Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track December 11, 2007
Expires: June 13, 2008
Reporting of DKIM Verification Failures
draft-kucherawy-dkim-reporting-01
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Copyright (C) The IETF Trust (2007).
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Abstract
This memo presents an extension to the DomainKeys Identified Mail
(DKIM) specifications to allow public keys for verification to
include a reporting address to be used to report message verification
issues, and extends an Internet Message reporting format to be
followed when generating such reports.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Optional Key Reporting Address for DKIM . . . . . . . . . . . 4
3. Optional Key Reporting Address for DKIM-SSP . . . . . . . . . 5
4. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Requested Reports for DKIM Failures . . . . . . . . . . . 6
4.2. Requested Reports for DKIM SSP Failures . . . . . . . . . 6
5. Reporting Formats . . . . . . . . . . . . . . . . . . . . . . 7
6. Extension ARF Fields for DKIM Reporting . . . . . . . . . . . 8
6.1. New ARF Feedback Type . . . . . . . . . . . . . . . . . . 8
6.2. New ARF Header Names . . . . . . . . . . . . . . . . . . . 8
6.3. DKIM Failure Types . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. DKIM Key Tag Registration . . . . . . . . . . . . . . . . 10
7.2. DKIM SSP Tag Registration . . . . . . . . . . . . . . . . 10
7.3. Updates to ARF Feedback Types . . . . . . . . . . . . . . 10
7.4. Updates to ARF Header Names . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8.1. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. Automatic Generation . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 15
B.1. Example Use of DKIM Key Extension Tags . . . . . . . . . . 15
B.2. Example Use of DKIM SSP Extension Tags . . . . . . . . . . 15
B.3. Example Use of ARF Extension Headers . . . . . . . . . . . 15
Appendix C. To-Do List . . . . . . . . . . . . . . . . . . . . . 16
Appendix D. Public Discussion . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
[DKIM] introduced a standard for digital signing of messages for the
purposes of sender authentication. There exist cases in which a
domain name owner might want to receive reports from verifiers that
determine DKIM-signed mail apparently from its domain is failing to
verify according to [DKIM] or is "Suspicious" according to
[I-D.DRAFT-IETF-DKIM-SSP].
This document extends [DKIM] and [I-D.DRAFT-IETF-DKIM-SSP] to add an
optional reporting address to selector records, an optional means of
specifying a desired report format, and furthermore extends
[I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT] to add features required for
DKIM reporting.
This memo presumes those specifications thus modified will issue as
RFCs without these modifications. If the modifications are adopted
prior to their publicatons, clearly those sections of this memo can
be removed.
1.1. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
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2. Optional Key Reporting Address for DKIM
There exist cases in which a domain name owner employing [DKIM] for
e-mail signing and authentication may want to know when signatures in
use by specific keys are not successfully verifying. Currently there
is no such mechanism defined.
This document adds the following two optional "tags" (as defined in
[DKIM]) to the DKIM key records, using the form defined in that
specification:
r= Reporting Address (plain-text; OPTIONAL; no default). The value
MUST be a dkim-quoted-printable string containing the local-part
of an e-mail address to which a report SHOULD be sent when mail
signed with this key fails verification because either (a) the
signature verification itself failed, or (b) the body hash test
failed. The format of this reply is selected by the value of the
"rf=" tag, defined below. To generate a complete address to which
the report is sent, the verifier simply appends to this value an
"@" followed by the domain found in the "d=" tag of the signature
whose validation failed.
rf= Reporting Format (plain-text; OPTIONAL; default is "arf"). The
value MUST be a colon-separated list of tokens representing
desired reporting formats in order of preference. Each element of
the list MUST be a token which is taken from the registered list
of DKIM report formats. See Section 7 for a description of the
registry and Section 5 for a description of recognized formats.
The verifier generating reports MUST generate a report using the
first token in the list which represents a report format it is
capable of generating.
ro= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing those
conditions under which a report is desired. See Section 4.1 for a
list of valid tags.
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3. Optional Key Reporting Address for DKIM-SSP
There also exist cases in which a domain name owner employing
[I-D.DRAFT-IETF-DKIM-SSP] for announcing signing practises with DKIM
may want to know when messages are received unsigned or signed with
third-party signatures contrary to the sender's published policy.
Currently there is no such mechanism defined.
