Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-07
| Document | Type | Active Internet-Draft (dmarc WG) | |
|---|---|---|---|
| Authors | Todd Herr , John R. Levine | ||
| Last updated | 2022-04-06 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-dmarc-dmarcbis-07
DMARC T. Herr (ed)
Internet-Draft Valimail
Obsoletes: 7489 (if approved) J. Levine (ed)
Intended status: Standards Track Standcore LLC
Expires: 8 October 2022 6 April 2022
Domain-based Message Authentication, Reporting, and Conformance (DMARC)
draft-ietf-dmarc-dmarcbis-07
Abstract
This document describes the Domain-based Message Authentication,
Reporting, and Conformance (DMARC) protocol.
DMARC permits the owner of an email author's domain name to enable
verification of the domain's use, to indicate the Domain Owner's or
Public Suffix Operator's message handling preference regarding failed
verification, and to request reports about use of the domain name.
Mail receiving organizations can use this information when evaluating
handling choices for incoming mail.
This document obsoletes RFC 7489.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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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 8 October 2022.
Copyright Notice
Copyright (c) 2022 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. High-Level Goals . . . . . . . . . . . . . . . . . . . . 6
2.2. Anti-Phishing . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 7
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 7
3.1. Conventions Used in This Document . . . . . . . . . . . . 7
3.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Authenticated Identifiers . . . . . . . . . . . . . . 8
3.2.2. Author Domain . . . . . . . . . . . . . . . . . . . . 8
3.2.3. Domain Owner . . . . . . . . . . . . . . . . . . . . 8
3.2.4. Identifier Alignment . . . . . . . . . . . . . . . . 8
3.2.5. Mail Receiver . . . . . . . . . . . . . . . . . . . . 8
3.2.6. Non-existent Domains . . . . . . . . . . . . . . . . 9
3.2.7. Organizational Domain . . . . . . . . . . . . . . . . 9
3.2.8. Public Suffix Domain (PSD) . . . . . . . . . . . . . 9
3.2.9. Public Suffix Operator (PSO) . . . . . . . . . . . . 9
3.2.10. PSO Controlled Domain Names . . . . . . . . . . . . . 9
3.2.11. Report Consumer . . . . . . . . . . . . . . . . . . . 10
4. Overview and Key Concepts . . . . . . . . . . . . . . . . . . 10
4.1. DMARC Basics . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Use of RFC5322.From . . . . . . . . . . . . . . . . . . . 11
4.3. Authentication Mechanisms . . . . . . . . . . . . . . . . 12
4.4. Identifier Alignment Explained . . . . . . . . . . . . . 12
4.4.1. DKIM-Authenticated Identifiers . . . . . . . . . . . 13
4.4.2. SPF-Authenticated Identifiers . . . . . . . . . . . . 14
4.4.3. Alignment and Extension Technologies . . . . . . . . 14
4.5. Flow Diagram . . . . . . . . . . . . . . . . . . . . . . 14
4.6. DNS Tree Walk . . . . . . . . . . . . . . . . . . . . . . 16
4.7. DMARC Policy Discovery . . . . . . . . . . . . . . . . . 17
4.8. Organizational Domain Discovery . . . . . . . . . . . . . 18
5. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1. DMARC Policy Record . . . . . . . . . . . . . . . . . . . 21
5.2. DMARC URIs . . . . . . . . . . . . . . . . . . . . . . . 21
5.3. General Record Format . . . . . . . . . . . . . . . . . . 22
5.4. Formal Definition . . . . . . . . . . . . . . . . . . . . 25
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5.5. Domain Owner Actions . . . . . . . . . . . . . . . . . . 27
5.5.1. Publish an SPF Policy for an Aligned Domain . . . . . 27
5.5.2. Configure Sending System for DKIM Signing Using an
Aligned Domain . . . . . . . . . . . . . . . . . . . 27
5.5.3. Setup a Mailbox to Receive Aggregate Reports . . . . 27
5.5.4. Publish a DMARC Policy for the Author Domain and
Organizational Domain . . . . . . . . . . . . . . . . 28
5.5.5. Collect and Analyze Reports . . . . . . . . . . . . . 28
5.5.6. Decide If and When to Update DMARC Policy . . . . . . 28
5.6. PSO Actions . . . . . . . . . . . . . . . . . . . . . . . 28
5.7. Mail Receiver Actions . . . . . . . . . . . . . . . . . . 29
5.7.1. Extract Author Domain . . . . . . . . . . . . . . . . 29
5.7.2. Determine Handling Policy . . . . . . . . . . . . . . 29
5.7.3. Store Results of DMARC Processing . . . . . . . . . . 30
5.7.4. Send Aggregate Reports . . . . . . . . . . . . . . . 31
5.8. Policy Enforcement Considerations . . . . . . . . . . . . 31
6. DMARC Feedback . . . . . . . . . . . . . . . . . . . . . . . 32
7. Other Topics . . . . . . . . . . . . . . . . . . . . . . . . 32
7.1. Issues Specific to SPF . . . . . . . . . . . . . . . . . 33
7.2. DNS Load and Caching . . . . . . . . . . . . . . . . . . 33
7.3. Rejecting Messages . . . . . . . . . . . . . . . . . . . 33
7.4. Identifier Alignment Considerations . . . . . . . . . . . 34
7.5. Interoperability Issues . . . . . . . . . . . . . . . . . 35
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
8.1. Authentication-Results Method Registry Update . . . . . . 35
8.2. Authentication-Results Result Registry Update . . . . . . 36
8.3. Feedback Report Header Fields Registry Update . . . . . . 37
8.4. DMARC Tag Registry . . . . . . . . . . . . . . . . . . . 38
8.5. DMARC Report Format Registry . . . . . . . . . . . . . . 39
8.6. Underscored and Globally Scoped DNS Node Names
Registry . . . . . . . . . . . . . . . . . . . . . . . . 40
9. Security Considerations . . . . . . . . . . . . . . . . . . . 40
9.1. Authentication Methods . . . . . . . . . . . . . . . . . 40
9.2. Attacks on Reporting URIs . . . . . . . . . . . . . . . . 41
9.3. DNS Security . . . . . . . . . . . . . . . . . . . . . . 41
9.4. Display Name Attacks . . . . . . . . . . . . . . . . . . 42
9.5. External Reporting Addresses . . . . . . . . . . . . . . 42
9.6. Secure Protocols . . . . . . . . . . . . . . . . . . . . 43
10. Normative References . . . . . . . . . . . . . . . . . . . . 43
11. Informative References . . . . . . . . . . . . . . . . . . . 45
Appendix A. Technology Considerations . . . . . . . . . . . . . 46
A.1. S/MIME . . . . . . . . . . . . . . . . . . . . . . . . . 46
A.2. Method Exclusion . . . . . . . . . . . . . . . . . . . . 47
A.3. Sender Header Field . . . . . . . . . . . . . . . . . . . 48
A.4. Domain Existence Test . . . . . . . . . . . . . . . . . . 48
A.5. Issues with ADSP in Operation . . . . . . . . . . . . . . 49
A.6. Organizational Domain Discovery Issues . . . . . . . . . 49
A.7. Removal of the "pct" Tag . . . . . . . . . . . . . . . . 51
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Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 52
B.1. Identifier Alignment Examples . . . . . . . . . . . . . . 52
B.1.1. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 52
B.1.2. DKIM . . . . . . . . . . . . . . . . . . . . . . . . 53
B.2. Domain Owner Example . . . . . . . . . . . . . . . . . . 54
B.2.1. Entire Domain, Monitoring Only . . . . . . . . . . . 54
B.2.2. Entire Domain, Monitoring Only, Per-Message
Reports . . . . . . . . . . . . . . . . . . . . . . . 55
B.2.3. Per-Message Failure Reports Directed to Third
Party . . . . . . . . . . . . . . . . . . . . . . . . 56
B.2.4. Subdomain, Testing, and Multiple Aggregate Report
URIs . . . . . . . . . . . . . . . . . . . . . . . . 57
B.3. Mail Receiver Example . . . . . . . . . . . . . . . . . . 59
B.3.1. SMTP Session Example . . . . . . . . . . . . . . . . 59
B.4. Utilization of Aggregate Feedback: Example . . . . . . . 61
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 61
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source for this draft is maintained in GitHub at:
https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis
(https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis)
Abusive email often includes unauthorized and deceptive use of a
domain name in the RFC5322.From header field. 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. DMARC leverages these two protocols,
providing a method for Domain Owners to publish a DNS record
describing the email authentication policies for the RFC5322.From
domain and to request specific handling for messages using that
domain that fail authentication checks.
As with SPF and DKIM, DMARC classes results as "pass" or "fail". In
order to get a DMARC result of "pass", a pass from either SPF or DKIM
is required. In addition, the passed domain must be "aligned" with
the RFC5322.From domain in one of two modes - "relaxed" or "strict".
The mode is expressed in the domain's DMARC policy record. Domains
are said to be "in relaxed alignment" if they have the same
"Organizational Domain", which is the domain at the top of the domain
hierarchy for the RFC5322.From domain while having the same
administrative authority as the RFC5322.From domain. Domains are "in
strict alignment" if and only if they are identical.
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A DMARC pass indicates only that the RFC5322.From domain has been
authenticated for that message. Authentication does not carry an
explicit or implicit value assertion about that message or about the
Domain Owner. Furthermore, a mail-receiving organization that
performs DMARC verification can choose to honor the Domain Owner's
requested message handling for authentication failures, but it is
under no obligation to do so; it might choose different actions
entirely.
For a mail-receiving organization supporting DMARC, a message that
passes verification is part of a message stream that is reliably
associated with the RFC5322.From field Domain Owner. Therefore,
reputation assessment of that stream by the mail-receiving
organization is not encumbered by accounting for unauthorized use of
that domain in the RFC5322.From field. A message that fails this
verification is not necessarily associated with the Domain Owner's
domain and its reputation.
DMARC policy records can also cover non-existent sub-domains, below
the "Organizational Domain", as well as domains at the top of the
name hierarchy, controlled by Public Suffix Operators (PSOs).
DMARC, in the associated [DMARC-Aggregate-Reporting] and
[DMARC-Failure-Reporting] documents, also specifies a reporting
framework. Using it, a mail-receiving domain can generate regular
reports about messages that claim to be from a domain publishing
DMARC policies, sending those reports to the address(es) specified by
the Domain Owner in the latter's DMARC policy record. Domain Owners
can use these reports, especially the aggregate reports, to identify
not only sources of mail attempting to fraudulently use their domain,
but also (and perhaps more importantly) gaps in their own
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.
