Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-01
The information below is for an old version of the document.
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| Authors | Todd Herr , John R. Levine | ||
| Last updated | 2021-04-23 | ||
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draft-ietf-dmarc-dmarcbis-01
DMARC T. Herr (ed)
Internet-Draft Valimail
Obsoletes: 7489 (if approved) J. Levine (ed)
Intended status: Standards Track Standcore LLC
Expires: 25 October 2021 23 April 2021
Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-01
Abstract
This document describes the Domain-based Message Authentication,
Reporting, and Conformance (DMARC) protocol.
_Tickets 75, 80, 85, 96, and 108_
DMARC permits the owner of an email author's domain name to enable
validation of the domain's use, to indicate the Domain Owner's or
Public Suffix Operator's severity of concern regarding failed
validation, and to request reports about use of the domain name.
Mail receiving organizations can use this information when evaluating
handling choices for incoming mail.
This document obsoletes RFC 7489.
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-
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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 25 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. High-Level Goals . . . . . . . . . . . . . . . . . . . . 7
2.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. Anti-Phishing . . . . . . . . . . . . . . . . . . . . . . 8
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 8
3.1. Conventions Used in This Document . . . . . . . . . . . . 8
3.2. Authenticated Identifiers . . . . . . . . . . . . . . . . 9
3.3. Author Domain . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Domain Owner . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Identifier Alignment . . . . . . . . . . . . . . . . . . 9
3.6. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 9
3.7. Mail Receiver . . . . . . . . . . . . . . . . . . . . . . 10
3.8. Non-existent Domains . . . . . . . . . . . . . . . . . . 10
3.9. Organizational Domain . . . . . . . . . . . . . . . . . . 10
3.10. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 10
3.11. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 10
3.12. PSO Controlled Domain Names . . . . . . . . . . . . . . . 10
3.13. Report Receiver . . . . . . . . . . . . . . . . . . . . . 10
3.14. More on Identifier Alignment . . . . . . . . . . . . . . 11
3.14.1. DKIM-Authenticated Identifiers . . . . . . . . . . . 12
3.14.2. SPF-Authenticated Identifiers . . . . . . . . . . . 12
3.14.3. Alignment and Extension Technologies . . . . . . . . 13
3.15. Determining The Organizational Domain . . . . . . . . . . 13
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Authentication Mechanisms . . . . . . . . . . . . . . . . 14
4.2. Key Concepts . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Flow Diagram . . . . . . . . . . . . . . . . . . . . . . 15
5. Use of RFC5322.From . . . . . . . . . . . . . . . . . . . . . 17
6. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. DMARC Policy Record . . . . . . . . . . . . . . . . . . . 18
6.2. DMARC URIs . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. General Record Format . . . . . . . . . . . . . . . . . . 19
6.4. Formal Definition . . . . . . . . . . . . . . . . . . . . 23
6.5. Domain Owner Actions . . . . . . . . . . . . . . . . . . 24
6.5.1. Publish an SPF Policy for an Aligned Domain . . . . . 24
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6.5.2. Configure Sending System for DKIM Signing Using an
Aligned Domain . . . . . . . . . . . . . . . . . . . 25
6.5.3. Setup a Mailbox to Receive Aggregate Reports . . . . 25
6.5.4. Publish a DMARC Policy for the Author Domain . . . . 25
6.5.5. Collect and Analyze Reports and Adjust
Authentication . . . . . . . . . . . . . . . . . . . 26
6.5.6. Decide If and When to Update DMARC Policy . . . . . . 26
6.6. PSO Actions . . . . . . . . . . . . . . . . . . . . . . . 26
6.7. Mail Receiver Actions . . . . . . . . . . . . . . . . . . 26
6.7.1. Extract Author Domain . . . . . . . . . . . . . . . . 26
6.7.2. Determine Handling Policy . . . . . . . . . . . . . . 27
6.7.3. Policy Discovery . . . . . . . . . . . . . . . . . . 28
6.7.4. Store Results of DMARC Processing . . . . . . . . . . 30
6.7.5. Send Aggregate Reports . . . . . . . . . . . . . . . 31
6.8. Policy Enforcement Considerations . . . . . . . . . . . . 31
7. DMARC Feedback . . . . . . . . . . . . . . . . . . . . . . . 32
8. Minimum Implementations . . . . . . . . . . . . . . . . . . . 33
9. Other Topics . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1. Issues Specific to SPF . . . . . . . . . . . . . . . . . 34
9.2. DNS Load and Caching . . . . . . . . . . . . . . . . . . 34
9.3. Rejecting Messages . . . . . . . . . . . . . . . . . . . 35
9.4. Identifier Alignment Considerations . . . . . . . . . . . 36
9.5. Interoperability Issues . . . . . . . . . . . . . . . . . 36
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
10.1. Authentication-Results Method Registry Update . . . . . 36
10.2. Authentication-Results Result Registry Update . . . . . 38
10.3. Feedback Report Header Fields Registry Update . . . . . 39
10.4. DMARC Tag Registry . . . . . . . . . . . . . . . . . . . 39
10.5. DMARC Report Format Registry . . . . . . . . . . . . . . 41
10.6. Underscored and Globally Scoped DNS Node Names
Registry . . . . . . . . . . . . . . . . . . . . . . . . 42
11. Security Considerations . . . . . . . . . . . . . . . . . . . 42
11.1. Authentication Methods . . . . . . . . . . . . . . . . . 42
11.2. Attacks on Reporting URIs . . . . . . . . . . . . . . . 42
11.3. DNS Security . . . . . . . . . . . . . . . . . . . . . . 43
11.4. Display Name Attacks . . . . . . . . . . . . . . . . . . 43
11.5. External Reporting Addresses . . . . . . . . . . . . . . 44
11.6. Secure Protocols . . . . . . . . . . . . . . . . . . . . 45
12. Normative References . . . . . . . . . . . . . . . . . . . . 45
13. Informative References . . . . . . . . . . . . . . . . . . . 46
Appendix A. Technology Considerations . . . . . . . . . . . . . 48
A.1. S/MIME . . . . . . . . . . . . . . . . . . . . . . . . . 48
A.2. Method Exclusion . . . . . . . . . . . . . . . . . . . . 49
A.3. Sender Header Field . . . . . . . . . . . . . . . . . . . 49
A.4. Domain Existence Test . . . . . . . . . . . . . . . . . . 50
A.5. Issues with ADSP in Operation . . . . . . . . . . . . . . 50
A.6. Organizational Domain Discovery Issues . . . . . . . . . 51
A.6.1. Public Suffix Lists . . . . . . . . . . . . . . . . . 52
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Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 52
B.1. Identifier Alignment Examples . . . . . . . . . . . . . . 52
B.1.1. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 52
B.1.2. DKIM . . . . . . . . . . . . . . . . . . . . . . . . 53
B.2. Domain Owner Example . . . . . . . . . . . . . . . . . . 54
B.2.1. Entire Domain, Monitoring Only . . . . . . . . . . . 55
B.2.2. Entire Domain, Monitoring Only, Per-Message
Reports . . . . . . . . . . . . . . . . . . . . . . . 56
B.2.3. Per-Message Failure Reports Directed to Third
Party . . . . . . . . . . . . . . . . . . . . . . . . 56
B.2.4. Subdomain and Multiple Aggregate Report URIs . . . . 58
B.3. Mail Receiver Example . . . . . . . . . . . . . . . . . . 60
B.4. Processing of SMTP Time . . . . . . . . . . . . . . . . . 60
B.5. Utilization of Aggregate Feedback: Example . . . . . . . 62
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 62
C.1. January 5, 2021 . . . . . . . . . . . . . . . . . . . . . 62
C.1.1. Ticket 80 - DMARCbis SHould Have Clear and Concise
Defintion of DMARC . . . . . . . . . . . . . . . . . 62
C.2. February 4, 2021 . . . . . . . . . . . . . . . . . . . . 63
C.2.1. Ticket 1 - SPF RFC 4408 vs 7208 . . . . . . . . . . . 63
C.3. February 10, 2021 . . . . . . . . . . . . . . . . . . . . 63
C.3.1. Ticket 84 - Remove Erroneous References to RFC3986 . 63
C.4. March 1, 2021 . . . . . . . . . . . . . . . . . . . . . . 63
C.4.1. Design Team Work Begins . . . . . . . . . . . . . . . 63
C.5. March 8, 2021 . . . . . . . . . . . . . . . . . . . . . . 63
C.5.1. Removed E. Gustafsson as editor . . . . . . . . . . 63
C.5.2. Ticket 3 - Two tiny nits . . . . . . . . . . . . . . 63
C.5.3. Ticket 4 - Definition of "fo" parameter . . . . . . . 64
C.6. March 16, 2021 . . . . . . . . . . . . . . . . . . . . . 64
C.6.1. Ticket 7 - ABNF for dmarc-record is slightly wrong . 64
C.6.2. Ticket 26 - ABNF for pct allows "999" . . . . . . . . 64
C.7. March 23, 2021 . . . . . . . . . . . . . . . . . . . . . 64
C.7.1. Ticket 75 - Using wording alternatives to
'disposition', 'dispose', and the like . . . . . . . 64
C.7.2. Ticket 72 - Remove absolute requirement for p= tag in
DMARC record . . . . . . . . . . . . . . . . . . . . 64
C.8. March 29, 2021 . . . . . . . . . . . . . . . . . . . . . 65
C.8.1. Ticket 54 - Remove or expand limits on number of
recipients per report . . . . . . . . . . . . . . . . 65
C.9. April 12, 2021 . . . . . . . . . . . . . . . . . . . . . 65
C.9.1. Ticket 50 - Remove ri= tag . . . . . . . . . . . . . 65
C.9.2. Ticket 66 - Define what it means to have implemented
DMARC . . . . . . . . . . . . . . . . . . . . . . . . 65
C.9.3. Ticket 96 - Tweaks to Abstract and Introduction . . . 65
C.10. April 13, 2021 . . . . . . . . . . . . . . . . . . . . . 65
C.10.1. Ticket 53 - Remove reporting message size
chunking . . . . . . . . . . . . . . . . . . . . . . 65
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C.10.2. Ticket 52 - Remove strict alignment (and adkim and
aspf tags) . . . . . . . . . . . . . . . . . . . . . 65
C.10.3. Ticket 47 - Remove pct= tag . . . . . . . . . . . . 66
C.10.4. Ticket 2 - Flow of operations text in dmarc-base . . 66
C.11. April 14, 2021 . . . . . . . . . . . . . . . . . . . . . 66
C.11.1. Ticket 107 - DMARCbis should take a stand on
multi-valued From fields . . . . . . . . . . . . . . 66
C.11.2. Ticket 82 - Deprecate rf= and maybe fo= tag . . . . 66
C.11.3. Ticket 85 - Proposed change to wording describing 'p'
tag and values . . . . . . . . . . . . . . . . . . . 66
C.12. April 15, 2021 . . . . . . . . . . . . . . . . . . . . . 66
C.12.1. Ticket 86 - A-R results for DMARC . . . . . . . . . 66
C.12.2. Ticket 62 - Make aggregate reporting a normative
MUST . . . . . . . . . . . . . . . . . . . . . . . . 67
C.13. April 19, 2021 . . . . . . . . . . . . . . . . . . . . . 67
C.13.1. Ticket 109 - Sanity Check DMARCbis Document . . . . 67
C.14. April 20, 2021 . . . . . . . . . . . . . . . . . . . . . 67
C.14.1. Ticket 108 - Changes to DMARCbis for PSD . . . . . . 67
C.15. April 22, 2021 . . . . . . . . . . . . . . . . . . . . . 67
C.15.1. Ticket 104 - Update the Security Considerations
section 11.3 on DNS . . . . . . . . . . . . . . . . . 67
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 67
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 68
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source for this draft is maintained in GitHub at:
https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis
(https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis)
_Tickets 80, 85, 96, and 108_
The Sender Policy Framework ([RFC7208]) and DomainKeys Identified
Mail ([RFC6376]) protocols provide domain-level authentication which
is not directly associated with the RFC5322.From domain, and DMARC
builds on those protocols. Using DMARC, Domain Owners that originate
email can publish a DNS TXT record with their email authentication
policies, state their level of concern for mail that fails
authentication checks, and request reports about email use of the
domain name. Similarly, Public Suffix Operators (PSOs) may do the
same for PSO Controlled Domain Names and non-existent subdomains of
the PSO Controlled Domain Name.
_Ticket 52_
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As with SPF and DKIM, DMARC authentication checks result in verdicts
of "pass" or "fail". A DMARC pass verdict requires not only that SPF
or DKIM pass for the message in question, but also that the domain
validated by the SPF or DKIM check is aligned with the RFC5322.From
domain. In the DMARC protocol, two domains are said to be "in
alignment" if they have the same Organizational Domain.
_Tickets 75, 80, 85, and 108_
A DMARC pass result indicates only that the RFC5322.From domain has
been authenticated in that message; there is no explicit or implied
value assertion attributed to a message that receives such a verdict.
A mail-receiving organization that performs a DMARC validation check
on inbound mail can choose to use the result and the published
severity of concern expresed by the Domain Owner or PSO for
authentication failures to inform its mail handling decision for that
message.
