Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-31
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| Authors | Todd Herr , John R. Levine | ||
| Last updated | 2024-05-20 (Latest revision 2024-02-28) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-dmarc-dmarcbis-31
DMARC T. Herr (ed)
Internet-Draft Valimail
Obsoletes: 7489, 9091 (if approved) J. Levine (ed)
Intended status: Standards Track Standcore LLC
Expires: 21 November 2024 20 May 2024
Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-31
Abstract
This document describes the Domain-based Message Authentication,
Reporting, and Conformance (DMARC) protocol.
DMARC permits the owner of an email's Author Domain (#author-domain)
to enable validation of the domain's use, to indicate the Domain
Owner's (#domain-owner) or Public Suffix Operator's (#public-suffix-
operator) message handling preference regarding failed validation,
and to request reports about the use of the domain name. Mail
receiving organizations can use this information when evaluating
handling choices for incoming mail.
This document obsoletes RFCs 7489 and 9091.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 21 November 2024.
Copyright Notice
Copyright (c) 2024 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
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. High-Level Goals . . . . . . . . . . . . . . . . . . . . 6
2.2. Anti-Phishing . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 7
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 8
3.1. Conventions Used in This Document . . . . . . . . . . . . 8
3.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Authenticated Identifiers . . . . . . . . . . . . . . 9
3.2.2. Author Domain . . . . . . . . . . . . . . . . . . . . 9
3.2.3. DMARC Policy Record . . . . . . . . . . . . . . . . . 9
3.2.4. Domain Owner . . . . . . . . . . . . . . . . . . . . 9
3.2.5. Domain Owner Assessment Policy . . . . . . . . . . . 9
3.2.6. Enforcement . . . . . . . . . . . . . . . . . . . . . 9
3.2.7. Identifier Alignment . . . . . . . . . . . . . . . . 10
3.2.8. Mail Receiver . . . . . . . . . . . . . . . . . . . . 10
3.2.9. Monitoring Mode . . . . . . . . . . . . . . . . . . . 10
3.2.10. Non-existent Domains . . . . . . . . . . . . . . . . 11
3.2.11. Organizational Domain . . . . . . . . . . . . . . . . 11
3.2.12. Public Suffix Domain (PSD) . . . . . . . . . . . . . 11
3.2.13. Public Suffix Operator (PSO) . . . . . . . . . . . . 11
3.2.14. PSO Controlled Domain Names . . . . . . . . . . . . . 11
3.2.15. Report Consumer . . . . . . . . . . . . . . . . . . . 11
4. Overview and Key Concepts . . . . . . . . . . . . . . . . . . 12
4.1. DMARC Basics . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Use of RFC5322.From . . . . . . . . . . . . . . . . . . . 13
4.3. Authentication Mechanisms . . . . . . . . . . . . . . . . 13
4.4. Identifier Alignment Explained . . . . . . . . . . . . . 13
4.4.1. DKIM-Authenticated Identifiers . . . . . . . . . . . 14
4.4.2. SPF-Authenticated Identifiers . . . . . . . . . . . . 15
4.4.3. Alignment and Extension Technologies . . . . . . . . 15
4.5. DMARC Policy Record Explained . . . . . . . . . . . . . . 15
4.6. DMARC Reporting URIs . . . . . . . . . . . . . . . . . . 16
4.7. DMARC Policy Record Format . . . . . . . . . . . . . . . 17
4.8. Formal Definition . . . . . . . . . . . . . . . . . . . . 21
4.9. Flow Diagram . . . . . . . . . . . . . . . . . . . . . . 23
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4.10. DNS Tree Walk . . . . . . . . . . . . . . . . . . . . . . 25
4.10.1. DMARC Policy Discovery . . . . . . . . . . . . . . . 27
4.10.2. Identifier Alignment Evaluation . . . . . . . . . . 28
5. DMARC Participation . . . . . . . . . . . . . . . . . . . . . 30
5.1. Domain Owner Actions . . . . . . . . . . . . . . . . . . 30
5.1.1. Publish an SPF Policy for an Aligned Domain . . . . . 30
5.1.2. Configure Sending System for DKIM Signing Using an
Aligned Domain . . . . . . . . . . . . . . . . . . . 30
5.1.3. Setup a Mailbox to Receive Aggregate Reports . . . . 31
5.1.4. Publish a DMARC Policy Record for the Author Domain and
Organizational Domain . . . . . . . . . . . . . . . . 31
5.1.5. Collect and Analyze Reports . . . . . . . . . . . . . 31
5.1.6. Remediate Unaligned or Unauthenticated Mail
Streams . . . . . . . . . . . . . . . . . . . . . . . 31
5.1.7. Decide Whether to Update Domain Owner Assessment Policy
to Enforcement . . . . . . . . . . . . . . . . . . . 32
5.1.8. A Note on Large, Complex Organizations and
Decentralized DNS Management . . . . . . . . . . . . 32
5.2. PSO Actions . . . . . . . . . . . . . . . . . . . . . . . 33
5.3. Mail Receiver Actions . . . . . . . . . . . . . . . . . . 33
5.3.1. Extract Author Domain . . . . . . . . . . . . . . . . 34
5.3.2. Determine If The DMARC Mechanism Applies . . . . . . 34
5.3.3. Determine If Authenticated Identifiers Exist . . . . 34
5.3.4. Conduct Identifier Alignment Checks If Necessary . . 34
5.3.5. Determine DMARC "Pass" or "Fail" . . . . . . . . . . 35
5.3.6. Apply Policy If Appropriate . . . . . . . . . . . . . 35
5.3.7. Store Results of DMARC Processing . . . . . . . . . . 35
5.3.8. Send Aggregate Reports . . . . . . . . . . . . . . . 35
5.3.9. Optionally Send Failure Reports . . . . . . . . . . . 36
5.4. Policy Enforcement Considerations . . . . . . . . . . . . 36
6. DMARC Feedback . . . . . . . . . . . . . . . . . . . . . . . 37
7. Other Topics . . . . . . . . . . . . . . . . . . . . . . . . 37
7.1. Issues Specific to SPF . . . . . . . . . . . . . . . . . 37
7.2. DNS Load and Caching . . . . . . . . . . . . . . . . . . 38
7.3. Rejecting Messages . . . . . . . . . . . . . . . . . . . 38
7.4. Interoperability Issues . . . . . . . . . . . . . . . . . 39
7.5. Interoperability Considerations . . . . . . . . . . . . . 40
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
8.1. Authentication-Results Method Registry Update . . . . . . 41
8.2. Authentication-Results Result Registry Update . . . . . . 42
8.3. Feedback Report Header Fields Registry Update . . . . . . 43
8.4. DMARC Tag Registry . . . . . . . . . . . . . . . . . . . 43
8.5. DMARC Report Format Registry . . . . . . . . . . . . . . 45
8.6. Underscored and Globally Scoped DNS Node Names
Registry . . . . . . . . . . . . . . . . . . . . . . . . 45
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 46
9.1. Aggregate Report Considerations . . . . . . . . . . . . . 46
9.2. Failure Report Considerations . . . . . . . . . . . . . . 46
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10. Security Considerations . . . . . . . . . . . . . . . . . . . 47
10.1. Authentication Methods . . . . . . . . . . . . . . . . . 47
10.2. Attacks on Reporting URIs . . . . . . . . . . . . . . . 47
10.3. DNS Security . . . . . . . . . . . . . . . . . . . . . . 48
10.4. Display Name Attacks . . . . . . . . . . . . . . . . . . 48
10.5. Denial of DMARC Processing Attacks . . . . . . . . . . . 49
10.6. External Reporting Addresses . . . . . . . . . . . . . . 49
10.7. Secure Protocols . . . . . . . . . . . . . . . . . . . . 50
10.8. Relaxed Alignment Considerations . . . . . . . . . . . . 50
11. Normative References . . . . . . . . . . . . . . . . . . . . 51
12. Informative References . . . . . . . . . . . . . . . . . . . 53
Appendix A. Technology Considerations . . . . . . . . . . . . . 55
A.1. S/MIME . . . . . . . . . . . . . . . . . . . . . . . . . 55
A.2. Method Exclusion . . . . . . . . . . . . . . . . . . . . 55
A.3. Sender Header Field . . . . . . . . . . . . . . . . . . . 56
A.4. Domain Existence Test . . . . . . . . . . . . . . . . . . 57
A.5. Organizational Domain Discovery Issues . . . . . . . . . 57
A.6. Removal of the "pct" Tag . . . . . . . . . . . . . . . . 58
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 59
B.1. Identifier Alignment Examples . . . . . . . . . . . . . . 59
B.1.1. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 59
B.1.2. DKIM . . . . . . . . . . . . . . . . . . . . . . . . 60
B.2. Domain Owner Example . . . . . . . . . . . . . . . . . . 61
B.2.1. Entire Domain, Monitoring Mode . . . . . . . . . . . 61
B.2.2. Entire Domain, Monitoring Mode, Per-Message
Reports . . . . . . . . . . . . . . . . . . . . . . . 62
B.2.3. Per-Message Failure Reports Directed to Third
Party . . . . . . . . . . . . . . . . . . . . . . . . 63
B.2.4. Subdomain, Testing, and Multiple Aggregate Report
URIs . . . . . . . . . . . . . . . . . . . . . . . . 64
B.3. Mail Receiver Example . . . . . . . . . . . . . . . . . . 66
B.3.1. SMTP Session Example . . . . . . . . . . . . . . . . 66
B.4. Organizational and Policy Domain Tree Walk Examples . . . 67
B.4.1. Simple Organizational and Policy Example . . . . . . 68
B.4.2. Deep Tree Walk Example . . . . . . . . . . . . . . . 68
B.4.3. Example with a PSD DMARC Record . . . . . . . . . . . 69
B.5. Utilization of Aggregate Feedback: Example . . . . . . . 70
Appendix C. Changes from RFC 7489 . . . . . . . . . . . . . . . 71
C.1. IETF Category . . . . . . . . . . . . . . . . . . . . . . 71
C.2. Changes to Terminology and Definitions . . . . . . . . . 71
C.2.1. Terms Added . . . . . . . . . . . . . . . . . . . . . 71
C.2.2. Definitions Updated . . . . . . . . . . . . . . . . . 72
C.3. Policy Discovery and Organizational Domain
Determination . . . . . . . . . . . . . . . . . . . . . 72
C.4. Reporting . . . . . . . . . . . . . . . . . . . . . . . . 72
C.5. Tags . . . . . . . . . . . . . . . . . . . . . . . . . . 72
C.5.1. Tags Added: . . . . . . . . . . . . . . . . . . . . . 72
C.5.2. Tags Removed: . . . . . . . . . . . . . . . . . . . . 72
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C.6. Expansion of Domain Owner Actions Section . . . . . . . . 73
C.7. Report Generator Recommendations . . . . . . . . . . . . 73
C.8. Removal of RFC 7489 Appendix A.5 . . . . . . . . . . . . 73
C.9. RFC 7489 Errata Summary . . . . . . . . . . . . . . . . . 73
C.10. General Editing and Formatting . . . . . . . . . . . . . 75
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 75
Acknowledgements - RFC 7489 . . . . . . . . . . . . . . . . . . . 75
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 76
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source for this draft is maintained on GitHub at:
https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis
(https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis)
Abusive email often includes unauthorized and deceptive use of a
domain name in the "From" header field defined in Section 3.6.2 of
[RFC5322] and referred to as RFC5322.From. The domain typically
belongs to an organization expected to be known to - and presumably
trusted by - the recipient. The Sender Policy Framework (SPF)
[RFC7208] and DomainKeys Identified Mail (DKIM) [RFC6376] protocols
provide domain-level authentication but are not directly associated
with the RFC5322.From domain, also known as the Author Domain
(#author-domain). DMARC leverages these two protocols, providing a
method for Domain Owners to publish a DNS TXT record describing the
email authentication policies for the Author Domain and to request
specific handling for messages using that domain that fail validation
checks. These DNS records are called DMARC Policy Records (#dmarc-
policy-record).
As with SPF and DKIM, DMARC valdation results are either "pass" or
"fail". A DMARC result of "pass" requires not only an SPF or DKIM
pass verdict for the email message, but also and more importantly
that the domain associated with the SPF or DKIM pass be "aligned"
with the Author Domain in one of two modes - "relaxed" or "strict".
Domains are said to be in "relaxed alignment" if they have the same
Organizational Domain (#organizational-domain); a domain's
Organizational Domain is the domain at the the top of the namespace
hierarchy for that domain while having the same administrative
authority as that domain. On the other hand, domains are in "strict
alignment" if and only if they are identical. The choice of required
alignment mode is left to the Domain Owner (#domain-owner) that
publishes a DMARC Policy Record.
A DMARC pass for a message indicates only that the use of the Author
Domain has been validated for that message as authorized by the
Domain Owner. Such authorization does not carry an explicit or
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implicit value assertion about that message or about the Domain
Owner, and so a DMARC pass by itself does not guarantee delivery of
the message to the recipient's Inbox. Furthermore, a mail-receiving
organization that performs DMARC validation can choose to honor the
Domain Owner's requested message handling for validation failures,
but it is not required to do so; it might choose different actions
entirely.
For a mail-receiving organization participating in DMARC, a message
that passes DMARC validation is part of a message stream reliably
associated with the Author Domain. Therefore, reputation assessment
of that stream by the mail-receiving organization can assume the use
of that Author Domain is authorized by the Domain Owner. A message
that fails this validation is not necessarily associated with the
Author Domain and should not affect its reputation.
DMARC, in the associated [I-D.ietf-dmarc-aggregate-reporting] and
[I-D.ietf-dmarc-failure-reporting] documents, also specifies a
reporting framework. Using it, a mail-receiving organization can
generate regular reports about messages that use Author Domains for
which a DMARC Policy Record exists; those reports are sent to the
address(es) specified by the Domain Owner in the DMARC Policy Record.
Domain Owners can use these reports, especially the aggregate
reports, not only to identify sources of mail attempting to
fraudulently use their domain, but also (and perhaps more
importantly) to flag and fix gaps in their authentication practices.
However, as with honoring the Domain Owner's stated mail handling
preference, a mail-receiving organization supporting DMARC is under
no obligation to send requested reports, although it is recommended
that they do send aggregate reports.
The use of DMARC creates some interoperability challenges that
require due consideration before deployment, particularly with
configurations that can cause mail to be rejected. These are
discussed in Section 7.
2. Requirements
The following sections describe topics that guide the specification
of DMARC.
2.1. High-Level Goals
DMARC has the following high-level goals:
* Allow Domain Owners (#domain-owner) and Public Suffix Operators
(PSOs) (#public-suffix-operator) to validate their email
authentication deployment.
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* Allow Domain Owners and PSOs to assert their desired message
handling for validation failures for messages purporting to have
authorship within the domain.
* 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 emails.
* Work at Internet scale.
2.2. Anti-Phishing
DMARC is designed to prevent the unauthorized use of the Author
Domain (#author-domain) of an email message, a technique known as
"spoofing", particularly in transactional email (official mail 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).
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 emails. 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, as defined in
Section 3.4 of [RFC5322].
2.3. Scalability
Scalability is a significant issue for systems that need to operate
in an environment 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. Out of Scope
Several topics and issues are specifically out of scope of this work.
These include the following:
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* Different treatment of messages that are not authenticated (e.g.,
those that have no DKIM signature or those sent using an Author
Domain (#author-domain) for which no DMARC Policy Record (#dmarc-
policy-record) exists) versus those that fail validation;
* 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 display-name portions of the RFC5322.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.
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] and [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 (#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. Definitions
The following sections define the terms used in this document.
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3.2.1. Authenticated Identifiers
Authenticated Identifiers are those domain-level identifiers for
which authorized use is validated using a supported authentication
mechanism (#authentication-mechanisms).
3.2.2. Author Domain
The domain name of the apparent author as extracted from the
RFC5322.From header field.
3.2.3. DMARC Policy Record
A DNS TXT record published by a Domain Owner (#domain-owner) or
Public Suffix Operator (PSO) (#public-suffix-operator) to enable
validation of an Author Domain's (#author-domain) use, to indicate
the Domain Owner's or PSO's message handling preference regarding
failed validation, and to request reports about the use of the Author
Domain.
3.2.4. Domain Owner
An entity or organization that has control of a given DNS domain,
usually by holding its registration. 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 Consumers
when those are outside of their immediate management domain.
3.2.5. Domain Owner Assessment Policy
The message handling preference expressed in a DMARC Policy Record
(#dmarc-policy-record) by the Domain Owner (#domain-owner) regarding
failed validation of the Author Domain (#author-domain) is called the
"Domain Owner Assessment Policy". Possible values are described in
Section 4.7.
3.2.6. Enforcement
Enforcement describes a state where the existing Domain Owner
Assessment Policy (#domain-owner-policy) for an Organizational Domain
(#organizational-domain) and all subdomains below it is not "p=none".
This state means that the Organizational Domain and its subdomains
can only be used as Author Domains (#author-domain) if they are
properly validated using the DMARC mechanism.
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Historically, Domain Owner Assessment Policies of "p=quarantine" or
"p=reject" have been higher value signals to Mail Receivers (#mail-
receiver). Messages with Author Domains for which such policies
exist that are not validated using the DMARC mechanism will not reach
the inbox at Mail Receivers that participate in DMARC and honor the
Domain Owner's expressed handling preference.
3.2.7. Identifier Alignment
DMARC describes the concept of alignment between the Author Domain
(#author-domain) and an Authenticated Identifier (#authenticated-
identifiers), and requires such Identifier Alignment between the two
for a message to achieve a DMARC pass. DMARC defines two states for
alignment.
3.2.7.1. Relaxed Alignment
When the Author Domain (#author-domain) has the same Organizational
Domain (#organizational-domain) as an Authenticated Identifier
(#authenticated-identifier), the two are said to be in relaxed
alignment.
3.2.7.2. Strict Alignment
When the Author Domain (#author-domain) is identical to an
Authenticated Identifier (#authenticated-identifier), the two are
said to be in strict alignment.
3.2.8. Mail Receiver
The entity or organization that receives and processes email. Mail
Receivers operate one or more Internet-facing Mail Transport Agents
(MTAs).
3.2.9. Monitoring Mode
Monitoring Mode describes a state where the existing Domain Owner
Assessment Policy (#domain-owner-policy) for an Organizational Domain
(#organizational-domain) and all subdomains below it is "p=none", and
the Domain Owner (#domain-owner) is receiving aggregate reports for
the Organizational Domain. While the use of the Organizational
Domain and all its subdomains as Author Domains (#author-domain) can
still be validated by a Mail Receiver (#mail-receiver) deploying the
DMARC mechanism, the Domain Owner expresses no handling preference
for messages that fail DMARC validation. The Domain Owner is,
however, using the content of the DMARC aggregate reports to make any
needed adjustments to the authentication practices for its mail
streams.
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3.2.10. Non-existent Domains
For DMARC purposes, a non-existent domain is consistent with the
term's meaning as described in [RFC8020]. That is, if the response
code received for a query for a domain name is NXDOMAIN, then the
domain name and any possible subdomains do not exist.
3.2.11. Organizational Domain
The Organizational Domain for any domain is akin to the ADMD
described in [RFC5598]. A domain's Organizational Domain is the
domain at the top of the namespace hierarchy for that domain while
having the same administrative authority as the domain. An
Organizational Domain is determined by applying the algorithm found
in Section 4.10
3.2.12. Public Suffix Domain (PSD)
Some domains allow the registration of subdomains that are "owned" by
independent organizations. Real-world examples of these domains are
".com", ".org", ".us", and ".co.uk", to name just a few. These
domains are called "Public Suffix Domains (PSDs)". For example,
"ietf.org" is a registered domain name, and ".org" is its PSD.
3.2.13. Public Suffix Operator (PSO)
A Public Suffix Operator is an organization that manages operations
within a PSD, particularly the DNS records published for names at and
under that domain name.
3.2.14. PSO Controlled Domain Names
PSO-Controlled Domain Names are names in the DNS that are managed by
a PSO. PSO-controlled Domain Names may have one label (e.g., ".com")
or more (e.g., ".co.uk"), depending on the PSD's policy.
3.2.15. Report Consumer
A Report Consumer is an operator that receives reports from another
operator implementing the reporting mechanisms described in the
documents [I-D.ietf-dmarc-aggregate-reporting] and
[I-D.ietf-dmarc-failure-reporting]. Such an operator might be
receiving reports about messages related to a domain for which it is
the Domain Owner (#domain-owner) or PSO (#public-suffix-operator) 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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4. Overview and Key Concepts
This section provides a general overview of the design and operation
of the DMARC environment.
4.1. DMARC Basics
DMARC permits a Domain Owner (#domain-owner) or PSO (#public-suffix-
operator) to enable validation of an Author Domain's (#author-domain)
use in an email message, to indicate the Domain Owner's or PSO's
message handling preference regarding failed validation, and to
request reports about use of the Author Domain. A domain's DMARC
Policy Record (#dmarc-policy-record) is published in DNS as a TXT
record at the name created by prepending the label "_dmarc" to the
domain name and is retrieved through normal DNS queries.
DMARC's validation mechanism produces a "pass" result if a DMARC
Policy Record exists for the Author Domain of an email message and
the Author Domain is aligned (#identifier-alignment) with an
Authenticated Identifier (#authenticated-identifiers) from that
message. When a DMARC Policy Record exists for the Author Domain and
the DMARC mechanism does not produce a "pass" result, the Mail
Receiver's (#mail-receiver) handling of that message can be
influenced by the Domain Owner Assessment Policy (#domain-owner-
policy) expressed in the DMARC Policy Record.
It is important to note that the authentication mechanisms employed
by DMARC only validate the usage of a DNS domain in an email message.
They do not validate the local-part of any email address identifier
found in that message, nor do such validations carry an explicit or
implicit value assertion about that message or about the Domain
Owner.
DMARC's reporting component involves the collection of information
about received messages using the Author Domain for periodic
aggregate reports to the Domain Owner or PSO. The parameters and
format for such reports are discussed in
[I-D.ietf-dmarc-aggregate-reporting]
A Mail Receiver participating in DMARC might also generate per-
message reports that contain information related to individual
messages that fail DMARC validation checks. Per-message failure
reports are a useful source of information when debugging deployments
(if messages can be determined to be legitimate even though failing
validation) or in analyzing attacks. The capability for such
services is enabled by DMARC but defined in other referenced material
such as [RFC6591] and [I-D.ietf-dmarc-failure-reporting]
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4.2. 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. This field is the one used by end
users to identify the source of the message, and so it has always
been a prime target for abuse through such forgery and other means.
That said, of all the identifiers that are part of the message
itself, this is the only one required to be present. A message
without a single, properly formed RFC5322.From header field does not
comply with [RFC5322], and handling such a message is outside of the
scope of this specification.
4.3. Authentication Mechanisms
The following mechanisms for determining Authenticated Identifiers
(#authenticated-identifiers) are supported in this version of DMARC:
* 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 validate the uses of both the domain
found in an SMTP [RFC5321] HELO/EHLO command (the HELO identity)
and the domain found in an SMTP MAIL command (the MAIL FROM
identity). DMARC relies solely on SPF validation of the MAIL FROM
identity.
4.4. Identifier Alignment Explained
DMARC validates the authorized use of the Author Domain (#author-
domain) by requiring either that it have the same Organizational
Domain (#organizational-domain) as an Authenticated Identifier
(#authenticated-identifier) (a condition known as "Relaxed Alignment
(#relaxed-alignment)") or that it be identical to the Authenticated
Identifier (a condition known as "Strict Alignment (#strict-
alignment)"). The choice of relaxed or strict alignment is left to
the Domain Owner (#domain-owner) and is expressed in the domain's
DMARC Policy Record (#dmarc-policy-record). In practice, nearly all
Domain Owners have found relaxed alignment sufficient to meet their
needs. Domain name comparisons in this context are case-insensitive,
per [RFC4343].
The following table is meant to illustrate possible alignment
conditions.
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+==================+==================+=========================+
| Authenticated | Author Domain | Identifier Alignment |
| Identifier | | |
+==================+==================+=========================+
| foo.example.com | news.example.com | relaxed; the two have |
| | | the same organizational |
| | | domain, example.com |
+------------------+------------------+-------------------------+
| news.example.com | news.example.com | strict; the two are |
| | | identical |
+------------------+------------------+-------------------------+
| foo.example.net | news.example.com | none; the two do not |
| | | share a common |
| | | organizational domain |
+------------------+------------------+-------------------------+
Table 1: "Alignment Examples"
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 Record
(#dmarc-policy-record) that applies to the message. Accordingly,
DMARC operation is predicated on the input being a valid RFC5322
message object. For non-compliant cases, handling is outside of the
scope of this specification. Further discussion of this can be found
in Section 10.5.
4.4.1. DKIM-Authenticated Identifiers
DKIM permits a Domain Owner to claim some responsibility for a
message by associating the domain to the message. This association
is done by inserting the domain as the value of the d= tag in a DKIM-
Signature header field, and the assertion of responsiblity is
validated through a crytographic signature in the header field. If
the cryptographic signature validates, then the signing domain (i.e.,
the value of the d= tag) is the Authenticated Identifier.
DMARC requires that Identifier Alignment is applied to the DKIM
Authenticated Identifier because a message can bear a valid signature
from any domain, even one used by a bad actor. Therefore, a DKIM-
Authenticated Identifier that does not have Identifier Alignment with
the Author Domain is not enough to validate whether the use of the
Author Domain has been authorized by its Domain Owner.
A single email can contain multiple DKIM signatures, and it is
considered to produce a DMARC "pass" result if any DKIM-Authenticated
Identifier aligns with the Author Domain.
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4.4.2. SPF-Authenticated Identifiers
SPF allows a Domain Owner to authorize hosts to use its domain in the
SMTP "MAIL FROM" or "HELO" identities. DMARC relies solely on SPF
validation of the MAIL FROM identity. If the authorization is
validated, the domain used in the MAIL FROM identity, which will also
be the RFC5321.MailFrom domain in the email message, is the
Authenticated Identifier.
DMARC requires that Identifier Alignment is applied to the SPF-
Authenticated Identifier, because any Domain Owner, even a bad actor,
can publish an SPF record for its domain and send email that will
obtain an SPF pass result. Therefore, an SPF-Authenticated
Identifier that does not have Identifier Alignment with the Author
Domain is not enough to validate whether the use of the Author Domain
has been authorized by its Domain Owner.
4.4.3. Alignment and Extension Technologies
If in the future DMARC is extended to include the use of other
authentication mechanisms, the extensions MUST allow for the
assignment of a domain as an Authenticated Identified so that
alignment with the Author Domain can be validated.
