Domain-based Message Authentication, Reporting, and Conformance (DMARC)

2.1. High-Level Goals

DMARC has the following high-level goals:¶

• Allow Domain Owners and Public Suffix Operators (PSOs) to validate their email authentication deployments.¶

• Allow Domain Owners and PSOs to assert their desired message handling for validation failures on messages purporting to have authorship within the domain.¶

• Minimize implementation complexity for both senders and receivers.¶

• Reduce the amount of successfully delivered spoofed emails.¶

• Work at Internet scale.¶

2.2. Anti-Phishing

DMARC is designed to prevent the unauthorized use of the Author Domain of an email message, a technique known as "spoofing". Such unauthorized usage can frequently be found in messages impersonating a domain belonging to a business entity, messages that are meant to entice the recipient to provide sensitive information, such as usernames, passwords, and financial account information. These spoofed messages are commonly referred to as "phishing".¶

DMARC can only be used to combat specific forms of exact-domain spoofing directly. DMARC does not attempt to solve all problems with spoofed or otherwise fraudulent emails. In particular, it does not address the use of visually similar domain names or abuse of the RFC5322.From human-readable display-name, as defined in Section 3.4 of [RFC5322].¶

2.3. Scalability

Scalability is a significant issue for systems that need to operate in an environment as widely deployed as current SMTP email. For this reason, DMARC seeks to avoid the need for third parties or pre-sending agreements between senders and receivers. This preserves the positive aspects of the current email infrastructure.¶

Although DMARC does not introduce third-party senders (namely external agents authorized to send on behalf of an operator) to the email-handling flow, it also does not preclude them. Such third parties are free to provide services in conjunction with DMARC.¶

2.4. Out of Scope

Several topics and issues are specifically out of scope of this work. These include the following:¶

• Different treatment of messages that are not authenticated (e.g., those that have no DKIM signature or those sent using an Author Domain for which no DMARC Policy Record exists) versus those that fail validation;¶

• Evaluation of anything other than RFC5322.From header field;¶

• Multiple reporting formats;¶

• Publishing policy other than via the DNS;¶

• Reporting or otherwise evaluating other than the last-hop IP address;¶

• Attacks in the display-name portions of the RFC5322.From header field, also known as "display name" attacks;¶

• Authentication of entities other than domains, since DMARC is built upon SPF and DKIM, which authenticate domains; and¶

• Content analysis.¶

3.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Readers are encouraged to be familiar with the contents of [RFC5598]. In particular, that document defines various roles in the messaging infrastructure that can appear the same or separate in various contexts. For example, a Domain Owner could, via the messaging security mechanisms on which DMARC is based, delegate the ability to send mail as the Domain Owner to a third party with another role. This document does not address the distinctions among such roles; the reader is encouraged to become familiar with that material before continuing.¶

3.2. Definitions

The following sections define the terms used in this document.¶

4.1. DMARC Basics

DMARC permits a Domain Owner or PSO to enable validation of an Author Domain's use in an email message, to indicate the Domain Owner's or PSO's message handling preference regarding failed validation, and to request reports about use of the Author Domain. A domain's DMARC Policy Record is published in the DNS as a TXT record at the name created by prepending the label "_dmarc" to the domain name and is retrieved through normal DNS queries.¶

DMARC's validation mechanism produces a "pass" result if a DMARC Policy Record exists for the Author Domain of an email message and the Author Domain is aligned with an Authenticated Identifier from that message. When a DMARC Policy Record exists for the Author Domain and the DMARC mechanism does not produce a "pass" result, the Mail Receiver's handling of that message can be influenced by the Domain Owner Assessment Policy expressed in the DMARC Policy Record.¶

It is important to note that the authentication mechanisms employed by DMARC only validate the usage of a DNS domain in an email message. They do not validate the local-part of any email address identifier found in that message, nor do such validations carry an explicit or implicit value assertion about that message or about the Domain Owner.¶

DMARC's reporting component involves the collection of information about received messages using the Author Domain for periodic aggregate reports to the Domain Owner or PSO. The parameters and format for such reports are discussed in [I-D.ietf-dmarc-aggregate-reporting].¶

A Mail Receiver participating in DMARC might also generate per-message failure reports that contain information related to individual messages that fail DMARC validation checks. Per-message failure reports are a useful source of information when debugging deployments (if messages can be determined to be legitimate even though failing validation) or in analyzing attacks. The capability for such services is enabled by DMARC but defined in other referenced material such as [RFC6591] and [I-D.ietf-dmarc-failure-reporting]¶

4.2. Use of RFC5322.From

One of the most obvious points of security scrutiny for DMARC is the choice to focus on an identifier, namely the RFC5322.From address, which is part of a body of data that has been trivially forged throughout the history of email. This field is the one used by end users to identify the source of the message, and so it has always been a prime target for abuse through such forgery and other means. That said, of all the identifiers that are part of the message itself, this is the only one required to be present. A message without a single, properly formed RFC5322.From header field does not comply with [RFC5322], and handling such a message is outside of the scope of this specification.¶

4.3. Authentication Mechanisms

The following mechanisms for determining Authenticated Identifiers are supported in this version of DMARC:¶

• DKIM, [RFC6376]. The DKIM Signing Domain from a validated DKIM-Signature header field is an Authenticated Identifier.¶

• SPF, [RFC7208]. The validated SPF Domain from the email message is the Authenticated Identifier.¶

4.4. Identifier Alignment Explained

DMARC validates the authorized use of the Author 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 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. In practice, nearly all Domain Owners have found relaxed alignment sufficient to meet their needs. Domain name comparisons in this context are case-insensitive, per [RFC4343].¶

The following table is meant to illustrate possible alignment conditions.¶

It is important to note that Identifier Alignment cannot occur with a message that is not valid per [RFC5322], particularly one with a malformed, absent, or repeated RFC5322.From header field, since in that case there is no reliable way to determine a DMARC Policy Record that applies to the message. Accordingly, DMARC operation is predicated on the input being a valid RFC5322 message object. For non-compliant cases, handling is outside of the scope of this specification. Further discussion of this can be found in Section 10.5.¶

4.5. DMARC Policy Record Explained

A Domain Owner or PSO advertises DMARC participation of one or more of its domains by publishing DMARC Policy Records that will apply to those domains. In doing so, Domain Owners and PSOs indicate their handling preference regarding failed validation for email messages using their domain in the RFC5322.From header field as well as their desire (if any) to receive feedback about such messages in the form of aggregate and/or failure reports.¶

DMARC Policy Records are stored as DNS TXT records with names starting with the label "_dmarc". For example, the Domain Owner of "example.com" would publish a DMARC Policy Record at the name "_dmarc.example.com", and a Mail Receiver wishing to find the DMARC Policy Record for mail with an Author Domain of "example.com" would issue a TXT query to the DNS for the name "_dmarc.example.com". A Domain Owner or PSO may choose not to participate in DMARC validation by Mail Receivers simply by not publishing a DMARC Policy Record for its Author Domain(s).¶

DMARC Policy Records can also apply to subdomains of the name at which they are published in the DNS, if the record is published at an Organizational Domain for the subdomains. The Domain Owner Assessment Policy that applies to the subdomains can be identical to the Domain Owner Assessment Policy that applies to the Organizational Domain or different, depending on the presence or absence of certain values in the DMARC Policy Record. See Section 4.7 for more details.¶

DMARC's use of the Domain Name Service is driven by DMARC's use of domain names and the nature of the query it performs. The query requirement matches with the DNS for obtaining simple parametric information. It uses an established method of storing the information associated with the domain name targeted by a DNS query, specifically an isolated TXT record that is restricted to the DMARC context. Using the DNS as the query service has the benefit of reusing an extremely well-established operations, administration, and management infrastructure, rather than creating a new one.¶

Per [RFC1035], a TXT record can comprise multiple "character-string" objects. Where this is the case, the module performing DMARC evaluation MUST concatenate these strings by joining together the objects in order and parsing the result as a single string.¶

