DKIM], which provides a domain-level identifier in the content of the "d=" tag of a validated DKIM-Signature header field. o [SPF], which can authenticate both the domain found in an [SMTP] HELO/EHLO command (the HELO identity) and the domain found in an SMTP MAIL command (the MAIL FROM identity). DMARC uses the result of SPF authentication of the MAIL FROM identity. Section 2.4 of [SPF] describes MAIL FROM processing for cases in which the MAIL command has a null path. RFC5322.From field domain is aligned with (matches) an authenticated domain name from SPF or DKIM. When a DMARC policy is published for the domain name found in the RFC5322.From field, and that domain name is not validated through SPF or DKIM, the disposition of that message can be affected by that DMARC policy when delivered to a participating receiver. It is important to note that the authentication mechanisms employed by DMARC authenticate only a DNS domain and do not authenticate the local-part of any email address identifier found in a message, nor do they validate the legitimacy of message content. DMARC's feedback component involves the collection of information about received messages claiming to be from the Organizational Domain for periodic aggregate reports to the Domain Owner. The parameters and format for such reports are discussed in later sections of this document. A DMARC-enabled Mail Receiver might also generate per-message reports that contain information related to individual messages that fail SPF and/or DKIM. Per-message failure reports are a useful source of information when debugging deployments (if messages can be determined
to be legitimate even though failing authentication) or in analyzing attacks. The capability for such services is enabled by DMARC but defined in other referenced material such as [AFRF]. A message satisfies the DMARC checks if at least one of the supported authentication mechanisms: 1. produces a "pass" result, and 2. produces that result based on an identifier that is in alignment, as defined in Section 3.
In essence, the steps are as follows: 1. Domain Owner constructs an SPF policy and publishes it in its DNS database as per [SPF]. Domain Owner also configures its system for DKIM signing as described in [DKIM]. Finally, Domain Owner publishes via the DNS a DMARC message-handling policy. 2. Author generates a message and hands the message to Domain Owner's designated mail submission service. 3. Submission service passes relevant details to the DKIM signing module in order to generate a DKIM signature to be applied to the message. 4. Submission service relays the now-signed message to its designated transport service for routing to its intended recipient(s). 5. Message may pass through other relays but eventually arrives at a recipient's transport service. 6. Recipient delivery service conducts SPF and DKIM authentication checks by passing the necessary data to their respective modules, each of which requires queries to the Author Domain's DNS data (when identifiers are aligned; see below). 7. The results of these are passed to the DMARC module along with the Author's domain. The DMARC module attempts to retrieve a policy from the DNS for that domain. If none is found, the DMARC module determines the Organizational Domain and repeats the attempt to retrieve a policy from the DNS. (This is described in further detail in Section 6.6.3.) 8. If a policy is found, it is combined with the Author's domain and the SPF and DKIM results to produce a DMARC policy result (a "pass" or "fail") and can optionally cause one of two kinds of reports to be generated (not shown). 9. Recipient transport service either delivers the message to the recipient inbox or takes other local policy action based on the DMARC result (not shown). 10. When requested, Recipient transport service collects data from the message delivery session to be used in providing feedback (see Section 7).
RFC5322.From address, which is part of a body of data that has been trivially forged throughout the history of email. Several points suggest that it is the most correct and safest thing to do in this context: o Of all the identifiers that are part of the message itself, this is the only one guaranteed to be present. o It seems the best choice of an identifier on which to focus, as most MUAs display some or all of the contents of that field in a manner strongly suggesting those data as reflective of the true originator of the message. The absence of a single, properly formed RFC5322.From field renders the message invalid. Handling of such a message is outside of the scope of this specification. Since the sorts of mail typically protected by DMARC participants tend to only have single Authors, DMARC participants generally operate under a slightly restricted profile of RFC5322 with respect to the expected syntax of this field. See Section 6.6 for details. Section 6.1) to those domains. In doing so, Domain Owners make specific requests of Mail Receivers regarding the disposition of messages purporting to be from one of the Domain Owner's domains and the provision of feedback about those messages. A Domain Owner may choose not to participate in DMARC evaluation by Mail Receivers. In this case, the Domain Owner simply declines to advertise participation in those schemes. For example, if the results of path authorization checks ought not be considered as part of the overall DMARC result for a given Author Domain, then the Domain Owner does not publish an SPF policy record that can produce an SPF pass result.
