MARF Working Group S. Kitterman
Internet-Draft Authentication Metrics
Intended status: Standards Track June 27, 2011
Expires: December 29, 2011
SPF Authentication Failure Reporting using the Abuse Report Format
draft-ietf-marf-spf-reporting-00
Abstract
This memo presents extensions to the Abuse Reporting Format (ARF),
and Sender Policy Framework (SPF) specifications to allow for
detailed reporting of message authentication failures in an on-demand
fashion.
Status of this Memo
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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 SPF . . . . . . . . . . . . . . 5
4. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Requested Reports for SPF Failures . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. SPF Modifier Registration . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Inherited Considerations . . . . . . . . . . . . . . . . . 9
6.2. Reports From Unrelated Domains . . . . . . . . . . . . . . 9
6.3. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 9
6.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 10
6.5. Envelope Sender Selection . . . . . . . . . . . . . . . . 10
6.6. Reporting Multiple Incidents . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 15
B.1. Minimal SPF DNS record change to add a reporting
address . . . . . . . . . . . . . . . . . . . . . . . . . 15
B.2. SPF DNS record with reporting address, report
interval, and requested report type . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
[ARF] defines a message format for sending reports of abuse in the
messaging infrastructure, with an eye toward automating both the
generating and consumption of those reports.
[SPF] is one mechanism for message sender authentication; it is
"path-based" meaning it authenticates the route that a message took
from origin to destination. As with other email authentication
methods, like [DKIM], the output is a verified domain name that can
then be subjected to some sort of evaluation process (e.g.,
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,
unauthorized path traversals (SPF failures), and conformance failures
involving the published signing practices (e.g., [ADSP]) of an
Administrative Mail Domain (ADMD; see [EMAIL-ARCH]).
This document extends [SPF] to add an optional reporting address and
an optional means of specifying a desired report format and other
parameters. Extension of [ARF] to add features required for the
reporting of these incidents is covered in [I-D.MARF-DKIM-REPORTING].
This document additionally creates a an IANA registry of [SPF] record
modifiers to avoid modidier namespace collisions.
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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].
"local-part" is defined in [MAIL].
"addr-spec" is defined in [MAIL].
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3. Optional Reporting Address for SPF
There exist cases in which a domain name owner employing [SPF] for
announcing sending practises may want to know when messages are
received via unauthorized routing. Currently there is no such
mechanism defined.
This document adds the following optional "modifier" (as defined in
Section 4.6.1 of [SPF]) to SPF records, using the form defined in
that specification:
r= Reporting Address (plain-text; OPTIONAL; no default). MUST be a
local-part or addr-spec (see Section 3.4.1 of [MAIL])\ containing
an e-mail address to which a report SHOULD be sent when mail
claiming to be from this domain (see Section 2.4 of [SPF] for a
description of how domains are identified for SPF checks) has
failed the evaluation algorithm described in [SPF], in particular
because a message arrived via an unauthorized route . 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 SPF record.
r= modifiers in a record that was reached by following an include:
mechanism MUST be ignored.
ABNF:
spf-report-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 report formats. See Section 5 for a
description of the registry and [I-D.MARF-DKIM-REPORTING] 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. If
no supported formats are requested, then no report is sent.
ABNF:
spf-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP
rep-format )
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ri= Requested Report Interval (plain-text; OPTIONAL; default is
"0"). The value is an unsigned 32-bit integer that specifies an
interval, i.e. a number of reports, 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. 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:
spf-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 4.1 for a
list of valid tags.
ABNF:
spf-ro-type = ( "all" / "e" / "f" / "s" )
spf-ro-tag = %x72 %x6f *WSP "=" *WSP spf-ro-type *WSP 0* ( ":"
*WSP spf-ro-type )
rs= Requested SMTP Error String (plain-text; OPTIONAL; no default).
The value is a string the sending domain requests be included in
[SMTP] error strings when messages are rejected.
ABNF:
spf-rs-tag = %x72 %x73 *WSP "=" qp-section
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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 that do
not match a requested report, and MUST ignore requests for reports
not included in this these lists.
4.1. Requested Reports for SPF Failures
The following report requests are defined for SPF keys:
all All reports are requested.
e Reports are requested for messages that produced an SPF result of
"TempError" or "PermError.
f Reports are requested for messages that produced an SPF result of
"Fail".
s Reports are requested for messages that produced an SPF result of
"SoftFail".
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5. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new [SPF] tags.
5.1. SPF Modifier Registration
IANA is requested to create the Sender Policy Framework Modifier
Registry, to include a list of all registered SPF modifier names and
their defining documents.
New registrations or updates MUST be published in accordance with the
"Specification Required" guidelines as described in
[IANA-CONSIDERATIONS]. New registrations and updates MUST contain
the following information:
1. Name of the modifier being registered or updated
2. The document in which the specification of the modifier is
published
3. New or updated status, which MUST be one of:
current: The field is in current use
deprecated: The field is in current use but its use is
discouraged
historic: The field is no longer in current use
An update may make a notation on an existing registration indicating
that a registered field is historic or deprecated if appropriate.
+------------+-----------------+---------+
| MODIFIER | REFERENCE | STATUS |
+------------+-----------------+---------+
| exp | RFC4408 | current |
| redirect | RFC4408 | current |
| r | (this document) | current |
| rf | (this document) | current |
| ri | (this document) | current |
| ro | (this document) | current |
| rs | (this document) | current |
+------------+-----------------+---------+
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6. Security Considerations
Security issues with respect to these reports are similar to those
found in [DSN].
6.1. Inherited Considerations
Implementors are advised to consider the Security Considerations
sections of [SPF] and [ARF].
6.2. Reports From Unrelated Domains
SPF records can be used by other domains via include mechanisms and
redirect modifiers. If reporting addresses included in these records
are specified with a full addr-spec then reports for other,
potentially unrelated, domains may be reported to this address. In
theory, malicious senders might use this as a path for generating
large numbers of feedback reports. To mitigate this issue, specify
reporting addresses with a local-part so that reports will be
directed to the original domain from which the message causing the
feedback report was sent.
6.3. 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
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 authentication information, such as result, domain,
etc.
Perhaps the simplest means of mitigating this threat is to assert
that these reports should themselves be signed with something like
DKIM or sent from sources authorized by SPF. 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.
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6.4. 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 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
discovered data in the DNS.
6.5. 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:<>". The HELO/EHLO command SHOULD pass [SPF] HELO checks.
6.6. 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.
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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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7. References
7.1. Normative References
[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.
[I-D.MARF-DKIM-REPORTING]
Kucherawy, M., "Extensions to DKIM for Failure Reporting",
May 2011.
[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.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[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.
7.2. Informative References
[ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
Sender Signing Practises", RFC 5617, August 2009.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
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for Delivery Status Notifications", RFC 3464,
January 2003.
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Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Murray Kucherawy and Tim
Draegen.
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Appendix B. Examples
B.1. Minimal SPF DNS record change to add a reporting address
v=spf1 mx:example.org r=postmaster -all
B.2. SPF DNS record with reporting address, report interval, and
requested report type
v=spf1 mx:example.org r=postmaster@example.net rf=arf ri=10 ro=e -all
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Author's Address
Scott Kitterman
Authentication Metrics
3611 Scheel Dr
Ellicott City, MD 21042
US
Phone: +1 301 325 5475
Email: skitterman@authmetrics.com
Kitterman Expires December 29, 2011 [Page 16]
