MARF Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track January 25, 2012
Expires: July 28, 2012
Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-06
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 28, 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 for DKIM . . . . . . . . . . . . . . . . . 5
3.1. Extension DKIM Signature Tag . . . . . . . . . . . . . . . 5
3.2. DKIM Reporting TXT Record . . . . . . . . . . . . . . . . 5
3.3. DKIM Reporting Algorithm . . . . . . . . . . . . . . . . . 7
4. Optional Reporting Address for DKIM-ADSP . . . . . . . . . . . 9
5. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Requested Reports for DKIM Failures . . . . . . . . . . . 11
5.2. Requested Reports for DKIM ADSP Failures . . . . . . . . . 11
6. Report Generation . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Report Format . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Envelope Sender Selection . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.1. DKIM Signature Tag Registration . . . . . . . . . . . . . 13
7.2. DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 13
7.3. DKIM Reporting Tag Registry . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. Inherited Considerations . . . . . . . . . . . . . . . . . 15
8.2. Deliberate Misuse . . . . . . . . . . . . . . . . . . . . 15
8.3. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 15
8.4. Amplification Attacks . . . . . . . . . . . . . . . . . . 15
8.5. Automatic Generation . . . . . . . . . . . . . . . . . . . 15
8.6. Reporting Multiple Incidents . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 20
B.1. Example Use of DKIM Signature Extension Tags . . . . . . . 20
B.2. Example Use of DKIM ADSP Extension Tags . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
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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 Management 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].
Numerous DKIM-specific terms used here are defined in [DKIM]. The
definition of the ABNF token "domain-name" can also be found there.
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3. Optional Reporting for DKIM
A domain name owner employing [DKIM] for email 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 optional "tags" (as defined in [DKIM]) to the
DKIM-Signature header field and the DKIM key record in the DNS, using
the formats defined in that specification.
3.1. Extension DKIM Signature Tag
The following tag is added to DKIM-Signature header fields when a
Signer wishes to request that reports of failed verifications be
generated by a Verifier:
rd= Reporting Domain (plain-text; OPTIONAL; no default). The value
MUST be a string containing a DNS domain name. This value will be
used by the Verifier to confirm that failed verification reports
are desired and collect report generation parameters. A complete
description and illustration of how this is applied can be found
in Section 3.3.
ABNF:
sig-rd-tag = %x72.64 *WSP "=" *WSP domain-name
3.2. DKIM Reporting TXT Record
When a Signer wishes to advertise that it wants to receive failed
verification reports, it also places in the DNS a TXT resource record
(RR) whose content follows the same general syntax as DKIM key
records, as defined in Section 3.6.1 of [DKIM], in that it is made up
of a sequence of tag-value objects. In this case, the tags and
values comprise the parameters to be used when generating the
reports. The Verifier (if it supports this extension) will request
the content of this record when it sees an "rd=" tag in a DKIM-
Signature header field.
The reassembly rules of Section 3.6.2.2 of [DKIM] also apply here if
the reporting TXT record consists of several string fragments.
Any tag found in the content of this record that is not registered
with IANA as described in Section 7.3 MUST be ignored.
The initial list of tags supported for the reporting TXT record is as
follows:
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ra= Reporting Address (plain-text; REQUIRED). A dkim-quoted-
printable string (see Section 2.11 of [DKIM]) containing the
local-part of an email address to which a report SHOULD be sent
when mail fails DKIM 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 name found in the "rd=" tag of the DKIM-Signature
header field. Therefore, an ADMD making use of this specification
MUST ensure that an email address thus constructed can receive
reports generated as described in Section 6. ABNF:
rep-ra-tag = %x72.61 *WSP "=" *WSP qp-section
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is an integer value from 0 to 100 inclusive
that indicates what percentage of incidents of signature
authentication failures, selected at random, are to 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:
rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
rr= 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:
rep-rr-type = ( "all" / "s" / "v" / "x" )
rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type
*WSP 0* ( ":" *WSP rep-rr-type )
rs= Requested SMTP Error String (text; OPTIONAL; no default). The
value is a dkim-quoted-printable string that the publishing ADMD
requests be included in [SMTP] error strings if messages are
rejected during the delivery SMTP session. ABNF:
rep-rs-tag = %x72.73 *WSP "=" qp-section
In the absence of an "ra=" tag, all other tags listed above MUST be
ignored. In other words, an implementation MUST NOT have a default
value for the "ra=" tag.
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3.3. DKIM Reporting Algorithm
The following algorithm, or one semantically equivalent to it, MUST
be applied when an implementation is equipped to generate reports in
compliance with this specification and its evaluation of the DKIM-
Signature header field (see [DKIM]) on a message fails for some
reason. Note that this processing is done as a reporting extension
only; the outcome of the specified DKIM evaluation is otherwise
unaffected.
1. If the DKIM-Signature field did not contain an "rd=" tag,
terminate.
2. Issue a [DNS] TXT query to the name that results from appending
the value of the "rd=" tag to the string "_report._domainkey".