4.5. DMARC Policy Record Explained
A Domain Owner (#domain-owner) or PSO (#public-suffix-operator)
advertises DMARC participation of one or more of its domains by
publishing DMARC Policy Records (#dmarc-policy-record) that will
apply to those domains. In doing so, Domain Owners and PSOs indicate
their handling preference regarding failed validation for email
messages using their domain in the RFC5322.From header field as well
as their desire to receive feedback about such messages in the form
of aggregate and/or failure reports.
DMARC Policy Records are stored as DNS TXT records in subdomains
named "_dmarc". For example, the Domain Owner of "example.com" would
publish a DMARC Policy Record at the name "_dmarc.example.com".
Similarly, a Mail Receiver (#mail-receiver) wishing to find the DMARC
Policy Record for mail with an Author Domain (#author-domain) of
"example.com" would issue a TXT query to the DNS for the subdomain of
"_dmarc.example.com". A Domain Owner or PSO may choose not to
participate in DMARC validation by Mail Receivers simply by not
publishing a DMARC Policy Record for its Author Domain(s).
DMARC Policy Records can also apply to subdomains of the name at
which they are published in the DNS, if the record is published at an
Organizational Domain (#organizational-domain) for the subdomains.
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The Domain Owner Assessment Policy (#domain-owner-policy) that
applies to the subdomains can be identical to the Domain Owner
Assessment Policy that applies to the Organizational Domain or
different, depending on the presence or absence of certain values in
the DMARC Policy Record. See Section 4.7 for more details.
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 domain name targeted by a DNS query,
specifically an isolated TXT record that is restricted to the DMARC
context. Using 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.
A Domain Owner can choose not to have some underlying authentication
mechanisms apply to DMARC evaluation of its Author Domain(s). For
example, if a Domain Owner only wants to use DKIM as the underlying
authentication mechanism, then the Domain Owner does not publish an
SPF policy record that can produce Identifier Alignment between an
SPF-Authenticated Identifier and the Author Domain. Alternatively,
if the Domain Owner wishes to rely solely on SPF, then it can send
email messages that have no DKIM-Signature header field that would
produce Identifier Alignment between a DKIM-Authenicated Identifier
and the Author Domain. Neither approach is recommended, however.
A Mail Receiver implementing the DMARC mechanism gets the Domain
Owner's or PSO's published Domain Owner Assessment Policy and can use
it to inform its handling decisions for messages that undergo DMARC
validation checks and do not produce a result of pass. Mail handling
considerations based on Domain Owner Assessment Policy enforcement
are discussed below in Section 5.4.
4.6. DMARC Reporting URIs
[RFC3986] defines a generic syntax for identifying a resource. The
DMARC mechanism uses this as the format by which a Domain Owner
(#domain-owner) or PSO (#public-suffix-organization) specifies the
destination for the two report types that are supported.
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The place such URIs are specified (see Section 4.7) allows a list of
these to be provided. The list of URIs is separated by commas (ASCII
0x2c). A report SHOULD be sent to each listed URI provided in the
DMARC record.
A formal definition is provided in Section 4.8.
4.7. DMARC Policy Record Format
DMARC records follow the extensible "tag-value" syntax for DNS-based
key records defined in DKIM [RFC6376].
Section 8 creates a registry for known DMARC tags and registers the
initial set defined in this document. Only tags defined in that
registry are to be processed; unknown tags MUST be ignored.
The following tags are valid DMARC tags:
adkim: (plain-text; OPTIONAL; default is "r".) Indicates whether
the Domain Owner (#domain-owner) or PSO (#public-suffix-
organization) requires strict or relaxed DKIM Identifier Alignment
mode. See Section 4.4.1 for details. Valid values are as
follows:
r: relaxed mode
s: strict mode
aspf: (plain-text; OPTIONAL; default is "r".) Indicates whether the
Domain Owner or PSO requires strict or relaxed SPF Identifier
Alignment mode. See Section 4.4.2 for details. Valid values are
as follows:
r: relaxed mode
s: strict mode
fo: Failure reporting options (plain-text; OPTIONAL; default is "0")
Provides requested options for the 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. The valid
values and their meanings are:
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
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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
evaluation, regardless of its alignment. SPF-specific
reporting is described in [RFC6652].
np: Domain Owner Assessment Policy (#domain-owner-policy) for non-
existent subdomains (plain-text; OPTIONAL). Indicates the message
handling preference of the Domain Owner or PSO for mail using non-
existent subdomains of its domain and not passing DMARC
validation. 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
Policy Records published on subdomains of Organizational Domains
and PSDs due to the effect of the DMARC Policy Discovery (#dmarc-
policy-discovery).
p: Domain Owner Assessment Policy (#domain-owner-policy) (plain-
text; RECOMMENDED for DMARC Policy Records). Indicates the
message handling preference of the Domain Owner or PSO for mail
using its domain but not passing DMARC validation. The policy
applies to the domain queried and to subdomains, unless the
subdomain policy is explicitly described using the "sp" or "np"
tags. If this tag is not present in an otherwise syntactically
valid DMARC record, then the record is treated as if it included
"p=none" (see Section 4.10.1). This tag is not applicable for
third-party reporting records (see
[I-D.ietf-dmarc-aggregate-reporting] and
[I-D.ietf-dmarc-failure-reporting]) Possible values are as
follows:
none: The Domain Owner offers no expression of preference.
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 7.3 for some discussion of SMTP rejection methods
and their implications.
psd: A flag indicating whether the domain is a PSD. (plain-text;
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OPTIONAL; default is 'u'). Possible values are:
y: PSOs include this tag with a value of 'y' to indicate that the
domain is a PSD. If a record containing this tag with a value
of 'y' is found during policy discovery, this information will
be used to determine the Organizational Domain and policy
domain applicable to the message in question.
n: The DMARC policy record is published for a domain that is not
a PSD, but it is the Organizational Domain for itself and its
subdomains.
u: The default indicates that the DMARC policy record is
published for a domain that is not a PSD, and may or may not be
an Organizational Domain for itself and its subdomains. Use
the mechanism described in Section 4.10 for determining the
Organizational Domain for this domain. There is no need to
explicitly publish psd=u in a DMARC record.
rua: Addresses to which aggregate feedback reports are to be sent
(comma-separated plain-text list of DMARC Reporting URIs;
OPTIONAL). If present, the Domain Owner is requesting Mail
Receivers to send aggregate feedback reports as defined in
[I-D.ietf-dmarc-aggregate-reporting] to the URIs listed. Any
valid URI can be specified. A Mail Receiver MUST implement
support for a "mailto:" URI, i.e., the ability to send a DMARC
report via electronic mail. If the tag is not provided, Mail
Receivers MUST NOT generate aggregate feedback reports for the
domain. URIs not supported by Mail Receivers MUST be ignored.
[I-D.ietf-dmarc-aggregate-reporting] also discusses considerations
that apply when the domain name of a URI differs from the domain
publishing the DMARC Policy Record. See Section 10.6 for
additional considerations.
ruf: Addresses to which message-specific failure information is to
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be reported (comma-separated plain-text list of DMARC URIs;
OPTIONAL). If present, the Domain Owner is requesting Mail
Receivers to send detailed failure reports about messages that
fail the DMARC evaluation in specific ways (see the "fo" tag
above) to the URIs listed. Depending on the value of the "fo"
tag, the format for such reports is described in
[I-D.ietf-dmarc-failure-reporting], [RFC6651], or [RFC6652]. Any
valid URI can be specified. A Mail Receiver MUST implement
support for a "mailto:" URI, i.e., the ability to send message-
specific failure information via electronic mail. If the tag is
not provided, Mail Receivers MUST NOT generate failure reports for
the domain. URIs not supported by Mail Receivers MUST be ignored.
[I-D.ietf-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 10.6 for additional
considerations.
sp: Domain Owner Assessment Policy for all subdomains (plain-text;
OPTIONAL). Indicates the message handling preference of the
Domain Owner or PSO for mail using an existing subdomain of its
domain and not passing DMARC validation. It applies only to
existing subdomains of the domain queried in the DNS hierarchy 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 Policy Records published on subdomains
of Organizational Domains and PSDs due to the effect of the DMARC
Policy Discovery.
t: DMARC policy test mode (plain-text; OPTIONAL; default is 'n').
For the Author Domain to which the DMARC Policy Record applies,
the "t" tag serves as a signal to the actor performing DMARC
validation checks as to whether or not the Domain Owner wishes the
Domain Owner Assessment Policy declared in the "p=", "sp=", and/or
"np=" tags to actually be applied. This parameter does not affect
the generation of DMARC reports. See Appendix A.6 for further
discussion of the use of this tag. Possible values are as
follows:
y: A request that the actor performing the DMARC validation check
not apply the policy, but instead apply any special handling
rules it might have in place, such as rewriting the
RFC5322.From header field. The Domain Owner is currently
testing its specified DMARC assessment policy.
n: The default is a request to apply the Domain Owner Assessment
Policy as specified to any message that produces a DMARC "fail"
result.
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v: Version (plain-text; REQUIRED). Identifies the record retrieved
as a DMARC Policy 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 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 4.8 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 MUST be discarded in favor of default values (if any)
or ignored outright.
Note that given the rules of the previous paragraph, the 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.
4.8. Formal Definition
The formal definition of the DMARC Policy Record format, using
[RFC5234] and [RFC7405], is as follows:
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dmarc-uri = URI
; "URI" is imported from [RFC3986];
; commas (ASCII 0x2C) and exclamation
; points (ASCII 0x21) MUST be
; encoded
dmarc-sep = *WSP ";" *WSP
equals = *WSP "=" *WSP
dmarc-record = dmarc-version *(dmarc-sep dmarc-tag) [dmarc-sep] *WSP
dmarc-tag = 1*ALPHA equals 1*dmarc-value
; any printing characters but semicolon
dmarc-value = %x20-3A / %x3C-7E
dmarc-version = "v" equals %s"DMARC1" ; case sensitive
; specialized syntax of DMARC values
dmarc-request = "none" / "quarantine" / "reject"
dmarc-yorn = "y" / "n"
dmarc-psd = "y" / "n" / "u"
dmarc-rors = "r" / "s"
dmarc-urilist = dmarc-uri *(*WSP "," *WSP dmarc-uri)
dmarc-fo = "0" / "1" / "d" / "s" / "d:s" / "s:d"
"Keyword" is imported from Section 4.1.2 of [RFC5321].
In each dmarc-tag, the dmarc-value has a syntax that depends on the
tag name. The ABNF rule for each dmarc-value is specified in the
following table:
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+==========+===============+
| Tag Name | Value Rule |
+==========+===============+
| p | dmarc-request |
+----------+---------------+
| t | dmarc-yorn |
+----------+---------------+
| psd | dmarc-psd |
+----------+---------------+
| np | dmarc-request |
+----------+---------------+
| sp | dmarc-request |
+----------+---------------+
| adkim | dmarc-rors |
+----------+---------------+
| aspf | dmarc-rors |
+----------+---------------+
| rua | dmarc-urilist |
+----------+---------------+
| ruf | dmarc-urilist |
+----------+---------------+
| fo | dmarc-fo |
+----------+---------------+
Table 2: "Tag Names and
Values"
4.9. Flow Diagram
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+---------------+ +--------------------+
| Author Domain |< . . . . . . . . . . . . | Return-Path Domain |
+---------------+ . +--------------------+
| . ^
V V .
+-----------+ +--------+ +----------+ v
| MSA |<***>| DKIM | | DMARC | +----------+
| Service | | Signer | | Validator|<***>| SPF |
+-----------+ +--------+ +----------+ * | Validator|
| ^ * +----------+
| * *
V v *
+------+ (~~~~~~~~~~~~) +------+ * +----------+
| sMTA |------->( other MTAs )----->| rMTA | **>| DKIM |
+------+ (~~~~~~~~~~~~) +------+ | Validator|
| +----------+
| ^
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. Dashed lines (e.g., -->) denote the actual message
flow, dotted lines (e.g., < . . >) represent DNS queries used to
retrieve message policy related to the supported message
authentication schemes, and starred lines (e.g., <**>) indicate data
exchange between message-handling modules and message authentication
modules. "sMTA" is the sending MTA, and "rMTA" is the receiving MTA.
Put simply, when a message reaches a DMARC-aware rMTA, a DNS query
will be initiated to determine if a DMARC Policy Record exists that
applies to the Author Domain. If a DMARC Policy Record is found, the
rMTA will use the results of SPF and DKIM validation checks to
determine DMARC validation status. The DMARC validation status can
then factor into the message handling decision made by the
recipient's mail system.
More details on specific actions for the parties involved can be
found in Section 5.1 and Section 5.3.
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4.10. DNS Tree Walk
An Organizational Domain (#organizational-domain) serves two
different purposes, depending on the context:
* The Organizational Domain of the Author Domain (#author-domain)
establishes the DMARC Policy Record (#dmarc-policy-record) for
that domain when no DMARC Policy Record is published specifically
for the Author Domain. (see Section 4.10.1)
* The Organizational Domains of an Authenticated Identifier
(#authenticated-identifiers) and the Author Domain are used in
determining Identifier Alignment between the two. (see
Section 4.10.2).
[RFC7489] defined an Organizational Domain as "The domain that was
registered with a domain name registrar." This update to that
document offers more flexibility to Domain Owners, especially those
with large, complex organizations that might want to applying
decentralized management to their DNS and their DMARC Policy Records.
Rather than just searching the Public Suffix List to identify an
Organizational Domain, this update defines a discovery technique
known colloquially as the "DNS Tree Walk". The target of any DNS
Tree Walk is a valid DMARC Policy Record, and its use in determing an
Organizational Domain allows for publishing DMARC Policy Records at
multiple points in the namespace.
This flexibility comes at a possible cost, however. Since the DNS
Tree Walk relies on the Mail Receiver making a series of DNS queries,
the potential exists for an ill-intentioned Domain Owner to send mail
with Author Domains with tens or even hundreds of labels for the
purpose of executing a Denial of Service Attack on the Mail Receiver.
To guard against such abuse of the DNS, a shortcut is built into the
process so that Author Domains with more than eight labels do not
result in more than eight DNS queries. Observed data at the time of
publication showed that Author Domains with up to seven labels were
in usage, and so eight was chosen as the query limit to allow for
some future expansion of the name space that did not require updating
this document.
The generic steps for a DNS Tree Walk are as follows:
1. Query the DNS for a DMARC Policy Record at the appropriate
starting point for the Tree Walk. A possibly empty set of
records is returned.
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2. Records that do not start with a "v=" tag that identifies the
current version of DMARC are discarded. If multiple DMARC Policy
Records are returned, they are all discarded. If a single record
remains and it contains a "psd=n" tag, stop.
3. Determine the target for additional queries (if needed; see the
note in Section 4.10.2), using steps 4 through 8 below.
4. Break the subject DNS domain name into a set of ordered labels.
Assign the count of labels to "x", and number the labels from
right to left; e.g., for "a.mail.example.com", "x" would be
assigned the value 4, "com" would be label 1, "example" would be
label 2, "mail" would be label 3, and so forth.
5. If x < 8, remove the left-most (highest-numbered) label from the
subject domain. If x >= 8, remove the left-most (highest-
numbered) labels from the subject domain until 7 labels remain.
The resulting DNS domain name is the new target for the next
lookup.
6. Query the DNS for a DMARC Policy Record at the DNS domain name
matching this new target. A possibly empty set of records is
returned.
7. Records that do not start with a "v=" tag that identifies the
current version of DMARC are discarded. If multiple DMARC Policy
Records are returned for a single target, they are all discarded.
If a single record remains and it contains a "psd=n" or "psd=y"
tag, stop.
8. Determine the target for additional queries by removing a single
label from the target domain as described in step 5 and repeating
steps 6 and 7 until the process stops or there are no more labels
remaining.
To illustrate, for a message with the arbitrary Author Domain of
"a.b.c.d.e.f.g.h.i.j.mail.example.com", a full DNS Tree Walk would
require the following eight queries to potentially locate the DMARC
Policy Record or Organizational Domain:
* _dmarc.a.b.c.d.e.f.g.h.i.j.mail.example.com
* _dmarc.g.h.i.j.mail.example.com
* _dmarc.h.i.j.mail.example.com
* _dmarc.i.j.mail.example.com
* _dmarc.j.mail.example.com
* _dmarc.mail.example.com
* _dmarc.example.com
* _dmarc.com
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4.10.1. DMARC Policy Discovery
The DMARC Policy Record to be applied to an email message will be the
record found at any of the following locations, listed from highest
preference to lowest:
* The Author Domain
* The Organizational Domain of the Author Domain
* The Public Suffix Domain of the Author Domain
Policy discovery starts first with a query for a valid DMARC Policy
Record at the name created by prepending the label "_dmarc" to the
Author Domain of the message being evaluated. If a valid DMARC
Policy Record is found there, then this is the DMARC Policy Record to
be applied to the message; however, this does not necessarily mean
that the Author Domain is the Organizational Domain to be used in
Identifier Alignment checks. Whether this is the also the
Organizational Domain is dependent on the value of the 'psd' tag, if
present, or some conditions described in Section 4.10.2.
If no valid DMARC Policy Record is found by the first query, then
perform a DNS Tree Walk to find the Author Domain's Organizational
Domain or its Public Suffix Domain. The starting point for this DNS
Tree Walk is determined as follows:
* If the Author Domain has eight or fewer labels, the starting point
will be the immediate parent domain of the Author Domain.
* Otherwise, the starting point will be the name produced by
shortening the Author Domain as described starting in step 3 of
the Section 4.10.
If the DMARC Policy Record to be applied is that of the Author
Domain, then the Domain Owner Assessment Policy is taken from the p=
tag of the record.
If the DMARC Policy Record to be applied is that of either the
Organizational Domain or the Public Suffix Domain and that domain is
different than the Author Domain, then the Domain Owner Assessment
Policy is taken from the sp= tag (if any) if the Author Domain
exists, or the np= tag (if any) if the Author Domain does not exist.
In the absence of applicable sp= or np= tags, the p= tag policy is
used for subdomains.
If a retrieved DMARC Policy Record does not contain a valid "p" tag,
or contains an "sp" or "np" tag that is not valid, then:
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* If a "rua" tag is present and contains at least one syntactically
valid reporting URI, the Mail Receiver MUST act as if a record
containing "p=none" was retrieved and continue processing;
* Otherwise, the Mail Receiver applies no DMARC processing to this
message.
If the set produced by the DNS Tree Walk contains no DMARC Policy
Record (i.e., any indication that there is no such record as opposed
to a transient DNS error), Mail Receivers MUST 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").
Note: PSD policy is not used for Organizational Domains that have
published a DMARC Policy Record. Specifically, this is not a
mechanism to provide feedback addresses (rua/ruf) when an
Organizational Domain has declined to do so.
4.10.2. Identifier Alignment Evaluation
It may be necessary to perform multiple DNS Tree Walks to determine
if an Authenticated Identifier and an Author Domain are in alignment,
meaning that they have either the same Organizational Domain (relaxed
alignment) or that they're identical (strict alignment). DNS Tree
Walks done to discover an Organizational Domain for use in Identifier
Alignment Evaluation might start at any of the following locations:
* The Author Domain of the message being evaluated.
* The SPF-Authenticated Identifier if there is an SPF pass result
for the message being evaluated.
* Any DKIM-Authenticated Identifier if one or more DKIM pass results
exist for the message being evaluated.
Note: There is no need to perform Tree Walk searches for
Organizational Domains under any of the following conditions:
* The Author Domain and the Authenticated Identifier(s) are all the
same domain, and there is a DMARC Policy Record published for that
domain. In this case, this common domain is treated as the
Organizational Domain. For example, if the common domain in
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question is "mail.example.com", and there is a valid DMARC Policy
Record published at _dmarc.mail.example.com, then mail.example.com
is the Organizational Domain.
* No applicable DMARC Policy Record is discovered for the Author
Domain. In this case, the DMARC mechanism does not apply to the
message in question.
* The DMARC Policy record for the Author Domain indicates strict
alignment. In this case, a simple string comparison of the Author
Domain and the Authenticated Identifier(s) is all that is
required.
To discover the Organizational Domain for a domain, perform the DNS
Tree Walk described in Section 4.10 as needed for any of the domains
in question.
For each Tree Walk that retrieved valid DMARC Policy Records, select
the Organizational Domain from the domains for which valid DMARC
Policy Records were retrieved from the longest to the shortest:
1. If a valid DMARC Policy Record contains the psd= tag set to 'n'
(psd=n), this is the Organizational Domain, and the selection
process is complete.
2. If a valid DMARC Policy Record, other than the one for the domain
where the tree walk started, contains the psd= tag set to 'y'
(psd=y), the Organizational Domain is the domain one label below
this one in the DNS hierarchy, and the selection process is
complete.
3. Otherwise, select the DMARC Policy Record found at the name with
the fewest number of labels. This is the Organizational Domain
and the selection process is complete.
If this process does not determine the Organizational Domain, then
the initial target domain is the Organizational Domain.
For example, given the starting domain "a.mail.example.com", a search
for the Organizational Domain would require a series of DNS queries
for DMARC Policy Records starting with "_dmarc.a.mail.example.com"
and finishing with "_dmarc.com". If there are DMARC Policy Records
published at "_dmarc.mail.example.com" and "_dmarc.example.com", but
not at "_dmarc.a.mail.example.com" or "_dmarc.com", then the
Organizational Domain for this domain would be "example.com".
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As another example, given the starting domain "a.mail.example.com",
if a search for the Organizational Domain yields a DMARC Policy
Record at "_dmarc.mail.example.com" with the psd= tag set to 'n',
then the Organizational Domain for this domain would be
"mail.example.com".
As a last example, given the starting domain "a.mail.example.com", if
a search for the Organizational Domain only yields a DMARC Policy
Record at "_dmarc.com" and that record contains the tag psd=y, then
the Organizational Domain for this domain would be "example.com".
5. DMARC Participation
This section describes the actions for participating in DMARC for
each of three unique entities - Domain Owners, PSOs, and Mail
Receivers.
5.1. Domain Owner Actions
A Domain Owner (#domain-owner) wishing to fully participate in DMARC
will publish a DMARC Policy Record (#dmarc-policy-record) to cover
each Author Domain (#author-domain) and corresponding Organizational
Domain (#organizational-domain) to which DMARC validation should
apply, send email that produces at least one, and preferably two,
Authenticated Identifiers (#authenticated-identifiers) that align
with the Author Domain, and will receive and monitor the content of
DMARC aggregate reports. The following sections describe how to
achieve this.
5.1.1. Publish an SPF Policy for an Aligned Domain
To configure SPF for DMARC, the Domain Owner MUST send mail that has
an RFC5321.MailFrom domain that will produce an SPF-Authenticated
Identifier (#spf-authenticated-identifier) that aligns with the
Author Domain. The SPF record for the RFC5321.MailFrom domain MUST
be constructed at a minimum to ensure an SPF pass verdict for all
sources of mail that are authorized to use RFC5321.MailFrom domain,
and SHOULD be constructed to ensure that only those authorized
sources can get an SPF pass verdict..
5.1.2. Configure Sending System for DKIM Signing Using an Aligned
Domain
To configure DKIM for DMARC, the Domain Owner MUST send mail that has
a DKIM-Signing domain (i.e., the d= domain in the DKIM-Signature
header field) that will produce a DKIM-Aligned Identifier (#dkim-
authenticated-identifier) that aligns with the Author Domain.
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5.1.3. Setup a Mailbox to Receive Aggregate Reports
Proper consumption and analysis of DMARC aggregate reports are
essential to any successful DMARC deployment for a Domain Owner.
DMARC aggregate reports, which are defined in
[I-D.ietf-dmarc-aggregate-reporting], contain valuable data for the
Domain Owner, showing sources of mail using the Author Domain.
5.1.4. Publish a DMARC Policy Record for the Author Domain and
Organizational Domain
Once SPF, DKIM, and the aggregate reports mailbox are all in place,
it's time to publish a DMARC Policy Record. For best results, Domain
Owners usually start with "p=none", (see Section 5.1.5) with the rua
tag containing a URI that references the mailbox created in the
previous step. This is commonly referred to as putting the Author
Domain into Monitoring Mode (#monitoring-mode). If the
Organizational Domain is different from the Author Domain, a record
also needs to be published for the Organizational Domain.
5.1.5. Collect and Analyze Reports
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, a Domain Owner can overlook a server here or be
unaware of a third party sending agreement 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' authentication configurations.
While it is possible for a human to read aggregate reports, they are
formatted in such a way that it is recommended that they be machine-
parsed, so setting up a mailbox involves more than just the physical
creation of that 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
consume these reports and parse the data contained in them will go a
long way to ensuring a successful deployment.
5.1.6. Remediate Unaligned or Unauthenticated Mail Streams
DMARC aggregate reports can reveal to the Domain Owner mail streams
using the Author Domain that should be passing DMARC validation
checks but are not. If the reason for the streams not passing is due
to Authenticated Identifiers being unaligned or missing entirely,
then the Domain Owner wishing to fully participate in DMARC MUST take
necessary steps to address these shortcomings.
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5.1.7. Decide Whether to Update Domain Owner Assessment Policy to
Enforcement
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 its Domain Owner Assessment Policy to Enforcement
(#enforcement). 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.
In making this decision it is important to understand the
interoperability issues involved and problems that can result for
mailing lists and for delivery of legitimate mail. Those issues are
discussed in detail in Section 7.5
5.1.8. A Note on Large, Complex Organizations and Decentralized DNS
Management
Large, complex organizations frequently adopt a decentralized model
for DNS management, whereby management of a subtree of the name space
is delegated to a local deparment by the central IT organization. In
such situations, the 'psd' tag makes it possible for those local
departments to declare any arbitrary node in their subtree as an
Organizational Domain. This would be accomplished by publishing a
DMARC record at that node with the psd tag set to 'n'. The reasons
that departments might declare their own Organizational Domains
include a desire to have different policy settings or reporting URIs
than the DMARC Policy Record published for the apex domain.
Such configurations would work in theory, and they might involve
domain names with many labels, reflecting the structure of the
organization, for example:
* Apex domain (DMARC Policy Record published here): example.com
* Zone cut domain (DMARC Policy Record with psd=n published here):
b.c.d.e.f.g.example.com
* Author Domain: mail.a.b.c.d.e.f.g.example.com
However, Domain Owners should be aware that due to the anti-abuse
protections built into the DNS Tree Walk (#dns-tree-walk), the DMARC
Policy Record published at the zone cut domain in this example will
never be discovered. A Mail Receiver performing a Tree Walk would
only perform queries for these names:
* _dmarc.mail.a.b.c.d.e.f.g.example.com
* _dmarc.c.d.e.f.g.example.com
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* _dmarc.d.e.f.g.example.com
* _dmarc.e.f.g.example.com
* _dmarc.f.g.example.com
* _dmarc.g.example.com
* _dmarc.example.com
* _dmarc.com
To avoid this circumstance, Domain Owners wishing to have a specific
DMARC Policy Record applied to a given [Author Domain]{#author-
domain) longer than eight labels MUST publish a DMARC Policy Record
at that domain's location in the DNS namespace, as such records are
always queried for by Mail Receivers that participate in DMARC. In
the above example, this would mean publishing a DMARC Policy Record
at the name _dmarc.mail.a.b.c.d.e.f.g.example.com.
5.2. PSO Actions
In addition to the DMARC Domain Owner actions, if a PSO (#public-
suffix-operator) publishes a DMARC Policy Record it MUST include the
psd tag (see Section 4.7) with a value of 'y' ("psd=y").
5.3. Mail Receiver Actions
Mail Receivers (#mail-receiver) wishing to fully participate in DMARC
will apply the DMARC mechanism to inbound email messages when a DMARC
Policy Record (#dmarc-policy-record) exists that applies to the
Author Domain (#author-domain), and will send aggregate reports to
Domain Owners that request them. Mail Receivers might also send
failure reports to Domain Owners that request them.
The steps for applying the DMARC mechanism to an email message can
take place during the SMTP transaction, and should do if the Mail
Receiver plans to honor Domain Owner Assessment Policies (#domain-
owner-policy) that are at the Enforcement (#enforcement) state.
Many Mail Receivers perform one or both of the underlying
Authentication Mechanisms (#authentication-mechanisms) on inbound
messages even in cases where no DMARC Policy Record exists for the
Author Domain of a given message, or where the Mail Receiver is not
participating in DMARC. Nothing in this section is intended to imply
that the underlying Authentication Mechanisms should only be
performed by Mail Receivers participating in DMARC.
The next sections describe the steps for a Mail Receiver wishing to
fully participate in DMARC.
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5.3.1. Extract Author Domain
Once the email message has been transmitted to the Mail Receiver, the
Mail Receiver extracts the domain in the RFC5322.From header field as
the Author Domain. If the domain is a U-label, the domain MUST be
converted to an A-label, as described in Section 2.3 of [RFC5890],
for further processing.
If zero or more than one domain is extracted, then DMARC validation
is not possible and the process terminates. In the case where more
than one domain is retrieved, the Mail Receiver MAY choose to go
forward with DMARC validation anyway. See Section 10.5 for further
discussion.
5.3.2. Determine If The DMARC Mechanism Applies
If precisely one Author Domain exists for the message, then perform
the step described in [DMARC Policy Discovery] to determine if the
DMARC mechanism applies. If a DMARC Policy Record (#dmarc-policy-
record) is not discovered during this step, then the DMARC mechanism
does not apply and DMARC validation terminates for the message.
5.3.3. Determine If Authenticated Identifiers Exist
For each Authentication Mechanism underlying DMARC, perform the
required check to determine if an Authenticated Identifier
(#authenticated-identifier) exists for the message if such check has
not already been performed. Results from each check must be
preserved for later use as follows:
* For SPF, the results 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.
* For DKIM signature validation checks, for each signature checked,
the results 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.
5.3.4. Conduct Identifier Alignment Checks If Necessary
For each Authenticated Identifier found in the message, the Mail
Receiver checks to see if the Authenticated Identifier is aligned
(#identifier-alignment-evaluation) with the Author Domain.
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5.3.5. Determine DMARC "Pass" or "Fail"
If one or more of the Authenticated Identifiers align with the Author
Domain, the message is considered to pass the DMARC mechanism check.
If no Authenticated Identifiers exist for the domain, or none of the
Authenticated Identifiers align with the Author Domain, the message
is considered to fail the DMARC mechanism check.
5.3.6. Apply Policy If Appropriate
Email messages that fail the DMARC mechanism check are handled in
accordance with the Mail Receiver's local policies. These local
policies may take into account the Domain Owner Assessment Policy for
the Author Domain at the Mail Receiver's discretion.
If one or more DNS queries required to perform DMARC validation on
the message do not complete due to temporary or permanent DNS errors,
the message cannot be considered to pass or fail the DMARC mechanism
check. In such cases, the Domain Owner Assessment Policy cannot be
applied to the message, and any other handling decisions for the
message are left to the discretion of the Mail Receiver.
See Section 7.3 for further discussion of topics regarding rejecting
messages.
5.3.7. Store Results of DMARC Processing
Results obtained from the application of the DMARC mechanism by the
Mail Receiver SHOULD be stored for eventual presentation back to the
Domain Owner in the form of aggregate feedback reports. Section 4.7
and [I-D.ietf-dmarc-aggregate-reporting] discuss aggregate feedback.
5.3.8. Send Aggregate Reports
To ensure maximum usefulness for DMARC across the email ecosystem,
Mail Receivers SHOULD generate and send aggregate reports with a
frequency of at least once every 24 hours. Such reports provide
Domain Owners with insight into all mail streams using Author Domains
under the Domain Owner's control, and aid the Domain Owner in
determining whether and when to transition from Monitoring Mode
(#monitoring-mode) to EnforcementSection 3.2.6.
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5.3.9. Optionally Send Failure Reports
Per-message failure reports can be a useful source of information for
a Domain Owner, either for debugging deployments or in analyzing
attacks, and so Mail Receivers MAY choose to send them. Experience
has shown, however, that Mail Receivers rightly concerned about
protecting user privacy have either chosen to heavily redact the
information in such reports (which can hinder their usefulness) or
not send them at all. See [I-D.ietf-dmarc-failure-reporting] for
further information.
5.4. Policy Enforcement Considerations
The final handling of any message is always a matter of local policy
and is left to the discretion of the Mail Receiver.
A DMARC pass for a message indicates only that the use of the Author
Domain (#author-domain) has been validated for that message as
authorized by the Domain Owner (#domain-owner). Such authorization
does not carry an explicit or implicit value assertion about that
message or the Domain Owner, and Mail Receivers MAY choose to reject
or quarantine a message even if it passes the DMARC validation check.
Mail Receivers are encouraged to maintain anti-abuse technologies to
combat the possibility of DMARC-enabled criminal campaigns.
Mail Receivers MAY choose to accept email that fails the DMARC
validation check even if the published Domain Owner Assessment Policy
is "reject". In particular, because of the considerations discussed
in [RFC7960] and in Section 7.5 of this document, it is important
that Mail Receivers not reject messages solely because of a published
policy of "reject", but that they apply other knowledge and analysis
to avoid situations such as rejection of legitimate messages sent in
ways that DMARC cannot describe, harm to the operation of mailing
lists, and similar.
If a Mail Receiver chooses not to honor the published Domain Owner
Assessment Policy to improve interoperability among mail systems, it
may increase the likelihood of accepting abusive mail. At a minimum,
Mail Receivers SHOULD add the Authentication-Results header field
(see [RFC8601]), and it is RECOMMENDED when delivering messages that
fail the DMARC validation check.
When Mail Receivers deviate from a published Domain Owner Assessment
Policy during message processing they 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 [I-D.ietf-dmarc-aggregate-reporting].
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To enable Domain Owners to receive DMARC feedback without impacting
existing mail processing, discovered policies of "p=none" MUST NOT
modify existing mail handling processes.
6. DMARC Feedback
DMARC Feedback is described in [I-D.ietf-dmarc-aggregate-reporting]
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 9 for discussion of Privacy Considerations
for PSD DMARC.
7. Other Topics
This section discusses some topics regarding choices made in the
development of DMARC, largely to commit the history to record.
7.1. 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 extensive
network ranges. Domain Owners (#domain-owner) 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. Furthermore, Domain Owners should
periodically review their SPF records to ensure that the
authorization conveyed by the records matches the domain's current
needs.
Some Mail Receiver architectures might implement SPF in advance of
any DMARC operations. This means that a SPF hard fail ("-") prefix
on a sender's SPF mechanism, such as "-all", could cause a message to
be rejected early in the SMTP transaction, before any DMARC
processing takes place, if the message fails SPF authentication
checks. Domain Owners choosing to use "-all" to terminate SPF
records should be aware of this, and should understand that messages
that might otherwise pass DMARC due to an aligned DKIM-Authenticated
Identifier (#dkim-authenticated-identifiers) could be rejected solely
due to an SPF fail. Domain Owners and Mail Receivers (#mail-
receiver) can consult the following two documents for more discussion
of the topic:
* M3AAWG Best Practices for Managing SPF Records
(https://www.m3aawg.org/Managing-SPF-Records)
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* M3AAWG Email Authentication Recommended Best Practices
(https://www.m3aawg.org/sites/default/files/m3aawg-email-
authentication-recommended-best-practices-09-2020.pdf)
7.2. DNS Load and Caching
DMARC Policy Records (#dmarc-policy-record) 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 (#mail-receiver) point of view.
If Domain Owners (#domain-owner) or PSOs (#public-suffix-operator)
publish a DMARC Policy Record with a very short TTL, the injection of
large volumes of messages could cause Mail Receivers to overwhelm the
Domain Owner's DNS hosting provider. Although this is not a concern
specific to DMARC, the implications of a very short TTL should be
considered when publishing DMARC Policy Records.
Conversely, long TTLs will cause records to be cached for long
periods. This can cause a critical change to a DMARC Policy Record
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 Policy Record changes while
preserving the benefits of DNS caching.
7.3. Rejecting Messages
The DMARC mechanism 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 the DMARC mechanism
would then result in the 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 a 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.
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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 Mail Receiver (#mail-
recevier) might indicate in plain text the reason for the rejection
by using the word "DMARC" somewhere in the reply text. For example:
550 5.7.1 Email rejected per DMARC policy for example.com
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.
If a Mail Receiver elects to defer delivery due to the inability to
retrieve or apply DMARC policy, this is best done with a 4xy SMTP
reply code.
7.4. Interoperability Issues
DMARC limits which end-to-end scenarios can achieve a "pass" result.
Because DMARC relies on SPF [RFC7208] and/or DKIM [RFC6376] to
achieve a "pass", their limitations also apply.
Issues specific to the use of policy mechanisms alongside DKIM are
further discussed in [RFC6377], particularly Section 5.2.
Mail that is sent by authorized, independent third parties might not
be sent with Identifier Alignment, also preventing a "pass" result.
A Domain Owner can use DMARC aggregate reports to identify this mail
and take steps to address authentication shortcomings.
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7.5. Interoperability Considerations
As discussed in "Interoperability Issues between DMARC and Indirect
Email Flows" [RFC7960], use of p=reject can be incompatible with and
cause interoperability problems to indirect message flows such as
"alumni forwarders", role-based email aliases, and mailing lists
across the Internet.
A domain that expects to send only targeted messages to account
holders - a bank, for example - could have account holders using
addresses such as jones@alumni.example.edu (an address that relays
the messages to another address with a real mailbox) or
finance@association.example (a role-based address that does similar
relaying for the current head of finance at the association). When
such mail is delivered to the actual recipient mailbox, it will
necessarily fail SPF checks, as the incoming IP address will be that
of example.edu or association.example, and not an address authorized
for the sending domain. DKIM signatures will generally remain valid
in these relay situations.
| It is therefore critical that domains that publish p=reject MUST
| NOT rely solely on SPF to secure a DMARC pass, and MUST apply
| valid DKIM signatures to their messages.
In the case of domains that have general users who send routine
email, those that publish a Domain Owner Assessment Policy (#domain-
owner-policy) of p=reject are likely to create significant
interoperability issues. In particular, if users in such domains
post messages to mailing lists on the Internet, those messages can
cause operational problems for the mailing lists and for the
subscribers to those lists, as explained below and in [RFC7960].
| It is therefore critical that domains that host users who might
| post messages to mailing lists SHOULD NOT publish Domain Owner
| Assessment Policies of p=reject. Any such domains wishing to
| publish p=reject SHOULD first take advantage of DMARC aggregate
| report data for their domain to determine the possible impact to
| their users, first by publishing p=none for at least a month,
| followed by publishing p=quarantine for an equally long period of
| time, and comparing the message disposition results. Domains that
| choose to publish p=reject SHOULD either implement policies that
| their users not post to Internet mailing lists and/or inform their
| users that their participation in mailing lists may be hindered.
As noted in Section 5.4, Mail Receivers (#mail-receivers) need to
apply more analysis than just DMARC validation in their disposition
of incoming messages. An example of the consequences of honoring a
Domain Owner Assessment Policy of p=reject without further analysis
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is that rejecting messages that have been relayed by a mailing list
can cause the Mail Receiver's users to have their subscriptions to
that mailing list canceled by the list software's automated handling
of such rejections - it looks to the list manager as though the
recipient's email address is no longer working, so the address is
automatically unsubscribed.
| It is therefore critical that Mail Receivers MUST NOT reject
| incoming messages solely on the basis of a p=reject policy by the
| sending domain. Mail Receivers must use the DMARC policy as part
| of their disposition decision, along with other knowledge and
| analysis.
Failure to understand and abide by these considerations can cause
legitimate, sometimes important email to be rejected, can cause
operational damage to mailing lists throughout the Internet, and can
result in trouble-desk calls and complaints from the Mail Receiver's
employees, customers, and clients.
As a final note, one possible mitigation to the problems caused by
Domain Owners publishing a Domain Owner Assessment Policy of p=reject
when they should not or Mail Receivers rejecting messages solely on
the basis of a p=reject policy is the Autheticated Received Chain
(ARC) protocol described in [RFC8617]. However, as of this writing,
use of ARC is nascent, as is industry experience with it in
connection with DMARC.
8. IANA Considerations
This section describes actions completed by IANA.
8.1. Authentication-Results Method Registry Update
IANA has added the following to the "Email Authentication Methods"
registry:
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+======+===========+======+==========+===============+======+=======+
|Method| Defined |ptype | Property | Value |Status|Version|
+======+===========+======+==========+===============+======+=======+
|dmarc | [RFC7489] |header| from | the domain |active|1 |
| | | | | portion of | | |
| | | | | the | | |
| | | | | RFC5322.From | | |
| | | | | header field | | |
+------+-----------+------+----------+---------------+------+-------+
|dmarc | [RFC7489] |policy| dmarc | Evaluated |active|1 |
| | | | | DMARC policy | | |
| | | | | applied/to | | |
| | | | | be applied | | |
| | | | | after policy | | |
| | | | | options | | |
| | | | | including | | |
| | | | | pct: and sp: | | |
| | | | | have been | | |
| | | | | processed. | | |
| | | | | Must be | | |
| | | | | none, | | |
| | | | | quarantine, | | |
| | | | | or reject. | | |
+------+-----------+------+----------+---------------+------+-------+
Table 3: "Authentication-Results Method Registry Update"
8.2. Authentication-Results Result Registry Update
IANA has added the following in the "Email Authentication Result
Names" registry:
+================+===========+========================+========+
| Auth Method(s) | Code | Specification | Status |
+================+===========+========================+========+
| dmarc | fail | [RFC7489] section 11.2 | active |
+----------------+-----------+------------------------+--------+
| dmarc | none | [RFC7489] section 11.2 | active |
+----------------+-----------+------------------------+--------+
| dmarc | pass | [RFC7489] section 11.2 | active |
+----------------+-----------+------------------------+--------+
| dmarc | permerror | [RFC7489] section 11.2 | active |
+----------------+-----------+------------------------+--------+
| dmarc | temperror | [RFC7489] | active |
+----------------+-----------+------------------------+--------+
Table 4: "Authentication-Results Result Registry Update"
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8.3. Feedback Report Header Fields Registry Update
The following has been added to the "Feedback Report Header Fields"
registry:
+=========+==============+==========+============+=========+=======+
|Field | Description |Multiple | Related |Reference|Status |
|Name | |Apperances| "Feedback- | | |
| | | | Type" | | |
+=========+==============+==========+============+=========+=======+
|Identity-| indicates |No | auth- |[RFC7489]|current|
|Alignment| whether the | | failure | | |
| | message | | | | |
| | about which | | | | |
| | a report is | | | | |
| | being | | | | |
| | generated | | | | |
| | had any | | | | |
| | identifiers | | | | |
| | in alignment | | | | |
| | as defined | | | | |
| | in [RFC7489] | | | | |
+---------+--------------+----------+------------+---------+-------+
Table 5: "Feedback Report Header Fields"
8.4. 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.
Names of DMARC tags are 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 includes the tag name;
the specification that defines it; a brief description; and its
status, which is 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.
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The initial set of entries in this registry is as follows:
+=======+===========+==========+=============================+
| Tag | Reference | Status | Description |
| Name | | | |
+=======+===========+==========+=============================+
| adkim | RFC 7489 | current | DKIM alignment mode |
+-------+-----------+----------+-----------------------------+
| aspf | RFC 7489 | current | SPF alignment mode |
+-------+-----------+----------+-----------------------------+
| fo | RFC 7489 | current | Failure reporting options |
+-------+-----------+----------+-----------------------------+
| np | RFC 9091 | current | Requested handling policy |
| | | | for non-existent subdomains |
+-------+-----------+----------+-----------------------------+
| p | RFC 7489 | current | Requested handling policy |
+-------+-----------+----------+-----------------------------+
| pct | RFC 7489 | historic | Sampling rate |
+-------+-----------+----------+-----------------------------+
| psd | [this | current | Indicates whether policy |
| | document] | | record is published by a |
| | | | Public Suffix Domain |
+-------+-----------+----------+-----------------------------+
| rf | RFC 7489 | historic | Failure reporting format(s) |
+-------+-----------+----------+-----------------------------+
| ri | RFC 7489 | historic | 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 |
+-------+-----------+----------+-----------------------------+
| t | [this | current | Test mode for the specified |
| | document] | | policy |
+-------+-----------+----------+-----------------------------+
| v | RFC 7489 | current | Specification version |
+-------+-----------+----------+-----------------------------+
Table 6: "DMARC Tag Registry"
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8.5. 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 are 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
includes 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.
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 7: "DMARC Report Format Registry"
8.6. 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 8: "Underscored and
Globally Scoped DNS Node Names"
registry
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9. Privacy Considerations
This section discusses issues specific to private data that may be
included if DMARC reports are requested. Issues associated with
sending aggregate reports and failure reports are addressed in
[I-D.ietf-dmarc-aggregate-reporting] and
[I-D.ietf-dmarc-failure-reporting] respectively.
9.1. Aggregate Report Considerations
Aggregate reports may, particularly for small organizations, provide
some limited insight into email sending patterns. As an example, in
a small organization, an aggregate report from a particular domain
may be sufficient to make the report receiver aware of sensitive
personal or business information. If setting an rua= tag in a DMARC
record, the reporting address needs controls appropriate to the
organizational requirements to mitigate any risk associated with
receiving and handling reports.
In the case of rua= requests for multi-organizational PSDs,
additional information leakage considerations exist. Multi-
organizational PSDs that do not mandate DMARC use by registrants risk
exposure of private data of registrant domains if they include the
rua= tag in their DMARC record.
9.2. Failure Report Considerations
Failure reports do provide insight into email sending patterns,
including specific users. If requesting failure reports, data
management controls are needed to support appropriate management of
this information. The additional detail available through failure
reports (relative to aggregate reports) can drive a need for
additional controls. As an example, a company may be legally
restricted from receiving data related to a specific subsidiary.
Before requesting failure reports, any such data spillage risks have
to be addressed through data management controls or publishing DMARC
records for relevant subdomains to prevent reporting on data related
to their emails.
Out of band agreements between failure report senders and receivers
may be required to address privacy concerns.
DMARC records for multi-organizational PSDs MUST NOT include the ruf=
tag.
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10. Security Considerations
This section discusses security issues and possible remediations
(where available) for DMARC.
10.1. Authentication Methods
Security considerations from the authentication methods used by DMARC
are incorporated here by reference.
Both of the email authentication methods that underlie DMARC provide
some assurance that an email was transmitted by an MTA which is
authorized to do so. SPF policies map domain names to sets of
authorized MTAs Section 11.4 of [RFC7208]. Validated DKIM signatures
indicate that an email was transmitted by an MTA with access to a
private key that matches the published DKIM key record.
Whenever mail is sent, there is a risk that an overly permissive
source may send mail that will receive a DMARC pass result that was
not, in fact, intended by the Domain Owner. These results may lead
to issues when systems interpret DMARC pass results to indicate a
message is in some way authentic. They also allow such unauthorized
senders to evade the Domain Owner's intended message handling for
DMARC validation failures.
To avoid this risk one must ensure that no unauthorized source can
add DKIM signatures to the domain's mail or transmit mail which will
evaluate as SPF pass. If, nonetheless, a Domain Owner wishes to
include a permissive source in a domain's SPF record, the source can
be excluded from DMARC consideration by using the '?' qualifier on
the SPF record mechanism associated with that source.
10.2. 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. Records found in the DNS such
as MX, NS, and others 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. This all means
that Domain Owners will need to harden these addresses against
various attacks, including but not limited to:
* high-volume denial-of-service attacks;
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* 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.
10.3. DNS Security
The DMARC mechanism and its underlying Authentication Mechanisms
(SPF, DKIM) depend on the security of the DNS. Examples of how
hostile parties can have an adverse impact on DNS traffic include:
* If they 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.
* 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, including, but not limited to:
* Signing DNS records with DNSSEC [RFC4033], which enables
recipients to validate the integrity of DNS data and detect and
discard forged responses.
* DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate
snooping and forged responses.
10.4. 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.
Such attacks, known as display name attacks, are out of scope for
DMARC.
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10.5. Denial of DMARC Processing Attacks
The declaration in Section 5.3.1 and elsewhere in this document that
messages that do not contain precisely one RFC5322.From domain are
outside the scope of this document exposes an attack vector that must
be taken into consideration.
Because such messages are outside the scope of this document, an
attacker can craft messages with multiple RFC5322.From domains,
including the spoofed domain, in an effort to bypass DMARC validation
and get the fraudulent message to be displayed by the victim's MUA
with the spoofed domain successfully shown to the victim. In those
cases where such messages are not rejected due to other reasons (for
example, many such messages would violate RFC5322's requirement that
there be precisely one From: header field), care must be taken by the
Mail Receiver to recognize such messages as the threats they might be
and handle them appropriately.
The case of a syntactically valid multi-valued RFC5322.From field
presents a particular challenge. Experience has shown that most such
messages are abusive and/or unwanted by their recipients, and given
this fact, a Mail Receiver may make a negative disposition decision
for the message prior to and instead of its being subjected to DMARC
processing. However, in a case where a Mail Receiver requires that
the message be subject to DMARC validation, a recommended approach as
per [RFC7489] is to apply the DMARC mechanism to each domain found in
the RFC5322.From field as the Author Domain and apply the most strict
policy selected among the checks that fail. Such an approach might
prove useful for a small number of Author Domains, but it is possible
that applying such logic to messages with a large number of domains
(where "large" is defined by each Mail Receiver) will expose the Mail
Receiver to a form of denial of service attack. Limiting the number
of Author Domains processed will avoid this risk. If not all Author
Domains are processed, then the DMARC evaluation is incomplete.
10.6. External Reporting Addresses
To avoid abuse by bad actors, reporting addresses generally have to
be inside the domains about which reports are requested. To
accommodate special cases such as a need to get reports about domains
that cannot actually receive mail, Section 3 of
[I-D.ietf-dmarc-aggregate-reporting] describes a DNS-based mechanism
for validating 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.
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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.
10.7. Secure Protocols
This document encourages the use of secure transport mechanisms to
prevent the 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.
10.8. Relaxed Alignment Considerations
The DMARC mechanism allows both [DKIM- and SPF-Authenticated
Identifiers]{#identifier-alignment-explained} to validate authorized
use of an Author Domain (#author-domain) on behalf of a Domain Owner
(#domain-owner). If malicious or unaware users can gain control of
the SPF record or DKIM selector records for a subdomain of the
Organizational Domain, the subdomain can be used to generate email
that achieves a DMARC pass on behalf of the Organizational Domain.
For example, an attacker who controls the SPF record for
"evil.example.com" can send mail that produces an SPF-Authenticated
Identifier of "evil.example.com" with an RFC5322.From header field
containing "foo@example.com" that can produce a DMARC pass for mail
using the Organizational Domain ("example.com") as the Author Domain.
The Organizational Domain Owner should be careful not to delegate
control of subdomains if this is an issue, and consider using the
Strict Alignment (#strict-alignment) option if appropriate.
DMARC evaluation for relaxed alignment is also highly sensitive to
errors in determining the Organizational Domain if the Author Domain
does not have a published DMARC Policy Record (#dmarc-policy-record).
If an incorrectly selected Organizational Domain is a parent of the
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correct Organizational Domain, then relaxed alignment could
potentially allow a malicious sender to send mail that achieves a
DMARC pass verdict. This potential exists for both the legacy
[RFC7489] and current methods for determining the organizational
domain, the latter described in Section 4.10.2.
This issue is entirely avoided by the use of Strict Alignment and
publishing explicit DMARC Policy Records for all Author Domains used
in an organization's email.
For cases where Strict Alignment is not appropriate, this issue can
be mitigated by periodically checking the DMARC Policy Records, if
any, of PSDs (#public-suffix-domain) above the Organizational Domain
in the DNS tree and (for legacy [RFC7489] checking that appropriate
PSL entries remain present). If a PSD publishes a DMARC Policy
Record without the appropriate psd=y tag, Organizational Domain
owners can add psd=n to their Organizational Domain's DMARC Policy
Record so that the PSD's DMARC Policy Record will not be incorrectly
interpreted to indicate that the PSD is the Organizational Domain.
11. Normative References
[I-D.ietf-dmarc-aggregate-reporting]
Brotman, A., "DMARC Aggregate Reporting", Work in
Progress, Internet-Draft, draft-ietf-dmarc-aggregate-
reporting-15, 25 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dmarc-
aggregate-reporting-15>.
[I-D.ietf-dmarc-failure-reporting]
Jones, S. M. and A. Vesely, "Domain-based Message
Authentication, Reporting, and Conformance (DMARC) Failure
Reporting", Work in Progress, Internet-Draft, draft-ietf-
dmarc-failure-reporting-10, 17 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dmarc-
failure-reporting-10>.
[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>.
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[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>.
[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>.
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[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>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8552] Crocker, D., "Scoped Interpretation of DNS Resource
Records through "Underscored" Naming of Attribute Leaves",
BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
<https://www.rfc-editor.org/info/rfc8552>.
[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>.
12. Informative References
[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>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.
[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>.
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[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>.
[RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen, T., Ed., Zwicky,
E., Ed., and K. Andersen, Ed., "Interoperability Issues
between Domain-based Message Authentication, Reporting,
and Conformance (DMARC) and Indirect Email Flows",
RFC 7960, DOI 10.17487/RFC7960, September 2016,
<https://www.rfc-editor.org/info/rfc7960>.
[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>.
[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>.
[RFC8617] Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
Kucherawy, Ed., "The Authenticated Received Chain (ARC)
Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
<https://www.rfc-editor.org/info/rfc8617>.
[RFC9091] Kitterman, S. and T. Wicinski, Ed., "Experimental Domain-
Based Message Authentication, Reporting, and Conformance
(DMARC) Extension for Public Suffix Domains", RFC 9091,
DOI 10.17487/RFC9091, July 2021,
<https://www.rfc-editor.org/info/rfc9091>.
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Appendix A. Technology Considerations
This section documents some design decisions made in the development
of DMARC. Specifically addressed here are some suggestions that were
considered but not included in the design, with explanatory text
regarding the decision.
A.1. S/MIME
S/MIME, or Secure Multipurpose Internet Mail Extensions, is a
standard for encrypting and signing 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
validate 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 are both deployed widely over the general
Internet, and their adoption rates continue to be positive.
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 instruct Mail 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.
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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 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
Sender field, if present. Accordingly, supporting the 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 validation algorithm in terms of which
policy is to be applied and when.
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A.4. Domain Existence Test
The presence of the "np" tag in this specification seemingly implies
that there would be an agreed-upon standard for determining a
domain's existence.
Since the DMARC mechanism is focused on email, one might think that
the definition of resolvable in [RFC5321] applies. Using that
definition, only names that resolve to MX Resource Records (RRs), A
RRs, or AAAA RRs are deemed to be resolvable and to exist in the DNS.
This is a common practice among Mail Receivers to determine whether
or not to accept a mail message before performing other more
expensive processing.
The DMARC mechanism makes no such requirement for the existence of
specific DNS RRs in order for a domain to exist; instead, if any RR
exists for a domain, then the domain exists. To use the terminology
from [RFC2308], an "NXDOMAIN" response (rcode "Name Error") to a DNS
query means that the domain name does not exist, while a "NODATA"
response (rcode "NOERROR") means that the given resource record type
queried for does not exist, but the domain name does.
Furthermore, in keeping with [RFC8020], if a query for a name returns
NXDOMAIN, then not only does the name not exist, every name below it
in the DNS hierarchy also does not exist.
A.5. Organizational Domain Discovery Issues
An earlier informational version of the DMARC mechanism [RFC7489]
noted that 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. That
version further mentioned suggestions that 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.
That previous version posited that one could "climb the tree" to find
the Organizational Domain, but expressed concern that an attacker
could exploit this for a denial-of-service attack through sending a
high number of messages each with a relatively large number of
nonsense labels, causing a Mail Receiver to perform a large number of
DNS queries in search of a DMARC Policy Record. This version defines
a method for performing a DNS Tree Walk (#dns-tree-walk), and further
mitigates the risk of the denial-of-service attack by expressly
limiting the number of DNS queries to execute regardless of the
number of labels in the domain name.
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Readers curious about the previous method for Organizational Domain
Discovery are directed to Section 3.2 of [RFC7489].
A.6. Removal of the "pct" Tag
An earlier informational version of the DMARC mechanism [RFC7489]
included a "pct" tag and specified all integers from 0 to 100
inclusive as valid values for the tag. The intent of the tag was to
provide domain owners with a method to gradually change their
preferred Domain Owner Assessment Policy (the p= tag) from 'none' to
'quarantine' or from 'quarantine' to 'reject' by requesting the
stricter treatment for just a percentage of messages that produced
DMARC results of "fail".
Operational experience showed that the pct tag was usually not
accurately applied, unless the value specified was either "0" or
"100" (the default), and the inaccuracies with other values varied
widely from implementation to implementation. The default value was
easily implemented, as it required no special processing on the part
of the Mail Receiver, while the value of "0" took on unintended
significance as a value used by some intermediaries and mailbox
providers as an indicator to deviate from standard handling of the
message, usually by rewriting the RFC5322.From header field in an
effort to avoid DMARC failures downstream.
These custom actions when the pct= tag was set to "0" proved valuable
to the email community. In particular, header field rewriting by an
intermediary meant that a Domain Owner's aggregate reports could
reveal to the Domain Owner how much of its traffic was routing
through intermediaries that don't rewrite the RFC5322.From header
field. It required work on the part of the Domain Owner to compare
aggregate reports from before and after the p= value was changed and
pct= was included in the DMARC Policy Record with a value of "0", but
the data was there. Consequently, knowing how much mail was subject
to possible DMARC failure due to a lack of RFC5322.From header field
rewriting by intermediaries could assist the Domain Owner in choosing
whether or not to move from Monitoring Mode (#monitoring-mode) to
Enforcement (#enforcement) Armed with this knowledge, the Domain
Owner could make an informed decision regarding subjecting its mail
traffic to possible DMARC failures based on the Domain Owner's
tolerance for such things.
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Because of the value provided by "pct=0" to Domain Owners, it was
logical to keep this functionality in the protocol; at the same time,
it didn't make sense to support a tag named "pct" that had only two
valid values. This version of the DMARC mechanism, therefore,
introduces the "t" tag as shorthand for "testing", with the valid
values of "y" and "n", which are meant to be analogous in their
application by mailbox providers and intermediaries to the "pct" tag
values "0" and "100", respectively.
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 header fields
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.
Alignment cannot exist if SPF does not produce a passing result.
Example 1: SPF in Strict Alignment:
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 domain and the Author Domain are
identical. Thus, the identifiers are in Strict Alignment.
Example 2: SPF in Relaxed Alignment:
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
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In this case, the Author Domain (example.com) is a parent of the
RFC5321.MailFrom domain. Thus, the identifiers are in relaxed
alignment because they both have the same Organizational Domain
(example.com).
Example 3: No SPF Identifier 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
In this case, the RFC5321.MailFrom domain that is neither the same
as, a parent of, nor a child of the Author Domain. Thus, the
identifiers are not in alignment.
B.1.2. DKIM
The examples below assume that the DKIM signatures pass validation.
Alignment cannot exist with a DKIM signature that does not validate.
Example 1: DKIM in Strict Alignment:
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 Author Domain have
identical DNS domains. Thus, the identifiers are in Strict
Alignment.
Example 2: DKIM in Relaxed Alignment:
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
In this case, the DKIM signature's "d=" parameter includes a DNS
domain that is a parent of the Author Domain. Thus, the identifiers
are in relaxed alignment, as they have the same Organizational Domain
(example.com).
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Example 3: No DKIM Identifier Alignment:
DKIM-Signature: v=1; ...; d=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
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
Author 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 DMARC Policy
Record for the Domain Owner's Organizational Domain.
B.2.1. Entire Domain, Monitoring Mode
The Domain Owner for "example.com" has deployed SPF and DKIM on its
messaging infrastructure. The Domain Owner wishes to begin using
DMARC with a policy that will solicit aggregate feedback from Mail
Receivers without affecting how the messages are processed in order
to:
* Confirm that its legitimate messages are authenticating correctly
* Validate that all authorized message sources have implemented
authentication measures
* Determine how many messages from other sources would be affected
by publishing a Domain Owner Assessment Policy at Enforcement
The Domain Owner accomplishes this by constructing a DMARC Policy
Record indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* Mail Receivers should not alter how they treat these messages
because of this DMARC Policy Record ("p=none")
* Aggregate feedback reports are sent via email to the address
"dmarc-feedback@example.com" ("rua=mailto:dmarc-
feedback@example.com" (mailto:dmarc-feedback@example.com"))
* All messages from this Organizational Domain are subject to this
policy (no "t" tag present, so the default of "n" applies).
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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 Policy Record for the domain example.com
_dmarc IN TXT ( "v=DMARC1; p=none; "
"rua=mailto:dmarc-feedback@example.com" )
B.2.2. Entire Domain, Monitoring Mode, 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. To diagnose these intermittent problems,
they wish to request per-message failure reports when authentication
failures occur.
Not all Mail 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
DMARC Policy Record from Appendix B.2.1:
* Per-message failure reports are sent via email to the address
"auth-reports@example.com" ("ruf=mailto:auth-reports@example.com"
(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):
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; DMARC Policy 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 serve as a Report
Consumer. Again, not all Mail 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.
The Domain Owner needs to alter its DMARC Policy Record from
Appendix B.2.2 as follows:
* Per-message failure reports are sent via email to the address
"auth-reports@thirdparty.example.net" ("ruf=mailto:auth-
reports@thirdparty.example.net" (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 Policy 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
Mail Receivers will implement additional checks as described in
Section 3 of [I-D.ietf-dmarc-aggregate-reporting]. To pass these
additional checks, the Report Consumer's Domain Owner will need to
publish an additional DMARC Policy Record as follows:
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* Given the DMARC Policy Record published by the Domain Owner at
"_dmarc.example.com", the DNS administrator for the Report
Consumer will need to publish a TXT resource record at
"example.com._report._dmarc.thirdparty.example.net" with the value
"v=DMARC1;".
The resulting DMARC Policy Record might look like this when retrieved
using a common command-line tool:
% 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):
; zone file for thirdparty.example.net
; Accept DMARC failure reports on behalf of example.com
example.com._report._dmarc IN TXT "v=DMARC1;"
B.2.4. Subdomain, Testing, and Multiple Aggregate Report URIs
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 handling preference for messages from this subdomain that
fail DMARC validation to indicate to participating Mail Receivers
that use of this domain is not valid.
As a first step, it will express that it considers messages using
this subdomain that fail DMARC validation to be suspicious. The goal
here will be to enable examination of messages sent to mailboxes
hosted by participating Mail Receivers as a 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 Report Consumer selected and authorized
to receive them by the Domain Owner.
The Domain Owner will accomplish this by constructing a DMARC Policy
Record indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* It is applied only to this subdomain (the DMARC Policy Record is
published at "_dmarc.test.example.com" and not
"_dmarc.example.com")
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* Mail Receivers are advised that the Domain Owner considers
messages that fail to authenticate to be suspicious
("p=quarantine")
* Aggregate feedback reports are sent via email to the addresses
"dmarc-feedback@example.com" and "example-tld-
test@thirdparty.example.net" ("rua=mailto:dmarc-
feedback@example.com (mailto:dmarc-feedback@example.com),
mailto:tld-test@thirdparty.example.net") (mailto:tld-
test@thirdparty.example.net"))
* The Domain Owner desires only that an actor performing a DMARC
validation check apply any special handling rules it might have in
place, such as rewriting the RFC53322.From header field; the
Domain Owner is testing its setup at this point and so does not
want the handling policy to be applied. ("t=y")
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=quarantine; rua=mailto:dmarc-feedback@example.com,
mailto:tld-test@thirdparty.example.net; t=y"
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 Policy 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;"
"t=y" )
Once enough time has passed to allow for collecting enough reports to
give the Domain Owner confidence that all authorized email sent using
the subdomain is properly authenticating and passing DMARC validation
checks, then the Domain Owner can update the DMARC Policy Record to
indicate that it considers validation failures to be a clear
indication that use of the subdomain is not valid. It would do this
by altering the record to advise Mail Receivers of its position on
such messages ("p=reject") and removing the testing flag ("t=y").
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):
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% dig +short TXT _dmarc.test.example.com
"v=DMARC1; p=reject; 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:
; DMARC Policy 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 participate in 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 Consumers).
B.3.1. SMTP Session Example
An optimal DMARC-enabled Mail Receiver performs validation and
Identifier Alignment checking during the SMTP [RFC5321] conversation.
Before 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 Record lookup.
The presence of an Author Domain DMARC Policy Record indicates that
the Mail Receiver should continue with DMARC-specific processing
before returning a reply to the DATA command.
Given a DMARC Policy Record and the set of Authenticated Identifiers,
the Mail Receiver checks to see if the Authenticated Identifiers
align with the Author Domain (taking into consideration any strict
versus relaxed options found in the DMARC Policy Record).
For example, the following sample data is considered to be from a
piece of email originating from the Domain Owner of "example.com":
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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; aspf=r;
rua=mailto:dmarc-feedback@example.com"
In the above sample, the SPF-Authenticated Identifier and the DKIM-
Authenticated Identifier both 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 the DMARC validation check.
If no Authenticated Identifiers align with the Author Domain, then
the Mail Receiver applies the Domain Owner Assessment Policy.
However, before this action is taken, the Mail Receiver can consult
external information to override the Domain Owner 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 Assessment 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 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 Policy Record requests AFRF-based
reporting. If so, then the Mail Receiver can emit an AFRF to the
reporting address supplied in the DMARC Policy 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 unnecessary if the Domain Owner
has not requested aggregate reports, i.e., no "rua" tag was found in
the policy record.
B.4. Organizational and Policy Domain Tree Walk Examples
If an Author Domain has no DMARC Policy Record, a Mail Receiver uses
a tree walk to find the DMARC Policy.
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If the DMARC Policy Record allows relaxed alignment and the SPF- or
DKIM-Authenticated Identifiers are different from the Author Domain,
a Mail Receiver uses a tree walk to discover the respective
Organizational Domains to determine Identifier Alignment.
B.4.1. Simple Organizational and Policy Example
A Mail Receiver receives an email with:
Author Domain example.com
RFC5321.MailFrom domain example.com
DKIM signature d= signing.example.com
In this example, _dmarc.example.com and _dmarc.signing.example.com
both have DMARC Policy Records while _dmarc.com does not. If SPF or
DKIM yield pass results, they still have to be aligned to support a
DMARC pass. Since not all domains are the same, if the alignment is
relaxed then the tree walk is performed to determine the
Organizational Domain for each:
For the Author Domain, query _dmarc.example.com and _dmarc.com;
example.com is the last element of the DNS tree with a DMARC record,
so it is the Organizational Domain for example.com.