For a mail-receiving organization supporting DMARC, a message that
passes validation is part of a message stream that is reliably
associated with the Domain Owner and/or any, some, or all of the
Authenticated Identifiers. Therefore, reputation assessment of that
stream by the mail-receiving organization does not need to be
encumbered by accounting for unauthorized use of any domains. A
message that fails this validation cannot reliably be associated with
the Domain Owner's domain and its reputation.
_Tickets 80 and 108_
DMARC, in the associated [DMARC-Aggregate-Reporting] and
[DMARC-Failure-Reporting] documents, also describes a reporting
framework in which mail-receiving domains can generate regular
reports containing data about messages seen that claim to be from
domains that publish DMARC policies, and send those reports to one or
more addresses as requested by the Domain Owner's or PSO's DMARC
policy record.
Experience with DMARC has revealed some issues of interoperability
with email in general that require due consideration before
deployment, particularly with configurations that can cause mail to
be rejected. These are discussed in Section 9.
2. Requirements
Specification of DMARC is guided by the following high-level goals,
security dependencies, detailed requirements, and items that are
documented as out of scope.
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2.1. High-Level Goals
DMARC has the following high-level goals:
_Tickets 85 and 108_
* Allow Domain Owners and PSOs to assert their severity of concern
for authentication failures for messages purporting to have
authorship within the domain.
* Allow Domain Owners and PSOs to verify their authentication
deployment.
* Minimize implementation complexity for both senders and receivers,
as well as the impact on handling and delivery of legitimate
messages.
* Reduce the amount of successfully delivered spoofed email.
* Work at Internet scale.
2.2. Out of Scope
_Ticket 109_
Several topics and issues are specifically out of scope for this
work. These include the following:
* different treatment of messages that are not authenticated versus
those that fail authentication;
* evaluation of anything other than RFC5322.From header field;
* multiple reporting formats;
* publishing policy other than via the DNS;
* reporting or otherwise evaluating other than the last-hop IP
address;
* attacks in the From: header field, also known as "display name"
attacks;
* authentication of entities other than domains, since DMARC is
built upon SPF and DKIM, which authenticate domains; and
* content analysis.
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2.3. Scalability
Scalability is a major issue for systems that need to operate in a
system as widely deployed as current SMTP email. For this reason,
DMARC seeks to avoid the need for third parties or pre-sending
agreements between senders and receivers. This preserves the
positive aspects of the current email infrastructure.
Although DMARC does not introduce third-party senders (namely
external agents authorized to send on behalf of an operator) to the
email-handling flow, it also does not preclude them. Such third
parties are free to provide services in conjunction with DMARC.
2.4. Anti-Phishing
DMARC is designed to prevent bad actors from sending mail that claims
to come from legitimate senders, particularly senders of
transactional email (official mail that is about business
transactions). One of the primary uses of this kind of spoofed mail
is phishing (enticing users to provide information by pretending to
be the legitimate service requesting the information). Thus, DMARC
is significantly informed by ongoing efforts to enact large-scale,
Internet-wide anti-phishing measures.
Although DMARC can only be used to combat specific forms of exact-
domain spoofing directly, the DMARC mechanism has been found to be
useful in the creation of reliable and defensible message streams.
DMARC does not attempt to solve all problems with spoofed or
otherwise fraudulent email. In particular, it does not address the
use of visually similar domain names ("cousin domains") or abuse of
the RFC5322.From human-readable <display-name>.
_Ticket 108_
3. Terminology and Definitions
This section defines terms used in the rest of the document.
3.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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Readers are encouraged to be familiar with the contents of [RFC5598].
In particular, that document defines various roles in the messaging
infrastructure that can appear the same or separate in various
contexts. For example, a Domain Owner could, via the messaging
security mechanisms on which DMARC is based, delegate the ability to
send mail as the Domain Owner to a third party with another role.
This document does not address the distinctions among such roles; the
reader is encouraged to become familiar with that material before
continuing.
3.2. Authenticated Identifiers
Domain-level identifiers that are validated using authentication
technologies are referred to as "Authenticated Identifiers". See
Section 4.1 for details about the supported mechanisms.
3.3. Author Domain
The domain name of the apparent author, as extracted from the From:
header field.
3.4. Domain Owner
An entity or organization that owns a DNS domain. The term "owns"
here indicates that the entity or organization being referenced holds
the registration of that DNS domain. Domain Owners range from
complex, globally distributed organizations, to service providers
working on behalf of non-technical clients, to individuals
responsible for maintaining personal domains. This specification
uses this term as analogous to an Administrative Management Domain as
defined in [RFC5598]. It can also refer to delegates, such as Report
Receivers, when those are outside of their immediate management
domain.
_Ticket 52_
3.5. Identifier Alignment
When the domain in the address in the From: header field has the same
Organizational Domain as a domain validated by SPF or DKIM (or both),
it has Identifier Alignment. (see below)
3.6. Longest PSD
The term Longest PSD is defined in [DMARC-PSD].
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3.7. Mail Receiver
The entity or organization that receives and processes email.
Mail Receivers operate one or more Internet- facing Mail Transport
Agents (MTAs).
3.8. Non-existent Domains
For DMARC purposes, a non-existent domain is a domain for which there
is an NXDOMAIN or NODATA response for A, AAAA, and MX records. This
is a broader definition than that in [RFC8020].
3.9. Organizational Domain
The domain that was registered with a domain name registrar. In the
absence of more accurate methods, heuristics are used to determine
this, since it is not always the case that the registered domain name
is simply a top-level DNS domain plus one component (e.g.,
"example.com", where "com" is a top-level domain). The
Organizational Domain is determined by applying the algorithm found
in Section 3.15.
3.10. Public Suffix Domain (PSD)
The term Public Suffix Domain is defined in [DMARC-PSD].
3.11. Public Suffix Operator (PSO)
The term Public Suffix Operator is defined in [DMARC-PSD].
3.12. PSO Controlled Domain Names
The term PSO Controlled Domain Names is defined in [DMARC-PSD].
_Tickets 108 and 109_
3.13. Report Receiver
An operator that receives reports from another operator implementing
the reporting mechanisms described in this document. Such an
operator might be receiving reports about messages related to a
domain for which it is the Domain Owner or PSO, or reports about
messages related to another operator's domain. This term applies
collectively to the system components that receive and process these
reports and the organizations that operate them.
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3.14. More on Identifier Alignment
_Ticket 109_
Email authentication technologies authenticate various (and
disparate) aspects of an individual message. For example, DKIM
[RFC6376] authenticates the domain that affixed a signature to the
message, while SPF [RFC7208] can authenticate either the domain that
appears in the RFC5321.MailFrom (MAIL FROM) portion of [RFC5322] or
the RFC5321.EHLO/ HELO domain, or both. These may be different
domains, and they are typically not visible to the end user.
_Ticket 52_
DMARC authenticates use of the RFC5322.From domain by requiring that
it have the same Organizational Domain (be aligned with) as an
Authenticated Identifier. The RFC5322.From domain was selected as
the central identity of the DMARC mechanism because it is a required
message header field and therefore guaranteed to be present in
compliant messages, and most Mail User Agents (MUAs) represent the
RFC5322.From header field as the originator of the message and render
some or all of this header field's content to end users.
Thus, this field is the one used by end users to identify the source
of the message and therefore is a prime target for abuse. Many high-
profile email sources, such as email service providers, require that
the sending agent have authenticated before email can be generated.
Thus, for these mailboxes, the mechanism described in this document
provides recipient end users with strong evidence that the message
was indeed originated by the agent they associate with that mailbox,
if the end user knows that these various protections have been
provided.
Domain names in this context are to be compared in a case-insensitive
manner, per [RFC4343].
It is important to note that Identifier Alignment cannot occur with a
message that is not valid per [RFC5322], particularly one with a
malformed, absent, or repeated RFC5322.From header field, since in
that case there is no reliable way to determine a DMARC policy that
applies to the message. Accordingly, DMARC operation is predicated
on the input being a valid RFC5322 message object, and handling of
such non-compliant cases is outside of the scope of this
specification. Further discussion of this can be found in
Section 6.7.1.
_Ticket 52_
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Each of the underlying authentication technologies that DMARC takes
as input yields authenticated domains as their outputs when they
succeed.
3.14.1. DKIM-Authenticated Identifiers
_Ticket 52_
DMARC requires Identifier Alignment based on the result of a DKIM
authentication because a message can bear a valid signature from any
domain, including domains used by a mailing list or even a bad actor.
Therefore, merely bearing a valid signature is not enough to infer
authenticity of the Author Domain.
To illustrate, if a validated DKIM signature successfully verifies
with a "d=" domain of "example.com", and the RFC5322.From address is
"alerts@news.example.com", the DKIM "d=" domain and the RFC5322.From
domain are considered to be "in alignment". However, a DKIM
signature bearing a value of "d=com" would never allow an "in
alignment" result, as "com" should appear on all public suffix lists
(see Appendix A.6.1) and therefore cannot be an Organizational
Domain.
Note that a single email can contain multiple DKIM signatures, and it
is considered to be a DMARC "pass" if any DKIM signature is aligned
and verifies.
3.14.2. SPF-Authenticated Identifiers
_Ticket 52_
DMARC permits Identifier Alignment based on the result of an SPF
authentication. As with DKIM, Identifier Alignement is determined
based on whether or not two domain's Organizational Domains are the
same.
For example, if a message passes an SPF check with an
RFC5321.MailFrom domain of "cbg.bounces.example.com", and the address
portion of the RFC5322.From header field contains
"payments@example.com", the Authenticated RFC5321.MailFrom domain
identifier and the RFC5322.From domain are considered to be "in
alignment" because they have the same Organizational Domain
("example.com").
_Ticket 1_
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The reader should note that SPF alignment checks in DMARC rely solely
on the RFC5321.MailFrom domain. This differs from section 2.3 of
[RFC7208], which recommends that SPF checks be done on not only the
"MAIL FROM" but also on a separate check of the "HELO" identity.
3.14.3. Alignment and Extension Technologies
If in the future DMARC is extended to include the use of other
authentication mechanisms, the extensions will need to allow for
domain identifier extraction so that alignment with the RFC5322.From
domain can be verified.
3.15. Determining The Organizational Domain
The Organizational Domain is determined using the following
algorithm:
1. Acquire a "public suffix" list, i.e., a list of DNS domain names
reserved for registrations. Some country Top-Level Domains
(TLDs) make specific registration requirements, e.g., the United
Kingdom places company registrations under ".co.uk"; other TLDs
such as ".com" appear in the IANA registry of top-level DNS
domains. A public suffix list is the union of all of these.
Appendix A.6.1 contains some discussion about obtaining a public
suffix list.
2. Break the subject DNS domain name into a set of "n" ordered
labels. Number these labels from right to left; e.g., for
"example.com", "com" would be label 1 and "example" would be
label 2.
3. Search the public suffix list for the name that matches the
largest number of labels found in the subject DNS domain. Let
that number be "x".
4. Construct a new DNS domain name using the name that matched from
the public suffix list and prefixing to it the "x+1"th label from
the subject domain. This new name is the Organizational Domain.
Thus, since "com" is an IANA-registered TLD, a subject domain of
"a.b.c.d.example.com" would have an Organizational Domain of
"example.com".
The process of determining a suffix is currently a heuristic one. No
list is guaranteed to be accurate or current.
Ticket 109, Original text: (Seems like these two paragraphs should be
moved elsewhere?)
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In addition to Mediators, mail that is sent by authorized,
independent third parties might not be sent with Identifier
Alignment, also preventing a "pass" result.
Issues specific to the use of policy mechanisms alongside DKIM are
further discussed in [@RFC6377], particularly Section 5.2.
4. Overview
This section provides a general overview of the design and operation
of the DMARC environment.
4.1. Authentication Mechanisms
The following mechanisms for determining Authenticated Identifiers
are supported in this version of DMARC:
_Ticket 109_
* DKIM, [RFC6376], which provides a domain-level identifier in the
content of the "d=" tag of a validated DKIM-Signature header
field.
* SPF, [RFC7208], which can authenticate both the domain found in an
[RFC5322] HELO/EHLO command (the HELO identity) and the domain
found in an SMTP MAIL command (the MAIL FROM identity).
Section 2.4 of [RFC7208] describes MAIL FROM processing for cases
in which the MAIL command has a null path.
4.2. Key Concepts
_Ticket 108_
DMARC policies are published by the Domain Owner or PSO, and
retrieved by the Mail Receiver during the SMTP session, via the DNS.
_Tickets 52 and 75_
DMARC's filtering function is based on whether the RFC5322.From
domain is aligned with (has the same Organizational Domain as) an
authenticated domain name from SPF or DKIM. When a DMARC policy is
published for the domain name found in the RFC5322.From header field,
and that domain name is not validated through SPF or DKIM, the
handling of that message can be affected by that DMARC policy when
delivered to a participating receiver.
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It is important to note that the authentication mechanisms employed
by DMARC authenticate only a DNS domain and do not authenticate the
local-part of any email address identifier found in a message, nor do
they validate the legitimacy of message content.
