Internet DRAFT - draft-davids-dmarc-fi-tag
draft-davids-dmarc-fi-tag
DMARC Working Group M. Davids
Internet-Draft SIDN Labs
Updates: 7489 (if approved) May 16, 2017
Intended status: Standards Track
Expires: November 17, 2017
DMARC Failure reporting Interval tag
draft-davids-dmarc-fi-tag-02
Abstract
This document extends the DMARC (RFC7489) record format by defining
an additional tag. This new tag, the "fi" tag, is to be used in
conjunction with the "ruf" tag used for message-specific failure
reporting. It provides a Domain Owner with a simple way of
requesting limitation of the rate at which such reports are sent.
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 17, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Extension to the General Record Format . . . . . . . . . . . 3
4. Formal Definition . . . . . . . . . . . . . . . . . . . . . . 4
5. Domain Owner Example . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
DMARC [RFC7489] enables Domain Owners to request for detailed failure
reports for individual messages by means of the "ruf" tag. There may
be various reasons to permanently configure such a "ruf" tag. For
example to facilitate reputation management, monitoring or simply for
research or operational purposes.
Failure reports are normally generated and sent almost immediately
after the Mail Receiver detects a DMARC failure. These reports are
useful for quickly notifying the Domain Owners of an authentication
failure, without waiting for an aggregate report. However, under
certain circumstances this property can potentially lead to an
undesirably high volume of reports. Especially when a Domain Owner's
name is spoofed and abused in a large-scale phishing or other
impersonation attack.
DMARC [RFC7489] Section 7.3 leaves it to the discretion of
participating Mail Receivers and report generators if and how they
take measures against sending high volumes of failure reports.
However, what a Mail Receiver or report generator considers
acceptable may exceed the capacity of the receiving Domain Owner.
The lack of a mechanism for a Domain Owner to influence the volume of
reports sent by any particular report generator constitutes an
obstacle to deployment of the "ruf" tag feature.
This document updates [RFC7489] by defining the "fi" tag, a mechanism
that allows the Domain Owner to request the limitation of failure
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reports of no more than one failure report per report generator per
time interval and discard the remainder.
2. Conventions Used In This Document
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 [RFC2119] when they
appear in ALL CAPS. These words may also appear in this document in
lower case as plain English words, absent their normative meanings.
The following terms are used, as defined in DMARC [RFC7489].
Domain Owner and Mail Receiver.
Also the term "report generator" is applied here the same way as in
DMARC [RFC7489].
3. Extension to the General Record Format
The following tag is introduced as an additional valid DMARC tag for
use in conjunction with the Reporting URI for Failure ("ruf") tag:
fi:
Interval requested between message-specific failure reports
(plain-text 32-bit unsigned integer; OPTIONAL; default is "60"; if
defined as "0", then there is no rate limitation requested,
thereby mimicking report generators that do not support this tag).
Indicates a request to report generators to send message-specific
failure reports at an interval of approximately the requested
number of seconds.
Any intermediate remaining reports SHOULD NOT be sent and MAY be
discarded, if generated at all. But discarding message-specific
failure reports as a consequence of this tag, SHALL NOT affect the
incident counts in the aggregated feedback reports.
A report generator MAY include in the message-specific failure report
an indication of the number of reports being discarded since the last
issued report. Where AFRF [RFC6650] is used, the Abuse Reporting
Format [RFC5965] optional "Incidents"-field may be used for this
purpose.
Report generators that choose to adhere to the "ruf" tag option,
SHOULD also adhere to the requested "fi" tag setting defined here.
This tag's content SHALL be ignored if a "ruf" tag is not also
specified, or if the syntax of the "fi" integer is invalid.
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Report generators that implement this feature MUST be able to support
the entire interval range from 0-86400 and MAY support longer
intervals. However, anything longer dan 86400 is understood to be
accommodated on a best-effort basis.
4. Formal Definition
The formal definition of the "fi" tag format, using ABNF [RFC5234],
is as follows:
Introduced:
dmarc-finterval = "fi" *WSP "=" *WSP 1*DIGIT
Which changes the dmarc-record definition to:
dmarc-record = dmarc-version dmarc-sep
[dmarc-request]
[dmarc-sep dmarc-srequest]
[dmarc-sep dmarc-auri]
[dmarc-sep dmarc-furi]
[dmarc-sep dmarc-adkim]
[dmarc-sep dmarc-aspf]
[dmarc-sep dmarc-ainterval]
[dmarc-sep dmarc-finterval]
[dmarc-sep dmarc-fo]
[dmarc-sep dmarc-rfmt]
[dmarc-sep dmarc-percent]
[dmarc-sep]
; components other than dmarc-version and
; dmarc-request may appear in any order
5. Domain Owner Example
The DMARC policy record with the "fi" tag might look like this when
retrieved using a common command-line tool:
% dig +short TXT _dmarc.example.com.
