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

Resolution of the Sender Policy Framework (SPF)

and Sender ID Experiments


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

Copyright Notice

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This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( 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:

  • 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.
  • 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. The absence of significant adoption of the RRTYPE 99 DNS Resource Record suggests that it has not attracted enough support to be useful.
  1. Unavailability of software implementing the protocols was not a gating factor in terms of the selection of which to use.
  1. 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",
   [SID-IMPL]    "Sender ID Framework Industry Support and Solutions",
                 October 2007, <
   [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;
  1. the substantial deployed base was already using RRTYPE 16, and it was working just fine, leading to inertia;
  1. [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
   128 King St., 2nd Floor
   San Francisco, CA  94107
   Phone: +1 415 946 3800