_Tickets 108 and 109_
DMARC's feedback component involves the collection of information
about received messages claiming to be from the Author Domain for
periodic aggregate reports to the Domain Owner or PSO. The
parameters and format for such reports are discussed in
[DMARC-Aggregate-Reporting]
A DMARC-enabled Mail Receiver might also generate per-message reports
that contain information related to individual messages that fail SPF
and/or DKIM. Per-message failure reports are a useful source of
information when debugging deployments (if messages can be determined
to be legitimate even though failing authentication) or in analyzing
attacks. The capability for such services is enabled by DMARC but
defined in other referenced material such as [RFC6591] and
[DMARC-Failure-Reporting]
A message satisfies the DMARC checks if at least one of the supported
authentication mechanisms:
1. produces a "pass" result, and
2. produces that result based on an identifier that is in alignment,
as defined in Section 3.
4.3. Flow Diagram
_Ticket 2_
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+---------------+ +--------------------+
| Author Domain |< . . . . . . . . . . . . | Return-Path Domain |
+---------------+ . +--------------------+
| . ^
V V .
+-----------+ +--------+ +----------+ v
| MSA |<***>| DKIM | | DMARC | +----------+
| Service | | Signer | | Verifier |<***>| SPF |
+-----------+ +--------+ +----------+ * | Verifier |
| ^ * +----------+
| * *
V v *
+------+ (~~~~~~~~~~~~) +------+ * +----------+
| sMTA |------->( other MTAs )----->| rMTA | **>| DKIM |
+------+ (~~~~~~~~~~~~) +------+ | Verifier |
| +----------+
| ^
V .
+-----------+ .
+---------+ | MDA | v
| User |<--| Filtering | +-----------+
| Mailbox | | Engine | | DKIM |
+---------+ +-----------+ | Signing |
| Domain(s) |
+-----------+
MSA = Mail Submission Agent
MDA = Mail Delivery Agent
The above diagram shows a simple flow of messages through a DMARC-
aware system. Solid lines denote the actual message flow, dotted
lines involve DNS queries used to retrieve message policy related to
the supported message authentication schemes, and asterisk lines
indicate data exchange between message-handling modules and message
authentication modules. "sMTA" is the sending MTA, and "rMTA" is the
receiving MTA.
_Ticket 2_
Put simply, when a message reaches a DMARC-aware rMTA, a DNS query
will be initiated to determine if the author domain has published a
DMARC policy. If a policy is found, the rMTA will use the results of
SPF and DKIM validation checks to determine the ultimate DMARC
authentication status. The DMARC status can then factor into the
message handling decision made by the recipient's mail sytsem.
More details on specific actions for the parties involved can be
found in Section 6.5 and Section 6.7.
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5. Use of RFC5322.From
One of the most obvious points of security scrutiny for DMARC is the
choice to focus on an identifier, namely the RFC5322.From address,
which is part of a body of data that has been trivially forged
throughout the history of email.
Several points suggest that it is the most correct and safest thing
to do in this context:
* Of all the identifiers that are part of the message itself, this
is the only one guaranteed to be present.
* It seems the best choice of an identifier on which to focus, as
most MUAs display some or all of the contents of that field in a
manner strongly suggesting those data as reflective of the true
originator of the message.
The absence of a single, properly formed RFC5322.From header field
renders the message invalid. Handling of such a message is outside
of the scope of this specification.
Since the sorts of mail typically protected by DMARC participants
tend to only have single Authors, DMARC participants generally
operate under a slightly restricted profile of RFC5322 with respect
to the expected syntax of this field. See Section 6.7 for details.
6. Policy
_Tickets 75, 85 and 108_
DMARC policies are published by Domain Owners and PSOs and can be
used by Mail Receivers to inform their message handling decisions.
A Domain Owner or PSO advertises DMARC participation of one or more
of its domains by adding a DNS TXT record (described in Section 6.1)
to those domains. In doing so, Domain Owners and PSOs indicate their
severity of concern regarding failed authentication for email
messages making use of their domain in the RFC5322.From header field
as well as the provision of feedback about those messages. Mail
Receivers in turn can take into account the Domain Owner's severity
of concern when making handling decisions about email messages that
fail DMARC authentication checks.
A Domain Owner or PSO may choose not to participate in DMARC
evaluation by Mail Receivers. In this case, the Domain Owner simply
declines to advertise participation in those schemes. For example,
if the results of path authorization checks ought not be considered
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as part of the overall DMARC result for a given Author Domain, then
the Domain Owner does not publish an SPF policy record that can
produce an SPF pass result.
A Mail Receiver implementing the DMARC mechanism SHOULD make a best-
effort attempt to adhere to the Domain Owner's or PSO's published
DMARC Domain Owner Assessment Policy when a message fails the DMARC
test.
Since email streams can be complicated (due to forwarding, existing
RFC5322.From domain-spoofing services, etc.), Mail Receivers MAY
deviate from a published Domain Owner Assessment Policy during
message processing and SHOULD make available the fact of and reason
for the deviation to the Domain Owner via feedback reporting,
specifically using the "PolicyOverride" feature of the aggregate
report defined in [DMARC-Aggregate-Reporting]
6.1. DMARC Policy Record
Domain Owner and PSO DMARC preferences are stored as DNS TXT records
in subdomains named "_dmarc". For example, the Domain Owner of
"example.com" would post DMARC preferences in a TXT record at
"_dmarc.example.com". Similarly, a Mail Receiver wishing to query
for DMARC preferences regarding mail with an RFC5322.From domain of
"example.com" would issue a TXT query to the DNS for the subdomain of
"_dmarc.example.com". The DNS-located DMARC preference data will
hereafter be called the "DMARC record".
DMARC's use of the Domain Name Service is driven by DMARC's use of
domain names and the nature of the query it performs. The query
requirement matches with the DNS, for obtaining simple parametric
information. It uses an established method of storing the
information, associated with the target domain name, namely an
isolated TXT record that is restricted to the DMARC context. Use of
the DNS as the query service has the benefit of reusing an extremely
well-established operations, administration, and management
infrastructure, rather than creating a new one.
Per [RFC1035], a TXT record can comprise several "character-string"
objects. Where this is the case, the module performing DMARC
evaluation MUST concatenate these strings by joining together the
objects in order and parsing the result as a single string.
6.2. DMARC URIs
[RFC3986] defines a generic syntax for identifying a resource. The
DMARC mechanism uses this as the format by which a Domain Owner or
PSO specifies the destination for the two report types that are
supported.
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_Ticket 54_
The place such URIs are specified (see Section 6.3) allows a list of
these to be provided. The list of URIs is separated by commas (ASCII
0x2c). A report is normally sent to each listed URI in the order
provided in the DMARC record.
_Ticket 53_
A formal definition is provided in Section 6.4.
6.3. General Record Format
DMARC records follow the extensible "tag-value" syntax for DNS-based
key records defined in DKIM [RFC6376].
Section 10 creates a registry for known DMARC tags and registers the
initial set defined in this document. Only tags defined in this
document or in later extensions, and thus added to that registry, are
to be processed; unknown tags MUST be ignored.
The following tags are introduced as the initial valid DMARC tags:
_Ticket 52_
_Tickets 4 and 109_
fo: Failure reporting options (plain-text; OPTIONAL; default is "0")
Provides requested options for generation of failure reports.
Report generators MAY choose to adhere to the requested options.
This tag's content MUST be ignored if a "ruf" tag (below) is not
also specified. Failure reporting options are shown below. The
value of this tag is either "0", "1", or a colon-separated list of
the options represented by alphabetic characters.
0: Generate a DMARC failure report if all underlying
authentication mechanisms fail to produce an aligned "pass"
result.
1: Generate a DMARC failure report if any underlying
authentication mechanism produced something other than an
aligned "pass" result.
d: Generate a DKIM failure report if the message had a signature
that failed evaluation, regardless of its alignment. DKIM-
specific reporting is described in [RFC6651].
s: Generate an SPF failure report if the message failed SPF
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evaluation, regardless of its alignment. SPF-specific
reporting is described in [RFC6652].
_Tickets 85 and 108_
np: Domain Owner Assessment Policy for non-existent subdomains
(plain-text; OPTIONAL). Indicates the severity of concern the
Domain Owner or PSO has for mail using non-existent subdomains of
the domain queried. It applies only to non-existent subdomains of
the domain queried and not to either existing subdomains or the
domain itself. Its syntax is identical to that of the "p" tag
defined below. If the "np" tag is absent, the policy specified by
the "sp" tag (if the "sp" tag is present) or the policy specified
by the "p" tag, if the "sp" tag is not present, MUST be applied
for non-existent subdomains. Note that "np" will be ignored for
DMARC records published on subdomains of Organizational Domains
and PSDs due to the effect of the DMARC policy discovery mechanism
described in Section 6.7.3.
_Tickets 72 and 85_
p: Domain Owner Assessment Policy (plain-text; RECOMMENDED for
policy records). Indicates the severity of concern the Domain
Owner or PSO has for mail using its domain but not passing DMARC
validation. Policy applies to the domain queried and to
subdomains, unless subdomain policy is explicitly described using
the "sp" or "np" tags. This tag is mandatory for policy records
only, but not for third-party reporting records (see
[DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting])
Possible values are as follows:
none: The Domain Owner offers no expression of concern.
quarantine: The Domain Owner considers such mail to be
suspicious. It is possible the mail is valid, although the
failure creates a significant concern.
reject: The Domain Owner considers all such failures to be a
clear indication that the use of the domain name is not valid.
See Section 9.3 for some discussion of SMTP rejection methods
and their implications.
_Ticket 47_
_Ticket 82_
rf (do not use): Format to be used for message-specific failure
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reports (colon- separated plain-text list of values; OPTIONAL;
default is "afrf"). This tag SHOULD NOT be used in a DMARC
record. See the note at the end for more information. The value
of this tag is a list of one or more report formats as requested
by the Domain Owner or PSO to be used when a message fails both
[RFC7208] and [RFC6376] tests to report details of the individual
failure. The values MUST be present in the registry of reporting
formats defined in Section 10; a Mail Receiver observing a
different value SHOULD ignore it or MAY ignore the entire DMARC
record. For this version, only "afrf" (the auth-failure report
type defined in [RFC6591]) is presently supported. See
[DMARC-Failure-Reporting] for details. For interoperability, the
Authentication Failure Reporting Format (AFRF) MUST be supported.
Note: Ever-broadening privacy laws in many governmental
jurisdictions have had the effect of receivers refusing to send
failure reports or at best redacting so much information from them
as to render them mostly useless to the Report Receiver. As such,
it is unlikely that there will ever be formats other than "afrf"
developed for failure reports, and so this tag should not be used.
_Ticket 50_
ri (do not use): Interval requested between aggregate reports
(plain-text 32-bit unsigned integer; OPTIONAL; default is 86400).
This tag SHOULD NOT be used in a DMARC record. See the note at
the end for more information. Indicates a request to Receivers to
generate aggregate reports separated by no more than the requested
number of seconds. DMARC implementations MUST be able to provide
daily reports and SHOULD be able to provide hourly reports when
requested. However, anything other than a daily report is
understood to be accommodated on a best- effort basis.
Note: In March, 2021, a survey of nearly 74,000 DMARC policy
records showed that fewer than 2% were publishing an ri tag with a
non-default value, with most of those set to a value of 3600.
There was no evidence that any of these requests for something
more frequent than once daily were being honored.
_Ticket 53_
rua: Addresses to which aggregate feedback is to be sent (comma-
separated plain-text list of DMARC URIs; OPTIONAL).
[DMARC-Aggregate-Reporting] discusses considerations that apply
when the domain name of a URI differs from that of the domain
advertising the policy.
See Section 11.5 for additional considerations. Any valid URI can
be specified. A Mail Receiver MUST implement support for a
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"mailto:" URI, i.e., the ability to send a DMARC report via
electronic mail. If not provided, Mail Receivers MUST NOT
generate aggregate feedback reports. URIs not supported by Mail
Receivers MUST be ignored. The aggregate feedback report format
is described in the DMARC reporting documents.
ruf: Addresses to which message-specific failure information is to
be reported (comma-separated plain-text list of DMARC URIs;
OPTIONAL). If present, the Domain Owner or PSO is requesting Mail
Receivers to send detailed failure reports about messages that
fail the DMARC evaluation in specific ways (see the "fo" tag
above). The format of the message to be generated MUST follow the
format specified for the "rf" tag. [DMARC-Failure-Reporting]
discusses considerations that apply when the domain name of a URI
differs from that of the domain advertising the policy. A Mail
Receiver MUST implement support for a "mailto:" URI, i.e., the
ability to send a DMARC report via electronic mail. If not
provided, Mail Receivers MUST NOT generate failure reports. See
Section 11.5 for additional considerations.
_Tickets 85 and 108_
sp: Domain Owner Assessment Policy for all subdomains (plain-text;
OPTIONAL). Indicates the severity of concern the Domain Owner or
PSO has for mail using an existing subdomain of the domain queried
but not passing DMARC validation. It applies only to subdomains
of the domain queried and not to the domain itself. Its syntax is
identical to that of the "p" tag defined above. If both the "sp"
tag is absent and the "np" tag is either absent or not applicable,
the policy specified by the "p" tag MUST be applied for
subdomains. Note that "sp" will be ignored for DMARC records
published on subdomains of Organizational Domains due to the
effect of the DMARC policy discovery mechanism described in
Section 6.7.3.
v: Version (plain-text; REQUIRED). Identifies the record retrieved
as a DMARC record. It MUST have the value of "DMARC1". The value
of this tag MUST match precisely; if it does not or it is absent,
the entire retrieved record MUST be ignored. It MUST be the first
tag in the list.