A Domain Owner can choose not to have some underlying authentication mechanisms apply to DMARC evaluation of its Author Domain(s). For example, if a Domain Owner only wants to use DKIM as the underlying authentication mechanism, then the Domain Owner does not publish an SPF record that can produce Identifier Alignment between an SPF-Authenticated Identifier and the Author Domain. Alternatively, if the Domain Owner wishes to rely solely on SPF, then it can send email messages that have no DKIM-Signature header field that would produce Identifier Alignment between a DKIM-Authenticated Identifier and the Author Domain. Neither approach is recommended, however.¶

A Mail Receiver implementing the DMARC mechanism gets the Domain Owner's or PSO's published Domain Owner Assessment Policy and can use it to inform its handling decisions for messages that undergo DMARC validation checks and do not produce a result of pass. Mail handling considerations based on Domain Owner Assessment Policy enforcement are discussed below in Section 5.4.¶

4.6. DMARC Reporting URIs

[RFC3986] defines a syntax for identifying a resource. The DMARC mechanism uses this as the format by which a Domain Owner or PSO specifies the destination(s) for the two report types that are supported. The DMARC Policy Record format allows for a list of these URIs to be provided, with each URI separated by commas (ASCII 0x2c).¶

A formal definition is provided in Section 4.8.¶

4.7. DMARC Policy Record Format

DMARC Policy Records follow the extensible "tag-value" syntax for DNS-based key records defined in DKIM [RFC6376].¶

Section 8 creates a registry for known DMARC tags and registers the initial set defined in this document. Only tags defined in that registry are to be processed; unknown tags MUST be ignored.¶

The following tags are valid DMARC tags:¶

adkim:

(plain-text; OPTIONAL; default is "r".) Indicates whether the Domain Owner or PSO requires strict or relaxed DKIM Identifier Alignment mode. See Section 4.4.1 for details. Valid values are as follows:¶

r:

relaxed mode¶

s:

strict mode¶

aspf:

(plain-text; OPTIONAL; default is "r".) Indicates whether the Domain Owner or PSO requires strict or relaxed SPF Identifier Alignment mode. See Section 4.4.2 for details. Valid values are as follows:¶

r:

relaxed mode¶

s:

strict mode¶

fo:

Failure reporting options (plain-text; OPTIONAL; default is "0") Provides requested options for the generation of failure reports. Report generators may choose to adhere to the requested options. This tag's content MUST be ignored if a "ruf" tag (below) is not also specified. This tag can include one or more of the values shown here, with the exception that "0" and "1" are mutually exclusive. If more than one value is assigned to the tag, the list of values should be separated by colons (e.g., fo=0:d), and the values may appear in the list in any order.¶

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 fails to produce an aligned "pass" result.

d:
: Generate a DKIM failure report if the message had a signature
  that failed evaluation, regardless of its alignment. DKIM-specific
  reporting is described in [@!RFC6651].

s:
: Generate an SPF failure report if the message failed SPF
  evaluation, regardless of its alignment. SPF-specific
  reporting is described in [@!RFC6652].

np:

Domain Owner Assessment Policy for non-existent subdomains of the given Organizational Domain (plain-text; OPTIONAL). For this tag, the definition of "non-existent subdomain" is the same as that used for "Non-existent Domains" in Section 3.2.13. The policy expressed by this tag indicates the message handling preference of the Domain Owner or PSO for mail using non-existent subdomains of the prevailing Organizational Domain and not passing DMARC validation. It applies only to non-existent subdomains of the Organizational 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.¶

p:

Domain Owner Assessment Policy (plain-text; RECOMMENDED for DMARC Policy Records). Indicates the message handling preference of the Domain Owner or PSO for mail using its domain but not passing DMARC validation. The policy applies to the domain queried and to subdomains, unless the subdomain policy is explicitly described using the "sp" or "np" tags. If this tag is not present in an otherwise syntactically valid DMARC Policy Record, then the record is treated as if it included "p=none" (see Section 4.10.1). This tag is not applicable for third-party reporting records (see [I-D.ietf-dmarc-aggregate-reporting] and [I-D.ietf-dmarc-failure-reporting]). Possible values are as follows:¶

none:

The Domain Owner offers no expression of preference.¶

quarantine:

The Domain Owner considers such mail to be suspicious. It is possible the mail is valid, although the failure creates a significant concern.¶

reject:

The Domain Owner considers all such failures to be a clear indication that the use of the domain name is not valid. See Section 7.2 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 include this tag with a value of "y" to indicate that the domain is a PSD. If a record containing this tag with a value of "y" is found during policy discovery, this information will be used to determine the Organizational Domain and DMARC Policy Domain applicable to the message in question.¶

n:

The DMARC Policy Record is published for a domain that is not a PSD, but it is the Organizational Domain for itself and its subdomains.¶

u:

The default indicates that the DMARC Policy Record is published for a domain that is not a PSD, and may or may not be an Organizational Domain for itself and its subdomains. Use the mechanism described in Section 4.10 for determining the Organizational Domain for this domain.¶

rua:

Addresses to which aggregate feedback reports are to be sent (comma-separated plain-text list of DMARC Reporting URIs; OPTIONAL). If present, the Domain Owner is requesting Mail Receivers to send aggregate feedback reports as defined in [I-D.ietf-dmarc-aggregate-reporting] to the URIs listed. Any valid URI can be specified. A Mail Receiver that sends aggregate feedback reports 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 involving schemes not supported by Mail Receivers MUST be ignored. [I-D.ietf-dmarc-aggregate-reporting] also discusses considerations that apply when the domain name of a URI differs from the domain publishing the DMARC Policy Record. See Section 10.6 for additional considerations.¶

ruf:

Addresses to which message-specific failure information is to be reported (comma-separated plain-text list of DMARC URIs; OPTIONAL). If present, the Domain Owner is requesting Mail Receivers to send detailed failure reports about messages that fail the DMARC evaluation in specific ways (see the "fo" tag above) to the URIs listed. Depending on the value of the "fo" tag, the format for such reports is described in [I-D.ietf-dmarc-failure-reporting], [RFC6651], or [RFC6652]. Any valid URI can be specified. A Mail Receiver sending failure reports MUST implement support for a "mailto:" URI, i.e., the ability to send message-specific failure information via electronic mail. If the tag is not provided, Mail Receivers MUST NOT generate failure reports for the domain. URIs involving schemes not supported by Mail Receivers MUST be ignored. [I-D.ietf-dmarc-aggregate-reporting] discusses considerations that apply when the domain name of a URI differs from that of the domain advertising the policy. See Section 10.6 for additional considerations.¶

sp:

Domain Owner Assessment Policy for all subdomains of the given Organizational Domain (plain-text; OPTIONAL). Indicates the message handling preference of the Domain Owner or PSO for mail using an existing subdomain of the prevailing Organizational Domain for and not passing DMARC validation. It applies only to existing subdomains of the message's Organizational Domain in the DNS hierarchy and not to the Organizational Domain itself. Its syntax is identical to that of the "p" tag defined above. If both the "sp" tag is absent, and the "np" tag is either absent or not applicable, the policy specified by the "p" tag MUST be applied for subdomains. Note that "sp" will be ignored for DMARC Policy Records published on subdomains of Organizational Domains and PSDs due to the effect of the DMARC Policy Discovery.¶

t:

DMARC policy test mode (plain-text; OPTIONAL; default is "n"). For the Author Domain to which the DMARC Policy Record applies, the "t" tag serves as a signal to the actor performing DMARC validation checks as to whether or not the Domain Owner wishes the Domain Owner Assessment Policy declared in the "p", "sp", and/or "np" tags to actually be applied. This tag does not affect the generation of DMARC reports, and it has no effect on any policy ("p", "sp", or "np") that is "none". See Appendix A.6 for further discussion of the use of this tag. Possible values are as follows:¶

y:

A request that the actor performing the DMARC validation check not apply the policy, but instead apply any special handling rules it might have in place, such as rewriting the RFC5322.From header field (see Appendix A.6). The Domain Owner is currently testing its specified DMARC assessment policy, and has an expectation that the policy applied to any failing messages will be one level below the specified policy. That is, if the policy is "quarantine" and the value of the "t" tag is "y", a policy of "none" will be applied to failing messages; if the policy is "reject" and the value of the "t" tag is "y", a policy of "quarantine" will be applied to failing messages, irrespective of any other special handling rules that might be triggered by the "t" tag having a value of "y".¶

n:

The default is a request to apply the Domain Owner Assessment Policy as specified to any message that produces a DMARC "fail" result.¶

v:

Version (plain-text; REQUIRED). Identifies the record retrieved as a DMARC Policy Record. It MUST have the value of "DMARC1". The value of this tag MUST match precisely; if it does not or it is absent, the entire record MUST be ignored. It MUST be the first tag in the list.¶

A DMARC Policy Record MUST comply with the formal specification found in Section 4.8 in that the "v" tag MUST be present and MUST appear first. Unknown tags MUST be ignored. Syntax errors in the remainder of the record MUST be discarded in favor of default values (if any) or ignored outright.¶

Note that given the rules of the previous paragraph, the addition of a new tag into the registered list of tags does not itself require a new version of DMARC to be generated (with a corresponding change to the "v" tag's value), but a change to any existing tags does require a new version of DMARC.¶

4.8. Formal Definition

The formal definition of the DMARC Policy Record format, using [RFC5234] and [RFC7405], is as follows:¶

  dmarc-uri     = URI
                ; "URI" is imported from [RFC3986];
                ; commas (ASCII 0x2C) and exclamation
                ; points (ASCII 0x21) MUST be
                ; encoded

  dmarc-sep     = *WSP ";" *WSP

  equals        = *WSP "=" *WSP

  dmarc-record  = dmarc-version *(dmarc-sep dmarc-tag) [dmarc-sep]

  dmarc-tag     = 1*ALPHA equals 1*dmarc-value

  ; any printing characters but semicolon
  dmarc-value   = %x20-3A / %x3C-7E

  dmarc-version = "v" equals %s"DMARC1" ; case sensitive

  ; specialized syntax of DMARC values
  dmarc-request = "none" / "quarantine" / "reject"

  dmarc-yorn    = "y" / "n"

  dmarc-psd     = "y" / "n" / "u"

  dmarc-rors    = "r" / "s"

  dmarc-urilist = dmarc-uri *(*WSP "," *WSP dmarc-uri)

  dmarc-fo      = "0" / "1" / "d" / "s" / "d:s" / "s:d" / "0:d" / "0:s" / "0:s:d" / "0:d:s" / "1:d" / "1:s" / "1:s:d" / "1:d:s"

In each dmarc-tag, the dmarc-value has a syntax that depends on the tag name. The ABNF rule for each dmarc-value is specified in the following table:¶

4.9. Flow Diagram

 +---------------+                             +--------------------+
 | Author Domain |< . . . . . . . . . . . .    | Return-Path Domain |
 +---------------+                        .    +--------------------+
     |                                    .               ^
     V                                    V               .
 +-----------+     +--------+       +----------+          v
 |   MSA     |<***>|  DKIM  |       |   DMARC  |     +----------+
 |  Service  |     | Signer |       | Validator|<***>|    SPF   |
 +-----------+     +--------+       +----------+  *  | Validator|
     |                                    ^       *  +----------+
     |                                    *       *
     V                                    v       *
  +------+        (~~~~~~~~~~~~)      +------+    *  +----------+
  | sMTA |------->( other MTAs )----->| rMTA |    **>|   DKIM   |
  +------+        (~~~~~~~~~~~~)      +------+       | Validator|
                                         |           +----------+
                                         |                ^
                                         V                .
                                  +-----------+           .
                    +---------+   |    MDA    |           v
                    |  User   |<--| Filtering |      +-----------+
                    | Mailbox |   |  Engine   |      |   DKIM    |
                    +---------+   +-----------+      |  Signing  |
                                                     | Domain(s) |
                                                     +-----------+

  MSA = Mail Submission Agent
  MDA = Mail Delivery Agent

The above diagram shows a typical flow of messages through a DMARC-aware system. Dashed lines (e.g., -->) denote the actual message flow, dotted lines (e.g., < . . >) represent DNS queries used to retrieve message policy related to the supported message authentication schemes, and starred lines (e.g., <**>) indicate data exchange between message-handling modules and message authentication modules. "sMTA" is the sending MTA, and "rMTA" is the receiving MTA.¶

Put simply, when a message reaches a DMARC-aware rMTA, a DNS query will be initiated to determine if a DMARC Policy Record exists that applies to the Author Domain. If a DMARC Policy Record is found, the rMTA will use the results of SPF and DKIM validation checks to determine DMARC validation status. The DMARC validation status can then factor into the message handling decision made by the recipient's mail system.¶

More details on specific actions for the parties involved can be found in Section 5.1 and Section 5.3.¶

4.10. DNS Tree Walk

An Organizational Domain serves two different purposes, depending on the context:¶

• The Organizational Domain of the Author Domain establishes the DMARC Policy Record for that domain when no DMARC Policy Record is published specifically for the Author Domain. (see Section 4.10.1)¶

• The Organizational Domains of an Authenticated Identifier and the Author Domain are used in determining Identifier Alignment between the two. (see Section 4.10.2).¶

[RFC7489] defined an Organizational Domain as "The domain that was registered with a domain name registrar." RFC 7489 discussed using a "public suffix" list (PSL) as the authoritative list of the parent domains for Organizational Domains, and further described a method for determining the Organizational Domain of an Author Domain or an Authenticated Identifier. However, RFC 7489 mandated no requirement for a specific PSL for Mail Receivers to use (though it did suggest the one found at https://publicsuffix.org/) nor did it provide any guidance for the frequency of regular retrieval of the PSL by Mail Receivers participating in DMARC. RFC 7489 acknowledged the possibility of interoperability issues caused by Mail Receivers choosing different PSLs, and even suggested that if a more reliable and secure method for determining the Organizational Domain could be created, that method should replace reliance on a public suffix list.¶

This update to DMARC offers more flexibility to Domain Owners, especially those with large, complex organizations that might want to apply decentralized management to their DNS and their DMARC Policy Records. Rather than just using a public suffix list to help identify an Organizational Domain, this update defines a discovery technique known colloquially as the "DNS Tree Walk". The target of any DNS Tree Walk is discovery of a valid DMARC Policy Record, and its use in determining an Organizational Domain allows for publishing DMARC Policy Records at multiple points in the namespace.¶

This flexibility comes at a possible cost, however. Since the DNS Tree Walk relies on the Mail Receiver making a series of DNS queries, the potential exists for an ill-intentioned Domain Owner to send mail with Author Domains with tens or even hundreds of labels for the purpose of executing a Denial of Service Attack on the Mail Receiver. To guard against such abuse of the DNS, a shortcut is built into the process so that Author Domains with more than eight labels do not result in more than eight DNS queries. Observed data at the time of publication showed that Author Domains with up to seven labels were in usage, and so eight was chosen as the query limit to allow for some future expansion of the name space that did not require updating this document.¶

The generic steps for a DNS Tree Walk are as follows:¶

1. Query the DNS for a TXT record that matches the format of a DMARC Policy Record at the starting point for the Tree Walk. The starting point for the DNS Tree Walk will depend on the ultimate target of the DNS Tree Walk. Section 4.10.1 and Section 4.10.2 describe the possible starting points. 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 Policy Records are returned, they are all discarded. If a single record remains and it contains a "psd=n" tag or "psd=y" tag, stop.¶

3. Break the subject DNS domain name into a set of ordered labels. Assign the count of labels to "x", and number the labels from right to left; e.g., for "a.mail.example.com", "x" would be assigned the value 4, "com" would be label 1, "example" would be label 2, "mail" would be label 3, and so forth.¶

4. If x < 8, remove the left-most (highest-numbered) label from the subject domain. If x >= 8, remove the left-most (highest-numbered) labels from the subject domain until 7 labels remain. The resulting DNS domain name is the new target for the next lookup.¶

5. Query the DNS for a DMARC Policy Record at the DNS domain name matching this new target. A possibly empty set of records is returned.¶

6. Records that do not start with a "v" tag that identifies the current version of DMARC are discarded. If multiple DMARC Policy Records are returned for a single target, they are all discarded. If a single record remains and it contains a "psd=n" or "psd=y" tag, stop.¶

7. Determine the target for the next query by removing the left-most label from the target of the previous query. Repeat steps 5, 6, and 7 until the process stops or there are no more labels remaining.¶

To illustrate, for a message with the arbitrary Author Domain of "a.b.c.d.e.f.g.h.i.j.mail.example.com", a full DNS Tree Walk would require the following eight queries to potentially locate the DMARC Policy Record or Organizational Domain:¶

• _dmarc.a.b.c.d.e.f.g.h.i.j.mail.example.com¶
• _dmarc.g.h.i.j.mail.example.com¶
• _dmarc.h.i.j.mail.example.com¶
• _dmarc.i.j.mail.example.com¶
• _dmarc.j.mail.example.com¶
• _dmarc.mail.example.com¶
• _dmarc.example.com¶
• _dmarc.com¶

5.1. Domain Owner Actions

A Domain Owner wishing to fully participate in DMARC will publish a DMARC Policy Record to cover each Author Domain and corresponding Organizational Domain to which DMARC validation should apply, send email that produces at least one, and preferably two, Authenticated Identifiers that align with the Author Domain, will receive and monitor the content of DMARC aggregate reports, and will correct any authentication shortcomings in mail making authorized use of its domains.¶

The following sections describe how to achieve this.¶

5.2. PSO Actions

In addition to the DMARC Domain Owner actions, if a PSO publishes a DMARC Policy Record it MUST include the "psd" tag (see Section 4.7) with a value of "y" ("psd=y").¶

5.3. Mail Receiver Actions

Mail Receivers wishing to fully participate in DMARC will apply the DMARC mechanism to inbound email messages when a DMARC Policy Record exists that applies to the Author Domain, and will send aggregate feedback reports to Domain Owners that request them. Mail Receivers might also send failure reports to Domain Owners that request them. The following sections describe how to achieve this.¶

The steps for applying the DMARC mechanism to an email message can take place during the SMTP transaction, and should do so if the Mail Receiver plans to honor Domain Owner Assessment Policies that are at the Enforcement state.¶

Many Mail Receivers perform one or both of the underlying Authentication Mechanisms on inbound messages even in cases where no DMARC Policy Record exists for the Author Domain of a given message, or where the Mail Receiver is not participating in DMARC. Nothing in this section is intended to imply that the underlying Authentication Mechanisms should only be performed by Mail Receivers participating in DMARC.¶

5.4. Policy Enforcement Considerations

The final handling of any message is always a matter of local policy and is left to the discretion of the Mail Receiver.¶

A DMARC pass for a message indicates only that the use of the Author Domain has been validated for that message as authorized by the Domain Owner. Such authorization does not carry an explicit or implicit value assertion about that message or the Domain Owner, and Mail Receivers MAY choose to reject or quarantine a message even if it passes the DMARC validation check. Mail Receivers are encouraged to maintain anti-abuse technologies to combat the possibility of DMARC-enabled criminal campaigns.¶

Mail Receivers MAY choose to accept email that fails the DMARC validation check even if the published Domain Owner Assessment Policy is "reject". In particular, because of the considerations discussed in [RFC7960] and in Section 7.4 of this document, it is important that Mail Receivers SHOULD NOT reject messages solely because of a published policy of "reject", but that they apply other knowledge and analysis to avoid situations such as rejection of legitimate messages sent in ways that DMARC cannot describe, harm to the operation of mailing lists, and similar.¶

If a Mail Receiver chooses not to honor the published Domain Owner Assessment Policy to improve interoperability among mail systems, it may increase the likelihood of accepting abusive mail. At a minimum, Mail Receivers SHOULD add the Authentication-Results header field (see [RFC8601]), and it is RECOMMENDED when delivering messages that fail the DMARC validation check.¶

When Mail Receivers deviate from a published Domain Owner Assessment Policy during message processing they SHOULD make available the fact of and reason for the deviation to the Domain Owner via feedback reporting, specifically using the "PolicyOverride" feature of the aggregate report defined in [I-D.ietf-dmarc-aggregate-reporting].¶

To enable Domain Owners to receive DMARC feedback without impacting existing mail processing, discovered policies of "p=none" MUST NOT modify existing mail handling processes.¶

7.1. Issues Specific to SPF

Though DMARC does not inherently change the semantics of an SPF policy record, historically lax enforcement of such policies has led many to publish extremely broad records containing many extensive network ranges. Domain Owners 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 Policy Record. Furthermore, Domain Owners should periodically review their SPF records to ensure that the authorization conveyed by the records matches the domain's current needs.¶

SPF was intended to be implemented early in the SMTP transaction, meaning it's possible for a message to fail SPF validation prior to any message content being transmitted, and so some Mail Receiver architectures might implement SPF in advance of any DMARC operations. This means that an SPF hard fail ("-") prefix on a sender's SPF mechanism, such as "-all", could cause a message to be rejected early in the SMTP transaction, before any DMARC processing takes place, if the message fails SPF authentication checks. Domain Owners choosing to use "-all" to terminate SPF records should be aware of this, and should understand that messages that might otherwise pass DMARC due to an aligned DKIM-Authenticated Identifier could be rejected solely due to an SPF fail. Moreover, messages rejected early in the SMTP transaction will never appear in aggregate DMARC reports, as the transaction will never proceed to the DATA phase and so the RFC5322.From domain will never be revealed and its DMARC policy will never be discovered. Domain Owners and Mail Receivers can consult [M3SPF] and [M3AUTH] for more discussion of the topic and best practices regarding publishing SPF records and when to reject based solely on SPF failure:¶

7.2. Rejecting Messages

The DMARC mechanism calls for rejection of a message during the SMTP session under certain circumstances. This is preferable to generation of a Delivery Status Notification [RFC3464], since fraudulent messages caught and rejected using the DMARC mechanism would then result in the annoying generation of such failure reports that go back to the RFC5321.MailFrom address.¶

This synchronous rejection is typically done in one of two ways:¶

• Full rejection, wherein the SMTP server issues a 5xy reply code to the DATA command as an indication to the SMTP client that the transaction failed; the SMTP client is then responsible for generating a notification that delivery failed (see Section 4.2.5 of [RFC5321]).¶

• A "silent discard", wherein the SMTP server returns a 2xy reply code implying to the client that delivery (or, at least, relay) was successfully completed, but then simply discards 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 the inability to retrieve or apply DMARC policy, this is best done with a 4xy SMTP reply code.¶

7.3. Interoperability Issues

DMARC limits which end-to-end scenarios can achieve a "pass" result.¶

Because DMARC relies on SPF [RFC7208] and/or DKIM [RFC6376] to achieve a "pass", their limitations also apply.¶

Issues specific to the use of policy mechanisms alongside DKIM are further discussed in [RFC6377], particularly Section 5.2.¶

Mail that is sent by authorized, independent third parties might not be sent with Identifier Alignment, also preventing a "pass" result. A Domain Owner can use DMARC aggregate reports to identify this mail and take steps to address authentication shortcomings.¶

7.4. Interoperability Considerations

As discussed in "Interoperability Issues between DMARC and Indirect Email Flows" [RFC7960], use of "p=reject" can be incompatible with and cause interoperability problems to indirect message flows such as "alumni forwarders", role-based email aliases, and mailing lists across the Internet.¶