"v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
ruf=mailto:auth-reports@example.com; fi=300;"
To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone file
(following the conventional zone file format):
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; DMARC record for the domain example.com
_dmarc IN TXT ( "v=DMARC1; p=none; "
"rua=mailto:dmarc-feedback@example.com; "
"ruf=mailto:auth-reports@example.com; fi=300; " )
The request implies that the Domain Owner is willing to accept no
more than one message-specific failure report every 5 minutes from
any report generator. Any optionally defined indications for the
maximum report size in the URI will continue to work as defined in
[RFC7489].
A report generator in this example would typically honour the "fi"
tag by sending out a report, storing a 'last report sent' timestamp
for example.com in memory and maintaining it as a 'do not sent' flag
for a minimum of 300 seconds during which period no consecutive
reports are to be sent. After the flag has cleared, a report may
again be sent. The cycle then repeats.
Intermediate, unsent reports are discarded. But they do add to
statistical counters as if they were sent. So their details are part
of any corresponding aggregated reports.
In AFRF, a message-specific report to the Domain Owner may contain
this output (some parts left out for readability):
Feedback-Type: auth-failure
User-Agent: SomeGenerator/1.0
Version: 1.0
Original-Mail-From: <somespammer@example.com>
Source-IP: 2001:db8::198:51:100:25
Auth-Failure: spf
Reported-Domain: example.com
Incidents: 600
As a result of the DMARC record above, when a spammer sends two
messages per second and fi=300, a report generator would produce only
one message-specific failure report per 5 minutes, instead of 600.
The "Incidents" field in the report shows that this report represents
599 other incidents as well. These will count in the daily
aggregated feedback report, but where disregarded and not sent out as
individual message-specific failure reports.
6. IANA Considerations
As per [RFC7489 p.17] Section 6.3 last paragraph, a new version of
DMARC is not required. Older implementations that consider the "fi"
tag as unknown, will ignore it.
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However, this document requires an update to the IANA [RFC5226] DMARC
Tag Registry [1]:
Tag Name | Description
---------+---------------------------
fi | Failure Reporting interval
7. Security Considerations
The Domain Owner should be aware that defining a "fi" tag involves a
trade-off between the benefit of preventing unmanageable incoming
report flows and the risk of not receiving potentially useful data.
A large scale attack that triggers reporting rate limitation, might
result in the non-dispatch of reports regarding other events
involving the same domain and the same Mail Receiver occuring at the
same.
An attack can involve many different report generators. The Domain
Owner should be aware that the "fi" tag limits reporting by each
individual report generator. Multiple report generators might still
collectively generate a large volume of reports. Mail Receivers with
a farm or cluster of several report generators might choose to
synchronise the 'last sent' timestamp value accross their machines in
order to better comply with the wishes of Domain Owners and to reduce
the risk described above.
An attack can also involve multiple domains belonging to a single
Domain Owner. The "fi" tag applies to an individual domain, so the
deliberate abuse of multiple spoofed domains belonging to Domain
Owner, might still generate a high volumes of message-specific
failure reports.
It therefore makes sense to define a relatively short TTL for DMARC-
records, to allow for the possibility of increasing the "fi"-value on
an ad hoc basis, or to remove the "ruf" (and "fi") tag altogether in
the event of a problem. This aspect is also mentiond in [RFC7489
p.40] Section 10.2, second paragraph.
The security of the DMARC TXT-record, which the "fi" tag part of,
depends on the security of the underlying DNS infrastructure. In
that respect it is advisable to make use of DNSSEC [RFC4033].
8. Discussion
The DMARC virtual verification draft [draft-akagiri-dmarc-virtual-
verification] discusses possible values for the "ruf" tag. The
authors of that draft are kindly requested to take this draft into
consideration as part of their discussions.
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9. Acknowledgments
The author would like to thank Elizabeth Zwicky, Moritz Mueller,
Maarten Wullink, Cristian Hesselman, Bart Knubben, Rolf Sonneveld and
Murray Kucherawy for their valuable feedback.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5965] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", RFC 5965,
DOI 10.17487/RFC5965, August 2010,
<http://www.rfc-editor.org/info/rfc5965>.
[RFC6650] Falk, J. and M. Kucherawy, Ed., "Creation and Use of Email
Feedback Reports: An Applicability Statement for the Abuse
Reporting Format (ARF)", RFC 6650, DOI 10.17487/RFC6650,
June 2012, <http://www.rfc-editor.org/info/rfc6650>.
10.2. Informative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<http://www.rfc-editor.org/info/rfc7489>.
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10.3. URIs
[1] https://www.iana.org/assignments/dmarc-parameters/dmarc-
parameters.xhtml
Author's Address
Marco Davids
SIDN
Meander 501
Arnhem 6825 MD
NL
Phone: +31 26 352 5500
Email: marco.davids@sidn.nl
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