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RFC 8601

Message Header Field for Indicating Message Authentication Status

Pages: 54
Proposed STD
Obsoletes:  7601
Part 2 of 4 – Pages 10 to 23
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Top   ToC   RFC8601 - Page 10   prevText
2.  Definition and Format of the Header Field

   This section gives a general overview of the format of the header
   field being defined and then provides a formal specification.

2.1.  General Description

   The header field specified here is called "Authentication-Results".
   It is a structured header field as defined in "Internet Message
   Format" [MAIL], and thus all of the related definitions in that
   document apply.

   This header field is added at the top of the message as it transits
   MTAs that do authentication checks, so some idea of how far away the
   checks were done can be inferred.  It is therefore considered to be a
   trace field as defined in [MAIL], and thus all of the related
   definitions in that document apply.

   The value of the header field (after removing comments) consists of
   an authentication service identifier, an optional version, and then a
   series of statements and supporting data.  The statements are of the
   form "method=result" and indicate which authentication method or
   methods were applied and their respective results.  For each such
   statement, the supporting data can include a "reason" string and one
   or more "property=value" statements indicating which message
   properties were evaluated to reach that conclusion.
Top   ToC   RFC8601 - Page 11
   The header field can appear more than once in a single message, more
   than one result can be represented in a single header field, or a
   combination of these can be applied.

2.2.  Formal Definition

   Formally, the header field is specified as shown below using
   Augmented Backus-Naur Form [ABNF].  Examples of valid header fields
   with explanations of their semantics can be found in Appendix B.

     authres-header-field = "Authentication-Results:" authres-payload

     authres-payload = [CFWS] authserv-id
              [ CFWS authres-version ]
              ( no-result / 1*resinfo ) [CFWS] CRLF

     authserv-id = value
                 ; see below for a description of this element

     authres-version = 1*DIGIT [CFWS]
             ; indicates which version of this specification is in use;
             ; this specification is version "1", and the absence of a
             ; version implies this version of the specification

     no-result = [CFWS] ";" [CFWS] "none"
               ; the special case of "none" is used to indicate that no
               ; message authentication was performed

     resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
               [ CFWS 1*propspec ]

     methodspec = [CFWS] method [CFWS] "=" [CFWS] result
                ; indicates which authentication method was evaluated
                ; and what its output was

     reasonspec = "reason" [CFWS] "=" [CFWS] value
                ; a free-form comment on the reason the given result
                ; was returned

     propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
              ; an indication of which properties of the message
              ; were evaluated by the authentication scheme being
              ; applied to yield the reported result
Top   ToC   RFC8601 - Page 12
     method = Keyword [ [CFWS] "/" [CFWS] method-version ]
            ; a method indicates which method's result is
            ; represented by "result"; it is one of the methods
            ; explicitly defined as valid in this document
            ; or is an extension method as defined below

     method-version = 1*DIGIT [CFWS]
            ; indicates which version of the method specification is
            ; in use, corresponding to the matching entry in the IANA
            ; "Email Authentication Methods" registry; a value of "1"
            ; is assumed if this version string is absent

     result = Keyword
            ; indicates the results of the attempt to authenticate
            ; the message; see below for details

     ptype = Keyword
           ; indicates whether the property being evaluated was
           ; a parameter to an SMTP command [SMTP], was a value taken
           ; from a message header field, was some property of
           ; the message body, or was some other property evaluated by
           ; the receiving MTA; expected to be one of the "property
           ; types" explicitly defined as valid, or an extension
           ; ptype, as defined below

     property = special-smtp-verb / Keyword
             ; indicates more specifically than "ptype" what the
             ; source of the evaluated property is; the exact meaning
             ; is specific to the method whose result is being reported
             ; and is defined more clearly below

     special-smtp-verb = "mailfrom" / "rcptto"
             ; special cases of SMTP commands [SMTP] that are made up
             ; of multiple words

     pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
              [CFWS]
            ; the value extracted from the message property defined
            ; by the "ptype.property" construction

   "local-part" is defined in Section 3.4.1 of [MAIL], as modified by
   [RFC6531].

   "CFWS" is defined in Section 3.2.2 of [MAIL].
Top   ToC   RFC8601 - Page 13
   "Keyword" is defined in Section 4.1.2 of [SMTP].  It is further
   constrained by the necessity of being registered in the Internet
   Assigned Numbers Authority (IANA) registry relevant to the context in
   which it is used.  See Sections 2.3, 2.7, and 6.

   The "value" is as defined in Section 5.1 of [MIME], with
   "quoted-string" updated as specified in [RFC6532].

   The "domain-name" is as defined in Section 3.5 of [DKIM].

   See Section 2.5 for a description of the authserv-id element.

   If the value portion of a "pvalue" construction identifies something
   intended to be an email identity, then it MUST use the right-hand
   portion of that ABNF definition.

   The list of commands eligible for use with the "smtp" ptype can be
   found in Section 4.1 of [SMTP].

   The "propspec" may be omitted if, for example, the method was unable
   to extract any properties to do its evaluation yet still has a result
   to report.  It may also be omitted if the agent generating this
   result wishes not to reveal such properties to downstream agents.

   Where an SMTP command name is being reported as a "property", the
   agent generating the header field represents that command by
   converting it to lowercase and dropping any spaces (e.g., "MAIL FROM"
   becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).

   A "ptype" value of "policy" indicates a policy decision about the
   message not specific to a property of the message that could be
   extracted.  See Section 2.4 for details.

   Examples of complete messages using this header field can be found in
   Appendix B.

2.3.  Property Types (ptypes) and Properties

   The "ptype" in the ABNF above indicates the general type of property
   being described by the result being reported, upon which the reported
   result was based.  Coupled with the "property", which is more
   specific, it indicates from where the reported "pvalue" was
   extracted.  This can include a particular part of the message header
   or body, some part of the SMTP session, a secondary output of an
   authentication method (apart from its pure result), or some other
   aspect of the message's handling.
Top   ToC   RFC8601 - Page 14
   Combinations of ptypes and properties are registered and described in
   the "Email Authentication Methods" registry, coupled with the
   authentication methods with which they are used.  This is further
   described in Section 6.

   Legal values of "ptype" are as defined in the IANA "Email
   Authentication Property Types" registry, created by [RFC7410].  The
   initial values and what they typically indicate are as follows, based
   on [RFC7001]:

   body:  Information that was extracted from the body of the message.
      This might be an arbitrary string of bytes, a hash of a string of
      bytes, a Uniform Resource Identifier, or some other content of
      interest.  The "property" is an indication of where within the
      message body the extracted content was found and can indicate an
      offset, identify a MIME part, etc.  (At the time of this revision,
      no properties matching this ptype have been registered.
      Accordingly, this ptype may be deprecated in the future.)

   header:  Indicates information that was extracted from the header of
      the message.  This might be the value of a header field or some
      portion of a header field.  The "property" gives a more precise
      indication of the place in the header from which the extraction
      took place.

   policy:  A local policy mechanism was applied that augments or
      overrides the result returned by the authentication mechanism.
      (See Section 2.4.)

   smtp:  Indicates information that was extracted from an SMTP command
      that was used to relay the message.  The "property" indicates
      which SMTP command included the extracted content as a parameter.

   Results reported using unknown ptypes MUST NOT be used in making
   handling decisions.  They can be safely ignored by consumers.

   Entries in the "Email Authentication Methods" registry can define
   properties that deviate from these definitions when appropriate.
   Such deviations need to be clear in the registry and/or in the
   defining document.  See Section 2.7.1 for an example.
Top   ToC   RFC8601 - Page 15
2.4.  The "policy" ptype

   A special ptype value of "policy" is also defined.  This ptype is
   provided to indicate that some local policy mechanism was applied
   that augments or even replaces (i.e., overrides) the result returned
   by the authentication mechanism.  The property and value in this case
   identify the local policy that was applied and the result it
   returned.

   For example, a DKIM signature is not required to include the Subject
   header field in the set of fields that are signed.  An ADMD receiving
   such a message might decide that such a signature is unacceptable,
   even if it passes, because the content of the Subject header field
   could be altered post-signing without invalidating the signature.
   Such an ADMD could replace the DKIM "pass" result with a "policy"
   result and then also include the following in the corresponding
   Authentication-Results field:

      ... dkim=policy policy.dkim-rules=unsigned-subject ...

   In this case, the property is "dkim-rules", indicating that some
   local check by that name took place and that check returned a result
   of "unsigned-subject".  These are arbitrary names selected by (and
   presumably used within) the ADMD making use of them, so they are not
   normally registered with IANA or otherwise specified apart from
   setting syntax restrictions that allow for easy parsing within the
   rest of the header field.

   This ptype existed in the original specification for this header
   field [RFC5451], but without a complete description or example of
   intended use.  As a result, it has not seen any practical use to date
   that matches its intended purpose.  These added details are provided
   to guide implementers toward proper use.

2.5.  Authentication Service Identifier Field

   Every Authentication-Results header field has an authentication
   service identifier field (authserv-id above).  Specifically, this is
   any string intended to identify the authentication service within the
   ADMD that conducted authentication checks on the message.  This
   identifier is intended to be machine-readable and not necessarily
   meaningful to users.

