DKIM Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Informational September 29, 2010
Expires: April 2, 2011
RFC4871 Implementation Report
draft-ietf-dkim-implementation-report-01
Abstract
This document contains an implementation report for the IESG covering
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 . . . . . . . . . 7
4.1. The OpenDKIM Project . . . . . . . . . . . . . . . . . . . 7
4.1.1. Details . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.2. Results . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.3. Conclusions . . . . . . . . . . . . . . . . . . . . . 9
4.2. AOL Data . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Informative References . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
[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
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, 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. Some 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 implementations implemented all of the required portions
of [DKIM] in terms of both signature and key features. Most of the
implementations implemented all of the optional features of both
signatures and keys. There were no notable or common exceptions.
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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.
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.
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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 is an open source project providing a DKIM
support library, an email filter for use with 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, the results of this effort
are as follows:
Reporting Hosts: six individual MTAs representing four distinct
ADMDs
Total Messages: 416393
Signatures: 304801 messages (73.2%) were not signed; 109367 (26.2%)
had one signature; 2198 (0.5%) had two signatures; the remainder
(less than 0.01%) had more.
Signing Algorithms: 48.7% of signatures used "rsa-sha1", while the
balance used "rsa-sha256".
Header Canonicalization Algorithms: 13.7% of signatures used
"simple", while the balance used "relaxed"; when grouped by
domains, the percentages were similar.
Body Canonicalization Algorithms: 25.4% of signatures used "simple",
while the balance used "relaxed"; when grouped by domains, the
percentages were similar.
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Keys in Test Mode: 55.8% of keys retrieved from the DNS were tagged
as being in test mode.
Keys with Specific Granularity: Four 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: 1.4% of the retrieved keys appeared to be
intended for use with the older DomainKeys proposal rather than
DKIM
Missing Keys: 2% 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 46.4% of the time (1% of
which included timestamps in the future, even after forgiving some
clock drift); "x=" was seen 4.4% of the time; "l=" was seen 4.6%
of the time; "z=" was seen 4.8% of the time.
Body Length Limits: Of the signatures for which "l=" was used, 6.4%
of them signed none of the body, and 100% of the rest had the body
extended after signing.
Signature Pass Rates: Overall, 89.9% of observed signatures were
successfully verified.
Pass Rates for Non-List Mail: Where "list mail" is defined as any
mail not 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 93.6%; selecting only for list mail produced a 84.7% success
rate.
Author vs. Third-Party: 73% of the signatures observed were author
signatures, meaning the "d=" value in the signature matched the
domain found in the From: header field. The remainder, therefore,
were third-party signatures.
DNSSEC: Only one reporting site is currently checking for DNSSEC on
keys retrieved from the DNS. It found no signed keys.
Common errors: The top five verification errors observed: Key not
found in DNS (18.7%), key granularity mismatch (13%), DNS
retrieval failure such as timeouts (2%), key revoked (1.9%),
unknown key type (1.8%).
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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 a factor of four or more.
Most Commonly Signed Fields: From: (100%), To: (95.5%), Subject:
(93.7%), Date: (92.3%), MIME-Version: (88.8%), Content-Type:
(80%), Message-Id: (75.9%), Received: (59.7%). All others are
below 50%.
Multiple-use Signing Domains: 9512 unique signing domains were
observed. Of these, 42.7% of them sent a single signed message in
the sample period, 18.6% sent two and 8.6% 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 MLM activity is
surprisingly low 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
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 Data
A one-day summary of observed traffic from America Online reports the
following:
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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
Signature refers to nonexistent domain: 10 million
Signature refers to nonexistent key: 36 million
Signature refers to revoked key: 138,000
Originator signatures: 1.2 billion
Third-party signatures: 184 million
Verified signatures: 1.2 billion
Failed signatures (body changed): 78 million
Failed signatures (other): 34 million
Expired signatures: less than 1 million
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5. Security Considerations
This document is an implementation report and thus has no security
considerations.
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6. Informative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[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.
[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: Tony Hansen
The working group expresses its 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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