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


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

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 17, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

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 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:


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.

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:


 dmarc-finterval = "fi" *WSP "=" *WSP 1*DIGIT

Which changes the dmarc-record definition to:

 dmarc-record    = dmarc-version dmarc-sep
                   [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]
                   ; 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
 "v=DMARC1; p=none;;; 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):

; DMARC record for the domain

_dmarc  IN TXT ( "v=DMARC1; p=none; "
                 "; "
                 "; 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 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: <>
Source-IP: 2001:db8::198:51:100:25
Auth-Failure: spf
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.

However, this document requires an update to the IANA [RFC5226] DMARC Tag Registry:

       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.

9. Acknowledgments

The author would like to thank Elizabeth Zwicky, Moritz Müller, 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.
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5965] Shafranovich, Y., Levine, J. and M. Kucherawy, "An Extensible Format for Email Feedback Reports", RFC 5965, DOI 10.17487/RFC5965, August 2010.
[RFC6650] Falk, J. and M. Kucherawy, "Creation and Use of Email Feedback Reports: An Applicability Statement for the Abuse Reporting Format (ARF)", RFC 6650, DOI 10.17487/RFC6650, June 2012.

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.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.

Author's Address

Marco Davids SIDN Meander 501 Arnhem, 6825 MD NL Phone: +31 26 352 5500 EMail: