DKIM Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Informational November 9, 2010
Expires: May 13, 2011
RFC4871 Implementation Report
draft-ietf-dkim-implementation-report-05
Abstract
This document contains an implementation report for the IESG covering
DomainKeys Identified Mail (DKIM) in support of the advancement of
that specification along the Standards Track.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DKIM Interoperability Event . . . . . . . . . . . . . . . . . 5
3.1. Participants . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Testing Methodology . . . . . . . . . . . . . . . . . . . 5
3.3. Observations . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Collected DKIM Interoperability and Use Data . . . . . . . . . 8
4.1. The OpenDKIM Project . . . . . . . . . . . . . . . . . . . 8
4.1.1. Details . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. Results . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.3. Conclusions . . . . . . . . . . . . . . . . . . . . . 10
4.2. AOL, Inc. Data . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Google Mail Data . . . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
DomainKeys Identified Mail (DKIM), published in May 2007, has reached
a level of maturity sufficient to consider its advancement along the
standards track. Enclosed is a summary of collected interoperability
data provided from sources that are aggregating such information as
well as from a more formal DKIM interoperability event that took
place in October 2007.
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2. Definitions
DomainKeys Identified Mail is defined in [DKIM].
Various terms specific to email are used in this document. Their
definitions and further discussion can be found in [EMAIL-ARCH].
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3. DKIM Interoperability Event
In October 2007, Alt-N Technologies of Dallas, Texas hosted an
interoperability and testing event at their headquarters. Twenty
organizations sent engineers and their various DKIM implementations
to connect to a private internal network and exchange test messages
and tabulate observed results.
3.1. Participants
The interoperability event included participants from all of the
following organizations: Alt-N Technologies, AOL Inc., AT&T
Laboratories, Bizanga Ltd., Brandenburg InternetWorking, Brandmail
Solutions, ColdSpark, Constant Contact, Inc., DKIMproxy, Domain
Assurance Council, Google Inc., ICONIX Inc., Internet Initiative
Japan (IIJ), Ironport Systems, Message Systems, Port25 Solutions,
Postfix, Sendmail, Inc., StrongMail Systems, and Yahoo! Inc. Most of
the participants traveled to Dallas and participated in person, but a
few operated remotely.
Nearly all of the implementations were based on disjoint code
development projects. A few were based on a common open source base
project.
3.2. Testing Methodology
Participants were encouraged before the event to craft a set of test
messages meant to exercise their own implementations as well as those
of the other participants, both in terms of successful verifications
as well as some expected to fail. Test cases were developed with the
intent of confounding verifiers that may not have implemented the
[ABNF] of [DKIM] correctly.
The participants set up Mail Transfer Agents (MTAs) equipped with
their own DKIM signing and verifying modules, and their own tools to
generate mail to be signed along with tools to analyze the results
post-verification. They then sent their own batteries of test
messages, looking for both expected and unexpected failures in
response. Some implementations included "auto-responders" that would
reply with verification results, while others simply collected the
results that would then be shared manually.
3.3. Observations
All of the packages implemented all of the required portions of
[DKIM] in terms of both signature and key features. Most of the
packages implemented all of the optional features of both signatures
and keys. There were at least two implementations of each optional
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feature.
The interoperability testing was largely successful. As might be
expected, there were many verification false negatives or false
positives that were the result of bugs in corner cases of some of the
implementations presented for testing. In such cases the developers
were able to identify the issue as resulting from their own mis-
reading of the specification and not an error in the specification
itself.
Several of the failures did occur as a result of specification
ambiguities. The participants discussed each of these in turn and
were able to come to consensus on how they believed the specification
should be changed to resolve them.
The participants agreed to keep the results about the specific tests
private. Accordingly, those data are not presented here.
3.4. Results
The handful of interoperability issues described above that referred
to weaknesses or ambiguities in [DKIM] resulted in several errata
being opened via the RFC Editor web site. These are being addressed
in an RFC4871bis draft effort that is now starting from within the
DKIM working group.
