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

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 emails.¶

• 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 transactional email (official mail about business transactions). One of the primary uses of this kind of spoofed mail is phishing (enticing users to provide information by pretending to be the legitimate service requesting the information). 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 emails. In particular, it does not address the use of visually similar domain names ("cousin domains") or abuse of the RFC5322.From human-readable <display-name>.¶

2.3. Scalability

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

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

2.4. 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 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 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] 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. 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 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. A domain's DMARC policy record is published in DNS as a TXT record at the name created by prepending the label "_dmarc" to the domain name and is retrieved through normal DNS queries.¶

DMARC's 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. They do not authenticate the local-part of any email address identifier found in a message, nor do they validate the legitimacy of message content.¶

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 [I-D.ietf-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 [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.¶

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 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.¶

• 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 has authenticated itself 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.¶

Since the sorts of mail typically protected by DMARC participants tend only to 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 domains may be different and are typically not visible to the end user.¶

DMARC authenticates the 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 name comparisons in this context are case-insensitive, per [RFC4343].¶

It is important to note that Identifier Alignment cannot occur with a message that is not valid per [RFC5322], particularly one with a malformed, absent, or repeated RFC5322.From header field, since in that case there is no reliable way to determine a DMARC policy that applies to the message. Accordingly, DMARC operation is predicated on the input being a valid RFC5322 message object. For non-compliant cases, handling is outside of the scope of this specification. Further discussion of this can be found in Section 11.5.¶

Each of the underlying authentication technologies that DMARC takes as input yields authenticated domains as their outputs when they succeed.¶

4.5. Flow Diagram

 +---------------+                             +--------------------+
 | 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 system.¶

More details on specific actions for the parties involved can be found in Section 5.5 and Section 5.7.¶

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 appropriate starting point for the Tree Walk. 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. If a single record remains and it contains a "psd=n" tag, stop.¶

3. Determine the target for additional queries (if needed; see the note in Section 4.8), using steps 4 through 8 below.¶

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

5. If x < 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 the next lookup.¶

6. Query the DNS for a DMARC TXT record at the DNS domain name matching this new target. A possibly empty set of records is returned.¶

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

8. Determine the target for additional queries by removing a single label from the target domain as described in step 5 and repeating steps 6 and 7 until the process stops or there are no more labels remaining.¶

To illustrate, for a message with the arbitrary RFC5322.From domain of "a.b.c.d.e.mail.example.com", a full DNS Tree Walk would require the following five queries 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 domain found in the RFC5322.From header of the message being evaluated. The DMARC policy to be applied to the message will be the record found at of the following locations, listed from highest preference to lowest:¶

• 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¶

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 to be applied is that of 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, or 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" or "np" tag that is not valid, then:¶

• If a "rua" tag is present and contains at least one syntactically valid reporting URI, the Mail Receiver MUST act as if a record containing "p=none" was retrieved and continue processing;¶

• Otherwise, the Mail Receiver applies no DMARC processing to this message.¶

If the set produced by the DNS Tree Walk contains no DMARC policy record (i.e., any indication that there is no such record as opposed to a transient DNS error), Mail Receivers MUST NOT apply the DMARC mechanism to the message.¶

Handling of DNS errors when querying for the DMARC policy record is left to the discretion of the Mail Receiver. For example, to ensure minimal disruption of mail flow, transient errors could result in delivery of the message ("fail open"), or they could result in the message being temporarily rejected (i.e., an SMTP 4yx reply), which invites the sending MTA to try again after the condition has possibly cleared, allowing a definite DMARC conclusion to be reached ("fail closed").¶

Note: PSD policy is not used for Organizational Domains that have published a DMARC policy. 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 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 found in the RFC5322.From header of the message being evaluated.¶
• The domain found in the RFC5321.MailFrom header if there is an SPF pass result for the message being evaluated.¶
• Any DKIM d= domain if there is a DKIM pass result for that domain for the message being evaluated.¶

Note: There is no need to perform Tree Walk searches for Organizational Domains under any of the following conditions:¶

• The 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 Tree Walk for that domain. In this case, the DMARC mechanism does not apply to the message in question.¶
• The record for the RFC5322.From domain indicates strict alignment. In this case, a simple string comparison of 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.¶

For each Tree Walk that retrieved valid DMARC records, 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, other than the one for the domain where the tree walk started, contains the psd= tag set to 'y' (psd=y), the Organizational Domain is the domain one label below this one in the DNS hierarchy, and the selection process is complete.¶

3. Otherwise, select the 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 yields a DMARC record at "_dmarc.mail.example.com" with the psd= tag set to 'n', then the Organizational Domain for this domain would be "mail.example.com".¶