For example, if a DKIM-Signature header field contains
"example.com", issue a DNS TXT query to
"_report._domainkey.example.com".
3. If the DNS query returns anything other than a success status
code (0), also known as NOERROR, the implementation MAY log this
locally; in either case, terminate.
4. If the resultant TXT is in several string fragments, reassemble
it as described in Section 3.6.2.2 of [DKIM].
5. If the TXT content is syntactically invalid, the implementation
MAY log this locally; in either case, terminate.
6. If the reason for the signature evaluation failure does not
match one of the report requests found in the "rr=" tag (or its
default value), terminate.
7. If a report percentage ("rp=") tag was present, select a random
number between 0 and 99, inclusive; if the selected number is
higher than the tag's value, terminate.
8. Determine the reporting address by extracting the value of the
"ra=" tag and appending to it "@" followed by the domain name
found in the "rd=" tag of the DKIM-Signature header field.
9. Construct and send a report in compliance with Section 6 of this
memo that includes as its intended recipient the address
constructed in the previous step.
10. If the [SMTP] session during which the DKIM signautre was
evaluated is still active and the SMTP server has not already
given its response to the DATA command that relayed the message,
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and an "rs=" tag was present in the TXT record, the SMTP server
SHOULD include the decoded string found in the "rs=" tag in its
SMTP reply to the DATA command.
This algorithm has the following advantages over previous
implementations:
a. If the DKIM signature fails to verify, no additional DNS check is
made to see if reporting is requested; the request is active in
that it is included in the DKIM-Signature header field.
(Previous implementations included the reporting address in the
DKIM key record, which is not queried for certain failure cases.
This meant, for full reporting, that the key record had to be
retrieved even when it was not otherwise necessary.)
b. The request is confirmed by the presence of a corresponding TXT
record in the DNS, since the Signer thus provides the parameters
required to construct and send the report. This means a
malicious Signer cannot falsely assert that someone else wants
failure reports. It can cause additional DNS traffic against the
domain listed in the "rd=" signature tag, but negative caching of
the requested DNS record will help to mitigate this issue.
c. It is not possible for a Signer to direct reports to an email
address outside of its own domain, preventing distributed email-
based denial-of-service attacks.
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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:
ra= Reporting Address (plain-text; OPTIONAL; no default). The value
MUST be a dkim-quoted-printable string containing the local-part
of an email 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-ra-tag = %x72.61 *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-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
rr= 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-rr-type = ( "all" / "s" / "u" )
adsp-rr-tag = %x72.72 *WSP "=" *WSP adsp-rr-type
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*WSP 0* ( ":" *WSP adsp-rr-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 = %x72.73 *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. The envelope sender address of the report
needs to be chosen so that these reports will not generate mail
loops.
Similar to Section 2 of [DSN], the envelope sender address of the
report SHOULD be chosen to ensure that no feedback reports will be
issued in response to the report itself.
Therefore, when an [SMTP] transaction is used to send a report, the
MAIL FROM command MUST either use the NULL return address, i.e.,
"MAIL FROM:<>", or one that will pass [SPF] MAIL FROM checks on
receipt. The HELO/EHLO command SHOULD also be selected so that it
wil pass [SPF] HELO checks.
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7. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new [DKIM] signature tags, and the new [ADSP]
tags. It also creates a DKIM reporting tag registry.
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 |
+------+-----------------+--------+
| rd | (this document) | active |
+------+-----------------+--------+
7.2. DKIM ADSP Tag Registration
IANA is requested to update the DKIM ADSP Specification Tag Registry
to include the following new items:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| ra | (this document) |
| rp | (this document) |
| rr | (this document) |
| rs | (this document) |
+------+-----------------+
7.3. DKIM Reporting Tag Registry
IANA is requested to create a sub-registry of the DKIM Parameters
registry called "DKIM Reporting Tags". Additions to this registry
follow the "Specification Required" rules, with the following columns
required for all registrations:
Type: The name of the tag being used in reporting records
Reference: The document that specifies the tag being defined
Status: The status of the tag's current use, either "active"
indicating active use, or "historic" indicating discontinued or
deprecated use
The initial registry entries are as follows:
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+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| ra | (this document) | active |
| rp | (this document) | active |
| rr | (this document) | active |
| rs | (this document) | active |
+------+-----------------+--------+
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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. Deliberate Misuse
Some threats caused by deliberate misuse of this mechanism are
discussed in Section 3.3.
8.3. 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.
8.4. 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.
8.5. Automatic Generation
The mechanisms described in this memo are primarily intended for use
in generating reports to aid implementers of [DKIM] and [ADSP] and
other related protocols in development and debugging. Therefore,
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they are not designed for prolonged forensic use. However, such uses
are possible by ADMDs that want to keep a close watch for fraud or
infrastructure problems.
Automatic generation of these reports by verifying agents can cause a
denial-of-service attack when a large volume of email 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.
In general ARF feedback loop terms, it is often 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.
8.6. 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.
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[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.
[SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
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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, John Levine, and
Scott Kitterman.
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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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