Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 6686 Cloudmark
Category: Informational 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
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
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Table of Contents
1. 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 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
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
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.
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
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
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.
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
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
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
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
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
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.
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
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.
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
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
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
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.
Murray S. Kucherawy
128 King St., 2nd Floor
San Francisco, CA 94107
Phone: +1 415 946 3800