This document adds the following two optional "tags" (as defined in
[I-D.DRAFT-IETF-DKIM-SSP]) to the DKIM SSP records, using the form
defined in that specification:
r= Reporting Address (plain-text; OPTIONAL; no default). The value
MUST be a dkim-quoted-printable string containing the local-part
of an e-mail address to which a report SHOULD be sent when mail
claiming to be from this domain failed the verification algorithm
described in [I-D.DRAFT-IETF-DKIM-SSP], in particular because a
message arrived unsigned or signed by a third-party signature in
violation of the sender's published policy. The format of this
reply MUST be in the format specified by the "rf=" tag defined
below. To generate a complete address to which the report is
sent, the verifier simply appends to this value an "@" followed by
the domain whose policy was queried in order to evaluate the
sender's SSP.
rf= Reporting Format (plain-text; OPTIONAL; default is "arf"). The
value MUST be a colon-separated list of tokens representing
desired reporting formats in decreasing order of preference. Each
element of the list MUST be a token which is taken from the
registered list of DKIM report formats. See Section 7 for a
description of the registry and Section 5 for a description of
recognized formats. The verifier generating reports MUST generate
a report using the first token in the list which represents a
report format it is capable of generating.
ro= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing those
conditions under which a report is desired. See Section 4.2 for a
list of valid tags.
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4. Requested Reports
This memo also includes, as the "ro" tags defined above, the means by
which the sender can request reports for specific circumstances of
interest. Verifiers MUST NOT generate reports for incidents which do
not match a requested report.
4.1. Requested Reports for DKIM Failures
The following report requests are defined for DKIM keys:
all All reports are requested.
s Reports are requested for signature or key syntax errors.
v Reports are requested for signature verification failures or body
hash mismatches.
x Reports are requested for signatures rejected by the verifier
because the expiration time has passed.
4.2. Requested Reports for DKIM SSP Failures
The following report requests are defined for DKIM keys:
all All reports are requested.
s Reports are requested for messages for which the SSP algorithm
determines the message is both signed and Suspicious.
u Reports are requested for messages for which the SSP algorithm
determines the message is both unsigned and Suspicious.
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5. Reporting Formats
This section lists reporting formats supported by this DKIM reporting
mechanism. Currently only one format is supported:
arf: Abuse Reporting Format, as defined in
[I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT], and as extended in
Section 6.
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6. Extension ARF Fields for DKIM Reporting
The current ARF format defined in
[I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT] lacks some specific features
required to do effective DKIM reporting. This section describes the
extensions required to do so and thus required to conform to this
specification.
6.1. New ARF Feedback Type
A new feedback type of "dkim" is defined as an extension to Section
8.2 of [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT]. See Section 7.3 for
details.
The header names listed in that draft which may appear for this new
feedback type include all shown in the draft except "Relevant-URI"
and "Removal-Recipient" as they have no semantics relating to DKIM.
6.2. New ARF Header Names
The following new ARF header names are defined as extensions to
Section 8.1 of [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT]:
DKIM-Canonicalized-Body: A base64 encoding of the canonicalized body
of the message as generated by the verifier. This header and
value MUST be present for reports using feedback type "dkim" when
reporting a "bodyhash" failure.
DKIM-Canonicalized-Headers: A base64 encoding of the canonicalized
header of the message as generated by the verifier. This header
and value MUST be present for reports using feedback type "dkim".
DKIM-Failure: Indicates the type of DKIM failure that is being
reported. The list of valid values is enumerated below. This
header and value MUST be present for reports using feedback type
"dkim".
DKIM-Identity: The identity of the signature which failed
verification, taken from the "i=" tag of the signature. This
header and value MUST be present for reports using feedback type
"dkim" when reporting anything other than an "ssp" failure.
DKIM-Selector: The selector of the signature which failed
verification, taken from the "s=" tag of the signature. This
header and value MUST be present for reports using feedback type
"dkim" when reporting anything other than an "ssp" failure.
The values which are base64 encodings may contain FWS for formatting
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purposes as per the usual header field wrapping defined in [MAIL].
During decoding, any characters not in the base64 alphabet are
ignored so that such line wrapping does not harm the value.
base64 is defined in [MIME].
6.3. DKIM Failure Types
The list of defined DKIM failure types, used in the "DKIM-Failure:"
header (defined above), is as follows:
bodyhash: The body hash in the signature and the body hash computed
by the verifier did not match.
granularity: The key referenced by the signature on the message was
not authorized for use by the sender.
revoked: The key referenced by the signature on the message has been
revoked.
signature: The signature on the message did not successfully verify
against the header hash and public key.
ssp: The sender's published signing practises determined the message
is suspicious.