A DMARC policy record MUST comply with the formal specification found
in Section 6.4 in that the "v" tag MUST be present and MUST appear
first. Unknown tags MUST be ignored. Syntax errors in the remainder
of the record SHOULD be discarded in favor of default values (if any)
or ignored outright.
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Note that given the rules of the previous paragraph, addition of a
new tag into the registered list of tags does not itself require a
new version of DMARC to be generated (with a corresponding change to
the "v" tag's value), but a change to any existing tags does require
a new version of DMARC.
_Ticket 109_ Question: Does removal of a tag or tags, as proposed
through other tickets, constitute "a change to any existing tags",
thus requiring "a new version of DMARC"?
6.4. Formal Definition
The formal definition of the DMARC format, using [RFC5234], is as
follows:
[FIXTHIS: Reference to [RFC3986] in code block]
dmarc-uri = URI [ "!" 1*DIGIT [ "k" / "m" / "g" / "t" ] ]
; "URI" is imported from [RFC3986]; commas (ASCII
; 0x2C) and exclamation points (ASCII 0x21)
; MUST be encoded; the numeric portion MUST fit
; within an unsigned 64-bit integer
_Ticket 7, 47, and 52_
dmarc-record = dmarc-version dmarc-sep *(dmarc-tag dmarc-sep)
dmarc-tag = dmarc-request /
dmarc-srequest /
dmarc-auri /
dmarc-furi /
dmarc-ainterval /
dmarc-fo /
dmarc-rfmt
; components other than dmarc-version and
; dmarc-request may appear in any order
dmarc-version = "v" *WSP "=" *WSP %x44 %x4d %x41 %x52 %x43 %x31
dmarc-sep = *WSP %x3b *WSP
dmarc-request = "p" *WSP "=" *WSP
( "none" / "quarantine" / "reject" )
dmarc-srequest = "sp" *WSP "=" *WSP
( "none" / "quarantine" / "reject" )
dmarc-auri = "rua" *WSP "=" *WSP
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dmarc-uri *(*WSP "," *WSP dmarc-uri)
dmarc-furi = "ruf" *WSP "=" *WSP
dmarc-uri *(*WSP "," *WSP dmarc-uri)
_Ticket 52_
dmarc-ainterval = "ri" *WSP "=" *WSP 1*DIGIT
dmarc-fo = "fo" *WSP "=" *WSP
( "0" / "1" / "d" / "s" )
*(*WSP ":" *WSP ( "0" / "1" / "d" / "s" ))
dmarc-rfmt = "rf" *WSP "=" *WSP Keyword *(*WSP ":" Keyword)
; registered reporting formats only
_Ticket 47_
"Keyword" is imported from Section 4.1.2 of [RFC5321].
_Ticket 53_
6.5. Domain Owner Actions
_Tickets 2, 108, and 109_
This section describes Domain Owner actions to fully implement the
DMARC mechanism.
6.5.1. Publish an SPF Policy for an Aligned Domain
Because DMARC relies on SPF [RFC7208] and DKIM [RFC6376], in order to
take full advantage of DMARC, a Domain Owner SHOULD first ensure that
SPF and DKIM authentication are properly configured. The easiest
first step here is to choose a domain to use as the RFC5321.From
domain (i.e., the Return-Path domain) for its mail, one that aligns
with the Author Domain, and then publish an SPF policy in DNS for
that domain.
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6.5.2. Configure Sending System for DKIM Signing Using an Aligned
Domain
While it is possible to secure a DMARC pass verdict based on only SPF
or DKIM, it is commonly accepted best practice to ensure that both
authentication mechanisms are in place in order to guard against
failure of just one of them. The Domain Owner SHOULD choose as a
DKIM-Signing domain (i.e., the d= domain in the DKIM-Signature
header) that aligns with the Author Domain and configure its system
to sign using that domain.
6.5.3. Setup a Mailbox to Receive Aggregate Reports
Proper consumption and analysis of DMARC aggregate reports is the key
to any successful DMARC deployment for a Domain Owner. DMARC
aggregate reports, which are XML documents and are defined in
[DMARC-Aggregate-Reporting], contain valuable data for the Domain
Owner, showing sources of mail using the Author Domain. Depending on
how mature the Domain Owner's DMARC rollout is, some of these sources
could be legitimate ones that were overlooked during the intial
deployment of SPF and/or DKIM.
Because the aggregate reports are XML documents, it is strongly
advised that they be machine-parsed, so setting up a mailbox involves
more than just the physical creation of the mailbox. Many third-
party services exist that will process DMARC aggregate reports, or
the Domain Owner can create its own set of tools. No matter which
method is chosen, the ability to parse these reports and consume the
data contained in them will go a long way to ensuring a successful
deployment.
6.5.4. Publish a DMARC Policy for the Author Domain
Once SPF, DKIM, and the aggregate reports mailbox are all in place,
it's time to publish a DMARC record. For best results, Domain Owners
SHOULD start with "p=none", with the rua tag containg the mailbox
created in the previous step.
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6.5.5. Collect and Analyze Reports and Adjust Authentication
The reason for starting at "p=none" is to ensure that nothing's been
missed in the initial SPF and DKIM deployments. In all but the most
trivial setups, it is possible for a Domain Owner to overlook a
server here or be unaware of a third party sending agreeement there.
Starting at "p=none", therefore, takes advantage of DMARC's aggregate
reporting function, with the Domain Owner using the reports to audit
its own mail streams. Should any overlooked systems be found in the
reports, the Domain Owner can adjust the SPF record and/or configure
DKIM signing for those systems.
6.5.6. Decide If and When to Update DMARC Policy
Once the Domain Owner is satisfied that it is properly authenticating
all of its mail, then it is time to decide if it is appropriate to
change the p= value in its DMARC record to p=quarantine or p=reject.
Depending on its cadence for sending mail, it may take many months of
consuming DMARC aggregate reports before a Domain Owner reaches the
point where it is sure that it is properly authenticating all of its
mail, and the decision on which p= value to use will depend on its
needs.
6.6. PSO Actions
In addition to the DMARC Domain Owner actions, PSOs that require use
of DMARC and participate in PSD DMARC ought to make that information
availablle to Mail Receivers. [DMARC-PSD] is an experimental method
for doing so, and the experiment is described in Appendix A of that
document.
6.7. Mail Receiver Actions
This section describes receiver actions in the DMARC environment.
6.7.1. Extract Author Domain
The domain in the RFC5322.From header field is extracted as the
domain to be evaluated by DMARC. If the domain is encoded with UTF-
8, the domain name must be converted to an A-label, as described in
Section 2.3 of [RFC5890], for further processing.
_Ticket 107_
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In order to be processed by DMARC, a message typically needs to
contain exactly one RFC5322.From domain (a single From: field with a
single domain in it). Not all messages meet this requirement, and
the handling of those that are forbidden under RFC 5322 [RFC5322] or
that contain no meaningful domains is outside the scope of this
document.
The case of a syntactically valid multi-valued RFC5322.From header
field presents a particular challenge. When a single RFC5322.From
header field contains multiple addresses, it is possible that there
may be multiple domains used in those addresses. The process in this
case is to only proceed with DMARC checking if the domain is
identical for all of the addresses in a multi-valued RFC5322.From
header field. Multi-valued RFC5322.From header fields with multiple
domains MUST be exempt from DMARC checking.
_Ticket 108_
Note that domain names that appear on a public suffix list are not
exempt from DMARC policy application and reporting.
6.7.2. Determine Handling Policy
To arrive at a policy for an individual message, Mail Receivers MUST
perform the following actions or their semantic equivalents. Steps
2-4 MAY be done in parallel, whereas steps 5 and 6 require input from
previous steps.
The steps are as follows:
1. Extract the RFC5322.From domain from the message (as above).
2. Query the DNS for a DMARC policy record. Continue if one is
found, or terminate DMARC evaluation otherwise. See
Section 6.7.3 for details.
_Ticket 3_
3. Perform DKIM signature verification checks. A single email could
contain multiple DKIM signatures. The results of this step are
passed to the remainder of the algorithm, MUST include "pass" or
"fail", and if "fail", SHOULD include information about the
reasons for failure. The results MUST further include the value
of the "d=" and "s=" tags from each checked DKIM signature.
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4. Perform SPF validation checks. The results of this step are
passed to the remainder of the algorithm, MUST include "pass" or
"fail", and if "fail", SHOULD include information about the
reasons for failure. The results MUST further include the domain
name used to complete the SPF check.
5. Conduct Identifier Alignment checks. With authentication checks
and policy discovery performed, the Mail Receiver checks to see
if Authenticated Identifiers fall into alignment as described in
Section 3. If one or more of the Authenticated Identifiers align
with the RFC5322.From domain, the message is considered to pass
the DMARC mechanism check. All other conditions (authentication
failures, identifier mismatches) are considered to be DMARC
mechanism check failures.
_Tickets 75 and 109_
6. Apply policy. Emails that fail the DMARC mechanism check are
handled in accordance with the discovered DMARC policy of the
Domain Owner and any local policy rules enforced by the Mail
Receiver. See Section 6.3 for details.
Heuristics applied in the absence of use by a Domain Owner of either
SPF or DKIM (e.g., [Best-Guess-SPF]) SHOULD NOT be used, as it may be
the case that the Domain Owner wishes a Message Receiver not to
consider the results of that underlying authentication protocol at
all.
DMARC evaluation can only yield a "pass" result after one of the
underlying authentication mechanisms passes for an aligned
identifier. If neither passes and one or both of them fail due to a
temporary error, the Receiver evaluating the message is unable to
conclude that the DMARC mechanism had a permanent failure; they
therefore cannot apply the advertised DMARC policy. When otherwise
appropriate, Receivers MAY send feedback reports regarding temporary
errors.
Handling of messages for which SPF and/or DKIM evaluation encounter a
permanent DNS error is left to the discretion of the Mail Receiver.
6.7.3. Policy Discovery
As stated above, the DMARC mechanism uses DNS TXT records to
advertise policy. Policy discovery is accomplished via a method
similar to the method used for SPF records. This method, and the
important differences between DMARC and SPF mechanisms, are discussed
below.
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To balance the conflicting requirements of supporting wildcarding,
allowing subdomain policy overrides, and limiting DNS query load, the
following DNS lookup scheme is employed:
1. Mail Receivers MUST query the DNS for a DMARC TXT record at the
DNS domain matching the one found in the RFC5322.From domain in
the message. A possibly empty set of records is returned.
2. Records that do not start with a "v=" tag that identifies the
current version of DMARC are discarded.
3. If the set is now empty, the Mail Receiver MUST query the DNS for
a DMARC TXT record at the DNS domain matching the Organizational
Domain in place of the RFC5322.From domain in the message (if
different). This record can contain policy to be asserted for
subdomains of the Organizational Domain. A possibly empty set of
records is returned.
_Ticket 109_
4. If the set is now empty and the longest PSD Section 3.6 of the
Organizational Domain is one that the receiver has determined is
acceptable for PSD DMARC (discussed in the [DMARC-PSD] experiment
description (Appendix A)), the Mail Receiver MUST query the DNS
for a DMARC TXT record at the DNS domain matching the [DMARC-PSD]
longest PSD Section 3.6 in place of the RFC5322.From domain in
the message (if different). A possibly empty set of records is
returned.
5. Records that do not start with a "v=" tag that identifies the
current version of DMARC are discarded.
6. If the remaining set contains multiple records or no records,
policy discovery terminates and DMARC processing is not applied
to this message.
7. If a retrieved policy record does not contain a valid "p" tag, or
contains an "sp" tag that is not valid, then:
1. if a "rua" tag is present and contains at least one
syntactically valid reporting URI, the Mail Receiver SHOULD
act as if a record containing a valid "v" tag and "p=none"
was retrieved, and continue processing;
2. otherwise, the Mail Receiver applies no DMARC processing to
this message.
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If the set produced by the mechanism above contains no DMARC policy
record (i.e., any indication that there is no such record as opposed
to a transient DNS error), Mail Receivers SHOULD NOT apply the DMARC
mechanism to the message.
Handling of DNS errors when querying for the DMARC policy record is
left to the discretion of the Mail Receiver. For example, to ensure
minimal disruption of mail flow, transient errors could result in
delivery of the message ("fail open"), or they could result in the
message being temporarily rejected (i.e., an SMTP 4yx reply), which
invites the sending MTA to try again after the condition has possibly
cleared, allowing a definite DMARC conclusion to be reached ("fail
closed").
_Ticket 108_
6.7.3.1. Longest PSD Example
As an example of step 4 above, for a message with the Organizational
Domain of "example.compute.cloudcompany.com.example", the query for
PSD DMARC would use "compute.cloudcompany.com.example" as the
[DMARC-PSD] longest PSD Section 3.6. The receiver would check to see
if that PSD is listed in the DMARC PSD Registry, and if so, perform
the policy lookup at "_dmarc.compute.cloudcompany.com.example".
Note: Because the PSD policy query comes after the Organizational
Domain policy query, PSD policy is not used for Organizational
domains that have published a DMARC policy. Specifically, this is
not a mechanism to provide feedback addresses (RUA/RUF) when an
Organizational Domain has declined to do so.
