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INFORMATIONAL
Errata Exist
Internet Engineering Task Force (IETF)                      M. KucherawyRequest for Comments: 6686                                     CloudmarkCategory: Informational                                        July 2012ISSN: 2070-1721Resolution of the Sender Policy Framework (SPF)and Sender ID ExperimentsAbstract   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,RFC4407, andRFC 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6686.Kucherawy                     Informational                     [Page 1]

RFC 6686                SPF/Sender ID Experiments              July 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. Definitions .....................................................33. Evidence of Deployment ..........................................33.1. DNS Resource Record Types ..................................33.2. Implementations ............................................53.3. The SUBMITTER SMTP Extension ...............................64. Evidence of Differences .........................................75. Analysis ........................................................76. Conclusions .....................................................87. Security Considerations .........................................98. References ......................................................98.1. Normative References .......................................98.2. Informative References .....................................9Appendix A. Background on the RRTYPE Issue ........................10Appendix B. Acknowledgments .......................................111.  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 observeKucherawy                     Informational                     [Page 2]

RFC 6686                SPF/Sender ID Experiments              July 2012   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.Kucherawy                     Informational                     [Page 3]

RFC 6686                SPF/Sender ID Experiments              July 2012   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.Kucherawy                     Informational                     [Page 4]

RFC 6686                SPF/Sender ID Experiments              July 2012   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.Kucherawy                     Informational                     [Page 5]

RFC 6686                SPF/Sender ID Experiments              July 20123.3.  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 thatKucherawy                     Informational                     [Page 6]

RFC 6686                SPF/Sender ID Experiments              July 2012   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 theRFC5321.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.Kucherawy                     Informational                     [Page 7]

RFC 6686                SPF/Sender ID Experiments              July 2012   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.Kucherawy                     Informational                     [Page 8]

RFC 6686                SPF/Sender ID Experiments              July 20127.  Security Considerations   This document contains information for the community, akin to an   implementation report, and does not introduce any new security   concerns.8.  References8.1.  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.Kucherawy                     Informational                     [Page 9]

RFC 6686                SPF/Sender ID Experiments              July 2012Appendix 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 (seeSection 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.Kucherawy                     Informational                    [Page 10]

RFC 6686                SPF/Sender ID Experiments              July 2012   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 betweenRFC5321.MailFrom andRFC5322.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 surveyKucherawy                     Informational                    [Page 11]

RFC 6686                SPF/Sender ID Experiments              July 2012   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.comKucherawy                     Informational                    [Page 12]

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