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
Specification of DMARC is guided by the following high-level goals,
security dependencies, detailed requirements, and items that are
documented as out of scope.
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2.1. High-Level Goals
DMARC has the following high-level goals:
* Allow Domain Owners and PSOs to assert their desired message
handling for authentication failures for messages purporting to
have authorship within the domain.
* Allow Domain Owners and PSOs to verify their authentication
deployment.
* Minimize implementation complexity for both senders and receivers,
as well as the impact on handling and delivery of legitimate
messages.
* Reduce the amount of successfully delivered spoofed email.
* Work at Internet scale.
2.2. Anti-Phishing
DMARC is designed to prevent bad actors from sending mail that claims
to come from legitimate senders, particularly senders of
transactional email (official mail that is about business
transactions). One of the primary uses of this kind of spoofed mail
is phishing (enticing users to provide information by pretending to
be the legitimate service requesting the information). Thus, DMARC
is significantly informed by ongoing efforts to enact large-scale,
Internet-wide anti-phishing measures.
Although DMARC can only be used to combat specific forms of exact-
domain spoofing directly, the DMARC mechanism has been found to be
useful in the creation of reliable and defensible message streams.
DMARC does not attempt to solve all problems with spoofed or
otherwise fraudulent email. In particular, it does not address the
use of visually similar domain names ("cousin domains") or abuse of
the RFC5322.From human-readable <display-name>.
2.3. Scalability
Scalability is a major issue for systems that need to operate in a
system as widely deployed as current SMTP email. For this reason,
DMARC seeks to avoid the need for third parties or pre-sending
agreements between senders and receivers. This preserves the
positive aspects of the current email infrastructure.
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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 for this
work. These include the following:
* Different treatment of messages that are not authenticated versus
those that fail authentication;
* Evaluation of anything other than RFC5322.From header field;
* Multiple reporting formats;
* Publishing policy other than via the DNS;
* Reporting or otherwise evaluating other than the last-hop IP
address;
* Attacks in the 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 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.
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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 terms used in this document.
3.2.1. Authenticated Identifiers
Domain-level identifiers that are verified using authentication
technologies are referred to as "Authenticated Identifiers". See
Section 4.3 for details about the supported mechanisms.
3.2.2. Author Domain
The domain name of the apparent author, as extracted from the
RFC5322.From header field.
3.2.3. Domain Owner
An entity or organization that owns a DNS domain. The term "owns"
here indicates that the entity or organization being referenced holds
the registration of that DNS domain. Domain Owners range from
complex, globally distributed organizations, to service providers
working on behalf of non-technical clients, to individuals
responsible for maintaining personal domains. This specification
uses this term as analogous to an Administrative Management Domain as
defined in [RFC5598]. It can also refer to delegates, such as Report
Consumers, when those are outside of their immediate management
domain.
3.2.4. Identifier Alignment
When the domain in the address in the RFC5322.From header field has
the same Organizational Domain as a domain verified by an
authenticated identifier, it has Identifier Alignment. (see
Section 3.2.7)
3.2.5. Mail Receiver
The entity or organization that receives and processes email. Mail
Receivers operate one or more Internet-facing Mail Transport Agents
(MTAs).
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3.2.6. Non-existent Domains
For DMARC purposes, a non-existent domain is consistent with the
meaning of the term 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 all the names under it do not exist.
3.2.7. Organizational Domain
The Organizational Domain is typically a domain that was registered
with a domain name registrar. More formally, it is any Public Suffix
Domain plus one label. The Organizational Domain for the domain in
the RFC5322.From domain is determined by applying the algorithm found
in Section 4.8.
3.2.8. Public Suffix Domain (PSD)
The global Internet Domain Name System (DNS) is documented in
numerous RFCs. It defines a tree of names starting with root, ".",
immediately below which are Top-Level Domain names such as ".com" and
".us". The domain name structure consists of a tree of names, each
of which is made of a sequence of words ("labels") separated by
period characters. The root of the tree is simply called ".". The
Internet community at large, through processes and policies external
to this work, selects points in this tree at which to register domain
names "owned" by independent organizations. Real-world examples of
these points are ".com", ".org", ".us", and ".gov.uk". Names at
which such registrations occur are called "Public Suffix Domains
(PSDs)", and a registration consists of a label selected by the
registrant to which a desirable PSD is appended. For example,
"ietf.org" is a registered domain name, and ".org" is its PSD.
3.2.9. 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.
3.2.10. PSO Controlled Domain Names
PSO-Controlled Domain Names are names in the DNS that are managed by
a PSO and are not available for use as Organizational Domains. PSO-
Controlled Domain Names may have one (e.g., ".com") or more (e.g.,
".co.uk") name components, depending on PSD policy.
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3.2.11. Report Consumer
An operator that receives reports from another operator implementing
the reporting mechanisms described in this document and/or the
documents [DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting].
Such an operator might be receiving reports about messages related to
a domain for which it is the Domain Owner or PSO, or reports about
messages related to another operator's domain. This term applies
collectively to the system components that receive and process these
reports and the organizations that operate them.
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 or PSO to enable verification of a
domain's use in an email message, to indicate the Domain Owner's or
PSO's message handling preference regarding failed verification, and
to request reports about use of the domain name. All information
about a Domain Owner's or PSO's DMARC policy is published and
retrieved via the DNS.
DMARC's verification function is based on whether the RFC5322.From
domain is aligned with a domain name used in a supported
authentication mechanism, as described in Section 4.3. When a DMARC
policy exists for the domain name found in the RFC5322.From header
field, and that domain name is not verified through an aligned
supported authentication mechanism, the handling of that message can
be affected based on the DMARC policy when delivered to a
participating Mail Receiver.
A message satisfies the DMARC checks if at least one of the supported
authentication mechanisms:
1. produces a "pass" result, and
2. produces that result based on an identifier that is in alignment,
as described in Section 4.4.
It is important to note that the authentication mechanisms employed
by DMARC authenticate only a DNS domain and do not authenticate the
local-part of any email address identifier found in a message, nor do
they validate the legitimacy of message content.
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DMARC's feedback component involves the collection of information
about received messages claiming to be from the Author Domain for
periodic aggregate reports to the Domain Owner or PSO. The
parameters and format for such reports are discussed in
[DMARC-Aggregate-Reporting]
A DMARC-enabled Mail Receiver might also generate per-message reports
that contain information related to individual messages that fail
authentication 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 authentication) or in
analyzing attacks. The capability for such services is enabled by
DMARC but defined in other referenced material such as [RFC6591] and
[DMARC-Failure-Reporting]
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.
Several points suggest that it is the most correct and safest thing
to do in this context:
* Of all the identifiers that are part of the message itself, this
is the only one guaranteed to be present.
* It seems the best choice of an identifier on which to focus, as
most MUAs display some or all of the contents of that field in a
manner strongly suggesting those data as reflective of the true
originator of the message.
* Many high-profile email sources, such as email service providers,
require that the sending agent have authenticated before email can
be generated. Thus, for these mailboxes, the mechanism described
in this document provides recipient end users with strong evidence
that the message was indeed originated by the agent they associate
with that mailbox, if the end user knows that these various
protections have been provided.
* The absence of a single, properly formed RFC5322.From header field
renders the message invalid. Handling of such a message is
outside of the scope of this specification.
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Since the sorts of mail typically protected by DMARC participants
tend to only have single Authors, DMARC participants generally
operate under a slightly restricted profile of RFC5322 with respect
to the expected syntax of this field. See Section 5.7 for details.
4.3. Authentication Mechanisms
The following mechanisms for determining 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 verified DKIM-Signature header field.
* SPF, [RFC7208], which can authenticate 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). As
noted earlier, however, DMARC relies solely on SPF authentication
of the domain found in SMTP MAIL FROM command. Section 2.4 of
[RFC7208] describes MAIL FROM processing for cases in which the
MAIL command has a null path.
4.4. Identifier Alignment Explained
Email authentication technologies authenticate various (and
disparate) aspects of an individual message. For example, DKIM
[RFC6376] authenticates the domain that affixed a signature to the
message, while SPF [RFC7208] can authenticate either the domain that
appears in the RFC5321.MailFrom (MAIL FROM) portion of an SMTP
[RFC5321] conversation or the RFC5321.EHLO/HELO domain, or both.
These may be different domains, and they are typically not visible to
the end user.
DMARC authenticates use of the RFC5322.From domain by requiring
either that it have the same Organizational Domain as an
Authenticated Identifier (a condition known as "relaxed alignment")
or that it be identical to the domain of the Authenticated Identifier
(a condition known as "strict alignment"). The choice of relaxed or
strict alignment is left to the Domain Owner and is expressed in the
domain's DMARC policy record. Domain names in this context are to be
compared in a case-insensitive manner, per [RFC4343].
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It is important to note that Identifier Alignment cannot occur with a
message that is not valid per [RFC5322], particularly one with a
malformed, absent, or repeated RFC5322.From header field, since in
that case there is no reliable way to determine a DMARC policy that
applies to the message. Accordingly, DMARC operation is predicated
on the input being a valid RFC5322 message object, and handling of
such non-compliant cases is outside of the scope of this
specification. Further discussion of this can be found in
Section 5.7.1.
Each of the underlying authentication technologies that DMARC takes
as input yields authenticated domains as their outputs when they
succeed.
4.4.1. DKIM-Authenticated Identifiers
DMARC requires Identifier Alignment based on the result of a DKIM
authentication because a message can bear a valid signature from any
domain, including domains used by a mailing list or even a bad actor.
Therefore, merely bearing a valid signature is not enough to infer
authenticity of the Author Domain.
DMARC permits Identifier Alignment based on the result of a DKIM
authentication to be strict or relaxed. (Note that these terms are
not related to DKIM's "simple" and "relaxed" canonicalization modes.)
In relaxed mode, the Organizational Domains of both the DKIM-
authenticated signing domain (taken from the value of the d= tag in
the signature) and that of the RFC5322.From domain must be equal if
the identifiers are to be considered to be aligned. In strict mode,
only an exact match between both Fully Qualified Domain Names (FQDNs)
is considered to produce Identifier Alignment.
To illustrate, in relaxed mode, if a verified DKIM signature
successfully verifies with a "d=" domain of "example.com", and the
RFC5322.From address is "alerts@news.example.com", the DKIM "d="
domain and the RFC5322.From domain are considered to be "in
alignment", because both domains have the same Organizational Domain
of "example.com". In strict mode, this test would fail because the
d= domain does not exactly match the RFC5322.From domain.