   Note that in an EAI-formatted message, this identifier may be
   expressed in UTF-8.
Top   ToC   RFC8601 - Page 16
   Since agents consuming this field will use this identifier to
   determine whether its contents are of interest (and are safe to use),
   the uniqueness of the identifier MUST be guaranteed by the ADMD that
   generates it and MUST pertain to that ADMD.  MUAs or downstream
   filters SHOULD use this identifier to determine whether or not the
   data contained in an Authentication-Results header field ought to be
   used or ignored.

   For simplicity and scalability, the authentication service identifier
   SHOULD be a common token used throughout the ADMD.  Common practice
   is to use the DNS domain name used by or within that ADMD, sometimes
   called the "organizational domain", but this is not strictly
   necessary.

   For tracing and debugging purposes, the authentication service
   identifier can instead be the specific hostname of the MTA performing
   the authentication check whose result is being reported.  Moreover,
   some implementations define a substructure to the identifier; such
   structures are outside of the scope of this specification.

   Note, however, that using a local, relative identifier like a flat
   hostname, rather than a hierarchical and globally unique ADMD
   identifier like a DNS domain name, makes configuration more difficult
   for large sites.  The hierarchical identifier permits aggregating
   related, trusted systems together under a single, parent identifier,
   which in turn permits assessing the trust relationship with a single
   reference.  The alternative is a flat namespace requiring
   individually listing each trusted system.  Since consumers will use
   the identifier to determine whether to use the contents of the header
   field:

   o  Changes to the identifier impose a large, centralized
      administrative burden.

   o  Ongoing administrative changes require constantly updating this
      centralized table, making it difficult to ensure that an MUA or
      downstream filter will have access to accurate information for
      assessing the usability of the header field's content.  In
      particular, consumers of the header field will need to know not
      only the current identifier(s) in use but previous ones as well to
      account for delivery latency or later reassessment of the header
      field's content.

   Examples of valid authentication service identifiers are
   "example.com", "mail.example.org", "ms1.newyork.example.com", and
   "example-auth".
Top   ToC   RFC8601 - Page 17
2.6.  Version Tokens

   The grammar above provides for the optional inclusion of versions on
   both the header field itself (attached to the authserv-id token) and
   on each of the methods being reported.  The method version refers to
   the method itself, which is specified in the documents describing
   those methods, while the authserv-id version refers to this document
   and thus the syntax of this header field.

   The purpose of including these is to avoid misinterpretation of the
   results.  That is, if a parser finds a version after an authserv-id
   that it does not explicitly know, it can immediately discontinue
   trying to parse, since what follows might not be in an expected
   format.  For a method version, the parser SHOULD ignore a method
   result if the version is not supported in case the semantics of the
   result have a different meaning than what is expected.  For example,
   if a hypothetical DKIM version 2 yielded a "pass" result for
   different reasons than version 1 does, a consumer of this field might
   not want to use the altered semantics.  Allowing versions in the
   syntax is a way to indicate this and let the consumer of the header
   field decide.

2.7.  Defined Methods and Result Values

   Each individual authentication method returns one of a set of
   specific result values.  The subsections below provide references to
   the documents defining the authentication methods specifically
   supported by this document, and their corresponding result values.
   Verifiers SHOULD use these values as described below.  New methods
   not specified in this document, but intended to be supported by the
   header field defined here, MUST include a similar result table either
   in their defining documents or in supplementary ones.

2.7.1.  DKIM

   DKIM is represented by the "dkim" method and is defined in [DKIM].

   A signature is "acceptable to the ADMD" if it passes local policy
   checks (or there are no specific local policy checks).  For example,
   an ADMD policy might require that the signature(s) on the message be
   added using the DNS domain present in the From header field of the
   message, thus making third-party signatures unacceptable even if they
   verify.
Top   ToC   RFC8601 - Page 18
   The DKIM result set is as follows:

   none:  The message was not signed.

   pass:  The message was signed, the signature or signatures were
      acceptable to the ADMD, and the signature(s) passed verification
      tests.

   fail:  The message was signed and the signature or signatures were
      acceptable to the ADMD, but they failed the verification test(s).

   policy:  The message was signed, but some aspect of the signature or
      signatures was not acceptable to the ADMD.

   neutral:  The message was signed, but the signature or signatures
      contained syntax errors or were not otherwise able to be
      processed.  This result is also used for other failures not
      covered elsewhere in this list.

   temperror:  The message could not be verified due to some error that
      is likely transient in nature, such as a temporary inability to
      retrieve a public key.  A later attempt may produce a final
      result.

   permerror:  The message could not be verified due to some error that
      is unrecoverable, such as a required header field being absent.  A
      later attempt is unlikely to produce a final result.

