MARF Working Group S. Kitterman
Internet-Draft Agari Data, Inc.
Intended status: Standards Track January 25, 2012
Expires: July 28, 2012
SPF Authentication Failure Reporting using the Abuse Report Format
draft-ietf-marf-spf-reporting-04
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. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Envelope Sender Selection . . . . . . . . . . . . . . . . 9
6.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 14
B.1. Minimal SPF DNS record change to add a reporting
address . . . . . . . . . . . . . . . . . . . . . . . . . 14
B.2. SPF DNS record with reporting address, report
percentage, and requested report type . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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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
other parameters. Extension of [ARF] to add features required for
the reporting of these incidents is covered in
[I-D.MARF-AUTHFAILURE-REPORT].
This document additionally creates a an IANA registry of [SPF] record
modifiers to avoid modifier 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 defined in [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 practices may want to know when messages are
received via unauthorized routing. Currently there is no such method
defined in conjunction with standardized approaches such as [ARF].
Similar information can be gathered using a specially crafted [SPF]
record and a special DNS server to track [SPF] record lookups.
This document defines the following optional "modifier" (as defined
in Section 4.6.1 of [SPF]) to SPF records, using the form defined in
that specification:
ra= Reporting Address (plain-text; OPTIONAL; no default). MUST be a
local-part (see Section 3.4.1 of [MAIL]) specifying 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. To generate a complete
address to which the report is sent, the verifier simply appends
to this value an "@" followed by the SPF domain per paragraph 4.1
of [SPF]. r= modifiers in a record that was reached by following
an include: mechanism MUST be ignored.
ABNF:
spf-report-tag = %x72.61 "=" 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 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.
ABNF:
spf-rp-tag = %x72.69 *WSP "=" *WSP 1*12DIGIT "/" 1*12DIGIT
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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ABNF:
spf-ro-type = ( "all" / "e" / "f" / "s" / "n" )
spf-ro-tag = %x72.6f "=" spf-ro-type 0* ( ":" spf-ro-type )
In the absence of an 'r=' tag in the SPF record, all other fields
defined above MUST be ignored.
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4. Requested Reports
This memo also includes, as the "ro" tokens 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 results:
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".
n Reports are requested for messages that produced an SPF result of
"Neutral".
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5. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new [SPF] modifiers.
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 |
| ra | (this document) | current |
| rp | (this document) | current |
| ro | (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], [ARF], and [I-D.MARF-AUTHFAILURE-REPORT].
6.2. 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 result information, such as [SPF] result type, domain,
etc.
Perhaps the simplest means of mitigating this threat is to assert
that these reports should themselves pass [SPF] checks and/or use
other email authentication technologies such as [DKIM].
6.3. 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
MUST 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 feed back reports will be
issued in response to the report itself.
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 will pass
[SPF] HELO checks.
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6.4. Automatic Generation
Automatic generation of these reports by checking 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. The spf-rp-tag provides a mechanism to
communicate appropriate limits.
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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.
[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", 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.
[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.
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[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, Tim
Draegen, Julian Mehnle, and John Levine.
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Appendix B. Examples
B.1. Minimal SPF DNS record change to add a reporting address
v=spf1 mx:example.org ra=postmaster -all
B.2. SPF DNS record with reporting address, report percentage, and
requested report type
v=spf1 mx:example.org -all ra=postmaster rp=10 ro=e
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Author's Address
Scott Kitterman
Agari Data, Inc.
3611 Scheel Dr
Ellicott City, MD 21042
US
Phone: +1 301 325 5475
Email: skitterman@agari.com
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