |
     | TXT replies      |   142,640 | 54.9% |
     | SPF replies      |     2,727 |  1.0% |
     | SPF+TXT replies  |     2,554 | <0.1% |
     | spf2.0/* replies |     6,972 |  2.7% |
     +------------------+-----------+-------+

   During this second survey, some domains were observed to provide
   immediate answers for RRTYPE 16 queries, but would time out waiting
   for replies to RRTYPE 99 queries.  For example, it was observed that
   4,179 (over 1.6%) distinct domains in the survey returned a result of
   some kind (a record or an error) for the TXT query in time N, while
   the SPF query ultimately failed after at least time 4N.

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   DNS Survey #3

     +------------------+-----------+-------+
     | Domains queried  |   100,000 |   -   |
     | TXT replies      |    46,221 | 46.2% |
     | SPF replies      |       954 | <0.1% |
     | SPF+TXT replies  |     1,383 |  1.4% |
     +------------------+-----------+-------+

   A survey was done of queries for RRTYPE 16 and RRTYPE 99 records by
   observing nameserver logs.  Only a few queries were ever received for
   RRTYPE 99 records, and those almost exclusively came from one large
   email service provider that queried for both RRTYPEs.  The vast
   majority of other querying agents only ever requested RRTYPE 16.

3.2.  Implementations

   It is likely impossible to determine from a survey which Mail
   Transfer Agents (MTAs) have SPF and/or Sender-ID checking enabled at
   message ingress since it does not appear, for example, in the reply
   to the EHLO command from extended [SMTP].  We therefore rely on
   evidence found via web searches, and observed the following:

   o  A web site [SID-IMPL] dedicated to highlighting Sender-ID
      implementations last updated in late 2007 listed 13 commercial
      implementations, which we assume means they implement the PRA
      checks.  At least one of them is known no longer to be supported
      by its vendor.  There were no free open source implementations
      listed.

   o  The [OPENSPF] web site maintains a list of implementations of SPF.
      At the time of this document's writing it listed six libraries, 22
      MTAs with built-in SPF implementations, and numerous patches for
      MTAs and mail clients.  The set included a mix of commercial and
      free open source implementations.

3.3.  The SUBMITTER SMTP Extension

   In a review of numerous MTAs in current or recent use, two
   (Santronics WinServer and McAfee MxLogic) were found to contain
   implementations of the SMTP SUBMITTER extension as part of the MTA
   service, which could act as an enabler to Sender-ID.

   An unknown number of SMTP clients implement SUBMITTER.  Although
   there is substantial activity showing its use in MTA logs, it is not
   possible to determine whether they are multiple instances of the same
   client, or separate client implementations.

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   An active survey was done of a approximately 170,00 running and
   publicly reachable MTAs.  Fewer than 4.5% of these advertised the
   SUBMITTER extension.  Based on the SMTP banner presented upon
   connection, the entire set of SUBMITTER-enabled MTAs consisted of the
   two found during the review (above) and a third whose identity could
   not be positively determined.

   Over 97% of the responding MTAs advertising the SUBMITTER SMTP
   extension were different instances of one MTA.  The service operating
   that MTA reported that about 11% of all observed SMTP sessions
   involved SMTP clients which make use of the SUBMITTER extension.

4.  Evidence of Differences

   Separate surveys compared the cases where the PRA (used by Sender-ID)
   and the RFC5321.MailFrom address (used by SPF) differed.  The results
   of these tests showed that at least 50% of the time the two addresses
   were the same, but beyond that the percentage varied substantially
   from one sampling location to the next due to the nature of the mail
   streams they each receive.

   Despite this, one working group contributor analyzed approximately
   150,000 messages and found that in more than 95% of those cases,
   Sender-ID and SPF reach the same conclusion about a message, meaning
   either both protocols return a "pass" result or both return a "fail"
   result.  The data set yielding this response could not further
   characterize the cases in which the answers differed.

5.  Analysis

   Given the six years that have passed since the publication of the
   experimental RFCs, and the evidence reported in the earlier sections
   of this document, the following analysis appears to be supported:

   1.  There has not been substantial adoption of the RRTYPE 99 (SPF)
       DNS resource record.  In all large-scale surveys performed for
       this work, less than 2% of responding domains published RRTYPE 99
       records, and almost no clients requested them.

   2.  Of the records retrieved, fewer than 3% requested processing of
       messages using the PRA algorithm, which was an essential part of
       Sender-ID.

   3.  Although the two mechanisms often used different email addresses
       as the subject being evaluated, no data collected showed any
       substantial operational benefit (e.g., cheaper processing,

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       improved accuracy) to using Sender-ID over SPF.

   4.  A review of known implementations shows significant support for
       both protocols, though there were more implementations in support
       of SPF than of Sender-ID.  Further, the SPF implementations
       showed better upkeep and current interest than the Sender-ID
       implemenations.

   5.  A survey of running MTAs shows fewer than 5% of them advertised
       the SUBMITTER extension, which is a Sender-ID enabler.  Only
       three implementations of it were found.

   6.  Although they may be marginal, there remain obstacles to
       deployment of protocols that use DNS RRTYPEs other than the most
       common ones, including firewalls and DNS servers that block or
       discard requests for unknown RRTYPEs.  Further, few if any web-
       based DNS configuration tools offer support for RRTYPE 99
       records.

6.  Conclusions

   It is standard procedure within the IETF to document as standard
   those protocols and practices that have come into sufficient common
   use as to become part of the basic infrastructure.