However, a DKIM signature bearing a value of "d=com" would never
allow an "in alignment" result, as "com" should be identified as a
PSD and therefore cannot be an Organizational Domain.
Note that a single email can contain multiple DKIM signatures, and it
is considered to produce a DMARC "pass" result if any DKIM signature
is aligned and verifies.
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4.4.2. SPF-Authenticated Identifiers
DMARC permits Identifier Alignment based on the result of an SPF
authentication. As with DKIM, Identifier Alignement can be either
strict or relaxed.
In relaxed mode, the Organizational Domains of the SPF-authenticated
domain and RFC5322.From domain must be equal if the identifiers are
to be considered to be aligned. In strict mode, the two FQDNs must
match exactly in order for them to be considered to be aligned.
For example, in relaxed mode, if a message passes an SPF check with
an RFC5321.MailFrom domain of "cbg.bounces.example.com", and the
address portion of the RFC5322.From header field contains
"payments@example.com", the Authenticated RFC5321.MailFrom domain
identifier and the RFC5322.From domain are considered to be "in
alignment" because they have the same Organizational Domain
("example.com"). In strict mode, this test would fail because the
two domains are not identical.
The reader should note that SPF alignment checks in DMARC rely solely
on the RFC5321.MailFrom domain. This differs from section 2.3 of
[RFC7208], which recommends that SPF checks be done on not only the
"MAIL FROM" but also on a separate check of the "HELO" identity.
4.4.3. Alignment and Extension Technologies
If in the future DMARC is extended to include the use of other
authentication mechanisms, the extensions will need to allow for
domain identifier extraction so that alignment with the RFC5322.From
domain can be verified.
4.5. Flow Diagram
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+---------------+ +--------------------+
| Author Domain |< . . . . . . . . . . . . | Return-Path Domain |
+---------------+ . +--------------------+
| . ^
V V .
+-----------+ +--------+ +----------+ v
| MSA |<***>| DKIM | | DMARC | +----------+
| Service | | Signer | | Verifier |<***>| SPF |
+-----------+ +--------+ +----------+ * | Verifier |
| ^ * +----------+
| * *
V v *
+------+ (~~~~~~~~~~~~) +------+ * +----------+
| sMTA |------->( other MTAs )----->| rMTA | **>| DKIM |
+------+ (~~~~~~~~~~~~) +------+ | Verifier |
| +----------+
| ^
V .
+-----------+ .
+---------+ | MDA | v
| User |<--| Filtering | +-----------+
| Mailbox | | Engine | | DKIM |
+---------+ +-----------+ | Signing |
| Domain(s) |
+-----------+
MSA = Mail Submission Agent
MDA = Mail Delivery Agent
The above diagram shows a simple flow of messages through a DMARC-
aware system. Solid lines denote the actual message flow, dotted
lines involve DNS queries used to retrieve message policy related to
the supported message authentication schemes, and asterisk lines
indicate data exchange between message-handling modules and message
authentication modules. "sMTA" is the sending MTA, and "rMTA" is the
receiving MTA.
Put simply, when a message reaches a DMARC-aware rMTA, a DNS query
will be initiated to determine if a DMARC policy exists that applies
to the author domain. If a policy is found, the rMTA will use the
results of SPF and DKIM verification checks to determine the ultimate
DMARC authentication status. The DMARC status can then factor into
the message handling decision made by the recipient's mail sytsem.
More details on specific actions for the parties involved can be
found in Section 5.5 and Section 5.7.
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4.6. DNS Tree Walk
The DMARC protocol defines a method for communicating information
through the publishing of records in DNS. Both the content of the
records and their location in the DNS hierarchy are used for two
purposes: policy discovery (see Section 4.7) and Organizational
Domain determination (see Section 4.8).
The relevant DMARC record for these purposes is not necessarily the
DMARC policy record found in DNS at the same level as the name label
for the domain in question. Instead, some domains will inherit their
DMARC policy records from parent domains one level or more above them
in the DNS hierarchy. Similarly, the Organizational Domain may be
found at a higher level in the DNS hierarchy.
These records are discovered through the technique described here,
known colloquially as the "DNS Tree Walk". The target of any DNS
Tree Walk is a valid DMARC policy record, but the rules defining
required content for that record depend on the reason for performing
the Tree Walk.
To prevent possible abuse of the DNS, a shortcut is built into the
process so that domains that have more than five labels do not result
in more than five DNS queries.
The generic steps for a DNS Tree Walk are as follows:
1. Query the DNS for a DMARC TXT record at the DNS domain matching
the one found in the domain(s) described above. A possibly empty
set of records is returned.
2. Records that do not start with a "v=" tag that identifies the
current version of DMARC are discarded. If multiple DMARC
records are returned, they are all discarded.
3. Determine the target for additional queries (if needed; see the
note in Section 4.8), using steps 4 through 8 below.
4. Break the subject DNS domain name into a set of "n" ordered
labels. Number these labels from right to left; e.g., for
"a.mail.example.com", "com" would be label 1, "example" would be
label 2, "mail.example.com" would be label 3, and so forth.
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5. Count the number of labels found in the subject DNS domain. Let
that number be "x". If x < 5, remove the left-most (highest-
numbered) label from the subject domain. If x >= 5, remove the
left-most (highest-numbered) labels from the subject domain until
4 labels remain. The resulting DNS domain name is the new target
for subsequent lookups.
6. Query the DNS for a DMARC TXT record at the DNS domain matching
this new target in place of the RFC5322.From domain in the
message. 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
records are returned for a single target, they are all discarded.
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 there are no more labels remaining.
To illustrate, for a message with the arbitrary RFC5322.From domain
of "a.b.c.d.e.mail.example.com", a full DNS Tree Walk would require
the following five queries, in order to locate the policy or
Organizational Domain:
* _dmarc.a.b.c.d.e.mail.example.com
* _dmarc.e.mail.example.com
* _dmarc.mail.example.com
* _dmarc.example.com
* _dmarc.com
4.7. DMARC Policy Discovery
For policy discovery, a DNS Tree Walk starts at the point in the DNS
hierarchy that matches the domain in the RFC5322.From header of the
message. The DMARC policy to be applied to the message will be the
record found at one of these three locations:
* The RFC5322.From domain
* The Organizational Domain (as determined by a separate DNS Tree
Walk) of the RFC5322.From domain
* The Public Suffix Domain of the RFC5322.From domain
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If the DMARC policy to be applied is that of the RFC5322.From domain,
then the DMARC policy is taken from the p= tag of the record. If the
DMARC policy is taken from either the Organizational Domain or the
Public Suffix Domain and that domain is different than the
RFC5322.From domain, then the DMARC policy is taken from the sp= tag
(if any) if the RFC5322.From domain exists and the np= tag (if any)
if the RFC5322.From domain does not exist. In the absence of
applicable sp= or np= tags, the p= tag policy is used for subdomains.
If a retrieved policy record does not contain a valid "p" tag, or
contains an "sp" tag that is not valid, then:
* If a "rua" tag is present and contains at least one syntactically
valid reporting URI, the Mail Receiver SHOULD act as if a record
containing a valid "v" tag and "p=none" was retrieved, and
continue processing;
* 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. Specifically, this is not a mechanism to
provide feedback addresses (rua/ruf) when an Organizational Domain
has declined to do so.
4.8. Organizational Domain Discovery
For Organizational Domain discovery, it may be necessary to perform
multiple DNS Tree Walks in order to determine if any two domains are
in alignment. This means that a DNS Tree Walk to discover an
Organizational Domain might start at any of the following locations:
* The domain in the RFC5322.From header of the message.
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* The RFC5321.MailFrom domain if there is an SPF pass result for the
message.
* Any DKIM d= domain if there is a DKIM pass result for the message
for that domain.
Note: There is no need to perform Tree Walk searches for
Organizational Domains under any of the following conditions:
* The RFC5322.From domain and the RFC5321.MailFrom domain (if SPF
authenticated), and/or the DKIM d= domain (if present and
authenticated) are all the same and that domain has a DMARC
record. In this case, this common domain is treated as the
Organizational Domain.
* No applicable DMARC policy is discovered for the RFC5322.From
domain during the first tree walk. In this case, the DMARC
mechanism does not apply to the message in question.
* There is no SPF pass result and no DKIM pass result for the
message. In this case, there can be no DMARC pass result, and so
the Organizational Domain of any domain is not required to be
discovered.
* The record for the RFC5322.From domain indicates strict alignment.
In this case, a simple string compare between the RFC5322.From
domain and the RFC5321.MailFrom domain (if SPF authenticated),
and/or the DKIM d= domain (if present and authenticated) is all
that is required.
To discover the Organizational Domain for a domain, perform the DNS
Tree Walk described in Section 4.6 as needed for any of the domains
in question.
Select the Organizational Domain from the domains for which valid
DMARC records were retrieved from the longest to the shortest:
1. If a valid DMARC 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 record 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.
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3. If the selection process completes and all records contain
(either explicitly or implicitly, since this is the default) the
psd= tag set to 'u' (psd=u), select the record for the domain
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 records starting with "_dmarc.a.mail.example.com" and
finishing with "_dmarc.com". If there are DMARC records for
"_dmarc.mail.example.com" and "_dmarc.example.com", but not for
"_dmarc.a.mail.example.com" or "_dmarc.com", then the Organizational
Domain for this domain would be "example.com".
As another example, given the starting domain "a.mail.example.com",
if a search for the Organizational Domain only yields a DMARC record
at "_dmarc.com" and that record contains the tag psd=y, then the
Organizational Domain for this domain would be "example.com".
5. Policy
A Domain Owner or PSO advertises DMARC participation of one or more
of its domains by adding a DNS TXT record (described in Section 5.1)
to those domains. In doing so, Domain Owners and PSOs indicate their
handling preference regarding failed authentication for email
messages making use of their domain in the RFC5322.From header field
as well as their desire for feedback about those messages. Mail
Receivers in turn can take into account the Domain Owner's stated
preference when making handling decisions about email messages that
fail DMARC authentication checks.
A Domain Owner or PSO may choose not to participate in DMARC
evaluation by Mail Receivers simply by not publishing an appropriate
DNS TXT record for its domain(s). A Domain Owner can also choose to
not have some underlying authentication technologies apply to DMARC
evaluation of its domain(s). In this case, the Domain Owner simply
declines to advertise participation in those schemes. For example,
if the results of path authorization checks ought not be considered
as part of the overall DMARC result for a given Author Domain, then
the Domain Owner does not publish an SPF policy record that can
produce an SPF pass result.