The errata items, in summary:
o explicit canonicalized forms of empty bodies for each
canonicalization method, along with their SHA1 and SHA256 hash
values (errata #1376 and #1377)
o clarification about normative text regarding the "a=" tag (errata
#1378)
o ABNF corrections regarding the "z=" tag (errata #1379)
o informative discussion regarding the "x=" tag (errata #1380)
o normative clarifications about "q=", "h=", "k=", "s=" and "t="
tags (errata #1381 and #1382)
o correction of "g=" description to match its ABNF (errata #1383)
o clarifications about "relaxed" body canonicalization (errata
#1384)
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o correction to the signature example (errata #1386)
o ABNF corrections regarding the "h=" tag (errata #1461)
o ABNF corrections regarding the "v=" tag (errata #1487)
o discussion of DomainKeys compatibility (errata #1532)
o discussion about what constitutes the actual value of the "b=" tag
(errata #1596)
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4. Collected DKIM Interoperability and Use Data
Several implementations are collecting private data about DKIM use,
signature survivability, which properties of the base specification
are observed in public use, etc. This section includes collection
methods and summary reports provided by those implementations.
4.1. The OpenDKIM Project
The OpenDKIM Project (http://www.opendkim.org) is an open source
project providing a DKIM support library, an email filter for use
with Mail Transfer Agents (MTAs), and a set of tools to assist with
deployment of DKIM.
4.1.1. Details
Recent releases have included an optional feature to record
statistics about messages with and without DKIM signatures. Sites
enabling this feature can choose to share the data with the project's
development team as part of this interoperability report work. The
data can be anonymized to conceal the sending domain and client IP
addresses, though these data are passed through a one-way hash to
enable collation of data from common sources.
4.1.2. Results
At the time of writing of this document, five weeks of data had been
collected. The results of this effort are as follows:
Reporting Hosts: six individual MTAs representing four distinct
ADMDs
Total Messages: 2558218
Signatures: 1869088 messages (73.0%) were not signed; 676133 (26.4%)
had one signature; 12906 (0.5%) had two signatures; the remainder
(less than 0.01%) had more.
Signing Algorithms: 50.5% of signatures used "rsa-sha1", while the
balance used "rsa-sha256".
Header Canonicalization Algorithms: 14.7% of signatures used
"simple", while the balance used "relaxed"; when grouped by
domains, the percentages were similar.
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Body Canonicalization Algorithms: 26.9% of signatures used "simple",
while the balance used "relaxed"; 18.9% of observed signing
domains used "simple" while the balance used "relaxed".
Keys in Test Mode: 46.6% of keys retrieved from the DNS were tagged
as being in test mode.
Keys with Specific Granularity: 14 keys were retrieved that had
specific names in their "g=" tags.
Keys with Syntax Errors: Less than 0.1% of keys retrieved from the
DNS had syntax errors.
DomainKeys Compatibility: 1.2% of the retrieved keys appeared to be
intended for use with the older DomainKeys proposal rather than
DKIM
AUID use: 52.4% of signatures did not contain an explicit AUID ("i="
value). Of those that did, 86.6% used a domain matching the SDID
("d=" value). Across all "i=" tags present, 42.8% provided no
local-part, 53.4% included a local-part matching the one found in
the From: field, and the remainder had a different local-part.
Missing Keys: 1.7% of signatures received referenced keys that were
not found in the DNS
Optional Signature Tags: Of the optional signature tags supported by
the base specification, "t=" was seen 46.6% of the time (1% of
which included timestamps in the future, even after forgiving some
clock drift); "x=" was seen 4.2% of the time; "l=" was seen 4% of
the time; "z=" was seen 7.2% of the time.
Body Length Limits: Of the signatures for which "l=" was used, 8.4%
of them signed none of the body, and 84.6% of the rest had the
body extended after signing.
Signature Pass Rates: Overall, 92% of observed signatures were
successfully verified.
Pass Rates for Non-List Mail: Where "list mail" is defined as any
mail bearing one of the header fields defined in [LIST-ID] or in
[LIST-URLS], or a "Precedence: list" field, selecting only for
mail that is not list mail revealed a successful verification rate
of 94.9%; selecting only for list mail produced a 87.8% success
rate.