As an example of this, a bank might send only targeted messages to account holders. Those account holders might have given their bank addresses such as "jones@alumni.example.edu" (an address that relays the messages to another address with a real mailbox) or "finance@association.example" (a role-based address that does similar relaying for the current head of finance at the association). When such mail is delivered to the actual recipient mailbox, it will most likely fail SPF checks unless the RFC5321.MailFrom address is rewritten by the relaying MTA, as the incoming IP address will be that of "example.edu" or "association.example", and not an IP address authorized by the originating RFC5321.MailFrom domain. DKIM signatures will generally remain valid in these relay situations.¶

It is therefore critical that domains that publish "p=reject" MUST NOT rely solely on SPF to secure a DMARC pass, and MUST apply valid DKIM signatures to their messages.¶

In the case of domains that have general users who send routine email, those that publish a Domain Owner Assessment Policy of "p=reject" are likely to create significant interoperability issues. In particular, if users in such domains post messages to mailing lists on the Internet, those messages can cause significant operational problems for the mailing lists and for the subscribers to those lists, as explained below and in [RFC7960].¶

It is therefore critical that domains that host users who might post messages to mailing lists SHOULD NOT publish Domain Owner Assessment Policies of "p=reject". Any such domains wishing to publish "p=reject" SHOULD first take advantage of DMARC aggregate report data for their domain to determine the possible impact to their users, first by publishing "p=none" for at least a month, followed by publishing "p=quarantine" for an equally long period of time, and comparing the message disposition results. Domains that choose to publish "p=reject" SHOULD either implement policies that their users not post to Internet mailing lists and/or inform their users that their participation in mailing lists may be hindered.¶

As noted in Section 5.4, Mail Receivers need to apply more analysis than just DMARC validation in their disposition of incoming messages. An example of the consequences of honoring a Domain Owner Assessment Policy of "p=reject" without further analysis is that rejecting messages that have been relayed by a mailing list can cause the Mail Receiver's users to have their subscriptions to that mailing list canceled by the list software's automated handling of such rejections - it looks to the list manager as though the recipient's email address is no longer working, so the address is automatically unsubscribed. An example of this scenario, albeit with DKIM Author Domain Signing Practices (ADSP) rather than DMARC, can be found in Section 5.2 of [RFC6377].¶

It is therefore critical that Mail Receivers MUST NOT reject incoming messages solely on the basis of a "p=reject" policy by the sending domain. Mail Receivers must use the DMARC policy as part of their disposition decision, along with other knowledge and analysis. "Other knowledge and analysis" here might refer to observed sending patterns for properly-authenticated mail using the sending domain, content filtering, etc. In the absence of other knowledge and analysis, Mail Receivers MUST treat such failing mail as if the policy were "p=quarantine" rather than "p=reject".¶

Failure to understand and abide by these considerations can cause legitimate, sometimes important email to be rejected, can cause operational damage to mailing lists throughout the Internet, and can result in trouble-desk calls and complaints from the Mail Receiver's employees, customers, and clients.¶

In practice, despite this advice, few Mail Receivers apply any mitigation techniques when receiving indirect mail flows, few organizations consider the effect of DMARC policies on their users' indirect mail, and it is unlikely that any advice in this document will change that. As a result, mail forwarded through mailing lists with unmodified From: header lines is frequently rejected due to a p=reject policy.¶

In the ten years since large consumer mail systems started publishing p=reject policies, mailing list software has all adopted workarounds to make the From: header line DMARC aligned. Some simply use the list's address, while others do per-address modifications intended to be reversible or to allow mail to be forwarded back to the original author, e.g., bob@example.com turned into bob=example.com@user.somelist.example. While these workarounds are far from ideal, they are firmly established and list operators treat them as a fact of life.¶

Mail developers have been trying for a decade to invent technical methods to allow mailing lists to continue to work without modifying the From: header line, with a prominent example being the Authenticated Received Chain (ARC) protocol described in [RFC8617]. While work continues, as of this document's publication, none of the methods have become widely used. Should such a technical method achieve widespread adoption in the future, this document can be updated to reflect that.¶

8.1. Authentication-Results Method Registry Update

IANA has added the following to the "Email Authentication Methods" registry:¶

8.2. Authentication-Results Result Registry Update

IANA has added the following in the "Email Authentication Result Names" registry:¶

8.3. Feedback Report Header Fields Registry Update

The following has been added to the "Feedback Report Header Fields" registry:¶

8.4. DMARC Tag Registry

A registry tree called "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters" exists, and it and any sub-registries thereunder should be updated to reference this document. Within it, a new sub-registry called the "DMARC Tag Registry" exists.¶

Names of DMARC tags are registered with IANA in this sub-registry. Entries are assigned only for values that have been documented in a manner that satisfies the terms of Specification Required, per [RFC8126]. Each registration includes the tag name; the specification that defines it; a brief description; and its status, which is one of "current", "experimental", or "historic". The Designated Expert needs to confirm that the provided specification adequately describes the new tag and clearly presents how it would be used within the DMARC context by Domain Owners and Mail Receivers.¶

To avoid version compatibility issues, tags added to the DMARC specification are to avoid changing the semantics of existing records when processed by implementations conforming to prior specifications.¶

The set of entries to be defined in this registry is as follows:¶

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" exists and should be updated to reference this document.¶

Names of DMARC failure reporting formats are registered with IANA in this registry. New entries are assigned only for values that satisfy the definition of Specification Required, per [RFC8126]. In addition to a reference to a permanent specification, each registration includes the format name, a brief description, and its status, which must be one of "current", "experimental", or "historic". The Designated Expert needs to confirm that the provided specification adequately describes the report format and clearly presents how it would be used within the DMARC context by Domain Owners and Mail Receivers.¶

The entry in this registry is as follows:¶

8.6. Underscored and Globally Scoped DNS Node Names Registry

Per [RFC8552], please update the following entry to the "Underscored and Globally Scoped DNS Node Names" registry:¶

9.1. Aggregate Report Considerations

Aggregate reports may, particularly for small organizations, provide some limited insight into email sending patterns. As an example, in a small organization, an aggregate report from a particular domain may be sufficient to make the report receiver aware of sensitive personal or business information. If setting an "rua" tag in a DMARC Policy Record, the reporting address needs controls appropriate to the organizational requirements to mitigate any risk associated with receiving and handling reports.¶

In the case of "rua" requests for multi-organizational PSDs, additional information leakage considerations exist. Multi-organizational PSDs that do not mandate DMARC use by registrants risk exposure of private data of registrant domains if they include the "rua" tag in their DMARC Policy Record.¶

9.2. Failure Report Considerations

Failure reports do provide insight into email sending patterns, including specific users. If requesting failure reports, data management controls are needed to support appropriate management of this information. The additional detail available through failure reports (relative to aggregate reports) can drive a need for additional controls. As an example, a company may be legally restricted from receiving data related to a specific subsidiary. Before requesting failure reports, any such data spillage risks have to be addressed through data management controls or publishing DMARC Policy Records for relevant subdomains to prevent reporting on data related to their emails.¶

Due to the nature of the email contents which may be shared through Failure Reports, most Mail Receivers refuse to send them out of privacy concerns. Out of band agreements between Report Consumers and Mail Receivers may be required to address these concerns.¶

DMARC Policy Records for multi-organizational PSDs MUST NOT include the "ruf" tag.¶

10.1. Authentication Methods

Security considerations from the authentication methods used by DMARC are incorporated here by reference.¶

Both of the email authentication methods that underlie DMARC provide some assurance that an email was transmitted by an MTA which is authorized to do so. SPF policies map domain names to sets of authorized MTAs (see Section 11.4 of [RFC7208]). Validated DKIM signatures indicate that an email was transmitted by an MTA with access to a private key that matches the published DKIM key record.¶