For the RFC5321.MailFrom domain, the Organizational Domain already
found for example.com is example.com, so SPF is aligned.
For the DKIM d= domain, query _dmarc.signing.example.com,
_dmarc.example.com, and _dmarc.com. Both signing.example.com and
example.com have DMARC records, but example.com is the highest
element in the tree with a DMARC Policy Record (it has the fewest
labels), so example.com is the Organizational Domain. Since this is
also the Organizational Domain for the Author Domain, DKIM is aligned
for relaxed alignment.
Since both SPF and DKIM are aligned, they can be used to determine if
the message has a DMARC pass result. If the result is not pass, then
the policy domain's DMARC record is used to determine the appropriate
policy. In this case, since the RFC5322.From domain has a DMARC
record, that is the policy domain.
B.4.2. Deep Tree Walk Example
A Mail Receiver receives an email with:
Author Domain a.b.c.d.e.f.g.h.i.j.k.example.com
RFC5321.MailFrom domain example.com
DKIM signature d= signing.example.com
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Both _dmarc.example.com and _dmarc.signing.example.com have DMARC
records, while _dmarc.com does not. If SPF or DKIM yield pass
results, they still have to be aligned to support a DMARC pass.
Since not all domains are the same, if the alignment is relaxed then
the tree walk is performed to determine the Organizational Domain for
each:
For the Author Domain, query
_dmarc.a.b.c.d.e.f.g.h.i.j.k.example.com, skip to
_dmarc.g.h.i.j.k.example.com, then query _dmarc.h.i.j.k.example.com,
_dmarc.i.j.k.example.com, _dmarc.j.k.example.com,
_dmarc.k.example.com, _dmarc.example.com, and _dmarc.com. None of
a.b.c.d.e.f.g.h.i.j.k.example.com, g.h.i.j.k.example.com,
h.i.j.k.example.com, i.j.k.example.com, j.k.example.com, or
k.example.com have a DMARC record.
Since example.com is the last element of the DNS tree with a DMARC
record, it is the Organizational Domain for
a.b.c.d.e.f.g.h.i.j.k.example.com.
For the RFC5321.MailFrom domain, the Organizational domain already
found for example.com is example.com. SPF is aligned.
For the DKIM d= domain, query _dmarc.signing.example.com,
_dmarc.example.com, and _dmarc.com. Both signing.example.com and
example.com have DMARC records, but example.com is the highest
element in the tree with a DMARC record, so example.com is the
Organizational Domain. Since this is also the Organizational Domain
for the Author Domain, DKIM is aligned for relaxed alignment.
Since both SPF and DKIM are aligned, they can be used to determine if
the message has a DMARC pass result. If the results for both are not
pass, then the policy domain's DMARC Policy Record is used to
determine the appropriate policy. In this case, the Author Domain
does not have a DMARC Policy Record, so the policy domain is the
highest element in the DNS tree with a DMARC Policy Record,
example.com.
B.4.3. Example with a PSD DMARC Record
In rare cases, a PSD publishes a DMARC Policy Record with a psd tag,
which the tree walk must take into account.
A Mail Receiver receives an email with:
Author Domain giant.bank.example
RFC5321.MailFrom domain mail.giant.bank.example
DKIM signature d= mail.mega.bank.example
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In this case, _dmarc.bank.example has a DMARC Policy Record which
includes the psd=y tag, and _dmarc.example does not have a DMARC
Policy Record. While _dmarc.giant.bank.example has a DMARC Policy
Record without a psd tag, _dmarc.mega.bank.example and
_mail.mega.bank.example have no DMARC Policy Records.
Since the three domains are all different, tree walks find their
Organizational Domains to see which are aligned.
For the Author Domain giant.bank.example, the tree walk finds the
DMARC Policy Record at _dmarc.giant.bank.example, then the DMARC
Policy Record at _dmarc.bank.example, and stops because of the psd=y
flag. The Organizational Domain is giant.bank.example because it is
the domain directly below the one with psd=y. Since the
Organizational Domain has a DMARC Policy Record, it is also the
policy domain.
For the RFC5321.MailFrom domain, the tree walk finds no DMARC Policy
Record at _dmarc.mail.giant.bank.example, the DMARC Policy Record at
_dmarc.giant.bank.example, then the DMARC Policy Record at
_dmarc.bank.example, and stops because of the psd=y flag. Again the
Organizational Domain is giant.bank.example because it is the domain
directly below the one with psd=y. Since this is the same
Organizational Domain as the Author Domain, SPF is aligned.
For the DKIM signature domain mail.mega.bank.example, the tree walk
finds no DMARC Policy Records at _dmarc.mail.mega.bank.example or
_dmarc.mega.bank.example, then finds the DMARC Policy Record at
_dmarc.bank.example and stops because of the psd=y flag. The
Organizational Domain is mega.bank.example, so DKIM is not aligned.
Since SPF is aligned, it can be used to determine if the message has
a DMARC pass result. If the result is not pass, then the policy
domain's DMARC Policy Record is used to determine the appropriate
policy.
B.5. Utilization of Aggregate Feedback: Example
Aggregate feedback is consumed by Domain Owners to enable their
understanding of how a given domain is being processed by the Mail
Receiver. Aggregate reporting data on emails that pass all
underlying authentication checks is used by Domain Owners to validate
that their 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 validate ongoing
authentication practices of the third party.
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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
produce an Authenticated Identifier, 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, if the mail that is failing the
checks was generated by or on behalf of the Domain Owner. Data
regarding failing authentication checks can also allow the Domain
Owner to come to an understanding of how its domain is being misused.
Appendix C. Changes from RFC 7489
This document is intended to render obsolete [RFC7489]. As one might
guess, that means there are significant differences between RFC 7489
and this document. This section will summarize those changes.
C.1. IETF Category
RFC 7489 was not an Internet Standards Track specification; it was
instead published in the Informational Category. This document, by
contrast, is intended to be Internet Standards Track.
C.2. Changes to Terminology and Definitions
The following changes were made to the Terminology and Definitions
section.
C.2.1. Terms Added
These terms were added:
* Domain Owner Assessment Policy
* Enforcement
* Monitoring Mode
* Non-existent Domains
* Public Suffix Domain (PSD)
* Public Suffix Operator (PSO)
* PSO Controlled Domain Names
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C.2.2. Definitions Updated
These definitions were updated:
* Organizational Domain
* Report Receiver (renamed to Report Consumer)
C.3. Policy Discovery and Organizational Domain Determination
The algorithms for DMARC policy discovery and for determining the
Organizational Domain have been changed. Specifically, reliance on
the Public Suffix List (PSL) has been replaced by a technique called
a "DNS Tree Walk", and the methodology for the DNS Tree Walk is
explained in detail in this document.
The DNS Tree Walk also incorporates PSD policy discovery, which was
introduced in [RFC9091]. [RFC9091] was an Experimental RFC, and the
results of that experiment were that the RFC was not implemented as
written. Instead, this document redefines the algorithm for PSD
policy discovery, and thus obsoletes [RFC9091].
C.4. Reporting
Discussion of both aggregate and failure reporting have been moved to
separate documents dedicated to the topics.
In addition, the ability to specify a maximum report size in the
DMARC URI has been removed.
C.5. Tags
Several tags have been added to the "DMARC Policy Record Format"
section of this document since RFC 7489 was published, and at the
same time, several others were removed.
C.5.1. Tags Added:
* np - Policy for non-existent domains (Imported from [RFC9091])
* psd - Flag indicating whether a domain is a Public Suffix Domain
* t - Replacement for some pct tag functionality. See Appendix A.6
for further discussion
C.5.2. Tags Removed:
* pct - Tag requesting application of DMARC policy to only a
percentage of messages
* rf - Tag specifying requested format of failure reports
* ri - Tag specifying requested interval between aggregate reports
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C.6. Expansion of Domain Owner Actions Section
This section has been expanded upon from RFC 7489.
RFC 7489 had just two paragraphs in its Domain Owner Actions section,
and while the content of those paragraphs was correct, it was
minimalist in its approach to providing guidance to domain owners on
just how to implement DMARC.
This document provides much more detail and explanatory text to a
Domain Owner, focusing not just on what to do to implement DMARC, but
also on the reasons for each step and the repercussions of each
decision.
In particular, this document makes explicit that domains for general-
purpose email SHOULD NOT deploy a DMARC policy of p=reject. See
Section 7.5 for further discussion of this topic.
C.7. Report Generator Recommendations
In the cases where a DMARC record specifies multiple destinations for
either aggregate reports or failure reports, RFC 7489 stated:
Receivers **MAY** impose a limit on the number of URIs to which they
will send reports but **MUST** support the ability to send to at least
two.
This document in Section 4.6 says:
A report **SHOULD** be sent to each listed URI provided in the DMARC
record.
C.8. Removal of RFC 7489 Appendix A.5
One of the appendices in RFC 7489, specifically Appendix A.5, has
been removed from the text with this update. The appendix was titled
"Issues with ADSP in Operation" and it contained a list of issues
associated with ADSP that influenced the direction of DMARC. The
ADSP protocol was moved to "Historic" status in 2013 and working
group consensus was that such a discussion of ADSP's influence on
DMARC was no longer relevant.
C.9. RFC 7489 Errata Summary
Remove this before final submission: (https://www.rfc-
editor.org/styleguide/part2/#ref_errata (https://www.rfc-
editor.org/styleguide/part2/#ref_errata) says errata in the Reported
state should not be referenced; they are not considered stable.)
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This document and its companion documents
([I-D.ietf-dmarc-aggregate-reporting] and
[I-D.ietf-dmarc-failure-reporting]) address the following errata
filed against [RFC7489] since that document's publication in March,
2015. More details on each of these can be found at https://www.rfc-
editor.org/errata_search.php?rfc=7489 (https://www.rfc-editor.org/
errata_search.php?rfc=7489)
[Err5365] RFC Errata, Erratum ID 5365, RFC 7489, Section 7.2.1.1
https://www.rfc-editor.org/errata/eid5365 (https://www.rfc-
editor.org/errata/eid5365): To be addressed in
[I-D.ietf-dmarc-aggregate-reporting]
[Err5371] RFC Errata, Erratum ID 5371, RFC 7489, Section 7.2.1.1
https://www.rfc-editor.org/errata/eid5371 (https://www.rfc-
editor.org/errata/eid5371): To be addressed in
[I-D.ietf-dmarc-aggregate-reporting]
[Err5440] RFC Errata, Erratum ID 5440, RFC 7489, Section 7.1
https://www.rfc-editor.org/errata/eid5440 (https://www.rfc-
editor.org/errata/eid5440): To be addressed in
[I-D.ietf-dmarc-aggregate-reporting]
[Err5440] RFC Errata, Erratum ID 5440, RFC 7489, Sections B.2.1,
B.2.3, and B.2.4 https://www.rfc-editor.org/errata/eid5440
(https://www.rfc-editor.org/errata/eid5440): Addressed in this
document.
[Err6439] RFC Errata, Erratum ID 6439, RFC 7489, Section 7.1
https://www.rfc-editor.org/errata/eid6439 (https://www.rfc-
editor.org/errata/eid6439): To be addressed in
[I-D.ietf-dmarc-aggregate-reporting]
[Err6485] RFC Errata, Erratum ID 6485, RFC 7489, Section 7.2.1.1
https://www.rfc-editor.org/errata/eid6485 (https://www.rfc-
editor.org/errata/eid6485): To be addressed in
[I-D.ietf-dmarc-aggregate-reporting]
[Err7835] RFC Errata, Erratum ID 7835, RFC 7489, Section 6.6.3
https://www.rfc-editor.org/errata/eid7835 (https://www.rfc-
editor.org/errata/eid7835): This erratum is in reference to the
description of the process documented in RFC 7489 for the
applicable DMARC policy for an email message. The process for
doing this has drastically changed in DMARCbis, and so the text
identified in this erratum no longer exists.
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[Err5151] RFC Errata, Erratum ID 5151, RFC 7489, Section 1
https://www.rfc-editor.org/errata/eid5151 (https://www.rfc-
editor.org/errata/eid5151): This erratum is in reference to the
Introduction section of RFC 7489. That section has been
substantially rewritten in DMARCbis, and the text at issue for
this erratum no longer exists.
C.10. General Editing and Formatting
A great deal of the content from RFC 7489 was preserved in this
document, but much of it was subject to either minor editing, re-
ordering of sections, and/or both.
Acknowledgements
This reworking of the DMARC mechanism specified in [RFC7489] is the
result of contributions from many participants in the IETF Working
Group dedicated to this effort. Although the contributors are too
numerous to mention, significant contributions were made by Kurt
Andersen, Laura Atkins, Seth Blank, Alex Brotman, Dave Crocker,
Douglas E. Foster, Ned Freed, Mike Hammer, Steven M. Jones, Scott
Kitterman, Murray S. Kucherawy, Barry Leiba, Alessandro Vesely, and
Tim Wicinski.
The authors and contributors also recognize that this document would
not have been possible without the work done by those who had a hand
in producing [RFC7489]. The Acknowledgements section from that
document is preserved in full below.
Acknowledgements - RFC 7489
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 https://dmarc.org
(https://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
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.
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Additional contributions within the IETF context were made by Kurt
Andersen, 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.
Authors' Addresses
Todd M. Herr
Valimail
Email: todd.herr@valimail.com
John Levine
Standcore LLC
Email: standards@standore.com
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