MARF Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track January 23, 2012
Expires: July 26, 2012
Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-04
Abstract
This memo presents extensions to the DomainKeys Identified Mail
(DKIM) specification to allow for detailed reporting of message
authentication failures in an on-demand fashion.
Status of this Memo
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This Internet-Draft will expire on July 26, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Imported Definitions . . . . . . . . . . . . . . . . . . . 4
3. Optional Reporting Address for DKIM . . . . . . . . . . . . . 5
4. Optional Reporting Address for DKIM-ADSP . . . . . . . . . . . 7
5. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Requested Reports for DKIM Failures . . . . . . . . . . . 9
5.2. Requested Reports for DKIM ADSP Failures . . . . . . . . . 9
6. Report Generation . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Report Format . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Envelope Sender Selection . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7.1. DKIM Signature Tag Registration . . . . . . . . . . . . . 11
7.2. DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8.1. Inherited Considerations . . . . . . . . . . . . . . . . . 12
8.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 12
8.3. Amplification Attacks . . . . . . . . . . . . . . . . . . 12
8.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 12
8.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 16
B.1. Example Use of DKIM Signature Extension Tags . . . . . . . 16
B.2. Example Use of DKIM ADSP Extension Tags . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
[DKIM] introduced a mechanism for message signing and authentication.
It uses digital signing to associate a domain name with a message in
a reliable (i.e. not forgeable) manner. The output is a verified
domain name that can then be subjected to some sort of evaluation
process (e.g., advertised sender policy, comparison to a known-good
list, submission to a reputation service, etc.).
Deployers of message authentication technologies are increasingly
seeking visibility into DKIM verification failures and conformance
failures involving the published signing practices (e.g., [ADSP]) of
an Administrative Mail Domain (ADMD; see [EMAIL-ARCH]).
This document extends [DKIM] and [ADSP] to add an optional reporting
address and some reporting parameters.
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2. Definitions
2.1. Keywords
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].
2.2. Imported Definitions
The ABNF token "qp-section" is imported from [MIME].
The base64 encoding method is defined in [MIME].
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3. Optional Reporting Address for DKIM
A domain name owner employing [DKIM] for e-mail signing and
authentication might 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 optional "tags" (as defined in
[DKIM]) to the DKIM-Signature header fields, 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
bearing this signature fails verification for one of the reasons
enumerated below. The value MUST be interpreted as a local-part
only. To construct the actual 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. Therefore, a signer making use of this extension tag MUST
ensure that an email address thus constructed can receive reports
generated as described in Section 6.
ABNF:
sig-r-tag = %x72 *WSP "=" *WSP qp-section
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 5.1 for a
list of valid tags.
ABNF:
sig-ro-type = ( "all" / "s" / "v" / "x" )
sig-ro-tag = %x72 %x6f *WSP "=" *WSP sig-ro-type
*WSP 0* ( ":" *WSP sig-ro-type )
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is a single integer value from 0 to 100
inclusive that indicates what percentage of incidents of signature
authentication failures, selected at random, should cause reports
to be generated. The report generator SHOULD NOT issue reports
for more than the requested percentage of incidents. Report
generators MAY make use of the "Incidents:" field in [ARF] to
indicate that there are more reportable incidents than there are
reports.
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ABNF:
sig-ri-tag = %x72 %x69 *WSP "=" *WSP 1*12DIGIT "/" 1*12DIGIT
rs= Requested SMTP Error String (text; OPTIONAL; no default). The
value is a dkim-quoted-printable string the signing domain
requests be included in [SMTP] error strings if messages are
rejected during a single SMTP session.
ABNF:
sig-rs-tag = %x72 %x73 *WSP "=" qp-section
In the absence of an "r=" tag, all other tags listed above MUST be
ignored.
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4. Optional Reporting Address for DKIM-ADSP
There also exist cases in which a domain name owner employing [ADSP]
for announcing signing practises with DKIM may want to know when
messages are received without valid author domain signatures.
Currently there is no such mechanism defined.