A Mail Receiver implementing the DMARC mechanism SHOULD make a best-
effort attempt to adhere to the Domain Owner's or PSO's published
DMARC Domain Owner Assessment Policy when a message fails the DMARC
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test. Since email streams can be complicated (due to forwarding,
existing RFC5322.From domain-spoofing services, etc.), Mail Receivers
MAY deviate from a published Domain Owner Assessment Policy during
message processing and SHOULD make available the fact of and reason
for the deviation to the Domain Owner via feedback reporting,
specifically using the "PolicyOverride" feature of the aggregate
report defined in [DMARC-Aggregate-Reporting]
5.1. DMARC Policy Record
Domain Owner and PSO DMARC preferences are stored as DNS TXT records
in subdomains named "_dmarc". For example, the Domain Owner of
"example.com" would post DMARC preferences in a TXT record at
"_dmarc.example.com". Similarly, a Mail Receiver wishing to query
for DMARC preferences regarding mail with an RFC5322.From domain of
"example.com" would issue a TXT query to the DNS for the subdomain of
"_dmarc.example.com". The DNS-located DMARC preference data will
hereafter be called the "DMARC record".
DMARC's use of the Domain Name Service is driven by DMARC's use of
domain names and the nature of the query it performs. The query
requirement matches with the DNS, for obtaining simple parametric
information. It uses an established method of storing the
information, associated with the target domain name, namely an
isolated TXT record that is restricted to the DMARC context. Use of
the DNS as the query service has the benefit of reusing an extremely
well-established operations, administration, and management
infrastructure, rather than creating a new one.
Per [RFC1035], a TXT record can comprise several "character-string"
objects. Where this is the case, the module performing DMARC
evaluation MUST concatenate these strings by joining together the
objects in order and parsing the result as a single string.
5.2. DMARC URIs
[RFC3986] defines a generic syntax for identifying a resource. The
DMARC mechanism uses this as the format by which a Domain Owner or
PSO specifies the destination for the two report types that are
supported.
The place such URIs are specified (see Section 5.3) 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 5.4.
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5.3. General 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
strict or relaxed DKIM Identifier Alignment mode is required by
the Domain Owner. 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
strict or relaxed SPF Identifier Alignment mode is required by the
Domain Owner. 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 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
authentication mechanism produced something other than an
aligned "pass" result.
d: Generate a DKIM failure report if the message had a signature
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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 for non-existent subdomains
(plain-text; OPTIONAL). Indicates the message handling preference
that the Domain Owner or PSO has for mail using non-existent
subdomains of the domain queried. It applies only to non-existent
subdomains of the domain queried and not to either existing
subdomains or the domain itself. Its syntax is identical to that
of the "p" tag defined below. If the "np" tag is absent, the
policy specified by the "sp" tag (if the "sp" tag is present) or
the policy specified by the "p" tag, if the "sp" tag is not
present, MUST be applied for non-existent subdomains. Note that
"np" will be ignored for DMARC records published on subdomains of
Organizational Domains and PSDs due to the effect of the DMARC
policy discovery mechanism described in Section 4.7.
p: Domain Owner Assessment Policy (plain-text; RECOMMENDED for
policy records). Indicates the message handling preference the
Domain Owner or PSO has for mail using its domain but not passing
DMARC verification. Policy applies to the domain queried and to
subdomains, unless 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 4.7). This tag is not
applicable for third-party reporting records (see
[DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting])
Possible values are as follows:
none: The Domain Owner offers no expression of 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;
OPTIONAL; default is 'u'). Possible values are:
y: PSOs MUSTinclude this tag with a value of 'y' to indicate that
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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 PSD, but it is the
Organizational Domain for itself and its subdomain. There is
no need to put psd=n in a DMARC record, except in the very
unusual case of a parent PSD publishing a DMARC record without
the requisite psd=y tag.
u: The default, indicating that the DMARC policy record is
published for a domain that is not a PSD. Use the mechanism
described in Section 4.8 for determining the Organizational
Domain. There is no need to explicitly publish psd=u in a
DMARC record.
rua: Addresses to which aggregate feedback is to be sent (comma-
separated plain-text list of DMARC URIs; OPTIONAL).
[DMARC-Aggregate-Reporting] discusses considerations that apply
when the domain name of a URI differs from that of the domain
advertising the policy. See Section 9.5 for additional
considerations. 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. The aggregate feedback report format is described in
[DMARC-Aggregate-Reporting]
ruf: Addresses to which message-specific failure information is to
be reported (comma-separated plain-text list of DMARC URIs;
OPTIONAL). If present, the Domain Owner or PSO is requesting Mail
Receivers to send detailed failure reports about messages that
fail the DMARC evaluation in specific ways (see the "fo" tag
above). The format of the message to be generated MUST follow the
format specified for the "rf" tag. [DMARC-Aggregate-Reporting]
discusses considerations that apply when the domain name of a URI
differs from that of the domain advertising the policy. A Mail
Receiver MUST implement support for a "mailto:" URI, i.e., the
ability to send a DMARC report via electronic mail. If the tag is
not provided, Mail Receivers MUST NOT generate failure reports for
the domain. See Section 9.5 for additional considerations.
sp: Domain Owner Assessment Policy for all subdomains (plain-text;
OPTIONAL). Indicates the message handling preference the Domain
Owner or PSO has for mail using an existing subdomain of the
domain queried but not passing DMARC verification. It applies
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only to subdomains of the domain queried and not to the domain
itself. Its syntax is identical to that of the "p" tag defined
above. If both the "sp" tag is absent and the "np" tag is either
absent or not applicable, the policy specified by the "p" tag MUST
be applied for subdomains. Note that "sp" will be ignored for
DMARC records published on subdomains of Organizational Domains
due to the effect of the DMARC policy discovery mechanism
described in Section 4.7.
t: DMARC policy test mode (plain-text; OPTIONAL; default is 'n').
For the RFC5322.From domain to which the DMARC record applies, the
"t" tag serves as a signal to the actor performing DMARC
verification checks as to whether or not the domain owner wishes
the 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. Possible values are as follows:
y: A request that the actor performing the DMARC verification
check not apply the policy, but instead apply any special
handling rules it might have in place, such as rewriting the
RFC5322.From header. The domain owner is currently testing its
specified DMARC assessment policy.
n: The default, a request to apply the policy as specified to any
message that produces a DMARC "fail" result.
v: Version (plain-text; REQUIRED). Identifies the record retrieved
as a DMARC record. It MUST have the value of "DMARC1". The value
of this tag MUST match precisely; if it does not or it is absent,
the entire retrieved record MUST be ignored. It MUST be the first
tag in the list.
A DMARC policy record MUST comply with the formal specification found
in Section 5.4 in that the "v" tag MUST be present and MUST appear
first. Unknown tags MUST be ignored. Syntax errors in the remainder
of the record SHOULD be discarded in favor of default values (if any)
or ignored outright.
Note that given the rules of the previous paragraph, addition of a
new tag into the registered list of tags does not itself require a
new version of DMARC to be generated (with a corresponding change to
the "v" tag's value), but a change to any existing tags does require
a new version of DMARC.
5.4. Formal Definition
The formal definition of the DMARC format, using [RFC5234], 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-record = dmarc-version dmarc-sep *(dmarc-tag dmarc-sep)
dmarc-tag = dmarc-request /
dmarc-test /
dmarc-psd /
dmarc-sprequest /
dmarc-nprequest /
dmarc-adkim /
dmarc-aspf /
dmarc-auri /
dmarc-furi /
dmarc-fo /
dmarc-rfmt
; components other than dmarc-version and
; dmarc-request may appear in any order
dmarc-version = "v" *WSP "=" *WSP %x44 %x4d %x41 %x52 %x43 %x31
dmarc-sep = *WSP %x3b *WSP
dmarc-request = "p" *WSP "=" *WSP
( "none" / "quarantine" / "reject" )
dmarc-test = "t" *WSP "=" ( "y" / "n" )
dmarc-psd = "psd" *WSP "=" ( "y" / "n" )
dmarc-sprequest = "sp" *WSP "=" *WSP
( "none" / "quarantine" / "reject" )
dmarc-nprequest = "np" *WSP "=" *WSP
( "none" / "quarantine" / "reject" )
dmarc-adkim = "adkim" *WSP "=" *WSP ( "r" / "s" )
dmarc-aspf = "aspf" *WSP "=" *WSP ( "r" / "s" )
dmarc-auri = "rua" *WSP "=" *WSP
dmarc-uri *(*WSP "," *WSP dmarc-uri)
dmarc-furi = "ruf" *WSP "=" *WSP
dmarc-uri *(*WSP "," *WSP dmarc-uri)
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dmarc-fo = "fo" *WSP "=" *WSP
( "0" / "1" / ( "d" / "s" / "d:s" / "s:d" ) )
dmarc-rfmt = "rf" *WSP "=" *WSP Keyword *(*WSP ":" Keyword)
; registered reporting formats only
"Keyword" is imported from Section 4.1.2 of [RFC5321].
5.5. Domain Owner Actions
This section describes Domain Owner actions to implement the DMARC
mechanism.
5.5.1. Publish an SPF Policy for an Aligned Domain
Because DMARC relies on SPF [RFC7208] and DKIM [RFC6376], in order to
take full advantage of DMARC, a Domain Owner SHOULD first ensure that
SPF and DKIM authentication are properly configured. As a first step
the Domain Owner SHOULD choose a domain to use as the
RFC5321.MailFrom domain (i.e., the Return-Path domain) for its mail,
one that aligns with the Author Domain, and then publish an SPF
policy in DNS for that domain. The SPF record SHOULD be constructed
at a minimum to ensure an SPF pass verdict for all known sources of
mail for the RFC5321.MailFrom domain.
5.5.2. Configure Sending System for DKIM Signing Using an Aligned
Domain
While it is possible to secure a DMARC pass verdict based on only SPF
or DKIM, it is commonly accepted best practice to ensure that both
authentication mechanisms are in place in order to guard against
failure of just one of them. The Domain Owner SHOULD choose a DKIM-
Signing domain (i.e., the d= domain in the DKIM-Signature header)
that aligns with the Author Domain.
5.5.3. Setup a Mailbox to Receive Aggregate Reports
Proper consumption and analysis of DMARC aggregate reports is the key
to any successful DMARC deployment for a Domain Owner. DMARC
aggregate reports, which are XML documents and are defined in
[DMARC-Aggregate-Reporting], contain valuable data for the Domain
Owner, showing sources of mail using the Author Domain. Depending on
how mature the Domain Owner's DMARC rollout is, some of these sources
could be legitimate ones that were overlooked during the initial
deployment of SPF and/or DKIM.
