SPFBIS Working Group M. Kucherawy Internet-Draft Cloudmark Intended status: Informational April 6, 2012 Expires: October 8, 2012 Resolution of The SPF/Sender-ID Experiment draft-ietf-spfbis-experiment-00 Abstract In 2006 the IETF published a suite of protocol documents comprising SPF and Sender-ID, two proposed email authentication protocols. Because of interoperability concerns created by simultaneous use of the two protocols by a receiver, and some concerns with Sender-ID and compatibility with existing standards, the IESG required them to have Experimental status and invited the community to observe their deployments for a period of time, hoping convergence would be possible later. 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 memo presents those findings. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 8, 2012. 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 Kucherawy Expires October 8, 2012 [Page 1] Internet-Draft SPF/Sender-ID Experiment April 2012 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The Need For Consensus . . . . . . . . . . . . . . . . . . . . 3 3. Evidence of Deployment . . . . . . . . . . . . . . . . . . . . 4 4. Evidence of Differences . . . . . . . . . . . . . . . . . . . . 5 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. From the Evidence . . . . . . . . . . . . . . . . . . . . . 5 5.2. Recommendations to the IESG . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 8. Informative References . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Experiences Developing SPF . . . . . . . . . . . . . . 8 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Kucherawy Expires October 8, 2012 [Page 2] Internet-Draft SPF/Sender-ID Experiment April 2012 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 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 or not the email client sending or relaying a message was authorized to do so. Because Sender-ID could use the same policy statement as 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 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.) Accordingly, this memo resolves the experiment by presenting evidence regarding both deployment and efficacy of the two protocols, and further discusses the increasing need for consensus. At the end it presents conclusions based on the data collected. 2. The Need For Consensus These two protocols fall into a family of protocols that provide domain-level email authentication services. For reference, another prominent one is [DKIM]. Various efforts exist that use these as building blocks to increased abuse filtering capabilities, and indeed this sort of work has spawned another working group in the Applications area, with still more of these incubating in associations and trade groups outside of the IETF. There is thus some palpable interest in having a path authorization scheme, as well as a domain-level signing scheme, on the Standards Track so that these newer technologies can develop with confidence. This is, in part, why the community has decided to expend the effort to bring this experiment to a conclusion and document the results, and then advance a single path authorization technology. Kucherawy Expires October 8, 2012 [Page 3] Internet-Draft SPF/Sender-ID Experiment April 2012 3. Evidence of Deployment Two large-scale DNS surveys were run that looked for the two supported kinds of resource records (RR) that can contain SPF policy statements. One data source for this report requested SPF records from one million domains. Approximately 287,267 domains included non-error and non-empty replies. Of these, 287,250 included type 16 (DNS RR TXT) replies, 4,613 included type 99 (DNS RR SPF) replies, and 4,596 included both types. Another source requested SPF records from 251,651 domains for which there was a history of previous observed SPF evaluations. Of these, 136,018 returned type 16 answers, 2,605 returned type 99 answers, 2,439 returned both types, and 115,465 returned neither. Of those answers retrieved, 6,479 included records that start with the string "spf2.0/pra" which are specific requests for Sender-ID processing by receivers. During this second survey, some domains were observed to provide immediate answers for type 16 queries, but would time out waiting for replies to type 99 queries. For example, it was observed that 3,953 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 but only after at least time 4N. It is likely impossible to determine from a survey which 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 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. o The [OPENSPF] web site maintains a list of known implementations of SPF. At the time of this memo's writing it listed six libraries, 22 MTAs with built-in SPF implementations, and numerous patches for MTAs and mail clients. In a survey of numerous MTAs in current or recent use, only 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 Kucherawy Expires October 8, 2012 [Page 4] Internet-Draft SPF/Sender-ID Experiment April 2012 number of clients implement it; although there is substantial activity showing its use in logs, it is unclear whether these are separate implementations by legitimate senders, or merely instances of distributed automated malware seeking to improve their odds of reaching the end user. A survey was done of queries to type 16 and type 99 records by observing nameserver logs. Only a few queries were ever received for type 99 records, and those almost exclusively came from one large email service provider that queried for both types. The vast majority of other querying agents only ever requested type 16. [pending: SPF query results from Hotmail] [other data TBD] 4. Evidence of Differences It is plain from inspection of the two protocols that they have much in common: For a single message, both require the same number of DNS queries, and both require the same code to parse the result. The PRA algorithm applied by Sender-ID is, however, more expensive than simply extracting the domain name from the omnipresent RFC5321.MailFrom. Thus, SPF is cheaper to apply to a message. One set of specific data collected by a working group contributor shows that in more than 95.5% of 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. [pending: MAIL FROM/PRA comparison report from Hotmail] [other data TBD] 5. 