As a last example, given the starting domain "a.mail.example.com", if a search for the Organizational Domain only yields a DMARC record at "_dmarc.com" and that record contains the tag psd=y, then the Organizational Domain for this domain would be "example.com".¶

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, specifically an isolated TXT record that is restricted to the DMARC context. Using the DNS as the query service has the benefit of reusing an extremely well-established operations, administration, and management infrastructure, rather than creating a new one.¶

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

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.¶

5.3. General Record Format

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

Section 9 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 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 requires strict or relaxed SPF Identifier Alignment mode. See Section 4.4.2 for details. Valid values are as follows:¶

r:

relaxed mode¶

s:

strict mode¶

fo:

Failure reporting options (plain-text; OPTIONAL; default is "0") Provides requested options for the generation of failure reports. Report generators may choose to adhere to the requested options. This tag's content MUST be ignored if a "ruf" tag (below) is not also specified. Failure reporting options are shown below. The value of this tag is either "0", "1", or a colon-separated list of the options represented by alphabetic characters. The valid values and their meanings are:¶

0:

Generate a DMARC failure report if all underlying authentication mechanisms fail to produce an aligned "pass" result.¶

1:

Generate a DMARC failure report if any underlying authentication mechanism produced something other than an aligned "pass" result.¶

d:

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

s:

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

np:

Domain Owner Assessment Policy for non-existent subdomains (plain-text; OPTIONAL). Indicates the message handling preference of the Domain Owner or PSO for mail using non-existent subdomains of the domain queried but not passing DMARC verification. 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 of the Domain Owner or PSO for mail using its domain but not passing DMARC verification. The policy applies to the domain queried and to subdomains, unless the subdomain policy is explicitly described using the "sp" or "np" tags. If this tag is not present in an otherwise syntactically valid DMARC record, then the record is treated as if it included "p=none" (see Section 4.7). 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 8.3 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 include this tag with a value of 'y' to indicate that the domain is a PSD. If a record containing this tag with a value of 'y' is found during policy discovery, this information will be used to determine the Organizational Domain and policy domain applicable to the message in question.¶

n:

The DMARC policy record is published for a 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 indicates 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). [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 11.6 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 [I-D.ietf-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 is requesting Mail Receivers to send detailed failure reports about messages that fail the DMARC evaluation in specific ways (see the "fo" tag above). [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. 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 11.6 for additional considerations.¶

sp:

Domain Owner Assessment Policy for all subdomains (plain-text; OPTIONAL). Indicates the message handling preference of the Domain Owner or PSO for mail using an existing subdomain of the domain queried but not passing DMARC verification. It applies only to subdomains of the domain queried and not to the domain itself. Its syntax is identical to that of the "p" tag defined above. If both the "sp" tag is absent, and the "np" tag is either absent or not applicable, the policy specified by the "p" tag MUST be applied for subdomains. Note that "sp" will be ignored for DMARC records published on subdomains of Organizational Domains due to the effect of the DMARC policy discovery mechanism described in Section 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 is 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 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.¶

5.4. Formal Definition

The formal definition of the DMARC 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] *WSP

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

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

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

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

  dmarc-yorn    = "y" / "n"

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

  dmarc-rors    = "r" / "s"

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

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

"Keyword" is imported from Section 4.1.2 of [RFC5321].¶

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

5.5. Domain Owner Actions

This section describes Domain Owner actions to implement the DMARC mechanism.¶

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").¶

5.7. Mail Receiver Actions

This section describes receiver actions in the DMARC environment.¶

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 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". In particular, because of the considerations discussed in [RFC7960] and in Section 8.6 of this document, it is important that Mail Receivers not reject messages solely because of a published policy of "reject", but that they apply other knowledge and analysis to avoid situations such as rejection of legitimate messages sent in ways that DMARC cannot describe, harm to the operation of mailing lists, and similar.¶

If they choose 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 failing mail.¶

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].¶

The 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) [RFC5617], 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 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" MUST 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 MUST NOT affect DMARC reporting.¶

7.1. IETF Category

RFC 7489 was not an Internet Standards Track specification; it was instead published in the Informational Category. This document, by contrast, is intended to be Internet Standards Track.¶

7.2. Changes to Terminology and Definitions

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

7.3. Policy Discovery and Organizational Domain Determination

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

The DNS Tree Walk also incorporates PSD policy discovery, which was introduced in [RFC9091]. [RFC9091] was an Experimental RFC, and the results of that experiment were that the RFC was not implemented as written. Instead, this document redefines the algorithm for PSD policy discovery, and thus obsoletes [RFC9091].¶

7.4. Reporting

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

7.5. Tags

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

7.6. Expansion of Domain Owner Actions Section

This section has been expanded upon from RFC 7489.¶

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

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

In particular, this document makes explicit that domains for general-purpose email MUST NOT deploy a DMARC policy of p=reject.¶

7.7. 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 5.2 says:¶

  A report SHOULD be sent to each listed URI provided in the DMARC
  record.

7.8. 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.¶

8.1. 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 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.¶

8.2. 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. If Domain Owners or PSOs publish a DNS record with a very short TTL, the injection of large volumes of messages could cause Receivers 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. 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.¶

8.3. 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 the annoying generation of such failure reports that go back to the RFC5321.MailFrom address.¶

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

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

• A "silent discard", wherein the SMTP server returns a 2xy reply code implying to the client that delivery (or, at least, relay) was successfully completed, but then simply discarding the message with no further action.¶

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.¶

8.4. Identifier Alignment Considerations

The DMARC mechanism allows both DKIM and SPF-authenticated identifiers to authenticate email on behalf of a Domain Owner and, possibly, on behalf of different subdomains. If malicious or unaware users can gain control of the SPF record or DKIM selector records for a subdomain, the subdomain can be used to generate DMARC-passing email on behalf of the Organizational Domain.¶

For example, an attacker who controls the SPF record for "evil.example.com" can send mail with an RFC5322.From header field containing "foo@example.com" that can pass both authentication and the DMARC check against "example.com".¶

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

8.5. 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.¶

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.¶

8.6. 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.¶

Even a domain that expects to send only targeted messages to account holders - a bank, for example - could have account holders using addresses such as jones@alumni.example.edu (an address that relays the messages to another address with a real mailbox) or finance@association.example (a role-based address that does similar relaying for the current head of finance at the association). When such mail is delivered to the actual recipient mailbox, it will necessarily fail SPF checks, as the incoming IP address will be that of example.edu or association.example, and not an address authorized for the sending domain. DKIM signatures will generally remain valid in these relay situations.¶

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

Domains that have general users who send routine email are particularly likely to create interoperability issues if they publish p=reject. For example, domains that serve as mailbox hosts and give out email addresses to the general public are best advised to delay adoption of p=reject until the authentication ecosystem becomes more mature and deliverability issues are better resolved.¶

In particular, if users in p=reject domains post messages to mailing lists on the Internet, those messages can cause operational problems for the mailing lists and for the subscribers to those lists, as explained below and in [RFC7960].¶

It is therefore critical that domains that host users who might post messages to mailing lists SHOULD NOT publish p=reject. Domains that choose to publish p=reject SHOULD implement policies that their users not post to Internet mailing lists.¶

As noted in Section 5.8, receiving domains need to apply more analysis than just DMARC evaluation in their disposition of incoming messages. An example of the consequences of honoring p=reject without further anaysis is that rejecting messages that have been relayed by a mailing list can cause your own users to have their subscriptions to that mailing list cancelled by the list software's automated handling of such rejections - it looks to the list manager as though the recipient's email address is no longer working, so the address is automatically unsubscribed.¶

It is therefore critical that receiving domains MUST NOT reject incoming messages solely on the basis of a p=reject policy by the sending domain. Receiving domains must use the DMARC policy as part of their disposition decision, along with other knowledge and analysis.¶

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

9.1. Authentication-Results Method Registry Update

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

9.2. Authentication-Results Result Registry Update

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

9.3. Feedback Report Header Fields Registry Update

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

Field Name: Identity-Alignment¶

Description:

indicates whether the message about which a report is being generated had any identifiers in alignment as defined in RFC 7489¶

Multiple Appearances: No¶

Related "Feedback-Type": auth-failure¶

Reference: RFC 7489¶

Status: current¶

9.4. DMARC Tag Registry

A new registry tree called "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters" has been created. Within it, a new sub-registry called the "DMARC Tag Registry" has been created.¶

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

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

The initial set of entries in this registry is as follows:¶

9.5. DMARC Report Format Registry

Also, within "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters", a new sub-registry called "DMARC Report Format Registry" has been created.¶

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

The initial entry in this registry is as follows:¶

9.6. 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:¶

10.1. Aggregate Report Considerations

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

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

10.2. Failure Report Considerations

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

Out of band agreements between failure report senders and receivers may be required to address privacy concerns.¶

DMARC records for multi-organizational PSDs MUST NOT include the ruf= tag.¶

11.1. Authentication Methods

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

11.2. Attacks on Reporting URIs

URIs published in DNS TXT records are well-understood possible targets for attack. Specifications such as [RFC1035] and [RFC2142] either expose or cause the exposure of email addresses that could be flooded by an attacker, for example. 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.¶