_Ticket 47_
6.7.4. Store Results of DMARC Processing
The results of Mail Receiver-based DMARC processing should be stored
for eventual presentation back to the Domain Owner in the form of
aggregate feedback reports. Section 6.3 and
[DMARC-Aggregate-Reporting] discuss aggregate feedback.
_Ticket 62_
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6.7.5. Send Aggregate Reports
For a Domain Owner, DMARC aggregate reports provide data about all
mailstreams making use of its domain in email, to include not only
illegitimate uses but also, and perhaps more importantly, all
legitimate uses. Domain Owners can use aggregate reports to ensure
that all legitimate uses of their domain for sending email are
properly authenticated, and once they are, increase the severity of
concern expressed in the p= tag in their DMARC policy records from
none to quarantine to reject, if appropriate. In turn, DMARC policy
records with p= tag values of 'quarantine' or 'reject' are higher
value signals to Mail Receivers, ones that can assist Mail Receivers
with handling decisions for a message in ways that p= tag values of
'none' cannot.
In order to ensure maximum usefulness for DMARC across the email
ecosystem, then, Mail Receivers MUST generate and send aggregate
reports with a frequency of at least once every 24 hours.
6.8. Policy Enforcement Considerations
Mail Receivers MAY choose to reject or quarantine email even if email
passes the DMARC mechanism check. The DMARC mechanism does not
inform Mail Receivers whether an email stream is "good". Mail
Receivers are encouraged to maintain anti-abuse technologies to
combat the possibility of DMARC-enabled criminal campaigns.
_Ticket 109_
Mail Receivers MAY choose to accept email that fails the DMARC
mechanism check even if the published Domain Owner Assessment Policy
is "reject". Mail Receivers need to make a best effort not to
increase the likelihood of accepting abusive mail if they choose not
to honor the published Domain Owner Assessment Policy. At a minimum,
addition of the Authentication-Results header field (see [RFC8601])
is RECOMMENDED when delivery of failing mail is done. When this is
done, the DNS domain name thus recorded MUST be encoded as an
A-label.
Mail Receivers are only obligated to report reject or quarantine
policy actions in aggregate feedback reports that are due to
published DMARC Domain Owner Assessment Policy. They are not
required to report reject or quarantine actions that are the result
of local policy. If local policy information is exposed, abusers can
gain insight into the effectiveness and delivery rates of spam
campaigns.
_Ticket 75_
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Final handling of a message is always a matter of local policy. An
operator that wishes to favor DMARC policy over SPF policy, for
example, will disregard the SPF policy, since enacting an SPF-
determined rejection prevents evaluation of DKIM; DKIM might
otherwise pass, satisfying the DMARC evaluation. There is a trade-
off to doing so, namely acceptance and processing of the entire
message body in exchange for the enhanced protection DMARC provides.
DMARC-compliant Mail Receivers typically disregard any mail-handling
directive discovered as part of an authentication mechanism (e.g.,
Author Domain Signing Practices (ADSP), SPF) where a DMARC record is
also discovered that specifies a policy other than "none". Deviating
from this practice introduces inconsistency among DMARC operators in
terms of handling of the message. However, such deviation is not
proscribed.
_Ticket 75_
To enable Domain Owners to receive DMARC feedback without impacting
existing mail processing, discovered policies of "p=none" SHOULD NOT
modify existing mail handling processes.
_Ticket 62_
Mail Receivers MUST also implement reporting instructions of DMARC,
even in the absence of a request for DKIM reporting [RFC6651] or SPF
reporting [RFC6652]. Furthermore, the presence of such requests
SHOULD NOT affect DMARC reporting.
7. DMARC Feedback
Providing Domain Owners with visibility into how Mail Receivers
implement and enforce the DMARC mechanism in the form of feedback is
critical to establishing and maintaining accurate authentication
deployments. When Domain Owners can see what effect their policies
and practices are having, they are better willing and able to use
quarantine and reject policies.
The details of this feedback are described in
[DMARC-Aggregate-Reporting]
_Ticket 108_
Operational note for PSD DMARC: For PSOs, feedback for non-existent
domains is desirable and useful, just as it is for org-level DMARC
operators. See Section 4 of [DMARC-PSD] for discussion of Privacy
Considerations for PSD DMARC
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8. Minimum Implementations
_Ticket 66_
Domain owners, mediators, and mail receivers can all claim to
implement DMARC, but what that means will depend on their role in the
transmission of mail. To remove any ambiguity from the claims, this
document specifies the following minimum criteria that must be met
for each agent to rightly claim to be "implementing DMARC".
Domain Owner: To implement DMARC, a Domain Owner MUST configure its
domain to convey its concern that unauthenticated mail be rejected or
at least treated with suspicion. This means that it MUST publish a
policy record that:
* Has a p tag with a value of 'quarantine' or 'reject'
* Has a rua tag with at least one valid URI
* If applicable, has an sp tag with a value of 'quarantine' or
'reject'
While 'none' is a syntactically valid value for both the p and sp
tags, the practical value of either the p tag or sp tag being 'none'
means that the Domain Owner is still gathering information about mail
flows for the domain or sub-domains. It is not yet ready to commit
to conveying a severity of concern for unauthenticated email using
its domain.
Mediator: To implement DMARC, a mediator MUST do the following before
passing the message to the next hop or rejecting it as appropriate:
* Perform DMARC validation checks on inbound mail
* Perform validation on any authentication checks recorded by
previous mediators.
* Record the results of its authentication checks in message headers
for consumption by later hosts.
Mail Receiver: To implement DMARC, a mail receiver MUST do the
following:
* Perform DMARC validation checks on inbound mail
* Perform validation checks on any authentication check results
recorded by mediators that handled the message prior to its
reaching the Mail Receiver.
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* Send aggregate reports to Domain Owners at least every 24 hours
when a minimum of 100 messages with that domain in the
RFC5322.From header field have been seen during the reporting
period
9. Other Topics
This section discusses some topics regarding choices made in the
development of DMARC, largely to commit the history to record.
9.1. Issues Specific to SPF
Though DMARC does not inherently change the semantics of an SPF
policy record, historically lax enforcement of such policies has led
many to publish extremely broad records containing many large network
ranges. Domain Owners are strongly encouraged to carefully review
their SPF records to understand which networks are authorized to send
on behalf of the Domain Owner before publishing a DMARC record.
Some receiver architectures might implement SPF in advance of any
DMARC operations. This means that a "-" prefix on a sender's SPF
mechanism, such as "-all", could cause that rejection to go into
effect early in handling, causing message rejection before any DMARC
processing takes place. Operators choosing to use "-all" should be
aware of this.
9.2. DNS Load and Caching
DMARC policies are communicated using the DNS and therefore inherit a
number of considerations related to DNS caching. The inherent
conflict between freshness and the impact of caching on the reduction
of DNS-lookup overhead should be considered from the Mail Receiver's
point of view. Should Domain Owners or PSOs publish a DNS record
with a very short TTL, Mail Receivers can be provoked through the
injection of large volumes of messages to overwhelm the publisher's
DNS. Although this is not a concern specific to DMARC, the
implications of a very short TTL should be considered when publishing
DMARC policies.
Conversely, long TTLs will cause records to be cached for long
periods of time. This can cause a critical change to DMARC
parameters advertised by a Domain Owner or PSO to go unnoticed for
the length of the TTL (while waiting for DNS caches to expire).
Avoiding this problem can mean shorter TTLs, with the potential
problems described above. A balance should be sought to maintain
responsiveness of DMARC preference changes while preserving the
benefits of DNS caching.
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9.3. Rejecting Messages
This proposal calls for rejection of a message during the SMTP
session under certain circumstances. This is preferable to
generation of a Delivery Status Notification ([RFC3464]), since
fraudulent messages caught and rejected using DMARC would then result
in annoying generation of such failure reports that go back to the
RFC5321.MailFrom address.
This synchronous rejection is typically done in one of two ways:
* Full rejection, wherein the SMTP server issues a 5xy reply code as
an indication to the SMTP client that the transaction failed; the
SMTP client is then responsible for generating notification that
delivery failed (see Section 4.2.5 of [RFC5321]).
* A "silent discard", wherein the SMTP server returns a 2xy reply
code implying to the client that delivery (or, at least, relay)
was successfully completed, but then simply discarding the message
with no further action.
Each of these has a cost. For instance, a silent discard can help to
prevent backscatter, but it also effectively means that the SMTP
server has to be programmed to give a false result, which can
confound external debugging efforts.
Similarly, the text portion of the SMTP reply may be important to
consider. For example, when rejecting a message, revealing the
reason for the rejection might give an attacker enough information to
bypass those efforts on a later attempt, though it might also assist
a legitimate client to determine the source of some local issue that
caused the rejection.
In the latter case, when doing an SMTP rejection, providing a clear
hint can be useful in resolving issues. A receiver might indicate in
plain text the reason for the rejection by using the word "DMARC"
somewhere in the reply text. Many systems are able to scan the SMTP
reply text to determine the nature of the rejection. Thus, providing
a machine-detectable reason for rejection allows the problems causing
rejections to be properly addressed by automated systems. For
example:
550 5.7.1 Email rejected per DMARC policy for example.com
If a Mail Receiver elects to defer delivery due to inability to
retrieve or apply DMARC policy, this is best done with a 4xy SMTP
reply code.
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9.4. Identifier Alignment Considerations
The DMARC mechanism allows both DKIM and SPF-authenticated
identifiers to authenticate email on behalf of a Domain Owner and,
possibly, on behalf of different subdomains. If malicious or unaware
users can gain control of the SPF record or DKIM selector records for
a subdomain, the subdomain can be used to generate DMARC-passing
email on behalf of the Organizational Domain.
For example, an attacker who controls the SPF record for
"evil.example.com" can send mail with an RFC5322.From header field
containing "foo@example.com" that can pass both authentication and
the DMARC check against "example.com".
_Ticket 52_
The Organizational Domain administrator should be careful not to
delegate control of subdomains if this is an issue.
9.5. Interoperability Issues
DMARC limits which end-to-end scenarios can achieve a "pass" result.
Because DMARC relies on [RFC7208] and/or [RFC6376] to achieve a
"pass", their limitations also apply.
Additional DMARC constraints occur when a message is processed by
some Mediators, such as mailing lists. Transiting a Mediator often
causes either the authentication to fail or Identifier Alignment to
be lost. These transformations may conform to standards but will
still prevent a DMARC "pass".
In addition to Mediators, mail that is sent by authorized,
independent third parties might not be sent with Identifier
Alignment, also preventing a "pass" result.
Issues specific to the use of policy mechanisms alongside DKIM are
further discussed in [RFC6377], particularly Section 5.2.
10. IANA Considerations
This section describes actions completed by IANA.
10.1. Authentication-Results Method Registry Update
IANA has added the following to the "Email Authentication Methods"
registry:
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Method: dmarc
Defined: RFC 7489
ptype: header
Property: from
Value: the domain portion of the RFC5322.From header field
Status: active
Version: 1
_Ticket 86_
Method: dmarc
Defined: RFC 7489
ptype: polrec
Property: p
Value: the p= value read from the discovered policy record
Status: active
Version: 1
Method: dmarc
Defined: RFC 7489
ptype: polrec
Property: domain
Value: the domain at which the policy record was discovered, if
different from the RFC5322.From domain
Status: active
Version: 1
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10.2. Authentication-Results Result Registry Update
IANA has added the following in the "Email Authentication Result
Names" registry:
Code: none
Existing/New Code: existing
Defined: [RFC8601]
Auth Method: dmarc (added)
Meaning: No DMARC policy record was published for the aligned
identifier, or no aligned identifier could be extracted.
Status: active
Code: pass
Existing/New Code: existing
Defined: [RFC8601]
Auth Method: dmarc (added)
Meaning: A DMARC policy record was published for the aligned
identifier, and at least one of the authentication mechanisms
passed.
Status: active
Code: fail
Existing/New Code: existing
Defined: [RFC8601]
Auth Method: dmarc (added)
Meaning: A DMARC policy record was published for the aligned
identifier, and none of the authentication mechanisms passed.
Status: active
Code: temperror
Existing/New Code: existing
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Defined: [RFC8601]
Auth Method: dmarc (added)
Meaning: A temporary error occurred during DMARC evaluation. A
later attempt might produce a final result.
Status: active
Code: permerror
Existing/New Code: existing
Defined: [RFC8601]
Auth Method: dmarc (added)
Meaning: A permanent error occurred during DMARC evaluation, such as
encountering a syntactically incorrect DMARC record. A later
attempt is unlikely to produce a final result.
Status: active
10.3. Feedback Report Header Fields Registry Update
The following has been added to the "Feedback Report Header Fields"
registry:
Field Name: Identity-Alignment
Description: indicates whether the message about which a report is
being generated had any identifiers in alignment as defined in RFC
7489
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Reference: RFC 7489
Status: current
10.4. DMARC Tag Registry
A new registry tree called "Domain-based Message Authentication,
Reporting, and Conformance (DMARC) Parameters" has been created.
Within it, a new sub-registry called the "DMARC Tag Registry" has
been created.