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Because the aggregate reports are XML documents, 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 parse these reports and consume the data
contained in them will go a long way to ensuring a successful
deployment.
5.5.4. Publish a DMARC Policy 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 record. For best results, Domain Owners
SHOULD start with "p=none", with the rua tag containg a URI that
references the mailbox created in the previous step. If the
Organizational Domain is different than the Author Domain, a record
also needs to be published for the Organizational Domain.
5.5.5. 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, it is possible for a Domain Owner to overlook a
server here or be unaware of a third party sending agreeement there.
Starting at "p=none", therefore, takes advantage of DMARC's aggregate
reporting function, with the Domain Owner using the reports to audit
its own mail streams' authentication configurations.
5.5.6. Decide If and When to Update DMARC Policy
Once the Domain Owner is satisfied that it is properly authenticating
all of its mail, then it is time to decide if it is appropriate to
change the p= value in its DMARC record to p=quarantine or p=reject.
Depending on its cadence for sending mail, it may take many months of
consuming DMARC aggregate reports before a Domain Owner reaches the
point where it is sure that it is properly authenticating all of its
mail, and the decision on which p= value to use will depend on its
needs.
5.6. PSO Actions
In addition to the DMARC Domain Owner actions, if a PSO publishes a
DMARC record it MUST include the psd tag (see Section 5.3) with a
value of 'y' ("psd=y").
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5.7. Mail Receiver Actions
This section describes receiver actions in the DMARC environment.
5.7.1. Extract Author Domain
The domain in the RFC5322.From header field is extracted as the
domain to be evaluated by DMARC. If the domain is encoded with UTF-
8, the domain name must be converted to an A-label, as described in
Section 2.3 of [RFC5890], for further processing.
In order to be processed by DMARC, a message typically needs to
contain exactly one RFC5322.From domain (a single From: field with a
single domain in it). Not all messages meet this requirement, and
the handling of those that are forbidden under [RFC5322] or that
contain no meaningful domains is outside the scope of this document.
The case of a syntactically valid multi-valued RFC5322.From header
field presents a particular challenge. When a single RFC5322.From
header field contains multiple addresses, it is possible that there
may be multiple domains used in those addresses. The process in this
case is to only proceed with DMARC checking if the domain is
identical for all of the addresses in a multi-valued RFC5322.From
header field. Multi-valued RFC5322.From header fields with multiple
domains MUST be exempt from DMARC checking.
Note that Public Suffix Domains are not exempt from DMARC policy
application and reporting.
5.7.2. Determine Handling Policy
To arrive at a policy for an individual message, Mail Receivers MUST
perform the following actions or their semantic equivalents. Steps
2-4 MAY be done in parallel, whereas steps 5 and 6 require input from
previous steps.
The steps are as follows:
1. Extract the RFC5322.From domain from the message (as above).
2. Query the DNS for a DMARC policy record. Continue if one is
found, or terminate DMARC evaluation otherwise. See Section 4.7
for details.
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3. Perform DKIM signature verification checks. A single email could
contain multiple DKIM signatures. The results of this step are
passed to the remainder of the algorithm, MUST include "pass" or
"fail", and if "fail", SHOULD include information about the
reasons for failure. The results MUST further include the value
of the "d=" and "s=" tags from each checked DKIM signature.
4. Perform SPF verification checks. The results of this step are
passed to the remainder of the algorithm, MUST include "pass" or
"fail", and if "fail", SHOULD include information about the
reasons for failure. The results MUST further include the domain
name used to complete the SPF check.
5. Conduct Identifier Alignment checks. With authentication checks
and policy discovery performed, the Mail Receiver checks to see
if Authenticated Identifiers fall into alignment as described in
Section 4.4. If one or more of the Authenticated Identifiers
align with the RFC5322.From domain, the message is considered to
pass the DMARC mechanism check. All other conditions
(authentication failures, identifier mismatches) are considered
to be DMARC mechanism check failures.
6. Apply policy, if appropriate. Emails that fail the DMARC
mechanism check are handled in accordance with the discovered
DMARC policy of the Domain Owner and any local policy rules
enforced by the Mail Receiver. See Section 5.3 for details.
DMARC evaluation can only yield a "pass" result after one of the
underlying authentication mechanisms passes for an aligned
identifier. If neither passes and one or both of them fail due to a
temporary error, the Mail Receiver evaluating the message is unable
to conclude that the DMARC mechanism had a permanent failure; they
therefore cannot apply the advertised DMARC policy. When otherwise
appropriate, Mail Receivers MAY send feedback reports regarding
temporary errors.
Handling of messages for which SPF and/or DKIM evaluation encounter a
permanent DNS error is left to the discretion of the Mail Receiver.
5.7.3. Store Results of DMARC Processing
The results of Mail Receiver-based DMARC processing should be stored
for eventual presentation back to the Domain Owner in the form of
aggregate feedback reports. Section 5.3 and
[DMARC-Aggregate-Reporting] discuss aggregate feedback.
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5.7.4. Send Aggregate Reports
For a Domain Owner, DMARC aggregate reports provide data about all
mailstreams making use of its domain in email, to include not only
illegitimate uses but also, and perhaps more importantly, all
legitimate uses. Domain Owners can use aggregate reports to ensure
that all legitimate uses of their domain for sending email are
properly authenticated, and once they are, express a stricter message
handling preference in the p= tag in their DMARC policy records from
none to quarantine to reject, if appropriate. In turn, DMARC policy
records with p= tag values of 'quarantine' or 'reject' are higher
value signals to Mail Receivers, ones that can assist Mail Receivers
with handling decisions for a message in ways that p= tag values of
'none' cannot.
Given the above, in order 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.
5.8. Policy Enforcement Considerations
Mail Receivers MAY choose to reject or quarantine email even if email
passes the DMARC mechanism check. The DMARC mechanism does not
inform Mail Receivers whether an email stream is "good"; a DMARC
result of "pass" only means that the domain in the RFC5322.From
header has been verified by the DMARC mechanism. 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
mechanism check even if the published Domain Owner Assessment Policy
is "reject". Mail Receivers need to make a best effort not to
increase the likelihood of accepting abusive mail if they choose not
to honor the published Domain Owner Assessment Policy. At a minimum,
addition of the Authentication-Results header field (see [RFC8601])
is RECOMMENDED when delivery of failing mail is done. When this is
done, the DNS domain name thus recorded MUST be encoded as an
A-label.
Mail Receivers are only obligated to report reject or quarantine
policy actions in aggregate feedback reports that are due to
published DMARC Domain Owner Assessment Policy. They are not
required to report reject or quarantine actions that are the result
of local policy. If local policy information is exposed, abusers can
gain insight into the effectiveness and delivery rates of spam
campaigns.
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Final handling of a message is always a matter of local policy. An
operator that wishes to favor DMARC policy over SPF policy, for
example, will disregard the SPF policy, since enacting an SPF-
determined rejection prevents evaluation of DKIM; DKIM might
otherwise pass, satisfying the DMARC evaluation. There is a trade-
off to doing so, namely acceptance and processing of the entire
message body in exchange for the enhanced protection DMARC provides.
DMARC-compliant Mail Receivers typically disregard any mail-handling
directive discovered as part of an authentication mechanism (e.g.,
Author Domain Signing Practices (ADSP), SPF) where a DMARC record is
also discovered that specifies a policy other than "none". Deviating
from this practice introduces inconsistency among DMARC operators in
terms of handling of the message. However, such deviation is not
proscribed.
To enable Domain Owners to receive DMARC feedback without impacting
existing mail processing, discovered policies of "p=none" SHOULD NOT
modify existing mail handling processes.
Mail Receivers MUST also implement reporting instructions of DMARC,
even in the absence of a request for DKIM reporting [RFC6651] or SPF
reporting [RFC6652]. Furthermore, the presence of such requests
SHOULD NOT affect DMARC reporting.
6. DMARC Feedback
Providing Domain Owners with visibility into how Mail Receivers
implement and enforce the DMARC mechanism in the form of feedback is
critical to establishing and maintaining accurate authentication
deployments. When Domain Owners can see what effect their policies
and practices are having, they are better willing and able to use
quarantine and reject policies.
The details of this feedback are described in
[DMARC-Aggregate-Reporting]
Operational note for PSD DMARC: For PSOs, feedback for non-existent
domains is desirable and useful, just as it is for org-level DMARC
operators. See Section 4 of [RFC9091] 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.
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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 large network
ranges. Domain Owners are strongly encouraged to carefully review
their SPF records to understand which networks are authorized to send
on behalf of the Domain Owner before publishing a DMARC record.
Some Mail Receiver architectures might implement SPF in advance of
any DMARC operations. This means that a "-" prefix on a sender's SPF
mechanism, such as "-all", could cause that rejection to go into
effect early in handling, causing message rejection before any DMARC
processing takes place. Operators choosing to use "-all" should be
aware of this.
7.2. DNS Load and Caching
DMARC policies are communicated using the DNS and therefore inherit a
number of considerations related to DNS caching. The inherent
conflict between freshness and the impact of caching on the reduction
of DNS-lookup overhead should be considered from the Mail Receiver's
point of view. Should Domain Owners or PSOs publish a DNS record
with a very short TTL, Mail Receivers can be provoked through the
injection of large volumes of messages to overwhelm the publisher's
DNS. Although this is not a concern specific to DMARC, the
implications of a very short TTL should be considered when publishing
DMARC policies.
Conversely, long TTLs will cause records to be cached for long
periods of time. This can cause a critical change to DMARC
parameters advertised by a Domain Owner or PSO to go unnoticed for
the length of the TTL (while waiting for DNS caches to expire).
Avoiding this problem can mean shorter TTLs, with the potential
problems described above. A balance should be sought to maintain
responsiveness of DMARC preference changes while preserving the
benefits of DNS caching.
7.3. Rejecting Messages
This protocol calls for rejection of a message during the SMTP
session under certain circumstances. This is preferable to
generation of a Delivery Status Notification [RFC3464], since
fraudulent messages caught and rejected using DMARC would then result
in annoying generation of such failure reports that go back to the
RFC5321.MailFrom address.
This synchronous rejection is typically done in one of two ways:
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* Full rejection, wherein the SMTP server issues a 5xy reply code as
an indication to the SMTP client that the transaction failed; the
SMTP client is then responsible for generating notification that
delivery failed (see Section 4.2.5 of [RFC5321]).
