MARF Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track May 15, 2011
Expires: November 16, 2011
Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-02
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 16, 2011.
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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. Reporting Formats . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7.1. DKIM Key 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. Automatic Generation . . . . . . . . . . . . . . . . . . . 12
8.4. Envelope Sender Selection . . . . . . . . . . . . . . . . 13
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 Key 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 sender 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 sender 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 an optional means of specifying a desired report format
and other parameters. Accompanying documents extend [ARF] and SPF
(RFC4408) to improve the reporting capabilities of those standards as
well, and are intended to be deployed together. The full suite of
documents is:
o RFCxxxx: Abuse Report Format (ARF) Extensions for Authentication
Failure Reporting
o RFCxxxx+1: DKIM and ADSP Extensions for Authentication Failure
Reporting (this memo)
o RFCxxxx+2: SPF Extensions for Authentication Failure Reporting
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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 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 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. If only a local-part is included, then 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.
ABNF:
key-r-tag = %x72 *WSP "=" *WSP qp-section
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 that is taken from the registered list of
report formats. See Section 7 for a description of the registry
and Section 6 for a description of recognized formats. The
verifier generating reports MUST generate a report using the first
token in the list that represents a report format it is capable of
generating.
ABNF:
rep-format = ( "arf" / "smtp" )
key-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP
rep-format )
ri= Requested Report Interval (plain-text; OPTIONAL; default is
"0"). The value is an unsigned 32-bit integer that specifies an
interval during which the report generator SHOULD NOT issue more
than one report about a given incident type. A value of "0"
requests a report for every incident. Where the requested
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interval is not zero, the agent generating a report SHOULD include
an "Incidents:" field in the generated report so the receiving
agent has some indication of how many reports were suppressed.
ABNF:
key-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT
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:
key-ro-type = ( "all" / "s" / "v" / "x" )
key-ro-tag = %x72 %x6f *WSP "=" *WSP key-ro-type *WSP 0* ( ":"
*WSP key-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.
ABNF:
key-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 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 format of this reply MUST be in the format specified
by the "rf=" tag defined below. If only a local-part is provided,
then 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
ADSP.
ABNF:
adsp-r-tag = %x72 *WSP "=" qp-section
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 that is taken from the
registered list of DKIM report formats. See Section 7 for a
description of the registry and Section 6 for a description of
recognized formats. The verifier generating reports MUST generate
a report using the first token in the list that represents a
report format it is capable of generating.
ABNF:
adsp-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP
rep-format )
ri= Requested Report Interval (plain-text; OPTIONAL; default is
"0"). The value is an unsigned 32-bit integer that specifies an
interval during which the report generator SHOULD NOT issue more
than one report about a given type of incident should be
generated. A value of "0" requests a report for every incident.
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Where the requested interval is not zero, the agent generating a
report SHOULD include an "Incidents:" field in the generated
report so the receiving agent has some indication of how many
reports were suppressed.
ABNF:
adsp-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT
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.
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 sender 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 but do not match the published [ADSP] policy.
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6. Reporting Formats
This section lists reporting formats supported by this reporting
mechanism. Currently only two formats are supported:
arf: Abuse Reporting Format, as defined in [ARF] and extended in
RFCxxxx.
smtp: An [SMTP] error with a string descriptive of the problem that
caused the sender authentication to fail. This explicitly
requests evaluation of sender authentication concurrent with the
SMTP session, and rejection (if appropriate) whenever possible
rather than acceptance of the message and later generation of a
feedback report of some kind (e.g. "arf" above) when verification
fails. The presence of an "rs" tag (see Section 3 and Section 4)
further requests a specific substring be included in the reply to
ease automatic handling of such errors by sending or relaying
MTAs.
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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 Key Tag Registration
IANA is requested to update the DKIM Key Tag Specification Registry
to include the following new items:
+------+-----------------+---------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+---------+
| r | (this document) | current |
| rf | (this document) | current |
| ri | (this document) | current |
| ro | (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 |
| rf | (this document) | current |
| ri | (this document) | current |
| ro | (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
Implementors 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
DSNs of any kind 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 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. 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 sender 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 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
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discovered data in the DNS.
8.4. Envelope Sender Selection
In the case of transmitted reports in the form of a new message, 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:<>".
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] 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,
October 2008.
[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: Monica Chew, Dave Crocker,
Tim Draegen and JD Falk.
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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
A DKIM key record including use of the extensions defined by this
memo:
v=DKIM1; k=rsa; t=y; r=dkim-errors; rf=arf; ro=v:x; p=MIGfMA0GCS
qGSIb3DQEBAQUAA4GNADCBiQKBgQDh2vbhJTijCs2qbyJcwRCa8WqDTxI+PisFJo
faPtoDJy0Qn41uNayCajfKADVcLqc87sXQS6GxfchPfzx7Vh9crYdxRbN/o/URCu
ZsKmym1i1IPTwRLcXSnuKS0XDs1eRW2WQHGYlXksUDqSHWOS3ZO1W5t/FLcZHpIl
l/80xs4QIDAQAB
Example 1: DKIM key record using these extensions
This example DKIM key record contains the following data in addition
to the basic DKIM key data:
o Reports about signature evaluation failures should be send to the
address "dkim-errors" at the sender's domain;
o The sender's domain requests reports in the "arf" format;
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; rf=arf; 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;
o The sender's domain requests reports in the "arf" format;
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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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