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RFC 6686 - Resolution of the Sender Policy Framework (SPF) and S


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Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 6686                                     Cloudmark
Category: Informational                                        July 2012
ISSN: 2070-1721

            Resolution of the Sender Policy Framework (SPF)
                       and Sender ID Experiments

Abstract

   In 2006, the IETF published a suite of protocol documents comprising
   the Sender Policy Framework (SPF) and Sender ID: two proposed email
   authentication protocols.  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 the domain name
   being queried.  There was concern that the two would conflict in some
   significant operational situations, interfering with message
   delivery.

   The IESG required all of these documents (RFC 4405, RFC 4406, RFC
   4407, and RFC 4408) to be published as Experimental RFCs and
   requested that the community observe deployment and operation of the
   protocols over a period of two years from the date of publication to
   determine a reasonable path forward.

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6686.

Copyright Notice

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

   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.

Table of Contents

   1. Introduction ....................................................2
   2. Definitions .....................................................3
   3. Evidence of Deployment ..........................................3
      3.1. DNS Resource Record Types ..................................3
      3.2. Implementations ............................................5
      3.3. The SUBMITTER SMTP Extension ...............................6
   4. Evidence of Differences .........................................7
   5. Analysis ........................................................7
   6. Conclusions .....................................................8
   7. Security Considerations .........................................9
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References .....................................9
   Appendix A. Background on the RRTYPE Issue ........................10
   Appendix B. Acknowledgments .......................................11

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 are authorized to send email on behalf of the
   selected domain name.

   Consensus did not clearly support one protocol over the other, and
   there was significant concern that the two would conflict in some
   significant operational situations, interfering with message
   delivery.  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.

   In line with the IESG's request to evaluate after a period of time,
   this document concludes the experiments 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 through its deployment choices.  Rather, the analysis presented
   here is merely an observation of what has been deployed and supported
   in the time since the protocols were published and lists conclusions
   based on those observations.

   The data collected and presented here are presumed to be a reasonable
   representative view of the global deployment data, which could never
   itself be fully surveyed within a reasonable period of time.

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 according to the procedures in
   [DNS-IANA] Assigned RRTYPEs also have names.  The two of interest in
   this work are the TXT RRTYPE (16) and the SPF RRTYPE (99).

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

   Three large-scale DNS surveys were run that looked for the two
   supported kinds of RRTYPEs that can contain SPF policy statements.
   These surveys selected substantial sets of distinct domain names from
   email headers and logs over long periods, regardless of whether the
   DNS data for those domains included A, MX, or any other RRTYPEs.  The
   nameservers for these domains were queried, asking for both of the
   RRTYPEs that could be used for SPF and/or Sender ID.

   In the tables below, replies were counted only if they included
   prefixes that indicated the record was intended to be of a form
   defined in either [SPF] or [SENDER-ID], though complete syntax
   validation of the replies was not done.  That is, the records started
   either "v=spf1" or "spf2.0/", or they were not counted as replies.

   The tables are broken down into three parts: (a) the size of the
   sample set, (b) a report about RRTYPE use independent of content, and
   (c) a report about content independent of RRTYPE.

   "SPF+TXT" indicates the count of domains where both types were in
   use.

   DNS Survey #1 (Cisco)

     +------------------+-----------+-------+
     | Domains queried  | 1,000,000 |   -   |
     +------------------+-----------+-------+
     | TXT replies      |   397,511 | 39.8% |
     | SPF replies      |     6,627 | <1.0% |
     | SPF+TXT replies  |     6,603 | <1.0% |
     +------------------+-----------+-------+
     | v=spf1 replies   |   395,659 | 39.6% |
     | spf2.0/* replies |     5,291 | <1.0% |
     +------------------+-----------+-------+

   Domains were selected as the top million domains as reported by
   Alexa, which monitors browser activity.

   DNS Survey #2 (The Trusted Domain Project)

     +------------------+-----------+-------+
     | Domains queried  |   278,353 |   -   |
     +------------------+-----------+-------+
     | TXT replies      |   156,894 | 56.4% |
     | SPF replies      |     2,876 |  1.0% |
     | SPF+TXT replies  |     2,689 | <1.0% |
     +------------------+-----------+-------+
     | v=spf1 replies   |   149,985 | 53.9% |
     | spf2.0/* replies |     7,285 |  2.7% |
     +------------------+-----------+-------+

   This survey selected its domains from data observed in email headers
   and previous SPF and Sender ID evaluations, collected from 23
   reporting hosts across a handful of unrelated operators over a period
   of 22 months.

   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
   4,360 (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.

   DNS Survey #3 (Hotmail)

     +------------------+-----------+-------+
     | Domains queried  |   100,000 |   -   |
     +------------------+-----------+-------+
     | TXT replies      |    46,221 | 46.2% |
     | SPF replies      |       954 | <1.0% |
     | SPF+TXT replies  |     1,383 |  1.4% |
     +------------------+-----------+-------+

   Hotmail's domain set was selected from live email traffic at the time
   the sample was extracted.  Only the RRTYPE portion of the report is
   available.