* A "silent discard", wherein the SMTP server returns a 2xy reply
code implying to the client that delivery (or, at least, relay)
was successfully completed, but then simply discarding the message
with no further action.
Each of these has a cost. For instance, a silent discard can help to
prevent backscatter, but it also effectively means that the SMTP
server has to be programmed to give a false result, which can
confound external debugging efforts.
Similarly, the text portion of the SMTP reply may be important to
consider. For example, when rejecting a message, revealing the
reason for the rejection might give an attacker enough information to
bypass those efforts on a later attempt, though it might also assist
a legitimate client to determine the source of some local issue that
caused the rejection.
In the latter case, when doing an SMTP rejection, providing a clear
hint can be useful in resolving issues. A Mail Receiver 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 inability to
retrieve or apply DMARC policy, this is best done with a 4xy SMTP
reply code.
7.4. Identifier Alignment Considerations
The DMARC mechanism allows both DKIM and SPF-authenticated
identifiers to authenticate email on behalf of a Domain Owner and,
possibly, on behalf of different subdomains. If malicious or unaware
users can gain control of the SPF record or DKIM selector records for
a subdomain, the subdomain can be used to generate DMARC-passing
email on behalf of the Organizational Domain.
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For example, an attacker who controls the SPF record for
"evil.example.com" can send mail with an RFC5322.From header field
containing "foo@example.com" that can pass both authentication and
the DMARC check against "example.com".
The Organizational Domain administrator should be careful not to
delegate control of subdomains if this is an issue, and to consider
using the "strict" Identifier Alignment option if appropriate.
7.5. 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.
Additional DMARC constraints occur when a message is processed by
some Mediators, such as mailing lists. Transiting a Mediator often
causes either the authentication to fail or Identifier Alignment to
be lost. These transformations may conform to standards but will
still prevent a DMARC "pass".
In addition to Mediators, mail that is sent by authorized,
independent third parties might not be sent with Identifier
Alignment, also preventing a "pass" result.
Issues specific to the use of policy mechanisms alongside DKIM are
further discussed in [RFC6377], particularly Section 5.2.
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] |polrec| p |the p= value |active|1 |
| | | | |read from the | | |
| | | | |discovered | | |
| | | | |policy record | | |
+------+-----------+------+----------+--------------+------+--------+
|dmarc | [RFC7489] |polrec| domain |the domain at |active|1 |
| | | | |which the | | |
| | | | |policy record | | |
| | | | |was | | |
| | | | |discovered, if| | |
| | | | |different from| | |
| | | | |the | | |
| | | | |RFC5322.From | | |
| | | | |domain | | |
+------+-----------+------+----------+--------------+------+--------+
Table 1: "Authentication-Results Method Registry Update"
8.2. Authentication-Results Result Registry Update
IANA has added the following in the "Email Authentication Result
Names" registry:
+===========+===========+===========+=======+================+======+
| Code | Existing/ | Defined |Auth |Meaning |Status|
| | New Code | |Method | | |
+===========+===========+===========+=======+================+======+
| none | existing | [RFC8601] |dmarc |No DMARC policy |active|
| | | |(added)|record was | |
| | | | |published for | |
| | | | |the aligned | |
| | | | |identifier, or | |
| | | | |no aligned | |
| | | | |identifier could| |
| | | | |be extracted. | |
+-----------+-----------+-----------+-------+----------------+------+
| pass | existing | [RFC8601] |dmarc |A DMARC policy |active|
| | | |(added)|record was | |
| | | | |published for | |
| | | | |the aligned | |
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| | | | |identifier, and | |
| | | | |at least one of | |
| | | | |the | |
| | | | |authentication | |
| | | | |mechanisms | |
| | | | |passed. | |
+-----------+-----------+-----------+-------+----------------+------+
| fail | existing | [RFC8601] |dmarc |A DMARC policy |active|
| | | |(added)|record was | |
| | | | |published for | |
| | | | |the aligned | |
| | | | |identifier, and | |
| | | | |none of the | |
| | | | |authentication | |
| | | | |mechanisms | |
| | | | |passed. | |
+-----------+-----------+-----------+-------+----------------+------+
| temperror | existing | [RFC8601] |dmarc |A temporary |active|
| | | |(added)|error occurred | |
| | | | |during DMARC | |
| | | | |evaluation. A | |
| | | | |later attempt | |
| | | | |might produce a | |
| | | | |final result. | |
+-----------+-----------+-----------+-------+----------------+------+
| permerror | existing | [RFC8601] |dmarc |A permanent |active|
| | | |(added)|error occurred | |
| | | | |during DMARC | |
| | | | |evaluation, such| |
| | | | |as encountering | |
| | | | |a syntactically | |
| | | | |incorrect DMARC | |
| | | | |record. A later| |
| | | | |attempt is | |
| | | | |unlikely to | |
| | | | |produce a final | |
| | | | |result. | |
+-----------+-----------+-----------+-------+----------------+------+
Table 2: "Authentication-Results Result Registry Update"
8.3. Feedback Report Header Fields Registry Update
The following has been added to the "Feedback Report Header Fields"
registry:
Field Name: Identity-Alignment
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Description: indicates whether the message about which a report is
being generated had any identifiers in alignment as defined in RFC
7489
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Reference: RFC 7489
Status: current
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 must be registered with IANA in this new sub-
registry. New entries are assigned only for values that have been
documented in a manner that satisfies the terms of Specification
Required, per [RFC8126]. Each registration must include the tag
name; the specification that defines it; a brief description; and its
status, which must be one of "current", "experimental", or
"historic". The Designated Expert needs to confirm that the provided
specification adequately describes the new tag and clearly presents
how it would be used within the DMARC context by Domain Owners and
Mail Receivers.
To avoid version compatibility issues, tags added to the DMARC
specification are to avoid changing the semantics of existing records
when processed by implementations conforming to prior specifications.
The initial set of entries in this registry is as follows:
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+=======+===========+==========+=============================+
| 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 7489 | current | Requested handling policy |
| | | | for non-existent subdomains |
+-------+-----------+----------+-----------------------------+
| p | RFC 7489 | current | Requested handling policy |
+-------+-----------+----------+-----------------------------+
| pct | RFC 7489 | historic | Sampling rate |
+-------+-----------+----------+-----------------------------+
| psd | RFC 7489 | current | Indicates whether policy |
| | | | 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 | RFC 7489 | current | Test mode for the specified |
| | | | policy |
+-------+-----------+----------+-----------------------------+
| v | RFC 7489 | current | Specification version |
+-------+-----------+----------+-----------------------------+
Table 3: "DMARC Tag Registry"
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.
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Names of DMARC failure reporting formats must be registered with IANA
in this registry. New entries are assigned only for values that
satisfy the definition of Specification Required, per [RFC8126]. In
addition to a reference to a permanent specification, each
registration must include the format name; a brief description; and
its status, which must be one of "current", "experimental", or
"historic". The Designated Expert needs to confirm that the provided
specification adequately describes the report format and clearly
presents how it would be used within the DMARC context by Domain
Owners and Mail Receivers.
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 4: "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 5: "Underscored and
Globally Scoped DNS Node Names"
registry
9. Security Considerations
This section discusses security issues and possible remediations
(where available) for DMARC.
9.1. Authentication Methods
Security considerations from the authentication methods used by DMARC
are incorporated here by reference.
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9.2. Attacks on Reporting URIs
URIs published in DNS TXT records are well-understood possible
targets for attack. Specifications such as [RFC1035] and [RFC2142]
either expose or cause the exposure of email addresses that could be
flooded by an attacker, for example; MX, NS, and other records found
in the DNS advertise potential attack destinations; common DNS names
such as "www" plainly identify the locations at which particular
services can be found, providing destinations for targeted denial-of-
service or penetration attacks.
Thus, Domain Owners will need to harden these addresses against
various attacks, including but not limited to:
* high-volume denial-of-service attacks;
* deliberate construction of malformed reports intended to identify
or exploit parsing or processing vulnerabilities;
* deliberate construction of reports containing false claims for the
Submitter or Reported-Domain fields, including the possibility of
false data from compromised but known Mail Receivers.
9.3. DNS Security
The DMARC mechanism and its underlying technologies (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, causing
recipients to assume p=none by default.
* If they can send forged response packets, they can make aligned
mail appear unaligned or vice-versa.
None of these threats are unique to DMARC, and they can be addressed
using a variety of techniques, including, but not limited to:
* Signing DNS records with DNSSEC [RFC4033], which enables
recipients to detect and discard forged responses.
* DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate
snooping and forged responses.
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9.4. Display Name Attacks
A common attack in messaging abuse is the presentation of false
information in the display-name portion of the RFC5322.From header
field. For example, it is possible for the email address in that
field to be an arbitrary address or domain name, while containing a
well-known name (a person, brand, role, etc.) in the display name,
intending to fool the end user into believing that the name is used
legitimately. The attack is predicated on the notion that most
common MUAs will show the display name and not the email address when
both are available.
Generally, display name attacks are out of scope for DMARC, as
further exploration of possible defenses against these attacks needs
to be undertaken.
There are a few possible mechanisms that attempt mitigation of these
attacks, such as the following:
* If the display name is found to include an email address (as
specified in [RFC5322]), execute the DMARC mechanism on the domain
name found there rather than the domain name discovered
originally. However, this addresses only a very specific attack
space, and spoofers can easily circumvent it by simply not using
an email address in the display name. There are also known cases
of legitimate uses of an email address in the display name with a
domain different from the one in the address portion, e.g.,
From: "user@example.org via Bug Tracker" support@example.com
(mailto:support@example.com)
* In the MUA, only show the display name if the DMARC mechanism
succeeds. This too is easily defeated, as an attacker could
arrange to pass the DMARC tests while fraudulently using another
domain name in the display name.
* In the MUA, only show the display name if the DMARC mechanism
passes and the email address thus verified matches one found in
the receiving user's list of known addresses.
9.5. External Reporting Addresses
To avoid abuse by bad actors, reporting addresses generally have to
be inside the domains about which reports are requested. In order to
accommodate special cases such as a need to get reports about domains
that cannot actually receive mail, Section 3 of
[DMARC-Aggregate-Reporting] describes a DNS-based mechanism for
verifying approved external reporting.
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The obvious consideration here is an increased DNS load against
domains that are claimed as external recipients. Negative caching
will mitigate this problem, but only to a limited extent, mostly
dependent on the default TTL in the domain's SOA record.