A Mail Receiver implementing the DMARC mechanism SHOULD make a best-effort attempt to adhere to the Domain Owner's published DMARC policy when a message fails the DMARC test. Since email streams can be complicated (due to forwarding, existing RFC5322.From domain-spoofing services, etc.), Mail Receivers MAY deviate from a Domain Owner's published policy during message processing and SHOULD make available the fact of and reason for the deviation to the Domain Owner via feedback reporting, specifically using the "PolicyOverride" feature of the aggregate report (see Section 7.2). RFC5322.From domain of "example.com" would issue a TXT query to the DNS for the subdomain of "_dmarc.example.com". The DNS-located DMARC preference data will hereafter be called the "DMARC record". DMARC's use of the Domain Name Service is driven by DMARC's use of domain names and the nature of the query it performs. The query requirement matches with the DNS, for obtaining simple parametric information. It uses an established method of storing the information, associated with the target domain name, namely an isolated TXT record that is restricted to the DMARC context. Use of the DNS as the query service has the benefit of reusing an extremely well-established operations, administration, and management infrastructure, rather than creating a new one. Per [DNS], 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. URI] defines a generic syntax for identifying a resource. The DMARC mechanism uses this as the format by which a Domain Owner specifies the destination for the two report types that are supported. The place such URIs are specified (see Section 6.3) allows a list of these to be provided. A report is normally sent to each listed URI in the order provided by the Domain Owner. 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. The list of URIs is separated by commas (ASCII 0x2C).
Each URI can have associated with it a maximum report size that may be sent to it. This is accomplished by appending an exclamation point (ASCII 0x21), followed by a maximum-size indication, before a separating comma or terminating semicolon. Thus, a DMARC URI is a URI within which any commas or exclamation points are percent-encoded per [URI], followed by an OPTIONAL exclamation point and a maximum-size specification, and, if there are additional reporting URIs in the list, a comma and the next URI. For example, the URI "mailto:email@example.com!50m" would request that a report be sent via email to "firstname.lastname@example.org" so long as the report payload does not exceed 50 megabytes. A formal definition is provided in Section 6.4. DKIM]. Section 11 creates a registry for known DMARC tags and registers the initial set defined in this document. Only tags defined in this document or in later extensions, and thus added to that registry, are to be processed; unknown tags MUST be ignored. The following tags are introduced as the initial valid DMARC tags: adkim: (plain-text; OPTIONAL; default is "r".) Indicates whether strict or relaxed DKIM Identifier Alignment mode is required by the Domain Owner. See Section 3.1.1 for details. Valid values are as follows: r: relaxed mode s: strict mode aspf: (plain-text; OPTIONAL; default is "r".) Indicates whether strict or relaxed SPF Identifier Alignment mode is required by the Domain Owner. See Section 3.1.2 for details. Valid values are as follows: r: relaxed mode s: strict mode
fo: Failure reporting options (plain-text; OPTIONAL; default is "0") Provides requested options for generation of failure reports. Report generators MAY choose to adhere to the requested options. This tag's content MUST be ignored if a "ruf" tag (below) is not also specified. The value of this tag is a colon-separated list of characters that indicate failure reporting options as follows: 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 [AFRF-DKIM]. s: Generate an SPF failure report if the message failed SPF evaluation, regardless of its alignment. SPF-specific reporting is described in [AFRF-SPF]. p: Requested Mail Receiver policy (plain-text; REQUIRED for policy records). Indicates the policy to be enacted by the Receiver at the request of the Domain Owner. Policy applies to the domain queried and to subdomains, unless subdomain policy is explicitly described using the "sp" tag. This tag is mandatory for policy records only, but not for third-party reporting records (see Section 7.1). Possible values are as follows: none: The Domain Owner requests no specific action be taken regarding delivery of messages. quarantine: The Domain Owner wishes to have email that fails the DMARC mechanism check be treated by Mail Receivers as suspicious. Depending on the capabilities of the Mail Receiver, this can mean "place into spam folder", "scrutinize with additional intensity", and/or "flag as suspicious". reject: The Domain Owner wishes for Mail Receivers to reject email that fails the DMARC mechanism check. Rejection SHOULD occur during the SMTP transaction. See Section 10.3 for some discussion of SMTP rejection methods and their implications. pct: (plain-text integer between 0 and 100, inclusive; OPTIONAL; default is 100). Percentage of messages from the Domain Owner's mail stream to which the DMARC policy is to be applied. However,
this MUST NOT be applied to the DMARC-generated reports, all of which must be sent and received unhindered. The purpose of the "pct" tag is to allow Domain Owners to enact a slow rollout enforcement of the DMARC mechanism. The prospect of "all or nothing" is recognized as preventing many organizations from experimenting with strong authentication-based mechanisms. See Section 6.6.4 for details. Note that random selection based on this percentage, such as the following pseudocode, is adequate: if (random mod 100) < pct then selected = true else selected = false rf: Format to be used for message-specific failure reports (colon- separated plain-text list of values; OPTIONAL; default is "afrf"). The value of this tag is a list of one or more report formats as requested by the Domain Owner to be used when a message fails both [SPF] and [DKIM] tests to report details of the individual failure. The values MUST be present in the registry of reporting formats defined in Section 11; a Mail Receiver observing a different value SHOULD ignore it or MAY ignore the entire DMARC record. For this version, only "afrf" (the auth-failure report type defined in [AFRF]) is presently supported. See Section 7.3 for details. For interoperability, the Authentication Failure Reporting Format (AFRF) MUST be supported. ri: Interval requested between aggregate reports (plain-text 32-bit unsigned integer; OPTIONAL; default is 86400). Indicates a request to Receivers to generate aggregate reports separated by no more than the requested number of seconds. DMARC implementations MUST be able to provide daily reports and SHOULD be able to provide hourly reports when requested. However, anything other than a daily report is understood to be accommodated on a best- effort basis. rua: Addresses to which aggregate feedback is to be sent (comma- separated plain-text list of DMARC URIs; OPTIONAL). A comma or exclamation point that is part of such a DMARC URI MUST be encoded per Section 2.1 of [URI] so as to distinguish it from the list delimiter or an OPTIONAL size limit. Section 7.1 discusses considerations that apply when the domain name of a URI differs from that of the domain advertising the policy. See Section 12.5 for additional considerations. Any valid URI can be specified. A Mail Receiver MUST implement support for a "mailto:" URI, i.e., the ability to send a DMARC report via electronic mail. If not
provided, Mail Receivers MUST NOT generate aggregate feedback reports. URIs not supported by Mail Receivers MUST be ignored. The aggregate feedback report format is described in Section 7.2. 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). The format of the message to be generated MUST follow the format specified for the "rf" tag. Section 7.1 discusses considerations that apply when the domain name of a URI differs from that of the domain advertising the policy. A Mail Receiver MUST implement support for a "mailto:" URI, i.e., the ability to send a DMARC report via electronic mail. If not provided, Mail Receivers MUST NOT generate failure reports. See Section 12.5 for additional considerations. sp: Requested Mail Receiver policy for all subdomains (plain-text; OPTIONAL). Indicates the policy to be enacted by the Receiver at the request of the Domain Owner. 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 absent, the policy specified by the "p" tag MUST be applied for subdomains. Note that "sp" will be ignored for DMARC records published on subdomains of Organizational Domains due to the effect of the DMARC policy discovery mechanism described in Section 6.6.3. v: Version (plain-text; REQUIRED). Identifies the record retrieved as a DMARC record. It MUST have the value of "DMARC1". The value of this tag MUST match precisely; if it does not or it is absent, the entire retrieved record MUST be ignored. It MUST be the first tag in the list. A DMARC policy record MUST comply with the formal specification found in Section 6.4 in that the "v" and "p" tags MUST be present and MUST appear in that order. Unknown tags MUST be ignored. Syntax errors in the remainder of the record SHOULD be discarded in favor of default values (if any) or ignored outright. Note that given the rules of the previous paragraph, addition of a new tag into the registered list of tags does not itself require a new version of DMARC to be generated (with a corresponding change to the "v" tag's value), but a change to any existing tags does require a new version of DMARC.
ABNF], is as follows: dmarc-uri = URI [ "!" 1*DIGIT [ "k" / "m" / "g" / "t" ] ] ; "URI" is imported from [URI]; commas (ASCII ; 0x2C) and exclamation points (ASCII 0x21) ; MUST be encoded; the numeric portion MUST fit ; within an unsigned 64-bit integer dmarc-record = dmarc-version dmarc-sep [dmarc-request] [dmarc-sep dmarc-srequest] [dmarc-sep dmarc-auri] [dmarc-sep dmarc-furi] [dmarc-sep dmarc-adkim] [dmarc-sep dmarc-aspf] [dmarc-sep dmarc-ainterval] [dmarc-sep dmarc-fo] [dmarc-sep dmarc-rfmt] [dmarc-sep dmarc-percent] [dmarc-sep] ; components other than dmarc-version and ; dmarc-request may appear in any order dmarc-version = "v" *WSP "=" *WSP %x44 %x4d %x41 %x52 %x43 %x31 dmarc-sep = *WSP %x3b *WSP dmarc-request = "p" *WSP "=" *WSP ( "none" / "quarantine" / "reject" ) dmarc-srequest = "sp" *WSP "=" *WSP ( "none" / "quarantine" / "reject" ) dmarc-auri = "rua" *WSP "=" *WSP dmarc-uri *(*WSP "," *WSP dmarc-uri) dmarc-furi = "ruf" *WSP "=" *WSP dmarc-uri *(*WSP "," *WSP dmarc-uri) dmarc-adkim = "adkim" *WSP "=" *WSP ( "r" / "s" ) dmarc-aspf = "aspf" *WSP "=" *WSP ( "r" / "s" )
dmarc-ainterval = "ri" *WSP "=" *WSP 1*DIGIT dmarc-fo = "fo" *WSP "=" *WSP ( "0" / "1" / "d" / "s" ) *(*WSP ":" *WSP ( "0" / "1" / "d" / "s" )) dmarc-rfmt = "rf" *WSP "=" *WSP Keyword *(*WSP ":" Keyword) ; registered reporting formats only dmarc-percent = "pct" *WSP "=" *WSP 1*3DIGIT "Keyword" is imported from Section 4.1.2 of [SMTP]. A size limitation in a dmarc-uri, if provided, is interpreted as a count of units followed by an OPTIONAL unit size ("k" for kilobytes, "m" for megabytes, "g" for gigabytes, "t" for terabytes). Without a unit, the number is presumed to be a basic byte count. Note that the units are considered to be powers of two; a kilobyte is 2^10, a megabyte is 2^20, etc. DKIM] (see also [DKIM-OVERVIEW] and [DKIM-DEPLOYMENT]) and [SPF].
RFC5322.From field is extracted as the domain to be evaluated by DMARC. If the domain is encoded with UTF-8, the domain name must be converted to an A-label, as described in Section 2.3 of [IDNA], for further processing. In order to be processed by DMARC, a message typically needs to contain exactly one RFC5322.From domain (a single From: field with a single domain in it). Not all messages meet this requirement, and handling of them is outside of the scope of this document. Typical exceptions, and the way they have been historically handled by DMARC participants, are as follows: o Messages with multiple RFC5322.From fields are typically rejected, since that form is forbidden under RFC 5322 [MAIL]; o Messages bearing a single RFC5322.From field containing multiple addresses (and, thus, multiple domain names to be evaluated) are typically rejected because the sorts of mail normally protected by DMARC do not use this format; o Messages that have no RFC5322.From field at all are typically rejected, since that form is forbidden under RFC 5322 [MAIL]; o Messages with an RFC5322.From field that contains no meaningful domains, such as RFC 5322 [MAIL]'s "group" syntax, are typically ignored. The case of a syntactically valid multi-valued RFC5322.From field presents a particular challenge. The process in this case is to apply the DMARC check using each of those domains found in the RFC5322.From field as the Author Domain and apply the most strict policy selected among the checks that fail.
RFC5322.From domain from the message (as above). 2. Query the DNS for a DMARC policy record. Continue if one is found, or terminate DMARC evaluation otherwise. See Section 6.6.3 for details. 3. Perform DKIM signature verification checks. A single email could contain multiple DKIM signatures. The results of this step are passed to the remainder of the algorithm and MUST include the value of the "d=" tag from each checked DKIM signature. 4. Perform SPF validation checks. The results of this step are passed to the remainder of the algorithm and MUST include the domain name used to complete the SPF check. 5. Conduct Identifier Alignment checks. With authentication checks and policy discovery performed, the Mail Receiver checks to see if Authenticated Identifiers fall into alignment as described in Section 3. If one or more of the Authenticated Identifiers align with the RFC5322.From domain, the message is considered to pass the DMARC mechanism check. All other conditions (authentication failures, identifier mismatches) are considered to be DMARC mechanism check failures. 6. Apply policy. Emails that fail the DMARC mechanism check are disposed of in accordance with the discovered DMARC policy of the Domain Owner. See Section 6.3 for details. Heuristics applied in the absence of use by a Domain Owner of either SPF or DKIM (e.g., [Best-Guess-SPF]) SHOULD NOT be used, as it may be the case that the Domain Owner wishes a Message Receiver not to consider the results of that underlying authentication protocol at all. DMARC evaluation can only yield a "pass" result after one of the underlying authentication mechanisms passes for an aligned identifier. If neither passes and one or both of them fail due to a temporary error, the Receiver evaluating the message is unable to conclude that the DMARC mechanism had a permanent failure; they
therefore cannot apply the advertised DMARC policy. When otherwise appropriate, Receivers MAY send feedback reports regarding temporary errors. Handling of messages for which SPF and/or DKIM evaluation encounter a permanent DNS error is left to the discretion of the Mail Receiver. RFC5322.From domain in the message. A possibly empty set of records is returned. 2. Records that do not start with a "v=" tag that identifies the current version of DMARC are discarded. 3. If the set is now empty, the Mail Receiver MUST query the DNS for a DMARC TXT record at the DNS domain matching the Organizational Domain in place of the RFC5322.From domain in the message (if different). This record can contain policy to be asserted for subdomains of the Organizational Domain. A possibly empty set of records is returned. 4. Records that do not start with a "v=" tag that identifies the current version of DMARC are discarded. 5. If the remaining set contains multiple records or no records, policy discovery terminates and DMARC processing is not applied to this message.
6. If a retrieved policy record does not contain a valid "p" tag, or contains an "sp" tag that is not valid, then: 1. if a "rua" tag is present and contains at least one syntactically valid reporting URI, the Mail Receiver SHOULD act as if a record containing a valid "v" tag and "p=none" was retrieved, and continue processing; 2. otherwise, the Mail Receiver applies no DMARC processing to this message. If the set produced by the mechanism above contains no DMARC policy record (i.e., any indication that there is no such record as opposed to a transient DNS error), Mail Receivers SHOULD NOT apply the DMARC mechanism to the message. Handling of DNS errors when querying for the DMARC policy record is left to the discretion of the Mail Receiver. For example, to ensure minimal disruption of mail flow, transient errors could result in delivery of the message ("fail open"), or they could result in the message being temporarily rejected (i.e., an SMTP 4yx reply), which invites the sending MTA to try again after the condition has possibly cleared, allowing a definite DMARC conclusion to be reached ("fail closed"). Section 10.3). If the email is not subject to the "reject" policy (due to the "pct" tag), the Mail Receiver SHOULD treat the email as though the "quarantine" policy applies. This behavior allows Domain Owners to experiment with progressively stronger policies without relaxing existing policy.
Mail Receivers implement "pct" via statistical mechanisms that achieve a close approximation to the requested percentage and provide a representative sample across a reporting period. Sections 6.3 and 7.2 discuss aggregate feedback. AUTH-RESULTS]) is RECOMMENDED when delivery of failing mail is done. When this is done, the DNS domain name thus recorded MUST be encoded as an A-label. Mail Receivers are only obligated to report reject or quarantine policy actions in aggregate feedback reports that are due to DMARC policy. They are not required to report reject or quarantine actions that are the result of local policy. If local policy information is exposed, abusers can gain insight into the effectiveness and delivery rates of spam campaigns. Final disposition of a message is always a matter of local policy. An operator that wishes to favor DMARC policy over SPF policy, for example, will disregard the SPF policy, since enacting an SPF-determined rejection prevents evaluation of DKIM; DKIM might otherwise pass, satisfying the DMARC evaluation. There is a trade-off to doing so, namely acceptance and processing of the entire message body in exchange for the enhanced protection DMARC provides. DMARC-compliant Mail Receivers typically disregard any mail-handling directive discovered as part of an authentication mechanism (e.g., Author Domain Signing Practices (ADSP), SPF) where a DMARC record is also discovered that specifies a policy other than "none". Deviating
from this practice introduces inconsistency among DMARC operators in terms of handling of the message. However, such deviation is not proscribed. To enable Domain Owners to receive DMARC feedback without impacting existing mail processing, discovered policies of "p=none" SHOULD NOT modify existing mail disposition processing. Mail Receivers SHOULD also implement reporting instructions of DMARC, even in the absence of a request for DKIM reporting [AFRF-DKIM] or SPF reporting [AFRF-SPF]. Furthermore, the presence of such requests SHOULD NOT affect DMARC reporting.