   DKIM results are reported using a ptype of "header".  The property,
   however, represents one of the tags found in the DKIM-Signature
   header field rather than a distinct header field.  For example, the
   ptype-property combination "header.d" refers to the content of the
   "d" (signing domain) tag from within the signature header field, and
   not a distinct header field called "d".

   Note that in an EAI-formatted message, the values of the "d" and "i"
   properties can be expressed in UTF-8.

   In addition to previous registrations, this document registers the
   DKIM tags "a" (cryptographic algorithm used to sign the message) and
   "s" (selector) as reportable properties.  These can be used to aid
   receivers during post-verification processing.  In particular,
   [RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is
   important to be able to distinguish such signatures from those using
   preferred algorithms.

   The ability to report different DKIM results for a message with
   multiple signatures is described in [RFC6008].
Top   ToC   RFC8601 - Page 19
   [DKIM] advises that if a message fails verification, it is to be
   treated as an unsigned message.  A report of "fail" here permits the
   receiver of the report to decide how to handle the failure.  A report
   of "neutral" or "none" preempts that choice, ensuring that the
   message will be treated as if it had not been signed.

2.7.2.  SPF

   SPF uses the "spf" method name.  The result values for SPF are
   defined in Section 2.6 of [SPF], and those definitions are included
   here by reference:

      +-----------+------------------------------+
      |    Code   | Meaning                      |
      +-----------+------------------------------+
      | none      | [SPF], Section 2.6.1         |
      +-----------+------------------------------+
      | pass      | [SPF], Section 2.6.3         |
      +-----------+------------------------------+
      | fail      | [SPF], Section 2.6.4         |
      +-----------+------------------------------+
      | softfail  | [SPF], Section 2.6.5         |
      +-----------+------------------------------+
      | policy    | RFC 8601, Section 2.4        |
      +-----------+------------------------------+
      | neutral   | [SPF], Section 2.6.2         |
      +-----------+------------------------------+
      | temperror | [SPF], Section 2.6.6         |
      +-----------+------------------------------+
      | permerror | [SPF], Section 2.6.7         |
      +-----------+------------------------------+

   These result codes are used in the context of this specification to
   reflect the result returned by the component conducting SPF
   evaluation.

   For SPF, the ptype used is "smtp", and the property is either
   "mailfrom" or "helo", since those values are the ones SPF can
   evaluate.  (If the SMTP client issued the EHLO command instead of
   HELO, the property used is "helo".)

   Note that in an EAI-formatted message, the local-part of the
   "mailfrom" can be expressed in UTF-8 and the domain part can be
   expressed as a U-label.

   For this method, an additional result of "policy" is defined, which
   means the client was authorized to inject or relay mail on behalf of
   the sender's DNS domain according to the authentication method's
Top   ToC   RFC8601 - Page 20
   algorithm, but local policy dictates that the result is unacceptable.
   For example, "policy" might be used if SPF returns a "pass" result,
   but a local policy check matches the sending DNS domain to one found
   in an explicit list of unacceptable DNS domains (e.g., spammers).

   If the retrieved sender policies used to evaluate SPF do not contain
   explicit provisions for authenticating the local-part (see
   Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported along
   with results for this mechanism SHOULD NOT include the local-part or
   the following "@" character.

2.7.3.  "iprev"

   The result values used by the "iprev" method, defined in Section 3,
   are as follows:

   pass:  The DNS evaluation succeeded, i.e., the "reverse" and
      "forward" lookup results were returned and were in agreement.

   fail:  The DNS evaluation failed.  In particular, the "reverse" and
      "forward" lookups each produced results, but they were not in
      agreement, or the "forward" query completed but produced no
      result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an
      RCODE of 0 (NOERROR) in a reply containing no answers, was
      returned.

   temperror:  The DNS evaluation could not be completed due to some
      error that is likely transient in nature, such as a temporary DNS
      error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or
      other error condition resulted.  A later attempt may produce a
      final result.

   permerror:  The DNS evaluation could not be completed because no PTR
      data are published for the connecting IP address, e.g., a DNS
      RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR)
      in a reply containing no answers, was returned.  This prevented
      completion of the evaluation.  A later attempt is unlikely to
      produce a final result.

   There is no "none" for this method, since any TCP connection
   delivering email has an IP address associated with it, so some kind
   of evaluation will always be possible.

   The result is reported using a ptype of "policy" (as this is not part
   of any established protocol) and a property of "iprev".

