DMARC Failure reporting Interval tag
draft-davids-dmarc-fi-tag-01

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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This Internet-Draft will expire on May 28, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Conventions Used In This Document
• 3. Extension to the General Record Format
• 4. Formal Definition
• 5. Domain Owner Example
• 6. IANA Considerations
• 7. Security Considerations
• 8. Discussion
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Author's Address

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 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.

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; if not defined or 0, then there is no rate limitation requested). 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 completeness of information in the aggregated feedback reports.

A report generator MAY include in the message-specific failure report an indication of the number of reports discarde since the last issued report. Where AFRF [RFC6591] is used, the Abuse Reporting Format [RFC5965] optional "Incidents"-field may be used to indicate the number of discarded reports.

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.

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):

; 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. 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.

Any optionally defined indications for the maximum report size in the URI will continue to work as defined in [RFC7489].

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 to the same Mail Receiver.

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 it 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 even of a problem.

[TODO: mention the part hereafter, or is it out of scope for this draft?] An attacker that enforces message-specific detailed failure ("ruf") reports that are larger than an optionally-defined maximum-size specification, may leave the Domain Owner in the darks, because no reports will be sent.

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.

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 and Bart Knubben for their valuable feedback.

10. References

10.1. Normative References

10.2. Informative References

Author's Address