This document adds the following optional "tags" (as defined in
[ADSP]) to the DKIM ADSP 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 [ADSP], in particular because a message arrived
without a signature that validates, which contradicts what the
ADSP record claims. The value MUST be interpreted as a local-part
only. To construct the actual 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 ADSP, i.e., the one taken from the RFC5322.From domain of
the message under evaluation. Therefore, a signer making use of
this extension tag MUST ensure that an email address thus
constructed can receive reports generated as described in
Section 6.
ABNF:
adsp-r-tag = %x72 *WSP "=" qp-section
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is a single integer value from 0 to 100
inclusive that indicates what percentage of incidents of ADSP
evaluation failures, selected at random, should cause reports to
be generated. The report generator SHOULD NOT issue reports for
more than the requested percentage of incidents. Report
generators MAY make use of the "Incidents:" field in [ARF] to
indicate that there are more reportable incidents than there are
reports.
ABNF:
adsp-ri-tag = %x72 %x69 *WSP "=" *WSP
1*12DIGIT "/" 1*12DIGIT
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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 5.2 for a
list of valid tags.
ABNF:
adsp-ro-type = ( "all" / "s" / "u" )
adsp-ro-tag = %x72 %x6f *WSP "=" *WSP adsp-ro-type
*WSP 0* ( ":" *WSP adsp-ro-type )
rs= Requested SMTP Error String (plain-text; OPTIONAL; no default).
The value is a string the signing domain requests be included in
[SMTP] error strings when messages are rejected during a single
SMTP session.
ABNF:
adsp-rs-tag = %x72 %x73 *WSP "=" qp-section
In the absence of an "r=" tag, all other tags listed above MUST be
ignored.
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5. Requested Reports
This memo also includes, as the "ro" tags defined above, the means by
which the signer can request reports for specific circumstances of
interest. Verifiers MUST NOT generate reports for incidents that do
not match a requested report, and MUST ignore requests for reports
not included in this these lists.
5.1. 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.
5.2. Requested Reports for DKIM ADSP Failures
The following report requests are defined for ADSP records:
all All reports are requested.
s Reports are requested for messages that have a valid [DKIM]
signature but do not match the published [ADSP] policy.
u Reports are requested for messages that have no valid [DKIM]
signature and do not match the published [ADSP] policy.
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6. Report Generation
This section describes the process for generating and sending reports
in accordance with the request of the signer and/or sender as
described above.
6.1. Report Format
All reports generated as a result of requests contained in these
extension parameters MUST be generated in compliance with [ARF] and
its extension specific to this work, [I-D.MARF-AUTHFAILURE-REPORT].
6.2. Envelope Sender Selection
In the case of transmitted reports in the form of a new message
(versus rejections during an [SMTP] session), it is necessary to
construct the message so as to avoid amplification attacks,
deliberate or otherwise. Thus, per Section 2 of [DSN], the envelope
sender address of the report SHOULD be chosen to ensure that no
delivery status reports will be issued in response to the report
itself, and MUST be chosen so that these reports will not generate
mail loops. Whenever an [SMTP] transaction is used to send a report,
the MAIL FROM command MUST use a NULL return address, i.e. "MAIL
FROM:<>".
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7. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new [DKIM] key tags, and the new [ADSP] tags.
7.1. DKIM Signature Tag Registration
IANA is requested to update the DKIM Signature Tag Specification
Registry to include the following new items:
+------+-----------------+---------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+---------+
| r | (this document) | current |
| ro | (this document) | current |
| rp | (this document) | current |
| rs | (this document) | current |
+------+-----------------+---------+
7.2. DKIM ADSP Tag Registration
IANA is requested to update the DKIM ADSP Tag Specification Registry
to include the following new items:
+------+-----------------+---------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+---------+
| r | (this document) | current |
| ro | (this document) | current |
| rp | (this document) | current |
| rs | (this document) | current |
+------+-----------------+---------+
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8. Security Considerations
Security issues with respect to these reports are similar to those
found in [DSN].