Supplementary data may be included in the form of [MAIL]-compliant
comments. For example, "Failure: ssp" could be augmented by a
comment to indicate that the failed message was rejected because it
was not signed when it should have been. See Appendix B for
examples.
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7. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new [DKIM] key tag, the new
[I-D.DRAFT-IETF-DKIM-SSP] tag, and the extensions to
[I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT].
7.1. DKIM Key Tag Registration
IANA is requested to update the DKIM Key Tag Specification Registry
to include the following new items:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| r | (this document) |
| rf | (this document) |
| ro | (this document) |
+------+-----------------+
7.2. DKIM SSP Tag Registration
IANA is requested to update the DKIM SSP Tag Specification Registry
to include the following new items:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| r | (this document) |
| rf | (this document) |
| ro | (this document) |
+------+-----------------+
7.3. Updates to ARF Feedback Types
The following feedback type is added to the Feedback Report Feedback
Type Registry:
Feedback Type: dkim
Description: DKIM failure report
Registration: (this document)
7.4. Updates to ARF Header Names
The following headers are added to the Feedback Report Header Names
Registry:
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Field Name: DKIM-Canonicalized-Body
Description: Canonicalized body, per DKIM
Multiple Appearances: No
Related "Feedback-Type": dkim
Field Name: DKIM-Canonicalized-Headers
Description: Canonicalized headers, per DKIM
Multiple Appearances: No
Related "Feedback-Type": dkim
Field Name: DKIM-Failure
Description: Type of DKIM failure
Multiple Appearances: No
Related "Feedback-Type": dkim
Field Name: DKIM-Identity
Description: Identity from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": dkim
Field Name: DKIM-Selector
Description: Selector from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": dkim
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8. Security Considerations
Security issues with respect to these DKIM reports are similar to
those found in [DSN].
8.1. Forgeries
These reports may be forged as easily as ordinary Internet electronic
mail. User agents and automatic mail handling facilities (such as
mail distribution list exploders) that wish to make automatic use of
DSNs should take appropriate precautions to minimize the potential
damage from denial-of-service attacks.
Security threats related to forged DSNs include the sending of:
a. A falsified DKIM failure notification when the message was in
fact delivered to the indicated recipient;
b. Falsified signature information, such as selector, domain, etc.
Perhaps the simplest means of mitigating this threat is to assert
that DKIM reports should themselves be signed. This is under
consideration.
8.2. Automatic Generation
Automatic generation of these reports by verifying agents can cause a
denial-of-service attack when a large volume of e-mail is sent that
causes DKIM verification failures for whatever reason.
It is unclear what a good solution for this issue is. Limiting the
rate of generation of these messages may be apropos but threatens to
inhibit the distribution of important and possibly time-sensitive
information.
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9. References
9.1. Normative References
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4817, May 2007.
[I-D.DRAFT-IETF-DKIM-SSP]
Allman, E., Delany, M., and J. Fenton, "DKIM Sender
Signing Practises", I-D DRAFT-IETF-DKIM-SSP-01.
[I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT]
Shafranovich, Y., Levine, J., and P. Hoffman, "An
Extensible Format for Email Feedback Reports", I-D DRAFT-
SHAFRANOVICH-FEEDBACK-REPORT-03, June 2007.
[IANA-CONSIDERATIONS]
Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
October 1998.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[MAIL] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
9.2. Informative References
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 1894,
January 1996.
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Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: (add names here)
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Appendix B. Examples
This section contains examples of the use of each of the extensions
defined by this memo.
B.1. Example Use of DKIM Key Extension Tags
B.2. Example Use of DKIM SSP Extension Tags
B.3. Example Use of ARF Extension Headers
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Appendix C. To-Do List
Things still to be done:
o ABNF
o Examples
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Appendix D. Public Discussion
[REMOVE BEFORE PUBLICATION]
Public discussion of this proposed specification is handled via the
mail-vet-discuss@mipassoc.org mailing list. The list is open.
Access to subscription forms and to list archives can be found at
http://mipassoc.org/mailman/listinfo/mail-vet-discuss.
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Author's Address
Murray S. Kucherawy
Sendmail, Inc.
6475 Christie Ave., Suite 350
Emeryville, CA 94608
US
Phone: +1 510 594 5400
Email: msk+ietf@sendmail.com
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