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Names of DMARC tags must be registered with IANA in this new sub-
registry. New entries are assigned only for values that have been
documented in a manner that satisfies the terms of Specification
Required, per [RFC8126]. Each registration must include the tag
name; the specification that defines it; a brief description; and its
status, which must be one of "current", "experimental", or
"historic". The Designated Expert needs to confirm that the provided
specification adequately describes the new tag and clearly presents
how it would be used within the DMARC context by Domain Owners and
Mail Receivers.
To avoid version compatibility issues, tags added to the DMARC
specification are to avoid changing the semantics of existing records
when processed by implementations conforming to prior specifications.
The initial set of entries in this registry is as follows:
_Ticket 47_
_Ticket 52_
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+==========+===========+==========+==============================+
| Tag Name | Reference | Status | Description |
+==========+===========+==========+==============================+
| adkim | RFC 7489 | historic | DKIM alignment mode |
+----------+-----------+----------+------------------------------+
| aspf | RFC 7489 | historic | SPF alignment mode |
+----------+-----------+----------+------------------------------+
| fo | RFC 7489 | current | Failure reporting options |
+----------+-----------+----------+------------------------------+
| p | RFC 7489 | current | Requested handling policy |
+----------+-----------+----------+------------------------------+
| pct | RFC 7489 | historic | Sampling rate |
+----------+-----------+----------+------------------------------+
| rf | RFC 7489 | current | Failure reporting format(s) |
+----------+-----------+----------+------------------------------+
| ri | RFC 7489 | current | Aggregate Reporting interval |
+----------+-----------+----------+------------------------------+
| rua | RFC 7489 | current | Reporting URI(s) for |
| | | | aggregate data |
+----------+-----------+----------+------------------------------+
| ruf | RFC 7489 | current | Reporting URI(s) for failure |
| | | | data |
+----------+-----------+----------+------------------------------+
| sp | RFC 7489 | current | Requested handling policy |
| | | | for subdomains |
+----------+-----------+----------+------------------------------+
| v | RFC 7489 | current | Specification version |
+----------+-----------+----------+------------------------------+
Table 1: "DMARC Tag Registry"
10.5. DMARC Report Format Registry
Also within "Domain-based Message Authentication, Reporting, and
Conformance (DMARC) Parameters", a new sub-registry called "DMARC
Report Format Registry" has been created.
Names of DMARC failure reporting formats must be registered with IANA
in this registry. New entries are assigned only for values that
satisfy the definition of Specification Required, per [RFC8126]. In
addition to a reference to a permanent specification, each
registration must include the format name; a brief description; and
its status, which must be one of "current", "experimental", or
"historic". The Designated Expert needs to confirm that the provided
specification adequately describes the report format and clearly
presents how it would be used within the DMARC context by Domain
Owners and Mail Receivers.
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The initial entry in this registry is as follows:
+========+===========+=========+==================================+
| Format | Reference | Status | Description |
| Name | | | |
+========+===========+=========+==================================+
| afrf | RFC 7489 | current | Authentication Failure Reporting |
| | | | Format (see [RFC6591]) |
+--------+-----------+---------+----------------------------------+
Table 2: "DMARC Report Format Registry"
10.6. Underscored and Globally Scoped DNS Node Names Registry
Per [!@RFC8552], please add the following entry to the "Underscored
and Globally Scoped DNS Node Names" registry:
+=========+============+===========+
| RR Type | _NODE NAME | Reference |
+=========+============+===========+
| TXT | _dmarc | RFC 7489 |
+---------+------------+-----------+
Table 3: "Underscored and
Globally Scoped DNS Node Names"
registry
11. Security Considerations
This section discusses security issues and possible remediations
(where available) for DMARC.
11.1. Authentication Methods
Security considerations from the authentication methods used by DMARC
are incorporated here by reference.
11.2. Attacks on Reporting URIs
URIs published in DNS TXT records are well-understood possible
targets for attack. Specifications such as [RFC1035] and [RFC2142]
either expose or cause the exposure of email addresses that could be
flooded by an attacker, for example; MX, NS, and other records found
in the DNS advertise potential attack destinations; common DNS names
such as "www" plainly identify the locations at which particular
services can be found, providing destinations for targeted denial-of-
service or penetration attacks.
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Thus, Domain Owners will need to harden these addresses against
various attacks, including but not limited to:
* high-volume denial-of-service attacks;
* deliberate construction of malformed reports intended to identify
or exploit parsing or processing vulnerabilities;
* deliberate construction of reports containing false claims for the
Submitter or Reported-Domain fields, including the possibility of
false data from compromised but known Mail Receivers.
_Ticket 104_
11.3. DNS Security
The DMARC mechanism and its underlying technologies (SPF, DKIM)
depend on the security of the DNS. If hostile parties can snoop on
DNS traffic, they can get an idea of who is sending mail. If they
can block outgoing or reply DNS messages, they can prevent systems
from discovering senders' DMARC policies, causing recipients to
assume p=none by default/ If they can send forged response packets,
they can make aligned mail appear unaligned or vice-versa.
None of these threats are unique to DMARC, and they can be addressed
using a variety of techniques. Signing DNS records with DNSSEC
[RFC4033] enables recipients to detect and discard forged responses.
DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate
snooping and forged responses.
11.4. Display Name Attacks
A common attack in messaging abuse is the presentation of false
information in the display-name portion of the RFC5322.From header
field. For example, it is possible for the email address in that
field to be an arbitrary address or domain name, while containing a
well-known name (a person, brand, role, etc.) in the display name,
intending to fool the end user into believing that the name is used
legitimately. The attack is predicated on the notion that most
common MUAs will show the display name and not the email address when
both are available.
Generally, display name attacks are out of scope for DMARC, as
further exploration of possible defenses against these attacks needs
to be undertaken.
There are a few possible mechanisms that attempt mitigation of these
attacks, such as the following:
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* If the display name is found to include an email address (as
specified in [RFC5322]), execute the DMARC mechanism on the domain
name found there rather than the domain name discovered
originally. However, this addresses only a very specific attack
space, and spoofers can easily circumvent it by simply not using
an email address in the display name. There are also known cases
of legitimate uses of an email address in the display name with a
domain different from the one in the address portion, e.g.,
From: "user@example.org via Bug Tracker" support@example.com
(mailto:support@example.com)
* In the MUA, only show the display name if the DMARC mechanism
succeeds. This too is easily defeated, as an attacker could
arrange to pass the DMARC tests while fraudulently using another
domain name in the display name.
* In the MUA, only show the display name if the DMARC mechanism
passes and the email address thus validated matches one found in
the receiving user's list of known addresses.
11.5. External Reporting Addresses
To avoid abuse by bad actors, reporting addresses generally have to
be inside the domains about which reports are requested. In order to
accommodate special cases such as a need to get reports about domains
that cannot actually receive mail, The DMARC reporting documents
describe a DNS-based mechanism for verifying approved external
reporting.
The obvious consideration here is an increased DNS load against
domains that are claimed as external recipients. Negative caching
will mitigate this problem, but only to a limited extent, mostly
dependent on the default TTL in the domain's SOA record.
Where possible, external reporting is best achieved by having the
report be directed to domains that can receive mail and simply having
it automatically forwarded to the desired external destination.
Note that the addresses shown in the "ruf" tag receive more
information that might be considered private data, since it is
possible for actual email content to appear in the failure reports.
The URIs identified there are thus more attractive targets for
intrusion attempts than those found in the "rua" tag. Moreover,
attacking the DNS of the subject domain to cause failure data to be
routed fraudulently to an attacker's systems may be an attractive
prospect. Deployment of [RFC4033] is advisable if this is a concern.
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The verification mechanism presented in the DMARC reporting docuemnts
is currently not mandatory ("MUST") but strongly recommended
("SHOULD"). It is possible that it would be elevated to a "MUST" by
later security review.
11.6. Secure Protocols
This document encourages use of secure transport mechanisms to
prevent loss of private data to third parties that may be able to
monitor such transmissions. Unencrypted mechanisms should be
avoided.
In particular, a message that was originally encrypted or otherwise
secured might appear in a report that is not sent securely, which
could reveal private information.
12. Normative References
[DMARC-Aggregate-Reporting]
Brotman, A., Ed., "DMARC Aggregate Reporting", February
2021, <https://datatracker.ietf.org/doc/draft-ietf-dmarc-
aggregate-reporting/>.
[DMARC-Failure-Reporting]
Jones, S.M., Ed. and A. Vesely, Ed., "DMARC Failure
Reporting", February 2021,
<https://datatracker.ietf.org/doc/draft-ietf-dmarc-
failure-reporting/>.
[DMARC-PSD]
Kitterman, S. and T. Wicinski, Ed., "Experimental DMARC
Extension For Public Suffix Domains", April 2021,
<https://datatracker.ietf.org/doc/draft-ietf-dmarc-
psd/?include_text=1>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
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[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/info/rfc4343>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6591] Fontana, H., "Authentication Failure Reporting Using the
Abuse Reporting Format", RFC 6591, DOI 10.17487/RFC6591,
April 2012, <https://www.rfc-editor.org/info/rfc6591>.
[RFC6651] Kucherawy, M., "Extensions to DomainKeys Identified Mail
(DKIM) for Failure Reporting", RFC 6651,
DOI 10.17487/RFC6651, June 2012,
<https://www.rfc-editor.org/info/rfc6651>.
[RFC6652] Kitterman, S., "Sender Policy Framework (SPF)
Authentication Failure Reporting Using the Abuse Reporting
Format", RFC 6652, DOI 10.17487/RFC6652, June 2012,
<https://www.rfc-editor.org/info/rfc6652>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
13. Informative References
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[Best-Guess-SPF]
Kitterman, S., "Sender Policy Framework: Best guess record
(FAQ entry)", May 2010,
<http://www.openspf.org/FAQ/Best_guess_record>.
[RFC2142] Crocker, D., "Mailbox Names for Common Services, Roles and
Functions", RFC 2142, DOI 10.17487/RFC2142, May 1997,
<https://www.rfc-editor.org/info/rfc2142>.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003,
<https://www.rfc-editor.org/info/rfc3464>.
[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,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, August
2009, <https://www.rfc-editor.org/info/rfc5617>.
[RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
September 2011, <https://www.rfc-editor.org/info/rfc6377>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <https://www.rfc-editor.org/info/rfc8020>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 8601,
DOI 10.17487/RFC8601, May 2019,
<https://www.rfc-editor.org/info/rfc8601>.
Appendix A. Technology Considerations
This section documents some design decisions that were made in the
development of DMARC. Specifically, addressed here are some
suggestions that were considered but not included in the design.
This text is included to explain why they were considered and not
included in this version.
A.1. S/MIME
S/MIME, or Secure Multipurpose Internet Mail Extensions, is a
standard for encryption and signing of MIME data in a message. This
was suggested and considered as a third security protocol for
authenticating the source of a message.
DMARC is focused on authentication at the domain level (i.e., the
Domain Owner taking responsibility for the message), while S/MIME is
really intended for user-to-user authentication and encryption. This
alone appears to make it a bad fit for DMARC's goals.
S/MIME also suffers from the heavyweight problem of Public Key
Infrastructure, which means that distribution of keys used to verify
signatures needs to be incorporated. In many instances, this alone
is a showstopper. There have been consistent promises that PKI
usability and deployment will improve, but these have yet to
materialize. DMARC can revisit this choice after those barriers are
addressed.
S/MIME has extensive deployment in specific market segments
(government, for example) but does not enjoy similar widespread
deployment over the general Internet, and this shows no signs of
changing. DKIM and SPF both are deployed widely over the general
Internet, and their adoption rates continue to be positive.
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Finally, experiments have shown that including S/MIME support in the
initial version of DMARC would neither cause nor enable a substantial
increase in the accuracy of the overall mechanism.
A.2. Method Exclusion
It was suggested that DMARC include a mechanism by which a Domain
Owner could tell Message Receivers not to attempt validation by one
of the supported methods (e.g., "check DKIM, but not SPF").
Specifically, consider a Domain Owner that has deployed one of the
technologies, and that technology fails for some messages, but such
failures don't cause enforcement action. Deploying DMARC would cause
enforcement action for policies other than "none", which would appear
to exclude participation by that Domain Owner.
The DMARC development team evaluated the idea of policy exception
mechanisms on several occasions and invariably concluded that there
was not a strong enough use case to include them. The specific
target audience for DMARC does not appear to have concerns about the
failure modes of one or the other being a barrier to DMARC's
adoption.
In the scenario described above, the Domain Owner has a few options:
1. Tighten up its infrastructure to minimize the failure modes of
the single deployed technology.
2. Deploy the other supported authentication mechanism, to offset
the failure modes of the first.
3. Deploy DMARC in a reporting-only mode.
A.3. Sender Header Field
It has been suggested in several message authentication efforts that
the Sender header field be checked for an identifier of interest, as
the standards indicate this as the proper way to indicate a re-
mailing of content such as through a mailing list. Most recently, it
was a protocol-level option for DomainKeys, but on evolution to DKIM,
this property was removed.
The DMARC development team considered this and decided not to include
support for doing so, for the following reasons:
1. The main user protection approach is to be concerned with what
the user sees when a message is rendered. There is no consistent
behavior among MUAs regarding what to do with the content of the
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Sender field, if present. Accordingly, supporting checking of
the Sender identifier would mean applying policy to an identifier
the end user might never actually see, which can create a vector
for attack against end users by simply forging a Sender field
containing some identifier that DMARC will like.