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DNSSEC: No signed keys were reported in the accumulated data to
date.
Common errors: The top five verification errors observed: Key not
found in DNS (21.2%), key granularity mismatch (16%), DNS
retrieval failure such as timeouts (2.1%), key type unknown
(2.0%), key syntax error (1.0%).
Detected Header Field Changes: A subset of the reporting sites are
capable of reporting header fields known to have been changed in
transit (e.g. when "z=" tags were used by the signer). In such
cases, changes to the "To:" field were more common than any other
by almost an order of magnitude.
Most Commonly Signed Fields: From: (100%), To: (95.4%), Subject:
(95.2%), Date: (94.6%), MIME-Version: (91.3%), Content-Type:
(82.9%), Message-Id: (75.6%), Received: (51.8%). All others are
below 50%.
Identities: 74.7% of the signatures observed included a "d=" value
matching the domain in the From: field.
Multiple-use Signing Domains: 24789 unique signing domains were
observed. Of these, 32.9% of them sent a single signed message in
the sample period, 16.6% sent two and 9.2% sent three.
4.1.3. Conclusions
The results of the OpenDKIM work are updated constantly as more data
feeds come online and more data are reported. Based on the data
available at the time of writing, some conclusions are possible.
At least some implementations of all of the optional signature
features, all of the canonicalization combinations and all of the
signing algorithms are in general use. None of the features had zero
use counts.
Overall signature pass rates are generally quite high. The impact of
signature survivability when correlated against Mailing List Manager
(MLM) activity is detectable based on observed data. More research
into this is recommended. The DKIM Working Group is already working
on an Informational draft to discuss those issues.
That the "To" field is the one most often associated with
verification failures suggests some MTAs handling the message are
correcting cases where the field is improperly formed. A common case
is failing to quote the comment portion of that field when required
to do so by [MAIL]. Such corrections cause signatures to become
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invalid.
The counts of low-use signing domains suggest that spammers, who
typically rotate domain names with high frequency, have adopted DKIM
as a tool to try to get through message filters.
4.2. AOL, Inc. Data
A one-day summary of observed traffic from AOL, Inc. reports the
following:
Ratio of DKIM-signed mail: 42%
Properly formed signatures: 1.4 billion
Malformed signatures: 3000
Unique signing domains: 50,000-90,000
Key retrieval errors: 14 million (1%)
Signature refers to nonexistent domain: 10 million (0.7%)
Signature refers to nonexistent key: 36 million (2.5%)
Signature refers to revoked key: 138,000 (~0% )
Verified signatures: 1.2 billion (85.7%)
AUID matches From: domain: 1.2 billion (85.7%)
Failed signatures (body changed): 78 million (5.6%)
Failed signatures (other): 34 million (2.4%)
Expired signatures: less than 1 million (~0%)
4.3. Google Mail Data
Google Mail reports the following:
Unsigned mail: 72.1%
AUID matches From: domain: 68.7%
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Signed mail that verified: 14.7%
Signed mail that verified in test mode: 11.7%
Signed mail that failed: 0.2%
Signed mail that failed in test mode: 0.2%
Body hash mismatch: 0.5%
Signature missing required parameters: 0.3%
Granularity mismatch: 0.2%
These data are reported based on an implementation that only
evaluates one signature per message.
All other reportable anomalies occurred in vanishingly small
percentages.
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5. IANA Considerations
This memo contains no actions for IANA.
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6. Security Considerations
This document is an implementation report and thus has no security
considerations.
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7. References
7.1. Normative References
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
7.2. Informative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[LIST-ID] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[LIST-URLS]
Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
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Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this document: Dave Crocker, Tony Hansen,
Jeff Macdonald, S. Moonesamy and Rolf Sonneveld.
The author also wishes to acknowledge Margot Koschier of AOL, Inc.,
Monica Chew of Google, and the members of The OpenDKIM Project for
providing data used in this report.
The working group expresses its profound thanks to Alt-N Technologies
for graciously hosting the 2007 DKIM interoperability event.
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Author's Address
Murray S. Kucherawy
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
US
Phone: +1 415 946 3800
Email: msk@cloudmark.com
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