   A separate survey was done of queries for RRTYPE 16 and RRTYPE 99
   records by observing nameserver traffic records.  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].  Therefore, we relied 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
      Purported Responsible Address (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

   The PRA is the output of a heuristic that seeks to scan a message
   header and extract from it the email address most likely to be the
   one responsible for injection of that message into the mail stream.
   The SUBMITTER extension to SMTP is a mechanism to provide an early
   hint (i.e., as part of the MAIL command in an SMTP session) to the
   receiving MTA of what the PRA would be on full receipt of the
   message.

   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 the SUBMITTER SMTP
   extension.  Although information from MTA logs indicates substantial
   use of the SMTP extension, it is not possible to determine whether
   the usage is from multiple instances of the same SMTP client or
   different SMTP client implementations.

   An active survey of MTAs accessible over the Internet was performed.
   The MTAs selected were found by querying for MX and A resource
   records of a subset of all domains observed by The Trusted Domain
   Project's data collection system in the preceding 20 months.  The
   results were as follows:

   SUBMITTER Survey (The Trusted Domain Project)

     +-------------------+-----------+-------+
     | MTAs selected     |   484,980 |   -   |
     | MTAs responding   |   371,779 | 76.7% |
     | SUBMITTER enabled |    17,425 |  4.7% |
     | MXLogic banner    |    16,914 |  4.6% |
     +-------------------+-----------+-------+

   Note: The bottom two rows indicate the percentage of responding MTAs
   with the stated property, not the percentage of selected MTAs.

   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.

   Of those few responding MTAs advertising the SUBMITTER SMTP
   extension, 97% were different instances of one MTA.  The service
   operating that MTA (MXLogic, a division of McAfee) reported that

   about 11% of all observed SMTP sessions involved SMTP clients that
   make use of the SUBMITTER extension.  Note that this represents about
   11% of the clients of 4.6% of the responding MTAs in the survey.

4.  Evidence of Differences

   Separate surveys from Hotmail and The Trusted Domain Project 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.

   Further, The Trusted Domain Project 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.
   Note that this does not include an evaluation of whether "fail" meant
   spam or other abusive mail was thus detected or that "pass" mail is
   good mail; it is merely a measure of how often the two protocols
   concurred.  The data set yielding this response could not further
   characterize the cases in which the answers differed.

   A second analysis of the same nature by Hotmail found that the two
   protocols yielded the same result approximately 80% of the time when
   evaluated across billions of messages.

   Anecdotally, the differences in conclusions have not been noted as
   causing significant operational problems by the email-receiving
   community.

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, fewer than 2% of responding domains published RRTYPE
       99 records, and almost no clients requested them.

   2.  Of the DNS resource records retrieved, fewer than 3% included
       specific requests for processing of messages using the PRA
       algorithm, which is an essential part of Sender ID.

   3.  Although the two protocols often used different email address
       fields as the subject being evaluated, no data collected showed
       any substantial operational benefit, in terms of improved
       accuracy, to using one mechanism over the other.

   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
       implementations.

   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.  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

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

   1.  The experiments comprising the series of RFCs defining the
       SUBMITTER SMTP extension (RFC4405), the Sender ID mechanism
       (RFC4406), the Purported Responsible Address algorithm (RFC4407),
       and SPF (RFC4408), 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.  Unavailability of software implementing the protocols was not a
       gating factor in terms of the selection of which to use.

   4.  The absence of significant adoption of the [SUBMITTER] extension,
       [SENDER-ID], and [PRA], indicates that there is not a strong
       community deploying and using these protocols.

   5.  [SPF] has widespread implementation and deployment, comparable to
       that of many Standards Track protocols.

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

7.  Security Considerations

   This document contains information for the community, akin to an
   implementation report, and does not introduce any new security
   concerns.

8.  References

8.1.  Normative References

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

   [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.

   [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.

8.2.  Informative References

   [DNS-EXPAND]  IAB, Faltstrom, P., Austein, R., and P. Koch, "Design
                 Choices When Expanding the DNS", RFC 5507, April 2009.

   [DNS-IANA]    Eastlake 3rd, D., "Domain Name System (DNS) IANA
                 Considerations", BCP 42, RFC 6195, March 2011.

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

   [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.

Appendix A.  Background on the RRTYPE Issue

   SPF was originally created 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 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
   [SPF], 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:

         An SPF-compliant domain name SHOULD have SPF records of both RR
         types.  A compliant domain name MUST have a record of at least
         one type.

       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
   for new RRTYPEs in DNS record provisioning tools.

   Fortunately, in the meantime, the requirements for new RRTYPE
   assignments was changed to be less stringent (see [DNS-IANA]).  Also,
   the publication of [DNS-EXPAND] has provided some useful guidance in
   this regard.  However, there is still a common perception that adding
   new types of data to the DNS will face resistance due to the lack of
   appropriate software support.

   There are DNS experts within the community that will undoubtedly
   point to DNS servers and firewalls that mistreat queries for unknown
   RRTYPEs, and to overly simplistic provisioning tools, and claim they
   are broken as a way of answering these concerns.  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 (DNS survey #1)

   Hotmail:  contributed data about the difference between
      RFC5321.MailFrom and RFC5322.From domains across large mail
      volumes, and a survey of DNS replies observed in response to
      incoming mail traffic (DNS survey #3)

   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 detailed
      TXT/SPF RRTYPE surveys including collecting timing data (DNS
      survey #2), 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.

Author's Address

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

   Phone: +1 415 946 3800
   EMail: superuser@gmail.com

 

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