Where possible, external reporting is best achieved by having the
report be directed to domains that can receive mail and simply having
it automatically forwarded to the desired external destination.
Note that the addresses shown in the "ruf" tag receive more
information that might be considered private data, since it is
possible for actual email content to appear in the failure reports.
The URIs identified there are thus more attractive targets for
intrusion attempts than those found in the "rua" tag. Moreover,
attacking the DNS of the subject domain to cause failure data to be
routed fraudulently to an attacker's systems may be an attractive
prospect. Deployment of [RFC4033] is advisable if this is a concern.
9.6. Secure Protocols
This document encourages use of secure transport mechanisms to
prevent loss of private data to third parties that may be able to
monitor such transmissions. Unencrypted mechanisms should be
avoided.
In particular, a message that was originally encrypted or otherwise
secured might appear in a report that is not sent securely, which
could reveal private information.
10. Normative References
[DMARC-Aggregate-Reporting]
Brotman, A., Ed., "DMARC Aggregate Reporting", February
2021, <https://datatracker.ietf.org/doc/draft-ietf-dmarc-
aggregate-reporting/>.
[DMARC-Failure-Reporting]
Jones, S.M., Ed. and A. Vesely, Ed., "DMARC Failure
Reporting", February 2021,
<https://datatracker.ietf.org/doc/draft-ietf-dmarc-
failure-reporting/>.
[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>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
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[RFC6652] Kitterman, S., "Sender Policy Framework (SPF)
Authentication Failure Reporting Using the Abuse Reporting
Format", RFC 6652, DOI 10.17487/RFC6652, June 2012,
<https://www.rfc-editor.org/info/rfc6652>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[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>.
[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>.
11. 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>.
[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>.
[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>.
[RFC8601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 8601,
DOI 10.17487/RFC8601, May 2019,
<https://www.rfc-editor.org/info/rfc8601>.
Appendix A. Technology Considerations
This section documents some design decisions that were made in the
development of DMARC. Specifically addressed here are some
suggestions that were considered but not included in the design, with
explanatory text regarding the decision.
A.1. S/MIME
S/MIME, or Secure Multipurpose Internet Mail Extensions, is a
standard for encryption and signing of MIME data in a message. This
was suggested and considered as a third security protocol for
authenticating the source of a message.
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DMARC is focused on authentication at the domain level (i.e., the
Domain Owner taking responsibility for the message), while S/MIME is
really intended for user-to-user authentication and encryption. This
alone appears to make it a bad fit for DMARC's goals.
S/MIME also suffers from the heavyweight problem of Public Key
Infrastructure, which means that distribution of keys used to verify
signatures needs to be incorporated. In many instances, this alone
is a showstopper. There have been consistent promises that PKI
usability and deployment will improve, but these have yet to
materialize. DMARC can revisit this choice after those barriers are
addressed.
S/MIME has extensive deployment in specific market segments
(government, for example) but does not enjoy similar widespread
deployment over the general Internet, and this shows no signs of
changing. DKIM and SPF both are deployed widely over the general
Internet, and their adoption rates continue to be positive.
Finally, experiments have shown that including S/MIME support in the
initial version of DMARC would neither cause nor enable a substantial
increase in the accuracy of the overall mechanism.
A.2. Method Exclusion
It was suggested that DMARC include a mechanism by which a Domain
Owner could tell Mail Receivers not to attempt verification by one of
the supported methods (e.g., "check DKIM, but not SPF").
Specifically, consider a Domain Owner that has deployed one of the
technologies, and that technology fails for some messages, but such
failures don't cause enforcement action. Deploying DMARC would cause
enforcement action for policies other than "none", which would appear
to exclude participation by that Domain Owner.
The DMARC development team evaluated the idea of policy exception
mechanisms on several occasions and invariably concluded that there
was not a strong enough use case to include them. The specific
target audience for DMARC does not appear to have concerns about the
failure modes of one or the other being a barrier to DMARC's
adoption.
In the scenario described above, the Domain Owner has a few options:
1. Tighten up its infrastructure to minimize the failure modes of
the single deployed technology.
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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 checking of
the Sender identifier would mean applying policy to an identifier
the end user might never actually see, which can create a vector
for attack against end users by simply forging a Sender field
containing some identifier that DMARC will like.
2. Although it is certainly true that this is what the Sender field
is for, its use in this way is also unreliable, making it a poor
candidate for inclusion in the DMARC evaluation algorithm.
3. Allowing multiple ways to discover policy introduces unacceptable
ambiguity into the DMARC evaluation algorithm in terms of which
policy is to be applied and when.
A.4. Domain Existence Test
A previous version of this protocol used the test specified in
[RFC5321] to determine a domain's existence. This test requires up
to three DNS lookups for the MX, A, and AAAA RRs for the name in
question.
This version of the protocol relies solely on the test for existence
as defined in [RFC8020]. If a query for a name returns NXDOMAIN,
then the name does not exist.
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A.5. Issues with ADSP in Operation
DMARC has been characterized as a "super-ADSP" of sorts.
Contributors to DMARC have compiled a list of issues associated with
ADSP, gained from operational experience, that have influenced the
direction of DMARC:
1. ADSP has no support for subdomains, i.e., the ADSP record for
example.com does not explicitly or implicitly apply to
subdomain.example.com. If wildcarding is not applied, then
spammers can trivially bypass ADSP by sending from a subdomain
with no ADSP record.
2. Nonexistent subdomains are explicitly out of scope in ADSP.
There is nothing in ADSP that states Mail Receivers should simply
reject mail from NXDOMAINs regardless of ADSP policy (which of
course allows spammers to trivially bypass ADSP by sending email
from nonexistent subdomains).
3. ADSP has no operational advice on when to look up the ADSP
record.
4. ADSP has no support for using SPF as an auxiliary mechanism to
DKIM.
5. ADSP has no support for a slow rollout, i.e., no way to configure
a percentage of email on which the Mail Receiver should apply the
policy. This is important for large-volume senders.
6. ADSP has no explicit support for an intermediate phase where the
Mail Receiver quarantines (e.g., sends to the recipient's "spam"
folder) rather than rejects the email.
7. The binding between the "From" header domain and DKIM is too
tight for ADSP; they must match exactly.
A.6. Organizational Domain Discovery Issues
An earlier informational version of the DMARC protocol [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.
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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 policy record. This version defines a
method for performing a DNS Tree Walk, described in Section 4.6, 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.
As a matter of historical record, the method for finding the
Organizational Domain described in [RFC7489] is preserved here:
1. Acquire a "public suffix" list (PSL), i.e., a list of DNS domain
names reserved for registrations. Some country Top-Level Domains
(TLDs) make specific registration requirements, e.g., the United
Kingdom places company registrations under ".co.uk"; other TLDs
such as ".com" appear in the IANA registry of top-level DNS
domains. A PSL is the union of all of these.
A PSL can be obtained from various sources. The most common one
is maintained by the Mozilla Foundation and made public at
http://publicsuffix.org (http://publicsuffix.org). License terms
governing the use of that list are available at that URI.
Note that if operators use a variety of public suffix lists,
interoperability will be difficult or impossible to guarantee.
2. Break the subject DNS domain name into a set of "n" ordered
labels. Number these labels from right to left; e.g., for
"example.com", "com" would be label 1 and "example" would be
label 2.
3. Search the public suffix list for the name that matches the
largest number of labels found in the subject DNS domain. Let
that number be "x".
4. Construct a new DNS domain name using the name that matched from
the public suffix list and prefixing to it the "x+1"th label from
the subject domain. This new name is the Organizational Domain.
Thus, since "com" is an IANA-registered TLD, a subject domain of
"a.b.c.d.example.com" would have an Organizational Domain of
"example.com".
The process of determining a suffix is currently a heuristic one. No
list is guaranteed to be accurate or current.
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A.7. Removal of the "pct" Tag
An earlier informational version of the DMARC protocol [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 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 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 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.
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 lack of RFC5322.From header rewriting
by intermediaries could assist the Domain Owner in choosing whether
or not to proceed from an applied policy of p=none to p=quarantine or
p=reject. 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.
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 protocol 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.
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Appendix B. Examples
This section illustrates both the Domain Owner side and the Mail
Receiver side of a DMARC exchange.
B.1. Identifier Alignment Examples
The following examples illustrate the DMARC mechanism's use of
Identifier Alignment. For brevity's sake, only message headers are
shown, as message bodies are not considered when conducting DMARC
checks.
B.1.1. SPF
The following SPF examples assume that SPF produces a passing result.
Alignment cannot exist if SPF does not produce a passing result.
Example 1: SPF in 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 parameter and the RFC5322.From
header field have identical DNS domains. Thus, the identifiers are
in strict alignment.
Example 2: SPF in alignment (parent):
MAIL FROM: <sender@child.example.com>
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
In this case, the RFC5322.From header parameter includes a DNS domain
that is a parent of the RFC5321.MailFrom domain. Thus, the
identifiers are in relaxed alignment, because they both have the same
Organizational Domain (example.com).
Example 3: SPF not in alignment:
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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 parameter includes a DNS domain
that is neither the same as, a parent of, nor a child of the
RFC5322.From domain. Thus, the identifiers are not in alignment.
B.1.2. DKIM
The examples below assume that the DKIM signatures pass verification.
Alignment cannot exist with a DKIM signature that does not verify.
Example 1: DKIM in alignment:
DKIM-Signature: v=1; ...; d=example.com; ...
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
In this case, the DKIM "d=" parameter and the RFC5322.From header
field have identical DNS domains. Thus, the identifiers are in
strict alignment.
Example 2: DKIM in alignment (parent):
DKIM-Signature: v=1; ...; d=example.com; ...
From: sender@child.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
In this case, the DKIM signature's "d=" parameter includes a DNS
domain that is a parent of the RFC5322.From domain. Thus, the
identifiers are in relaxed alignment, as they have the same
Organizational Domain (example.com).
Example 3: DKIM not in alignment:
DKIM-Signature: v=1; ...; d=sample.net; ...
From: sender@child.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Subject: here's a sample
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In this case, the DKIM signature's "d=" parameter includes a DNS
domain that is neither the same as, a parent of, nor a child of the
RFC5322.From domain. Thus, the identifiers are not in alignment.
B.2. Domain Owner Example
A Domain Owner that wants to use DMARC should have already deployed
and tested SPF and DKIM. The next step is to publish a DNS record
that advertises a DMARC policy for the Domain Owner's Organizational
Domain.