Whenever mail is sent, there is a risk that an overly permissive source may send mail that will receive a DMARC pass result that was not, in fact, intended by the Domain Owner. These results may lead to issues when systems interpret DMARC pass results to indicate a message is in some way authentic. They also allow such unauthorized senders to evade the Domain Owner's intended message handling for DMARC validation failures.¶

To avoid this risk one must ensure that no unauthorized source can add DKIM signatures to the domain's mail or transmit mail which will evaluate as SPF pass. If, nonetheless, a Domain Owner wishes to include a permissive source in a domain's SPF record, the source can be excluded from DMARC consideration by using the "?" qualifier on the SPF record mechanism associated with that source. The DMARC working group had a lively discussion about possibly eliminating SPF entirely as an underlying Authentication Mechanism for DMARC, but consensus was not reached, and the suggestion to use the "?" qualifier for permissive sources is presented here instead.¶

10.2. Attacks on Reporting URIs

URIs published in DNS TXT records are well-understood possible targets for attack. Specifications such as [RFC1035] and [RFC2142] either expose or cause the exposure of email addresses that could be flooded by an attacker, for example. Records found in the DNS such as MX, NS, and others advertise potential attack destinations. Common DNS names such as "www" plainly identify the locations at which particular services can be found, providing destinations for targeted denial-of-service or penetration attacks. This all means that Domain Owners will need to harden these addresses against various attacks, including but not limited to:¶

• high-volume denial-of-service attacks;¶

• deliberate construction of malformed reports intended to identify or exploit parsing or processing vulnerabilities;¶

• deliberate construction of reports containing false claims for the Submitter or Reported-Domain fields, including the possibility of false data from compromised but known Mail Receivers.¶

10.3. DNS Security

The DMARC mechanism and its underlying Authentication Mechanisms (SPF and 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 receiving mail using the domain(s) in question.¶

• If they can block outgoing or reply DNS messages, they can prevent systems from discovering senders' DMARC policies.¶

• If they can send forged response packets, they can make aligned mail appear unaligned or vice-versa.¶

None of these threats are unique to DMARC, and they can be addressed using a variety of techniques, including, but not limited to:¶

• Signing DNS records with Domain Name System Security Extensions (DNSSEC) [RFC4033], which enables recipients to validate the integrity of DNS data and detect and discard forged responses.¶

• DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate snooping and forged responses.¶

10.4. Display Name Attacks

An increasingly common attack in messaging abuse is the presentation of false information in the display-name portion of the RFC5322.From header field. For example, it is possible for the email address in that field to be an arbitrary address or domain name while containing a well-known name (a person, brand, role, etc.) in the display name, intending to fool the end user into believing that the name is used legitimately.¶

Such attacks, known as display name attacks, are out of scope for DMARC.¶

10.5. Denial of DMARC Processing Attacks

The declaration in Section 5.3.1 and elsewhere in this document that messages that do not contain precisely one RFC5322.From domain are outside the scope of this document exposes an attack vector that must be taken into consideration.¶

Because such messages are outside the scope of this document, an attacker can craft messages with multiple RFC5322.From domains, including the spoofed domain, in an effort to bypass DMARC validation and get the fraudulent message to be displayed by the victim's MUA with the spoofed domain successfully shown to the victim. In those cases where such messages are not rejected due to other reasons (for example, many such messages would violate RFC5322's requirement that there be precisely one From: header field), care must be taken by the Mail Receiver to recognize such messages as the threats they might be and handle them appropriately.¶

The case of a syntactically valid multi-valued RFC5322.From field presents a particular challenge. Experience has shown that most such messages are abusive and/or unwanted by their recipients, and given this fact, a Mail Receiver may make a negative disposition decision for the message prior to and instead of its being subjected to DMARC processing. However, in a case where a Mail Receiver requires that the message be subject to DMARC validation, a recommended approach as per [RFC7489] is to apply the DMARC mechanism to each domain found in the RFC5322.From field as the Author Domain and apply the most strict policy selected among the checks that fail. Such an approach might prove useful for a small number of Author Domains, but it is possible that applying such logic to messages with a large number of domains (where "large" is defined by each Mail Receiver) will expose the Mail Receiver to a form of denial of service attack. Limiting the number of Author Domains processed will avoid this risk. If not all Author Domains are processed, then the DMARC evaluation is incomplete.¶

10.6. External Reporting Addresses

To avoid abuse by bad actors, reporting addresses generally have to be inside the domains about which reports are requested. To accommodate special cases such as a need to get reports about domains that cannot actually receive mail, Section 3 of [I-D.ietf-dmarc-aggregate-reporting] describes a DNS-based mechanism for validating approved external reporting.¶

The obvious consideration here is an increased DNS load against domains that are claimed as external recipients. Negative caching will mitigate this problem, but only to a limited extent, mostly dependent on the default TTL in the domain's SOA record.¶

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 DNSSEC [RFC4033] is advisable if this is a concern.¶

10.7. Secure Protocols

This document encourages the use of secure transport mechanisms to prevent the loss of private data to third parties that may be able to monitor such transmissions. Unencrypted mechanisms should be avoided.¶

In particular, a message that was originally encrypted or otherwise secured might appear in a report that is not sent securely, which could reveal private information.¶

10.8. Relaxed Alignment Considerations

The DMARC mechanism allows both DKIM- and SPF-Authenticated Identifiers to validate authorized use of an Author Domain on behalf of a Domain Owner. If malicious or unaware users can gain control of the SPF record or DKIM selector records for a subdomain of the Organizational Domain, the subdomain can be used to generate email that achieves a DMARC pass on behalf of the Organizational Domain.¶

A scenario such as this could occur under the following conditions:¶

• A DMARC Policy Record exists for the domain "example.com", such that "example.com" is an Organizational Domain¶
• An attacker controls DNS for the domain "evil.example.com" and publishes an SPF record for that domain¶
• The attacker sends email with RFC5322.From header field containing "foo@example.com" and an SPF-Authenticated Identifier of "evil.example.com"¶

Although this email was not authorized by the Domain Owner, it can produce a DMARC pass because the SPF-Authenticated Identifier ("evil.example.com") has Identifier Alignment with the Author Domain ("example.com").¶

The Organizational Domain Owner should be careful not to delegate control of subdomains if this is an issue, and consider using the Strict Alignment option if appropriate.¶

DMARC evaluation for relaxed alignment is also highly sensitive to errors in determining the Organizational Domain if the Author Domain does not have a published DMARC Policy Record. If an incorrectly selected Organizational Domain is a parent of the correct Organizational Domain, then relaxed alignment could potentially allow a malicious sender to send mail that achieves a DMARC pass verdict. This potential exists for both the legacy [RFC7489] and current methods for determining the organizational domain, the latter described in Section 4.10.2.¶

The following example illustrates this possibility:¶

• Mail is sent with an Author Domain of "a.mail.example.com" and Authenticated Identifiers of "mail.example.com"¶
• There is no DMARC Policy Record published at "_dmarc.a.mail.example.com"¶
• There is one published at "_dmarc.mail.example.com" and this is intended to be the Organizational Domain for this message¶
• There is also a DMARC Policy Record published at "_dmarc.example.com", with default alignment (relaxed)¶
• An is able to send mail with the Author Domain of "evil.example.com" and an Authenticated Identifier of "mail.example.com"¶

In this scenario, if a Mail Receiver incorrectly determines the Organizational Domain to be "example.com", then the attacker's mail will pass DMARC validation checks.¶

This issue is entirely avoided by the use of Strict Alignment and publishing explicit DMARC Policy Records for all Author Domains used in an organization's email.¶

For cases where Strict Alignment is not appropriate, this issue can be mitigated by the Domain Owner periodically (perhaps weekly, or whatever frequency might be appropriate for a given organization's operational needs) checking the DMARC Policy Records, if any, of PSDs above the Organizational Domain in the DNS tree and (for legacy [RFC7489] checking that appropriate PSL entries remain present). If a PSD publishes a DMARC Policy Record without the appropriate "psd=y" tag, Organizational Domain owners can add "psd=n" to their Organizational Domain's DMARC Policy Record so that the PSD's DMARC Policy Record will not be incorrectly interpreted to indicate that the PSD is the Organizational Domain.¶