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. 5.1. From the Evidence Given the six years that have passed since the publication of the experimental RFCs, and the evidence reported in the earlier sections Kucherawy Expires October 8, 2012 [Page 5] Internet-Draft SPF/Sender-ID Experiment April 2012 of this document, the following conclusions are supported: 1. There has not been substantial adoption of the type 99 (SPF) DNS resource record. In both large-scale surveys performed for this work, less than 2% of responding domains published type 99 records, and almost no clients requested them. 2. Of the records retrieved, fewer than 5% requested processing of messages using the PRA algorithm that was an integral part of Sender-ID. 3. No data collected showed any substantial operational benefit (e.g., cheaper processing, improved accuracy) to using Sender-ID over SPF. 4. A survey of 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 implementations shows no significant use of the SUBMITTER extension by servers, but some by clients. 5.2. Recommendations to the IESG In light of the above, the working group recommends to the IESG the following: 1. that the experiment comprising the series of RFCs defining the SUBMITTER SMTP extension, the Sender-ID mechanism, the Purported Responsible address algorithm, and SPF, be considered concluded; 2. that [SPF] be advanced to the standards track after appropriate technical review with respect to the deployed base; 3. that this revision to SPF deprecate the use of the type 99 DNS resource record by both clients and servers; 4. that [SUBMITTER], [SENDER-ID], and [PRA] have failed to gain any substantial adoption and are thus, de facto, obsolete (and thus could be marked "Obsoleted" and "Historic" by some future action). Appendix A is offered as a cautionary review of problems that affected the process of developing SPF and Sender-ID in terms of use of the DNS. Kucherawy Expires October 8, 2012 [Page 6] Internet-Draft SPF/Sender-ID Experiment April 2012 6. IANA Considerations This memo presents no actions for IANA. [RFC Editor: Please remove this section prior to publication.] 7. Security Considerations This memo contains information for the community only, akin to an implementation report, and does not introduce any new security concerns. Its implications could, in fact, resolve some. 8. Informative References [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, September 2011. [OPENSPF] "Sender Policy Framework: Project Overview", . [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, . [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. Kucherawy Expires October 8, 2012 [Page 7] Internet-Draft SPF/Sender-ID Experiment April 2012 Appendix A. Experiences Developing SPF SPF was originally developed by a community of interested developers outside the IETF. It was brought to the IETF for standardization only after the specification was relatively mature and ready for the rigors of the IETF publication process. At the time, the prospect of getting a DNS resource record (RR) type allocated for SPF was not seriously considered, partly because it was perceived to have high barriers to entry. As a result, by the time the working group was formed, there was already a substantial and growing installed base that had SPF running using TXT RRs. Eventually the application was made for the new RR type as a result of pressure from the DNS experts in the community, who encouraged doing so as the preferred path toward using the DNS for storing such things as policy data. Later, after type 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 type 99 instead of using type 16. This plan failed to take effect for four primary reasons: 1. there was hesitation to make the transition because of concerns that nameservers (and, in fact, DNS-aware firewalls) would drop or reject requests for unknown RR types (see Section 3 for evidence of this), which means successful rollout of a new RR type 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 RR types; 3. the substantial deployed base was already using type 16, and it was working just fine, leading to inertia; 4. [SPF] itself included a faulty transition plan: It said a server SHOULD publish both types 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. It is likely that this will happen again if the bar to creating new RR types even for experimental development purposes is not lowered, and handling of unknown RR types becomes generally more graceful. There are DNS experts within the community that will undoubtedly point to DNS servers and firewalls that mistreat queries for unknown RR types, and claim they are broken, as a way of answering this Kucherawy Expires October 8, 2012 [Page 8] Internet-Draft SPF/Sender-ID Experiment April 2012 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 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, and conducted one of the two detailed TXT/SPF record surveys including collecting timing data The author would also like to thank the following for their contributions to the development of the text in this memo: Dave Crocker, and Scott Kitterman 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 Kucherawy Expires October 8, 2012 [Page 9]