B.2.1. Entire Domain, Monitoring Only
The owner of the domain "example.com" has deployed SPF and DKIM on
its messaging infrastructure. The owner wishes to begin using DMARC
with a policy that will solicit aggregate feedback from Mail
Receivers without affecting how the messages are processed, in order
to:
* Confirm that its legitimate messages are authenticating correctly
* Verify that all authorized message sources have implemented
authentication measures
* Determine how many messages from other sources would be affected
by a blocking policy
The Domain Owner accomplishes this by constructing a policy record
indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* Mail Receivers should not alter how they treat these messages
because of this DMARC policy record ("p=none")
* Aggregate feedback reports should be sent via email to the address
"dmarc-feedback@example.com" ("rua=mailto:dmarc-
feedback@example.com")
* All messages from this Organizational Domain are subject to this
policy (no "t" tag present, so the default of "n" applies).
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"
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To publish such a record, the DNS administrator for the Domain Owner
creates an entry like the following in the appropriate zone file
(following the conventional zone file format):
; DMARC record for the domain example.com
_dmarc IN TXT ( "v=DMARC1; p=none; "
"rua=mailto:dmarc-feedback@example.com" )
B.2.2. Entire Domain, Monitoring Only, Per-Message Reports
The Domain Owner from the previous example has used the aggregate
reporting to discover some messaging systems that had not yet
implemented DKIM correctly, but they are still seeing periodic
authentication failures. In order to diagnose these intermittent
problems, they wish to request per-message failure reports when
authentication failures occur.
Not all 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
policy record from Appendix B.2.1:
* Per-message failure reports should be sent via email to the
address "auth-reports@example.com" ("ruf=mailto:auth-
reports@example.com")
The DMARC policy record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT _dmarc.example.com.
"v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
ruf=mailto:auth-reports@example.com"
To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone file
(following the conventional zone file format):
; DMARC record for the domain example.com
_dmarc IN TXT ( "v=DMARC1; p=none; "
"rua=mailto:dmarc-feedback@example.com; "
"ruf=mailto:auth-reports@example.com" )
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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 verify that the
third party wishes to receive the failure reports for this domain.
The Domain Owner needs to alter its policy record from Appendix B.2.2
as follows:
* Per-message failure reports should be sent via email to the
address "auth-reports@thirdparty.example.net" ("ruf=mailto:auth-
reports@thirdparty.example.net")
The DMARC policy record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT _dmarc.example.com.
"v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
ruf=mailto:auth-reports@thirdparty.example.net"
To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone file
(following the conventional zone file format):
; DMARC record for the domain example.com
_dmarc IN TXT ( "v=DMARC1; p=none; "
"rua=mailto:dmarc-feedback@example.com; "
"ruf=mailto:auth-reports@thirdparty.example.net" )
Because the address used in the "ruf" tag is outside the
Organizational Domain in which this record is published, conforming
Mail Receivers will implement additional checks as described in
Section 3 of [DMARC-Aggregate-Reporting]. In order to pass these
additional checks, the Report Consumer's Domain Owner will need to
publish an additional DNS record as follows:
* Given the DMARC record published by the Domain Owner at
"_dmarc.example.com", the DNS administrator for the Report
Consumer will need to publish a TXT resource record at
"example.com._report._dmarc.thirdparty.example.net" with the value
"v=DMARC1;".
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The resulting DNS record might look like this when retrieved using a
common command-line tool (the output shown would appear on a single
line but is wrapped here for publication):
% dig +short TXT example.com._report._dmarc.thirdparty.example.net
"v=DMARC1;"
To publish such a record, the DNS administrator for example.net might
create an entry like the following in the appropriate zone file
(following the conventional zone file format):
; zone file for thirdparty.example.net
; Accept DMARC failure reports on behalf of example.com
example.com._report._dmarc IN TXT "v=DMARC1;"
Mediators and other third parties should refer to Section 3 of
[DMARC-Aggregate-Reporting] for the full details of this mechanism.
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 to authenticate to indicate to participating Mail Receivers that
use of this domain is not valid.
As a first step, it will express that it considers to be suspicious
messages using this subdomain that fail authentication. The goal
here will be to enable examination of messages sent to mailboxes
hosted by participating Mail Receivers as method for troubleshooting
any existing authentication issues. Aggregate feedback reports will
be sent to a mailbox within the Organizational Domain, and to a
mailbox at a Report Consumer selected and authorized to receive same
by the Domain Owner.
The Domain Owner will accomplish this by constructing a policy record
indicating that:
* The version of DMARC being used is "DMARC1" ("v=DMARC1;")
* It is applied only to this subdomain (record is published at
"_dmarc.test.example.com" and not "_dmarc.example.com")
* Mail Receivers are advised that the Domain Owner considers
messages that fail to authenticate to be suspicious
("p=quarantine")
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* Aggregate feedback reports should be sent via email to the
addresses "dmarc-feedback@example.com" and "example-tld-
test@thirdparty.example.net" ("rua=mailto:dmarc-
feedback@example.com, mailto:tld-test@thirdparty.example.net")
* The Domain Owner desires only that an actor performing a DMARC
verification check apply any special handling rules it might have
in place, such as rewriting the RFC53322.From header; 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 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 legitimate email sent using
the subdomain is properly authenticating and passing DMARC checks,
then the Domain Owner can update the policy record to indicate that
it considers authentication failures to be a clear indication that
use of the subdomain is not valid. It would do this by altering the
DNS record to advise 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):
% dig +short TXT _dmarc.test.example.com
"v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com,
mailto:tld-test@thirdparty.example.net"
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To publish such a record, the DNS administrator for the Domain Owner
might create an entry like the following in the appropriate zone
file:
; DMARC record for the domain test.example.com
_dmarc IN TXT ( "v=DMARC1; p=reject; "
"rua=mailto:dmarc-feedback@example.com,"
"mailto:tld-test@thirdparty.example.net" )
B.3. Mail Receiver Example
A Mail Receiver that wants to use DMARC should already be checking
SPF and DKIM, and possess the ability to collect relevant information
from various email-processing stages to provide feedback to Domain
Owners (possibly via Report Consumers).
B.3.1. SMTP Session Example
An optimal DMARC-enabled Mail Receiver performs authentication and
Identifier Alignment checking during the SMTP [RFC5321] conversation.
Prior to returning a final reply to the DATA command, the Mail
Receiver's MTA has performed:
1. An SPF check to determine an SPF-authenticated Identifier.
2. DKIM checks that yield one or more DKIM-authenticated
Identifiers.
3. A DMARC policy lookup.
The presence of an Author Domain DMARC record indicates that the Mail
Receiver should continue with DMARC-specific processing before
returning a reply to the DATA command.
Given a DMARC record and the set of Authenticated Identifiers, the
Mail Receiver checks to see if the Authenticated Identifiers align
with the Author Domain (taking into consideration any strict versus
relaxed options found in the DMARC 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, both the SPF-authenticated Identifier and the
DKIM-authenticated Identifier align with the Author Domain. The Mail
Receiver considers the above email to pass the DMARC check, avoiding
the "reject" policy that is requested to be applied to email that
fails to pass the DMARC check.
If no Authenticated Identifiers align with the Author Domain, then
the Mail Receiver applies the DMARC-record-specified policy.
However, before this action is taken, the Mail Receiver can consult
external information to override the Domain Owner's Assessment
Policy. For example, if the Mail Receiver knows that this particular
email came from a known and trusted forwarder (that happens to break
both SPF and DKIM), then the Mail Receiver may choose to ignore the
Domain Owner's policy.
The Mail Receiver is now ready to reply to the DATA command. If the
DMARC check yields that the message is to be rejected, then the Mail
Receiver replies with a 5xy code to inform the sender of failure. If
the DMARC check cannot be resolved due to transient network errors,
then the Mail Receiver replies with a 4xy code to inform the sender
as to the need to reattempt delivery later. If the DMARC check
yields a passing message, then the Mail Receiver continues on with
email processing, perhaps using the result of the DMARC check as an
input to additional processing modules such as a domain reputation
query.
Before exiting DMARC-specific processing, the Mail Receiver checks to
see if the Author Domain DMARC record requests AFRF-based reporting.
If so, then the Mail Receiver can emit an AFRF to the reporting
address supplied in the DMARC record.
At the exit of DMARC-specific processing, the Mail Receiver captures
(through logging or direct insertion into a data store) the result of
DMARC processing. Captured information is used to build feedback for
Domain Owner consumption. This is not necessary if the Domain Owner
has not requested aggregate reports, i.e., no "rua" tag was found in
the policy record.
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B.4. Utilization of Aggregate Feedback: Example
Aggregate feedback is consumed by Domain Owners to verify their
understanding of how a given domain is being processed by the Mail
Receiver. Aggregate reporting data on emails that pass all DMARC-
supporting authentication checks is used by Domain Owners to verify
that 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 verify ongoing authentication
practices of the third party.
Data on email that only partially passes underlying authentication
checks provides visibility into problems that need to be addressed by
the Domain Owner. For example, if either SPF or DKIM fails to pass,
the Domain Owner is provided with enough information to either
directly correct the problem or understand where authentication-
breaking changes are being introduced in the email transmission path.
If authentication-breaking changes due to email transmission path
cannot be directly corrected, then the Domain Owner at least
maintains an understanding of the effect of DMARC-based policies upon
the Domain Owner's email.
Data on email that fails all underlying authentication checks
provides baseline visibility on how the Domain Owner's domain is
being received at the Mail Receiver. Based on this visibility, the
Domain Owner can begin deployment of authentication technologies
across uncovered email sources, 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.
Acknowledgements
DMARC and the draft version of this document submitted to the
Independent Submission Editor were the result of lengthy efforts by
an informal industry consortium: DMARC.org (see http://dmarc.org
(http://dmarc.org)). Participating companies included Agari,
American Greetings, AOL, Bank of America, Cloudmark, Comcast,
Facebook, Fidelity Investments, Google, JPMorgan Chase & Company,
LinkedIn, Microsoft, Netease, PayPal, ReturnPath, The Trusted Domain
Project, and Yahoo!. Although the contributors and supporters are
too numerous to mention, notable individual contributions were made
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
Anderson, Michael Jack Assels, Les Barstow, Anne Bennett, Jim Fenton,
J. Gomez, Mike Jones, Scott Kitterman, Eliot Lear, John Levine, S.
Moonesamy, Rolf Sonneveld, Henry Timmes, and Stephen J. Turnbull.
Authors' Addresses
Todd M. Herr
Valimail
Email: todd.herr@valimail.com
John Levine
Standcore LLC
Email: standards@standore.com
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