   In light of the this and the analysis in the previous section, the
   following conclusions are supported:

   1.  The experiment comprising the series of RFCs defining the
       SUBMITTER SMTP extension, the Sender-ID mechanism, the Purported
       Responsible address algorithm, and SPF, should be considered
       concluded.

   2.  The absence of significant adoption of the RRTYPE 99 DNS Resource
       Record suggests that it has not attracted enough support to be
       useful.

   3.  The absence of significant adoption of the [SUBMITTER] extension,
       [SENDER-ID], and [PRA], indicates that there is not a strong
       community prepared to develop those mechanisms beyond
       experimental status.

   4.  Continued widespread use of [SPF] indicates it is worthy of
       consideration for the Standards Track.

   Appendix A is offered as a cautionary review of problems that
   affected the process of developing SPF and Sender-ID in terms of

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   their use of the DNS.

7.  IANA Considerations

   This document presents no actions for IANA.  [RFC Editor: Please
   remove this section prior to publication.]

8.  Security Considerations

   This document contains information for the community, akin to an
   implementation report, and does not introduce any new security
   concerns.  Its implications could, in fact, resolve some.

9.  Informative References

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [OPENSPF]  "Sender Policy Framework: Project Overview",
              <http://www.openspf.net>.

   [PRA]      Lyon, J., "Purported Responsible Address in E-Mail
              Messages", RFC 4407, April 2006.

   [SENDER-ID]
              Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, April 2006.

   [SID-IMPL]
              "Sender ID Framework Industry Support and Solutions",
              October 2007, <http://www.microsoft.com/mscorp/safety/
              technologies/senderid/support.mspx>.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [SUBMITTER]
              Allman, E. and H. Katz, "SMTP Service Extension for
              Indicating the Responsible Submitter of an E-Mail
              Message", RFC 4405, April 2006.

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Appendix A.  Experiences Developing SPF

   SPF was originally developed by a community of interested developers
   outside the IETF, with the intent of bringing it to the IETF for
   standardization after it had become relatively mature and ready for
   the IETF standards track process.

   At the time of SPF's initial development, the prospect of getting an
   RRTYPE allocated for SPF was not seriously considered, partly because
   doing so had high barriers to entry.  As a result, at the time it was
   brought to the IETF for development and publication, there was
   already a substantial and growing installed base that had SPF running
   using TXT RRs.  Eventually the application was made for the new
   RRTYPE as a result of pressure from the DNS experts in the community,
   who insisted upon doing so as the preferred path toward using the DNS
   for storing such things as policy data.

   Later, after RRTYPE 99 was assigned (long after IESG approval of the
   document, in fact), a plan was put into place to effect a gradual
   transition to using RRTYPE 99 instead of using RRTYPE 16.  This plan
   failed to take effect for four primary reasons:

   1.  there was hesitation to make the transition because existing
       nameservers (and, in fact, DNS-aware firewalls) would drop or
       reject requests for unknown RRTYPEs (see Section 3 for evidence
       of this), which means successful rollout of a new RRtype is
       contingent upon widespread adoption of updated nameservers and
       resolver functions;

   2.  many DNS provisioning tools (e.g., web interfaces to controlling
       DNS zone data) were, and still are, typically lethargic about
       adding support for new RRTYPEs;

   3.  the substantial deployed base was already using RRTYPE 16, and it
       was working just fine, leading to inertia;

   4.  [SPF] itself included a faulty transition plan, likely because of
       the late addition of a requirement to develop one: It said a
       server SHOULD publish both RRTYPEs and MUST publish at least one,
       while a client can query either or both, which means both can
       claim to be fully compliant while failing utterly to
       interoperate.  Publication occurred without proper IETF review,
       so this was not detected prior to publication.

   It is likely that this will happen again if the bar to creating new
   RRTYPEs even for experimental development purposes is not lowered,
   and handling of unknown RRTYPEs in software becomes generally more
   graceful.  Also important in this regard is encouragement of support

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   for new RRTYPEs in DNS record provisioning tools.

   There are DNS experts within the community that will undoubtedly
   point to DNS servers and firewalls that mistreat queries for unknown
   RRTYPEs, and claim they are broken, as a way of answering this
   concern.  This is undoubtedly correct, but the reality is that they
   are among us and likely will be for some time, and this needs to be
   considered as new protocols and IETF procedures are developed.

Appendix B.  Acknowledgments

   The following provided operational data that contributed to the
   evidence presented above:

   Cisco:  contributed data about observed Sender-ID and SPF records in
      the DNS for a large number of domains

   Hotmail:  contributed data about the difference between
      RFC5321.MailFrom and RFC5322.From domains across large mail
      volumes, and a survey of DNS queries observed in response to
      outgoing mail traffic

   John Levine:  conducted a survey of DNS server logs to evaluate SPF-
      related query traffic

   McAfee:  provided details about their SUBMITTER implementation and
      usage statistics

   Santronics:  contributed data about the use of the SUBMITTER
      extension in aggregate SMTP client traffic

   The Trusted Domain Project:  contributed data about the difference
      between Sender-ID and SPF results, conducted one of the two
      detailed TXT/SPF RRTYPE surveys including collecting timing data,
      and conducted the MTA SUBMITTER survey

   The author would also like to thank the following for their
   contributions to the development of the text in this document: Dave
   Crocker, Scott Kitterman, Barry Leiba, John Leslie, John Levine,
   Hector Santos, and Alessandro Vesely.

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Author's Address

   Murray S. Kucherawy
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   USA

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com

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