2. Although it is certainly true that this is what the Sender field
is for, its use in this way is also unreliable, making it a poor
candidate for inclusion in the DMARC evaluation algorithm.
3. Allowing multiple ways to discover policy introduces unacceptable
ambiguity into the DMARC evaluation algorithm in terms of which
policy is to be applied and when.
A.4. Domain Existence Test
A common practice among MTA operators, and indeed one documented in
[RFC5617], is a test to determine domain existence prior to any more
expensive processing. This is typically done by querying the DNS for
MX, A, or AAAA resource records for the name being evaluated and
assuming that the domain is nonexistent if it could be determined
that no such records were published for that domain name.
The original pre-standardization version of this protocol included a
mandatory check of this nature. It was ultimately removed, as the
method's error rate was too high without substantial manual tuning
and heuristic work. There are indeed use cases this work needs to
address where such a method would return a negative result about a
domain for which reporting is desired, such as a registered domain
name that never sends legitimate mail and thus has none of these
records present in the DNS.
A.5. Issues with ADSP in Operation
DMARC has been characterized as a "super-ADSP" of sorts.
Contributors to DMARC have compiled a list of issues associated with
ADSP, gained from operational experience, that have influenced the
direction of DMARC:
1. ADSP has no support for subdomains, i.e., the ADSP record for
example.com does not explicitly or implicitly apply to
subdomain.example.com. If wildcarding is not applied, then
spammers can trivially bypass ADSP by sending from a subdomain
with no ADSP record.
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2. Nonexistent subdomains are explicitly out of scope in ADSP.
There is nothing in ADSP that states receivers should simply
reject mail from NXDOMAINs regardless of ADSP policy (which of
course allows spammers to trivially bypass ADSP by sending email
from nonexistent subdomains).
3. ADSP has no operational advice on when to look up the ADSP
record.
4. ADSP has no support for using SPF as an auxiliary mechanism to
DKIM.
5. ADSP has no support for a slow rollout, i.e., no way to configure
a percentage of email on which the receiver should apply the
policy. This is important for large-volume senders.
6. ADSP has no explicit support for an intermediate phase where the
receiver quarantines (e.g., sends to the recipient's "spam"
folder) rather than rejects the email.
7. The binding between the "From" header domain and DKIM is too
tight for ADSP; they must match exactly.
A.6. Organizational Domain Discovery Issues
Although protocols like ADSP are useful for "protecting" a specific
domain name, they are not helpful at protecting subdomains. If one
wished to protect "example.com" by requiring via ADSP that all mail
bearing an RFC5322.From domain of "example.com" be signed, this would
"protect" that domain; however, one could then craft an email whose
RFC5322.From domain is "security.example.com", and ADSP would not
provide any protection. One could use a DNS wildcard, but this can
undesirably interfere with other DNS activity; one could add ADSP
records as fraudulent domains are discovered, but this solution does
not scale and is a purely reactive measure against abuse.
The DNS does not provide a method by which the "domain of record", or
the domain that was actually registered with a domain registrar, can
be determined given an arbitrary domain name. Suggestions have been
made that attempt to glean such information from SOA or NS resource
records, but these too are not fully reliable, as the partitioning of
the DNS is not always done at administrative boundaries.
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When seeking domain-specific policy based on an arbitrary domain
name, one could "climb the tree", dropping labels off the left end of
the name until the root is reached or a policy is discovered, but
then one could craft a name that has a large number of nonsense
labels; this would cause a Mail Receiver to attempt a large number of
queries in search of a policy record. Sending many such messages
constitutes an amplified denial-of-service attack.
The Organizational Domain mechanism is a necessary component to the
goals of DMARC. The method described in Section 3.15 is far from
perfect but serves this purpose reasonably well without adding undue
burden or semantics to the DNS. If a method is created to do so that
is more reliable and secure than the use of a public suffix list,
DMARC should be amended to use that method as soon as it is generally
available.
A.6.1. Public Suffix Lists
A public suffix list for the purposes of determining the
Organizational Domain can be obtained from various sources. The most
common one is maintained by the Mozilla Foundation and made public at
http://publicsuffix.org (http://publicsuffix.org). License terms
governing the use of that list are available at that URI.
Note that if operators use a variety of public suffix lists,
interoperability will be difficult or impossible to guarantee.
Appendix B. Examples
This section illustrates both the Domain Owner side and the Mail
Receiver side of a DMARC exchange.
B.1. Identifier Alignment Examples
The following examples illustrate the DMARC mechanism's use of
Identifier Alignment. For brevity's sake, only message headers are
shown, as message bodies are not considered when conducting DMARC
checks.
B.1.1. SPF
The following SPF examples assume that SPF produces a passing result.
Example 1: SPF in alignment:
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MAIL FROM: <sender@example.com>
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
In this case, the RFC5321.MailFrom parameter and the RFC5322.From
header field have identical DNS domains. Thus, the identifiers are
in alignment.
Example 2: SPF in alignment (parent):
MAIL FROM: <sender@child.example.com>
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
_Ticket 52_
In this case, the RFC5322.From header parameter includes a DNS domain
that is a parent of the RFC5321.MailFrom domain. Thus, the
identifiers are in alignment.
Example 3: SPF not in alignment:
MAIL FROM: <sender@example.net>
From: sender@child.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
_Ticket 109_
In this case, the RFC5321.MailFrom parameter includes a DNS domain
that is neither the same as, a parent of, nor a child of the
RFC5322.From domain. Thus, the identifiers are not in alignment.
B.1.2. DKIM
The examples below assume that the DKIM signatures pass verification.
Alignment cannot exist with a DKIM signature that does not verify.
Example 1: DKIM in alignment:
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DKIM-Signature: v=1; ...; d=example.com; ...
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
In this case, the DKIM "d=" parameter and the RFC5322.From header
field have identical DNS domains. Thus, the identifiers are in
alignment.
Example 2: DKIM in alignment (parent):
DKIM-Signature: v=1; ...; d=example.com; ...
From: sender@child.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
_Ticket 52_
In this case, the DKIM signature's "d=" parameter includes a DNS
domain that is a parent of the RFC5322.From domain. Thus, the
identifiers are in alignment.
Example 3: DKIM not in alignment:
DKIM-Signature: v=1; ...; d=sample.net; ...
From: sender@child.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
_Ticket 109_
In this case, the DKIM signature's "d=" parameter includes a DNS
domain that is neither the same as, a parent of, nor a child of the
RFC5322.From domain. Thus, the identifiers are not in alignment.
B.2. Domain Owner Example
A Domain Owner that wants to use DMARC should have already deployed
and tested SPF and DKIM. The next step is to publish a DNS record
that advertises a DMARC policy for the Domain Owner's Organizational
Domain.
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B.2.1. Entire Domain, Monitoring Only
The owner of the domain "example.com" has deployed SPF and DKIM on
its messaging infrastructure. The owner wishes to begin using DMARC
with a policy that will solicit aggregate feedback from receivers
without affecting how the messages are processed, in order to:
* Confirm that its legitimate messages are authenticating correctly
* Verify that all authorized message sources have implemented
authentication measures
* Determine how many messages from other sources would be affected
by a blocking policy
The Domain Owner accomplishes this by constructing a policy record
indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* Receivers should not alter how they treat these messages because
of this DMARC policy record ("p=none")
* Aggregate feedback reports should be sent via email to the address
"dmarc-feedback@example.com" ("rua=mailto:dmarc-
feedback@example.com")
_Ticket 47_
The DMARC policy record 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"
To publish such a record, the DNS administrator for the Domain Owner
creates 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" )
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B.2.2. Entire Domain, Monitoring Only, Per-Message Reports
The Domain Owner from the previous example has used the aggregate
reporting to discover some messaging systems that had not yet
implemented DKIM correctly, but they are still seeing periodic
authentication failures. In order to diagnose these intermittent
problems, they wish to request per-message failure reports when
authentication failures occur.
Not all Receivers will honor such a request, but the Domain Owner
feels that any reports it does receive will be helpful enough to
justify publishing this record. The default per-message report
format ([RFC6591]) meets the Domain Owner's needs in this scenario.
The Domain Owner accomplishes this by adding the following to its
policy record from Appendix B.2:
* Per-message failure reports should be sent via email to the
address "auth-reports@example.com" ("ruf=mailto:auth-
reports@example.com")
The DMARC policy record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT _dmarc.example.com.
"v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
ruf=mailto:auth-reports@example.com"
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" )
B.2.3. Per-Message Failure Reports Directed to Third Party
The Domain Owner from the previous example is maintaining the same
policy but now wishes to have a third party receive and process the
per-message failure reports. Again, not all Receivers will honor
this request, but those that do may implement additional checks to
validate that the third party wishes to receive the failure reports
for this domain.
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The Domain Owner needs to alter its policy record from Appendix B.2.2
as follows:
* Per-message failure reports should be sent via email to the
address "auth-reports@thirdparty.example.net" ("ruf=mailto:auth-
reports@thirdparty.example.net")
The DMARC policy record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT _dmarc.example.com.
"v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
ruf=mailto:auth-reports@thirdparty.example.net"
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@thirdparty.example.net" )
Because the address used in the "ruf" tag is outside the
Organizational Domain in which this record is published, conforming
Receivers will implement additional checks as described in the DMARC
reporting documents. In order to pass these additional checks, the
third party will need to publish an additional DNS record as follows:
* Given the DMARC record published by the Domain Owner at
"_dmarc.example.com", the DNS administrator for the third party
will need to publish a TXT resource record at
"example.com._report._dmarc.thirdparty.example.net" with the value
"v=DMARC1;".
The resulting DNS record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT example.com._report._dmarc.thirdparty.example.net
"v=DMARC1;"
To publish such a record, the DNS administrator for example.net might
create an entry like the following in the appropriate zone file
(following the conventional zone file format):
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; zone file for thirdparty.example.net
; Accept DMARC failure reports on behalf of example.com
example.com._report._dmarc IN TXT "v=DMARC1;"
Mediators and other third parties should refer to the DMARC reporting
documents for the full details of this mechanism.
_Tickets 47 and 53_
B.2.4. Subdomain and Multiple Aggregate Report URIs
_Tickets 85 and 109_
The Domain Owner has implemented SPF and DKIM in a subdomain used for
pre-production testing of messaging services. It now wishes to
express a severity of concern for messages from this subdomain that
fail to authenticate to indicate to participating receivers that use
of this domain is not valid.
_Tickets 47, 53, 85, and 109_
As a first step, it will express that it considers to be suspicious
messages using this subdomain that fail authentication. The goal
here will be to enable examination of messages sent to mailboxes
hosted by participating receivers as method for troubleshooting any
existing authentication issues. Aggregate feedback reports will be
sent to a mailbox within the Organizational Domain, and to a mailbox
at a third party selected and authorized to receive same by the
Domain Owner.
The Domain Owner will accomplish this by constructing a policy record
indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* It is applied only to this subdomain (record is published at
"_dmarc.test.example.com" and not "_dmarc.example.com")
_Ticket 109_
* Receivers are advised that the Domain Owner considers messages
that fail to authenticate to be suspicious ("p=quarantine")
_Ticket 53_
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* Aggregate feedback reports should be sent via email to the
addresses "dmarc-feedback@example.com" and "example-tld-
test@thirdparty.example.net" ("rua=mailto:dmarc-
feedback@example.com, mailto:tld-test@thirdparty.example.net")
_Ticket 47_
The DMARC policy record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
_Ticket 47_
% dig +short TXT _dmarc.test.example.com
"v=DMARC1; p=quarantine; rua=mailto:dmarc-feedback@example.com,
mailto:tld-test@thirdparty.example.net"
To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone
file:
_Tickets 47 and 109_
; DMARC record for the domain test.example.com
_dmarc IN TXT ( "v=DMARC1; p=quarantine; "
"rua=mailto:dmarc-feedback@example.com,"
"mailto:tld-test@thirdparty.example.net" )
_Ticket 109_
Once enough time has passed to allow for collecting enough reports to
give the Domain Owner confidence that all legitimate email sent using
the subdomain is properly authenticating and passing DMARC checks,
then the Domain Owner can update the policy record to indicate that
it considers authentication failures to be a clear indication that
use of the subdomain is not valid. It would do this by altering the
DNS record to advise receivers of its position on such messages
("p=reject").
After alteration, the DMARC policy record might look like this when
retrieved using a common command-line tool (the output shown would
appear on a single line but is wrapped here for publication):
% dig +short TXT _dmarc.test.example.com
"v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com,
mailto:tld-test@thirdparty.example.net"
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To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone
file:
; DMARC record for the domain test.example.com
_dmarc IN TXT ( "v=DMARC1; p=reject; "
"rua=mailto:dmarc-feedback@example.com,"
"mailto:tld-test@thirdparty.example.net" )
B.3. Mail Receiver Example
A Mail Receiver that wants to use DMARC should already be checking
SPF and DKIM, and possess the ability to collect relevant information
from various email-processing stages to provide feedback to Domain
Owners (possibly via Report Receivers).
B.4. Processing of SMTP Time
_Ticket 109_
An optimal DMARC-enabled Mail Receiver performs authentication and
Identifier Alignment checking during the [RFC5321] conversation.
