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RFC 7001 - Message Header Field for Indicating Message Authentic

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Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 7001                                September 2013
Obsoletes: 5451, 6577
Category: Standards Track
ISSN: 2070-1721

   Message Header Field for Indicating Message Authentication Status


   This document specifies a message header field called Authentication-
   Results for use with electronic mail messages to indicate the results
   of message authentication efforts.  Any receiver-side software, such
   as mail filters or Mail User Agents (MUAs), can use this header field
   to relay that information in a convenient and meaningful way to users
   or to make sorting and filtering decisions.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Purpose ....................................................4
      1.2. Trust Boundary .............................................5
      1.3. Processing Scope ...........................................6
      1.4. Requirements ...............................................6
      1.5. Definitions ................................................6
           1.5.1. Key Words ...........................................6
           1.5.2. Security ............................................6
           1.5.3. Email Architecture ..................................7
           1.5.4. Other Terms .........................................8
      1.6. Trust Environment ..........................................8
   2. Definition and Format of the Header Field .......................9
      2.1. General Description ........................................9
      2.2. Formal Definition ..........................................9
      2.3. The "policy" ptype ........................................12
      2.4. Authentication Identifier Field ...........................13
      2.5. Version Tokens ............................................14
      2.6. Defined Methods and Result Values .........................14
           2.6.1. DKIM and DomainKeys ................................14
           2.6.2. SPF and Sender ID ..................................15
           2.6.3. "iprev" ............................................17
           2.6.4. SMTP AUTH ..........................................17
           2.6.5. Other Registered Codes .............................18
           2.6.6. Extension Methods ..................................18
           2.6.7. Extension Result Codes .............................19
   3. The "iprev" Authentication Method ..............................19
   4. Adding the Header Field to a Message ...........................20
      4.1. Header Field Position and Interpretation ..................22
      4.2. Local Policy Enforcement ..................................23
   5. Removing Existing Header Fields ................................23
   6. IANA Considerations ............................................24
      6.1. The Authentication-Results Header Field ...................25
      6.2. "Email Authentication Methods" Registry ...................25
      6.3. "Email Authentication Result Names" Registry ..............26
   7. Security Considerations ........................................26
      7.1. Forged Header Fields ......................................26
      7.2. Misleading Results ........................................28
      7.3. Header Field Position .....................................28
      7.4. Reverse IP Query Denial-of-Service Attacks ................28
      7.5. Mitigation of Backscatter .................................29
      7.6. Internal MTA Lists ........................................29
      7.7. Attacks against Authentication Methods ....................29
      7.8. Intentionally Malformed Header Fields .....................29
      7.9. Compromised Internal Hosts ................................29
      7.10. Encapsulated Instances ...................................30
      7.11. Reverse Mapping ..........................................30

   8. References .....................................................30
      8.1. Normative References ......................................30
      8.2. Informative References ....................................31
   Appendix A.  Acknowledgements .....................................33
   Appendix B.  Legacy MUAs ..........................................33
   Appendix C.  Authentication-Results Examples ......................33
     C.1.  Trivial Case; Header Field Not Present ....................34
     C.2.  Nearly Trivial Case; Service Provided, but No
           Authentication Done .......................................34
     C.3.  Service Provided, Authentication Done .....................35
     C.4.  Service Provided, Several Authentications Done, Single
           MTA .......................................................36
     C.5.  Service Provided, Several Authentications Done,
           Different MTAs ............................................37
     C.6.  Service Provided, Multi-Tiered Authentication Done ........38
     C.7.  Comment-Heavy Example .....................................40
   Appendix D.  Operational Considerations about Message
                Authentication .......................................40
   Appendix E.  Changes since RFC 5451 ...............................42

1.  Introduction

   This document describes a header field called Authentication-Results
   for electronic mail messages that presents the results of a message
   authentication effort in a machine-readable format.  The intent of
   the header field is to create a place to collect such data when
   message authentication mechanisms are in use so that a Mail User
   Agent (MUA) and downstream filters can make filtering decisions
   and/or provide a recommendation to the user as to the validity of the
   message's origin and possibly the safety and integrity of its

   This document revises the original definition found in [RFC5451]
   based upon various authentication protocols in current use and
   incorporates errata logged since the publication of the original

   End users are not expected to be direct consumers of this header
   field.  This header field is intended for consumption by programs
   that will then use such data or render it in a human-usable form.

   This document specifies the format of this header field and discusses
   the implications of its presence or absence.  However, it does not
   discuss how the data contained in the header field ought to be used,
   such as what filtering decisions are appropriate or how an MUA might
   render those results, as these are local policy and/or user interface
   design questions that are not appropriate for this document.