A.1. S/MIME

S/MIME, or Secure Multipurpose Internet Mail Extensions [RFC8551], is a standard for encrypting and signing MIME data in a message. This was suggested and considered as a third security protocol for authenticating the source of a message.¶

DMARC is focused on authentication at the domain level (i.e., the Domain Owner taking responsibility for the message), while S/MIME is really intended for user-to-user authentication and encryption. This alone appears to make it a bad fit for DMARC's goals.¶

S/MIME also suffers from the heavyweight problem of Public Key Infrastructure, which means that distribution of keys used to validate signatures needs to be incorporated. In many instances, this alone is a showstopper. There have been consistent promises that PKI usability and deployment will improve, but these have yet to materialize. DMARC can revisit this choice after those barriers are addressed.¶

S/MIME has extensive deployment in specific market segments (government, for example) but does not enjoy similar widespread deployment over the general Internet, and this shows no signs of changing. DKIM and SPF are both deployed widely over the general Internet, and their adoption rates continue to be positive.¶

Finally, experiments have shown that including S/MIME support in the initial version of DMARC would neither cause nor enable a substantial increase in the accuracy of the overall mechanism.¶

A.2. Method Exclusion

It was suggested that DMARC include a mechanism by which a Domain Owner could instruct Mail Receivers not to attempt validation by one of the supported methods (e.g., "check DKIM, but not SPF").¶

Specifically, consider a Domain Owner that has deployed one of the technologies and that technology fails for some messages, but such failures don't cause enforcement action. Deploying DMARC would cause enforcement action for policies other than "none", which would appear to exclude participation by that Domain Owner.¶

The DMARC development team evaluated the idea of policy exception mechanisms on several occasions and invariably concluded that there was not a strong enough use case to include them. The target audience for DMARC does not appear to have concerns about the failure modes of one or the other being a barrier to DMARC's adoption.¶

In the scenario described above, the Domain Owner has a few options:¶

1. Tighten up its infrastructure to minimize the failure modes of the single deployed technology.¶

2. Deploy the other supported authentication mechanism, to offset the failure modes of the first.¶

3. Deploy DMARC in a reporting-only mode.¶

A.3. Sender Header Field

It has been suggested in several message authentication efforts that the Sender header field be checked for an identifier of interest, as the standards indicate this as the proper way to indicate a re-mailing of content such as through a mailing list. Most recently, it was a protocol-level option for DomainKeys [RFC4870], but on evolution to DKIM, this property was removed.¶

The DMARC development team considered this and decided not to include support for doing so for the following reasons:¶

1. The main user protection approach is to be concerned with what the user sees when a message is rendered. There is no consistent behavior among MUAs regarding what to do with the content of the Sender field, if present. Accordingly, supporting the checking of the Sender identifier would mean applying policy to an identifier the end user might never actually see, which can create a vector for attack against end users by simply forging a Sender field containing some identifier that DMARC will like.¶

2. Although it is certainly true that this is what the Sender field is for, its use in this way is also unreliable, making it a poor candidate for inclusion in the DMARC evaluation algorithm.¶

3. Allowing multiple ways to discover policy introduces unacceptable ambiguity into the DMARC validation algorithm in terms of which policy is to be applied and when.¶

A.4. Domain Existence Test

The presence of the "np" tag in this specification seemingly implies that there would be an agreed-upon standard for determining a domain's existence.¶

Since the DMARC mechanism is focused on email, one might think that the definition of "resolvable" in [RFC5321] applies. Using that definition, only names that resolve to MX Resource Records (RRs), A RRs, or AAAA RRs are deemed to be resolvable and to exist in the DNS. This is a common practice among Mail Receivers to determine whether or not to accept a mail message before performing other more expensive processing.¶

The DMARC mechanism makes no such requirement for the existence of specific DNS RRs in order for a domain to exist; instead, if any RR exists for a domain, then the domain exists. To use the terminology from [RFC2308], an "NXDOMAIN" response (rcode "Name Error") to a DNS query means that the domain name does not exist, while a "NODATA" response (rcode "NOERROR") means that the given resource record type queried for does not exist, but the domain name does.¶

Furthermore, in keeping with [RFC8020], if a query for a name returns NXDOMAIN, then not only does the name not exist, every name below it in the DNS hierarchy also does not exist.¶

A.5. Organizational Domain Discovery Issues

An earlier informational version of the DMARC mechanism [RFC7489] noted that the DNS does not provide a method by which the "domain of record", or the domain that was actually registered with a domain registrar, can be determined given an arbitrary domain name. That version further mentioned suggestions that have been made that attempt to glean such information from SOA or NS resource records, but these too are not fully reliable, as the partitioning of the DNS is not always done at administrative boundaries.¶

That previous version posited that one could "climb the tree" to find the Organizational Domain, but expressed concern that an attacker could exploit this for a denial-of-service attack through sending a high number of messages each with a relatively large number of nonsense labels, causing a Mail Receiver to perform a large number of DNS queries in search of a DMARC Policy Record. This version defines a method for performing a DNS Tree Walk, 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.¶

Readers curious about the previous method for Organizational Domain Discovery are directed to Section 3.2 of [RFC7489].¶

A.6. Removal of the "pct" Tag

An earlier informational version of the DMARC mechanism [RFC7489] included a "pct" tag and specified all integers from 0 to 100 inclusive as valid values for the tag. The intent of the tag was to provide domain owners with a method to gradually change their preferred Domain Owner Assessment Policy (the "p" tag) from "none" to "quarantine" or from "quarantine" to "reject" by requesting the stricter treatment for just a percentage of messages that produced DMARC results of "fail".¶

Operational experience showed that the pct tag was usually not accurately applied, unless the value specified was either 0 or 100 (the default), and the inaccuracies with other values varied widely from one implementation to another. The default value was easily implemented, as it required no special processing on the part of the Mail Receiver, while the value of 0 took on unintended significance as a value used by some intermediaries and mailbox providers as an indicator to deviate from standard handling of the message, usually by rewriting the RFC5322.From header field in an effort to avoid DMARC failures downstream.¶

These custom actions when the "pct" tag was set to 0 proved valuable to the email community. In particular, header field rewriting by an intermediary meant that a Domain Owner's aggregate reports could reveal to the Domain Owner how much of its traffic was routing through intermediaries that don't rewrite the RFC5322.From header field. Such information wasn't explicit in the aggregate reports received; rather, sussing it out required work on the part of the Domain Owner to compare aggregate reports from before and after the "p" value was changed and "pct=0" was included in the DMARC Policy Record, but the data was there. Consequently, knowing how much mail was subject to possible DMARC failure due to a lack of RFC5322.From header field rewriting by intermediaries could assist the Domain Owner in choosing whether to move from Monitoring Mode to Enforcement. Armed with this knowledge, the Domain Owner could make an informed decision regarding subjecting its mail traffic to possible DMARC failures based on the Domain Owner's tolerance for such things.¶

Because of the value provided by "pct=0" to Domain Owners, it was logical to keep this functionality in the protocol; at the same time, it didn't make sense to support a tag named "pct" that had only two valid values. This version of the DMARC mechanism, therefore, introduces the "t" tag as shorthand for "testing", with the valid values of "y" and "n", which are meant to be analogous in their application by mailbox providers and intermediaries to the "pct" tag values "0" and "100", respectively.¶

B.1. Identifier Alignment Examples

The following examples illustrate the DMARC mechanism's use of Identifier Alignment. For brevity's sake, only message header fields and relevant SMTP commands are shown, as message bodies are not considered when conducting DMARC checks.¶

     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

     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

     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

     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

     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

     DKIM-Signature: v=1; ...; d=example.net; ...
     From: sender@child.example.com
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: receiver@example.org
     Subject: here's a sample