   For discussion of the format of DNS replies, see "Domain names -
   implementation and specification" [DNS].
Top   ToC   RFC8601 - Page 21
2.7.4.  SMTP AUTH

   SMTP AUTH (defined in [AUTH]) is represented by the "auth" method.
   Its result values are as follows:

   none:  SMTP authentication was not attempted.

   pass:  The SMTP client authenticated to the server reporting the
      result using the protocol described in [AUTH].

   fail:  The SMTP client attempted to authenticate to the server using
      the protocol described in [AUTH] but was not successful (such as
      providing a valid identity but an incorrect password).

   temperror:  The SMTP client attempted to authenticate using the
      protocol described in [AUTH] but was not able to complete the
      attempt due to some error that is likely transient in nature, such
      as a temporary directory service lookup error.  A later attempt
      may produce a final result.

   permerror:  The SMTP client attempted to authenticate using the
      protocol described in [AUTH] but was not able to complete the
      attempt due to some error that is likely not transient in nature,
      such as a permanent directory service lookup error.  A later
      attempt is not likely to produce a final result.

   The result of AUTH is reported using a ptype of "smtp" and a property
   of either:

   o  "auth", in which case the value is the authorization identity
      generated by the exchange initiated by the AUTH command; or

   o  "mailfrom", in which case the value is the mailbox identified by
      the AUTH parameter used with the MAIL FROM command.

   Note that in an EAI-formatted message, the local-part can be
   expressed in UTF-8 and the domain can be expressed as a U-label.

   If both identities are available, both can be reported.  For example,
   consider this command issued by a client that has completed session
   authentication with the AUTH command resulting in an authorized
   identity of "client@c.example":

     MAIL FROM:<alice@a.example> AUTH=<bob@b.example>

   This could result in a "resinfo" construction like so:

     ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example
Top   ToC   RFC8601 - Page 22
   Note that in all cases other than "pass", the message was sent by an
   unauthenticated client.  All non-"pass" cases SHOULD thus be treated
   as equivalent with respect to this method.

2.7.5.  Other Registered Codes

   Result codes were also registered in other RFCs as follows:

   o  Vouch By Reference (in [AR-VBR], represented by "vbr").

   o  Authorized Third-Party Signatures (in [ATPS], represented by
      "dkim-atps").

   o  Author Domain Signing Practices (in [ADSP], represented by
      "dkim-adsp").

   o  Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs").

   o  S/MIME (in [SMIME-REG], represented by "smime").

   Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which
   are already registered, can be expressed as U-labels.

2.7.6.  Extension Methods

   Additional authentication method identifiers (extension methods) may
   be defined in the future by later revisions or extensions to this
   specification.  These method identifiers are registered with IANA
   and, preferably, published in an RFC.  See Section 6 for further
   details.

   Extension methods can be defined for the following reasons:

   1.  To allow additional information from new authentication systems
       to be communicated to MUAs or downstream filters.  The names of
       such identifiers ought to reflect the name of the method being
       defined but ought not be needlessly long.

   2.  To allow the creation of "sub-identifiers" that indicate
       different levels of authentication and differentiate between
       their relative strengths, e.g., "auth1-weak" and "auth1-strong".
Top   ToC   RFC8601 - Page 23
   Authentication method implementers are encouraged to provide adequate
   information, via message header field comments if necessary, to allow
   an MUA developer to understand or relay ancillary details of
   authentication results.  For example, if it might be of interest to
   relay what data were used to perform an evaluation, such information
   could be relayed as a comment in the header field, such as:

        Authentication-Results: example.com;
                  foo=pass bar.baz=blob (2 of 3 tests OK)

   Experimental method identifiers MUST only be used within ADMDs that
   have explicitly consented to use them.  These method identifiers and
   the parameters associated with them are not documented formally.
   Therefore, they are subject to change at any time and not suitable
   for production use.  Any MTA, MUA, or downstream filter intended for
   production use SHOULD ignore or delete any Authentication-Results
   header field that includes an experimental (unknown) method
   identifier.

2.7.7.  Extension Result Codes

   Additional result codes (extension results) might be defined in the
   future by later revisions or extensions to this specification.
   Non-experimental result codes MUST be registered with IANA (and,
   preferably, published in an RFC).  See Section 6 for further details.

   Experimental results MUST only be used within ADMDs that have
   explicitly consented to use them.  These results and the parameters
   associated with them are not formally documented.  Therefore, they
   are subject to change at any time and not suitable for production
   use.  Any MTA, MUA, or downstream filter intended for production use
   SHOULD ignore or delete any Authentication-Results header field that
   includes an extension result.



(page 23 continued on part 3)

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