11.3. 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 verify 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.¶

11.4. Display Name Attacks

A common attack in messaging abuse is the presentation of false information in the display-name portion of the RFC5322.From header field. For example, it is possible for the email address in that field to be an arbitrary address or domain name while containing a well-known name (a person, brand, role, etc.) in the display name, intending to fool the end user into believing that the name is used legitimately. The attack is predicated on the notion that most common MUAs will show the display name and not the email address when both are available.¶

Generally, display name attacks are out of scope for DMARC, as further exploration of possible defenses against these attacks needs to be undertaken.¶

There are a few possible mechanisms that attempt mitigation of these attacks, such as the following:¶

• If the display name includes an email address (as specified in [RFC5322]), execute the DMARC mechanism on the domain name found there rather than the original domain name. 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¶

• 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.¶

11.5. Denial of DMARC Processing Attacks

The declaration in Section 5.7.1 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), care must be taken by the receiving MTA to recognize such messages as the threats they might be and handle them appropriately.¶

11.6. 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 verifying approved external reporting.¶

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

Where possible, external reporting is best achieved by having the report be directed to domains that can receive mail and simply having it automatically forwarded to the desired external destination.¶

Note that the addresses shown in the "ruf" tag receive more information that might be considered private data since it is possible for actual email content to appear in the failure reports. The URIs identified there are thus more attractive targets for intrusion attempts than those found in the "rua" tag. Moreover, attacking the DNS of the subject domain to cause failure data to be routed fraudulently to an attacker's systems may be an attractive prospect. Deployment of [RFC4033] is advisable if this is a concern.¶

11.7. 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.¶

11.8. Determination of the Organizational Domain For Relaxed Alignment

DMARC evaluation for relaxed alignment is highly sensitive to errors in determining the organizational domain if the RFC5322.From domain does not have a published policy. If an incorrectly selected organizational domain is a parent of the correct organizational domain, then relaxed alignment could potentially allow a malicious sender to obtain DMARC PASS. This potential exists for both the legacy [RFC7489] and current methods for determining the organizational domain, the latter described in Section 4.8.¶

This issue is entirely avoided by the use of strict alignment and publishing DMARC records for all domains/sub-domains used as RFC5322.From domain in an organization's email.¶

For cases where strict alignment is not appropriate, this issue can be mitigated by periodically checking the DMARC records, if any, of PSDs above the organization's domains in the DNS tree and (for legacy [RFC7489] checking that appropriate PSL entries remain present). If a PSD domain publishes a DMARC record without the appropriate psd=y tag, organizational domain owners can add psd=n to their organizational domain's DMARC record so that the PSD record will not be incorrectly evaluated to be the organizational domain¶

A.1. S/MIME

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

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

S/MIME also suffers from the heavyweight problem of Public Key Infrastructure, which means that distribution of keys used to 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 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 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 target audience for DMARC does not appear to have concerns about the failure modes of one or the other being a barrier to DMARC's adoption.¶

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

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

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

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

A.3. Sender Header Field

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

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

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

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

3. Allowing multiple ways to discover policy introduces unacceptable ambiguity into the DMARC evaluation 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 protocol 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 also consistent with the process documented in [RFC5617] (ADSP), and is a common practice among MTA operators to determine whether or not to accept a mail message before performing other more expensive processing.¶

The DMARC protocol 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. 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, ones 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.¶

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. 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.¶

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 a 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.¶

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.¶

     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=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

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.¶

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com"

  ; DMARC record for the domain example.com

  _dmarc  IN TXT ( "v=DMARC1; p=none; "
                   "rua=mailto:dmarc-feedback@example.com" )

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
   ruf=mailto:auth-reports@example.com"

  ; 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" )

  % dig +short TXT _dmarc.example.com.
  "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
   ruf=mailto:auth-reports@thirdparty.example.net"

  ; 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" )

  % dig +short TXT example.com._report._dmarc.thirdparty.example.net
  "v=DMARC1;"

  ; zone file for thirdparty.example.net
  ; Accept DMARC failure reports on behalf of example.com

  example.com._report._dmarc   IN   TXT    "v=DMARC1;"

  % 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"

  ; 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" )

  % dig +short TXT _dmarc.test.example.com
  "v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com,
   mailto:tld-test@thirdparty.example.net"

  ; 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).¶

  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"

B.4. Organizational and Policy Domain Tree Walk Examples

If an RFC5322.From domain has no DMARC record, a receiver uses a tree walk to find the policy domain.¶

If the policy in a policy domain allows relaxed alignment and the SPF or DKIM domains are different from the RFC5322.From domain, a receiver uses a tree walk to discover the respective Organizational domains.¶

B.5. 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.¶