B.2. Domain Owner Example

A Domain Owner that wants to use DMARC should have already deployed and tested SPF and DKIM. The next step is to publish a DMARC Policy Record for the Domain Owner's Organizational Domain.¶

  ; DMARC Policy Record for the domain example.com
  _dmarc  IN TXT ( "v=DMARC1; p=none; "
                   "rua=mailto:dmarc-feedback@example.com" )

  ; DMARC Policy Record for the domain example.com
  _dmarc  IN TXT ( "v=DMARC1; p=none; "
                   "rua=mailto:dmarc-feedback@example.com; "
                   "ruf=mailto:auth-reports@example.com" )

  ; DMARC Policy Record for the domain example.com
  _dmarc IN TXT ( "v=DMARC1; p=none; "
                  "rua=mailto:dmarc-feedback@example.com; "
                  "ruf=mailto:auth-reports@thirdparty.example.net" )

  ; zone file for thirdparty.example.net
  ; Accept DMARC reports on behalf of example.com
  example.com._report._dmarc   IN   TXT    "v=DMARC1;"

  ; zone file for thirdparty.example.net
  ; Accept DMARC reports on behalf of example.com
  ; Override destination mailboxes
  example.com._report._dmarc   IN   TXT    (
          "v=DMARC1; "
          "rua=mailto:aggregate-reports@thirdparty.example.net; "
          "ruf=mailto:failure-reports@thirdparty.example.net" )

  ; DMARC Policy Record for the domain test.example.com
  _dmarc IN  TXT  ( "v=DMARC1; p=quarantine; "
                    "rua=mailto:dmarc-feedback@example.com,"
                    "mailto:tld-test@thirdparty.example.net; "
                    "t=y" )

  ; DMARC Policy Record for the domain test.example.com
  _dmarc IN  TXT  ( "v=DMARC1; p=reject; "
                    "rua=mailto:dmarc-feedback@example.com,"
                    "mailto:tld-test@thirdparty.example.net" )

B.3. Mail Receiver Example

A Mail Receiver that wants to participate in DMARC should already be checking SPF and DKIM, and possess the ability to collect relevant information from various email-processing stages to provide feedback to Domain Owners (possibly via Report Consumers).¶

  Author Domain: example.com
  SPF-authenticated Identifier: mail.example.com
  DKIM-authenticated Identifier: example.com
  DMARC Policy Record:
    "v=DMARC1; p=reject; aspf=r;
     rua=mailto:dmarc-feedback@example.com"

B.4. Organizational and Policy Domain Tree Walk Examples

If an Author Domain has no DMARC Policy Record, a Mail Receiver uses a tree walk to find the DMARC Policy.¶

If the DMARC Policy Record allows relaxed alignment and the SPF- or DKIM-Authenticated Identifiers are different from the Author Domain, a Mail Receiver uses a tree walk to discover the respective Organizational Domains to determine Identifier Alignment.¶

B.5. Utilization of Aggregate Feedback: Example

Aggregate feedback is consumed by Domain Owners to enable their understanding of how a given domain is being processed by the Mail Receiver. Aggregate reporting data on emails that pass all underlying authentication checks is used by Domain Owners to validate that their authentication practices remain accurate. For example, if a third party is sending on behalf of a Domain Owner, the Domain Owner can use aggregate report data to validate ongoing authentication practices of the third party.¶

Data on email that only partially passes underlying authentication checks provides visibility into problems that need to be addressed by the Domain Owner. For example, if either SPF or DKIM fails to produce an Authenticated Identifier, the Domain Owner is provided with enough information to either directly correct the problem or understand where authentication-breaking changes are being introduced in the email transmission path. If authentication-breaking changes due to email transmission path cannot be directly corrected, then the Domain Owner at least maintains an understanding of the effect of DMARC-based policies upon the Domain Owner's email.¶

Data on email that fails all underlying authentication checks provides baseline visibility on how the Domain Owner's domain is being received at the Mail Receiver. Based on this visibility, the Domain Owner can begin deployment of authentication technologies across uncovered email sources, if the mail that is failing the checks was generated by or on behalf of the Domain Owner. Data regarding failing authentication checks can also allow the Domain Owner to come to an understanding of how its domain is being misused.¶

C.1. Informational vs. Standards Track

RFC 7489 was not the product of any IETF work stream, but was instead published into the RFC series by the Independent Submissions Editor and is classified as an Informational RFC.¶

This document, by contrast, is intended to be Internet Standards Track.¶

C.2. Changes to Terminology and Definitions

The following changes were made to the Terminology and Definitions section.¶

C.3. Policy Discovery and Organizational Domain Determination

The algorithms for DMARC policy discovery and for determining the Organizational Domain have been changed. Specifically, reliance on a Public Suffix List (PSL) has been replaced by a technique called a "DNS Tree Walk", and the methodology for the DNS Tree Walk is explained in detail in this document.¶

The DNS Tree Walk also incorporates PSD policy discovery, which was introduced in [RFC9091]. That RFC was an Experimental RFC, and the results of that experiment were that the RFC was not implemented as written. Instead, this document redefines the algorithm for PSD policy discovery, and thus obsoletes [RFC9091]. Specifically, the DNS Tree Walk defined in this document obviates the need for a PSD DMARC registry, and that PSD DMARC regisry is what made RFC 9091 and Experimental RFC.¶

These algorithm changes introduce the possibility of interoperability issues where a Domain Owner expects a DMARC Policy Record or an Organizational Domain to be identified by the Tree Walk process, but a Mail Receiver using an RFC 7489-based implementation of DMARC and relying on a PSL might arrive at a different answer.¶

This issue is entirely avoided by the use of Strict Alignment and publishing explicit DMARC Policy Records for all Author Domains used in an organization's email.¶

C.4. Reporting

Discussion of both aggregate and failure reporting have been moved to separate documents dedicated to the topics.¶

In addition, the ability to specify a maximum report size in the DMARC URI has been removed.¶

C.5. Tags

Several tags have been added to the "DMARC Policy Record Format" section of this document since RFC 7489 was published, and at the same time, several others were removed.¶

C.6. Expansion of Domain Owner Actions Section

RFC 7489 had just two paragraphs in its Domain Owner Actions section, and while the content of those paragraphs was correct, it was minimalist in its approach to providing guidance to domain owners on just how to implement DMARC.¶

This document provides much more detail and explanatory text to a Domain Owner, focusing not just on what to do to implement DMARC, but also on the reasons for each step and the repercussions of each decision.¶

In particular, this document makes explicit that domains for general-purpose email SHOULD NOT deploy a DMARC policy of p=reject. See Section 7.4 for further discussion of this topic.¶

C.7. Report Generator Recommendations

In the cases where a DMARC Policy Record specifies multiple destinations for either aggregate reports or failure reports, RFC 7489 stated:¶

  Receivers **MAY** impose a limit on the number of URIs to which they
  will send reports but **MUST** support the ability to send to at least
  two.

This document in Section 4.6 says:¶

  A report **SHOULD** be sent to each listed URI provided in the DMARC
  Policy Record.

C.8. Removal of RFC 7489 Appendix A.5

One of the appendices in RFC 7489, specifically Appendix A.5, has been removed from the text with this update. The appendix was titled "Issues with ADSP in Operation" and it contained a list of issues associated with ADSP that influenced the direction of DMARC. The ADSP protocol was moved to "Historic" status in 2013 and working group consensus was that such a discussion of ADSP's influence on DMARC was no longer relevant.¶

C.9. RFC 7489 Errata Summary

This document and its companion documents ([I-D.ietf-dmarc-aggregate-reporting] and [I-D.ietf-dmarc-failure-reporting]) address the following errata filed against [RFC7489] since that document's publication in March, 2015. More details on each of these can be found at https://www.rfc-editor.org/errata_search.php?rfc=7489¶

C.10. General Editing and Formatting

A great deal of the content from RFC 7489 was preserved in this document, but much of it was subject to either minor editing, re-ordering of sections, and/or both.¶