8.1. Inherited Considerations
Implementers are advised to consider the Security Considerations
sections of [DKIM] and [ADSP].
8.2. 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
reports of any kind should take appropriate precautions to minimize
the potential damage from denial-of-service attacks.
Security threats related to forged reports include the sending of:
a. A falsified authentication 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 these reports should themselves be signed with something like
DKIM. On the other hand, if there's a problem with the DKIM
infrastructure at the verifier, signing DKIM failure reports may
produce reports that aren't trusted or even accepted by their
intended recipients.
8.3. Amplification Attacks
Failure to compile with the normative statements in Section 6.2 can
lead to amplification denial-of-service attacks. See that section
for details.
8.4. 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 authentication failures for whatever reason.
Limiting the rate of generation of these messages may be appropriate
but threatens to inhibit the distribution of important and possibly
time-sensitive information.
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In general ARF feedback loop terms, it is suggested that report
generators only create these (or any) ARF reports after an out-of-
band arrangement has been made between two parties. This mechanism
then becomes a way to adjust parameters of an authorized abuse report
feedback loop that is configured and activated by private agreement
rather than starting to send them automatically based solely on data
found in the DKIM signatures, which could have been fraudulently
inserted.
8.5. Reporting Multiple Incidents
If it is known that a particular host generates abuse reports upon
certain incidents, an attacker could forge a high volume of messages
that will trigger such a report. The recipient of the report could
then be innundated with reports. This could easily be extended to a
distributed denial-of-service attack by finding a number of report-
generating servers.
The incident count referenced in [ARF] provides a limited form of
mitigation. The host generating reports may elect to send reports
only periodically, with each report representing a number of
identical or near-identical incidents. One might even do something
inverse-exponentially, sending reports for each of the first ten
incidents, then every tenth incident up to 100, then every 100th
incident up to 1000, etc. until some period of relative quiet after
which the limitation resets.
The use of this for "near-identical" incidents in particular causes a
degradation in reporting quality, however. If for example a large
number of pieces of spam arrive from one attacker, a reporting agent
may decide only to send a report about a fraction of those messages.
While this averts a flood of reports to a system administrator, the
precise details of each incident are similarly not sent.
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9. References
9.1. Normative References
[ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
Sender Signing Practises", RFC 5617, August 2009.
[ARF] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", RFC 5965,
August 2010.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
September 2011.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
October 2008.
[I-D.MARF-AUTHFAILURE-REPORT]
Fontana, H., "Authentication Failure Reporting using the
Abuse Report Format",
I-D draft-ietf-marf-authfailure-report, January 2012.
[IANA-CONSIDERATIONS]
Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226, May 2008.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
9.2. Informative References
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
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Appendix A. Acknowledgements
The authors wish to acknowledge the following for their review and
constructive criticism of this proposal: Steve Atkins, Monica Chew,
Dave Crocker, Tim Draegen, Frank Ellermann, JD Falk, and John Levine.
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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 Signature Extension Tags
A DKIM-Signature field including use of the extensions defined by
this memo:
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
d=example.com; s=jan2012;
r=dkim-errors; ro=v:x;
h=from:to:subject:date:message-id;
bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8
Example 1: DKIM-Signature field using these extensions
This example DKIM-Signature field contains the following data in
addition to the basic DKIM signature data:
o Reports about signature evaluation failures should be send to the
address "dkim-errors" at the signer's domain;
o Only reports about signature verification failures and expired
signatures should be generated.
B.2. Example Use of DKIM ADSP Extension Tags
A DKIM ADSP record including use of the extensions defined by this
memo:
dkim=all; r=dkim-adsp-errors; ro=u
Example 2: DKIM ADSP record using these extensions
This example ADSP record makes the following assertions:
o The sending domain (i.e. the one that is advertising this policy)
signs all mail it sends;
o Reports about ADSP evaluation failures should be send to the
address "dkim-adsp-errors" at the sender's domain;
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o Only reports about unsigned messages should be generated.
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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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