Prior to returning a final reply to the DATA command, the Mail
Receiver's MTA has performed:
1. An SPF check to determine an SPF-authenticated Identifier.
2. DKIM checks that yield one or more DKIM-authenticated
Identifiers.
3. A DMARC policy lookup.
The presence of an Author Domain DMARC record indicates that the Mail
Receiver should continue with DMARC-specific processing before
returning a reply to the DATA command.
_Ticket 52_
Given a DMARC record and the set of Authenticated Identifiers, the
Mail Receiver checks to see if the Authenticated Identifiers align
with the Author Domain.
For example, the following sample data is considered to be from a
piece of email originating from the Domain Owner of "example.com":
_Ticket 52_
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Author Domain: example.com
SPF-authenticated Identifier: mail.example.com
DKIM-authenticated Identifier: example.com
DMARC record:
"v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com"
_Ticket 109_
In the above sample, both the SPF-authenticated Identifier and the
DKIM-authenticated Identifier align with the Author Domain. The Mail
Receiver considers the above email to pass the DMARC check, avoiding
the "reject" policy that is requested to be applied to email that
fails to pass the DMARC check.
_Ticket 85_
If no Authenticated Identifiers align with the Author Domain, then
the Mail Receiver applies the DMARC-record-specified policy.
However, before this action is taken, the Mail Receiver can consult
external information to override the Domain Owner's Assessment
Policy.
For example, if the Mail Receiver knows that this particular email
came from a known and trusted forwarder (that happens to break both
SPF and DKIM), then the Mail Receiver may choose to ignore the Domain
Owner's policy.
The Mail Receiver is now ready to reply to the DATA command. If the
DMARC check yields that the message is to be rejected, then the Mail
Receiver replies with a 5xy code to inform the sender of failure. If
the DMARC check cannot be resolved due to transient network errors,
then the Mail Receiver replies with a 4xy code to inform the sender
as to the need to reattempt delivery later. If the DMARC check
yields a passing message, then the Mail Receiver continues on with
email processing, perhaps using the result of the DMARC check as an
input to additional processing modules such as a domain reputation
query.
Before exiting DMARC-specific processing, the Mail Receiver checks to
see if the Author Domain DMARC record requests AFRF-based reporting.
If so, then the Mail Receiver can emit an AFRF to the reporting
address supplied in the DMARC record.
At the exit of DMARC-specific processing, the Mail Receiver captures
(through logging or direct insertion into a data store) the result of
DMARC processing. Captured information is used to build feedback for
Domain Owner consumption. This is not necessary if the Domain Owner
has not requested aggregate reports, i.e., no "rua" tag was found in
the policy record.
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B.5. Utilization of Aggregate Feedback: Example
Aggregate feedback is consumed by Domain Owners to verify a Domain
Owner's understanding of how the Domain Owner's domain is being
processed by the Mail Receiver. Aggregate reporting data on emails
that pass all DMARC-supporting authentication checks is used by
Domain Owners to verify that authentication practices remain
accurate. For example, if a third party is sending on behalf of a
Domain Owner, the Domain Owner can use aggregate report data to
verify ongoing authentication practices of the third party.
Data on email that only partially passes underlying authentication
checks provides visibility into problems that need to be addressed by
the Domain Owner. For example, if either SPF or DKIM fails to pass,
the Domain Owner is provided with enough information to either
directly correct the problem or understand where authentication-
breaking changes are being introduced in the email transmission path.
If authentication-breaking changes due to email transmission path
cannot be directly corrected, then the Domain Owner at least
maintains an understanding of the effect of DMARC-based policies upon
the Domain Owner's email.
Data on email that fails all underlying authentication checks
provides baseline visibility on how the Domain Owner's domain is
being received at the Mail Receiver. Based on this visibility, the
Domain Owner can begin deployment of authentication technologies
across uncovered email sources. Additionally, the Domain Owner may
come to an understanding of how its domain is being misused.
_Ticket 109_
(Aggregate report example should be moved to
[DMARC-Aggregate-Reporting])
Appendix C. Change Log
C.1. January 5, 2021
C.1.1. Ticket 80 - DMARCbis SHould Have Clear and Concise Defintion of
DMARC
* Updated text for Abstract and Introduction sections.
* Diffs are recorded here - https://github.com/ietf-wg-dmarc/draft-
ietf-dmarc-dmarcbis/pull/1/files (https://github.com/ietf-wg-
dmarc/draft-ietf-dmarc-dmarcbis/pull/1/files)
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C.2. February 4, 2021
C.2.1. Ticket 1 - SPF RFC 4408 vs 7208
* Some rearranging of text in the "SPF-Authenticated Identifiers"
section
* Clarification of the term "in alignment" in that same section
* Diffs are here - https://github.com/ietf-wg-dmarc/draft-ietf-
dmarc-dmarcbis/pull/3/files (https://github.com/ietf-wg-dmarc/
draft-ietf-dmarc-dmarcbis/pull/3/files)
C.3. February 10, 2021
C.3.1. Ticket 84 - Remove Erroneous References to RFC3986
* Several references to RFC3986 changed to RFC7208
* Diffs are here - https://github.com/ietf-wg-dmarc/draft-ietf-
dmarc-dmarcbis/pull/4/files (https://github.com/ietf-wg-dmarc/
draft-ietf-dmarc-dmarcbis/pull/4/files)
C.4. March 1, 2021
C.4.1. Design Team Work Begins
* Added change log section to document
C.5. March 8, 2021
C.5.1. Removed E. Gustafsson as editor
* He withdrew as editor after a job change.
C.5.2. Ticket 3 - Two tiny nits
* Changes to wording in section 6.6.2, Determine Handling Policy,
steps 3 and 4.
* New text documented here - https://trac.ietf.org/trac/dmarc/
ticket/3#comment:6 (https://trac.ietf.org/trac/dmarc/
ticket/3#comment:6)
* No change to section 6.6.3, Policy Discovery; ticket seems to pre-
date current text, which appears to have answered the concern
raised.
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C.5.3. Ticket 4 - Definition of "fo" parameter
* Changes to wording in section 6.3, to bring clarity to use of
colon-separated list as possible value to "fo"
* New text documented here - https://trac.ietf.org/trac/dmarc/
ticket/4#comment:4 (https://trac.ietf.org/trac/dmarc/
ticket/4#comment:4)
C.6. March 16, 2021
C.6.1. Ticket 7 - ABNF for dmarc-record is slightly wrong
* New text documented here - https://trac.ietf.org/trac/dmarc/
ticket/7 (https://trac.ietf.org/trac/dmarc/ticket/7)
C.6.2. Ticket 26 - ABNF for pct allows "999"
* Updated ABNF for dmarc-percent
* New text documented here - https://trac.ietf.org/trac/dmarc/
ticket/26#comment:6 (https://trac.ietf.org/trac/dmarc/
ticket/26#comment:6)
* Ticket 47, Remove pct= tag, rendered change obsolete
C.7. March 23, 2021
C.7.1. Ticket 75 - Using wording alternatives to 'disposition',
'dispose', and the like
* Changed disposition/dispose to "handling"
* Diffs documented here - https://trac.ietf.org/trac/dmarc/
ticket/75#comment:3 (https://trac.ietf.org/trac/dmarc/
ticket/75#comment:3)
C.7.2. Ticket 72 - Remove absolute requirement for p= tag in DMARC
record
* Changed from REQUIRED to RECOMMENDED, noted default with forward
reference to discussion
* Diffs documented here - https://trac.ietf.org/trac/dmarc/
ticket/72#comment:3 (https://trac.ietf.org/trac/dmarc/
ticket/72#comment:3)
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C.8. March 29, 2021
C.8.1. Ticket 54 - Remove or expand limits on number of recipients per
report
* Removed limit
* Diffs documented here - https://trac.ietf.org/trac/dmarc/
ticket/54#comment:5 (https://trac.ietf.org/trac/dmarc/
ticket/54#comment:5)
C.9. April 12, 2021
C.9.1. Ticket 50 - Remove ri= tag
* Updated text to recommend against its usage, a la the ptr
mechanism in RFC 7208
* Diffs documented here - https://trac.ietf.org/trac/dmarc/
ticket/50#comment:5 (https://trac.ietf.org/trac/dmarc/
ticket/50#comment:5)
C.9.2. Ticket 66 - Define what it means to have implemented DMARC
* Proposed new text (taken straight from
https://trac.ietf.org/trac/dmarc/ticket/66
(https://trac.ietf.org/trac/dmarc/ticket/66) as replacement for
current text in "Minimum Implemenatations"
C.9.3. Ticket 96 - Tweaks to Abstract and Introduction
* Changed phrase in Abstract to "an email author's domain name"
* Changed phrase in Introduction to "reports about email use of the
domain name"
C.10. April 13, 2021
C.10.1. Ticket 53 - Remove reporting message size chunking
* Proposed text to remove all references to message size chunking
* Data demonstrating lack of use of feature entered into ticket -
https://trac.ietf.org/trac/dmarc/ticket/53#comment:4
(https://trac.ietf.org/trac/dmarc/ticket/53#comment:4)
C.10.2. Ticket 52 - Remove strict alignment (and adkim and aspf tags)
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* Proposed text to remove all references to strict alignment
* Data demonstrating lack of use of feature entered into ticket -
https://trac.ietf.org/trac/dmarc/ticket/52#comment:2
(https://trac.ietf.org/trac/dmarc/ticket/52#comment:2)
C.10.3. Ticket 47 - Remove pct= tag
* Proposed text to remove all references to pct and message sampling
* Data demonstrating lack of use of feature entered into ticket -
https://trac.ietf.org/trac/dmarc/ticket/47#comment:4
(https://trac.ietf.org/trac/dmarc/ticket/47#comment:4)
C.10.4. Ticket 2 - Flow of operations text in dmarc-base
* Update ASCII Art
* Proposed text to replace description of ASCII Art
* Proposed text to update Domain Owner Actions section
C.11. April 14, 2021
C.11.1. Ticket 107 - DMARCbis should take a stand on multi-valued From
fields
* Proposed text that limits processing to only those times when all
domains are the same.
C.11.2. Ticket 82 - Deprecate rf= and maybe fo= tag
* Proposed text to deprecate rf= tag, while leaving fo= tag as is
C.11.3. Ticket 85 - Proposed change to wording describing 'p' tag and
values
* The language expressing the semantics is proposed to be changed to
be, in a sense, egocentric. How do I, the domain owner feel about
(assess) the meaning of a DMARC failure?
C.12. April 15, 2021
C.12.1. Ticket 86 - A-R results for DMARC
* Proposed text to add for polrec.p and polrec.domain methods for
registry update.
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* Did not include polrec.pct due to proposal to remove pct tag
(Ticket 47)
C.12.2. Ticket 62 - Make aggregate reporting a normative MUST
* Proposed text to do just that in Mail Receiver Actions, section
titled "Send Aggregate Reports"
C.13. April 19, 2021
C.13.1. Ticket 109 - Sanity Check DMARCbis Document
* Updated document to remove all "original text"/"proposed text"
couplets in favor of one (hopefully coherent) document full of
proposed text changes.
* Noted which tickets were the cause of whatever rfcdiff output will
show in tracker
C.14. April 20, 2021
C.14.1. Ticket 108 - Changes to DMARCbis for PSD
* Incorporating requests for changes to DMARCbis made in text of
"Experimental DMARC Extension for Public Suffix Domains"
(https://datatracker.ietf.org/doc/draft-ietf-dmarc-psd/
(https://datatracker.ietf.org/doc/draft-ietf-dmarc-psd/))
C.15. April 22, 2021
C.15.1. Ticket 104 - Update the Security Considerations section 11.3 on
DNS
* Updated text. Diffs are here - https://github.com/ietf-wg-dmarc/
draft-ietf-dmarc-dmarcbis/pull/31/files (https://github.com/ietf-
wg-dmarc/draft-ietf-dmarc-dmarcbis/pull/31/files)
Acknowledgements
DMARC and the draft version of this document submitted to the
Independent Submission Editor were the result of lengthy efforts by
an informal industry consortium: DMARC.org (see http://dmarc.org
(http://dmarc.org)). Participating companies included Agari,
American Greetings, AOL, Bank of America, Cloudmark, Comcast,
Facebook, Fidelity Investments, Google, JPMorgan Chase & Company,
LinkedIn, Microsoft, Netease, PayPal, ReturnPath, The Trusted Domain
Project, and Yahoo!. Although the contributors and supporters are
too numerous to mention, notable individual contributions were made
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by J. Trent Adams, Michael Adkins, Monica Chew, Dave Crocker, Tim
Draegen, Steve Jones, Franck Martin, Brett McDowell, and Paul Midgen.
The contributors would also like to recognize the invaluable input
and guidance that was provided early on by J.D. Falk.
Additional contributions within the IETF context were made by Kurt
Anderson, Michael Jack Assels, Les Barstow, Anne Bennett, Jim Fenton,
J. Gomez, Mike Jones, Scott Kitterman, Eliot Lear, John Levine, S.
Moonesamy, Rolf Sonneveld, Henry Timmes, and Stephen J. Turnbull.
_Ticket 108_
Authors' Addresses
Todd M. Herr
Valimail
Email: todd.herr@valimail.com
John Levine
Standcore LLC
Email: standards@standore.com
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