DMARC Working Group M. Davids Internet-Draft SIDN Labs Updates: 7489 (if approved) October 27, 2016 Intended status: Standards Track Expires: April 30, 2017 DMARC Failure reporting Interval tag draft-davids-dmarc-fi-tag-00 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. It enables a simple way of rate limiting the message-specific failure reporting on the request of a Domain Owner. 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 http://datatracker.ietf.org/drafts/current/. 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 April 30, 2017. Copyright Notice Copyright (c) 2016 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 (http://trustee.ietf.org/license-info) 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 Davids Expires April 30, 2017 [Page 1]
Internet-Draft DMARC-fi October 2016 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 . . . . . . . . . . . . . . . . . . . . . . 3 5. Domain Owner Example . . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 10.1. Normative References . . . . . . . . . . . . . . . . . . 5 10.2. Informative References . . . . . . . . . . . . . . . . . 6 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Domain Owners may have various reasons to permanently configure a "ruf" tag. For example in the case of reputation management, monitoring or research this can be seen as useful functionality. DMARC [RFC7489] per-message failure reports ("ruf") are sent almost immediately and in a non-aggregated manner. Under certain circumstances this can potentially lead to an undesirable high volume of reports. Especially in the case where the 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 to take measures against sending high volumes of failure reports. But their mileage may vary and the measures they take, if any, may not meet the criteria of the Domain Owner at the receiving end. The lack of a mechanism to influence the volume of reports from a Domain Owners' perspective, constitutes an obstacle for deployment of this feature. This document updates [RFC7489] by defining the "fi" tag, a mechanism that allows the Domain Owner to request a limitation of no more than one failure report per report generator per time interval. Davids Expires April 30, 2017 [Page 2]
Internet-Draft DMARC-fi October 2016 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 also used, as defined in DMARC [RFC7489]. Domain Owner and Mail Receiver. 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 limiting requested). Indicates a request to report generators to send per-message failure reports with an interval of approximately the requested number in seconds. But no more than that. Any intermediate remaining reports SHOULD NOT be sent and MAY be disgarded if generated at all. But disregarding per-message failure reports as a consequence of this tag, SHALL NOT affect the statistical information in aggregated feedback repports. 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 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 Updated: Davids Expires April 30, 2017 [Page 3]
Internet-Draft DMARC-fi October 2016 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 implicates that the Domain Owner is willing to accept no more than one per-message failure report every 5 minutes from any report generator. 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 of the IANA [RFC5226] DMARC Tag Registry [1]: Davids Expires April 30, 2017 [Page 4]
Internet-Draft DMARC-fi October 2016 Tag Name | Description ---------+--------------------------- fi | Failure Reporting interval 7. Security Considerations The Domain Owner should be aware that defining a "fi" tag is a trade- off between too much reports and possibly missing out on some details. A large scale attack that triggers the rate limiting, might block the generation of reports for other events on the same domain to the same Mail Receiver. An attack could involve many different report generators. The Domain Owner should be aware that the "fi" tag is on a per report generator basis. Combined, multiple report generators might still generate a considerable amount of reports. An attack could also involve multiple domains of one particular Domain Owner. The "fi" tag is on a per domain basis, so a deliberate abuse of multiple spoofed domains of one Domain Owner, might still generate high volumes of per-message failure reports. Therefore it makes sense to define a relatively short TTL on 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 case of a problem. The security of the DMARC TXT-record of which the "fi" tag is a part, rests on the security of the underlying DNS infrastructure. In that respect is 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 TBD 10. References 10.1. Normative References Davids Expires April 30, 2017 [Page 5]
Internet-Draft DMARC-fi October 2016 [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>. 10.2. Informative References [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>. 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 Davids Expires April 30, 2017 [Page 6]