   At the time of publication of this document, the following are
   published, domain-level email authentication methods in common use:

   o  Author Domain Signing Practices ([ADSP])

   o  SMTP Service Extension for Authentication ([AUTH])

   o  DomainKeys Identified Mail Signatures ([DKIM])

   o  Sender Policy Framework ([SPF])

   o  Vouch By Reference ([VBR])

   o  reverse IP address name validation ("iprev", defined in Section 3)

   In addition, the following are non-standard methods recognized by
   this specification that are no longer common:

   o  DomainKeys ([DOMAINKEYS]) (Historic)

   o  Sender ID ([SENDERID]) (Experimental)

   This specification is not intended to be restricted to domain-based
   authentication schemes, but the existing schemes in that family have
   proven to be a good starting point for implementations.  The goal is
   to give current and future authentication schemes a common framework
   within which to deliver their results to downstream agents and
   discourage the creation of unique header fields for each.

   Although SPF defined a header field called "Received-SPF" and the
   historic DomainKeys defined one called "DomainKey-Status" for this
   purpose, those header fields are specific to the conveyance of their
   respective results only and thus are insufficient to satisfy the
   requirements enumerated below.  In addition, many SPF implementations
   have adopted the header field specified here at least as an option,
   and DomainKeys has been obsoleted by DKIM.

1.1.  Purpose

   The header field defined in this document is expected to serve
   several purposes:

   1.  Convey the results of various message authentication checks,
       which are applied by upstream filters and Mail Transfer Agents
       (MTAs) and then passed to MUAs and downstream filters within the
       same "trust domain".  Such agents might wish to render those
       results to end users or to use those data to apply more or less
       stringent content checks based on authentication results;

   2.  Provide a common location within a message for this data;

   3.  Create an extensible framework for reporting new authentication
       methods as they emerge.

   In particular, the mere presence of this header field does not mean
   its contents are valid.  Rather, the header field is reporting
   assertions made by one or more authentication schemes (supposedly)
   applied somewhere upstream.  For an MUA or downstream filter to treat
   the assertions as actually valid, there must be an assessment of the
   trust relationship among such agents, the validating MTA, and the
   mechanism for conveying the information.

1.2.  Trust Boundary

   This document makes several references to the "trust boundary" of an
   administrative management domain (ADMD).  Given the diversity among
   existing mail environments, a precise definition of this term isn't

   Simply put, a transfer from the producer of the header field to the
   consumer must occur within a context that permits the consumer to
   treat assertions by the producer as being reliable and accurate
   (trustworthy).  How this trust is obtained is outside the scope of
   this document.  It is entirely a local matter.

   Thus, this document defines a "trust boundary" as the delineation
   between "external" and "internal" entities.  Services that are
   internal -- within the trust boundary -- are provided by the ADMD's
   infrastructure for its users.  Those that are external are outside of
   the authority of the ADMD.  By this definition, hosts that are within
   a trust boundary are subject to the ADMD's authority and policies,
   independent of their physical placement or their physical operation.
   For example, a host within a trust boundary might actually be
   operated by a remote service provider and reside physically within
   its data center.

   It is possible for a message to be evaluated inside a trust boundary
   but then depart and re-enter the trust boundary.  An example might be
   a forwarded message such as a message/rfc822 attachment (see
   Multipurpose Internet Mail Extensions [MIME]) or one that is part of
   a multipart/digest.  The details reported by this field cannot be
   trusted in that case.  Thus, this field found within one of those
   media types is typically ignored.

1.3.  Processing Scope

   The content of this header field is meant to convey to message
   consumers that authentication work on the message was already done
   within its trust boundary, and those results are being presented.  It
   is not intended to provide message parameters to consumers so that
   they can perform authentication protocols on their own.

1.4.  Requirements

   This document establishes no new requirements on existing protocols
   or servers.

   In particular, this document establishes no requirement on MTAs to
   reject or filter arriving messages that do not pass authentication
   checks.  The data conveyed by the specified header field's contents
   are for the information of MUAs and filters and are to be used at
   their discretion.

1.5.  Definitions

   This section defines various terms used throughout this document.

1.5.1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

1.5.2.  Security

   "Guidelines for Writing RFC Text on Security Considerations"
   ([SECURITY]) discusses authentication and authorization and the
   conflation of the two concepts.  The use of those terms within the
   context of recent message security work has given rise to slightly
   different definitions, and this document reflects those current
   usages, as follows:

   o  "Authorization" is the establishment of permission to use a
      resource or represent an identity.  In this context, authorization
      indicates that a message from a particular ADMD arrived via a
      route the ADMD has explicitly approved.

   o  "Authentication" is the assertion of validity of a piece of data
      about a message (such as the sender's identity) or the message in
      its entirety.

   As examples: SPF and Sender ID are authorization mechanisms in that
   they express a result that shows whether or not the ADMD that
   apparently sent the message has explicitly authorized the connecting
   Simple Mail Transfer Protocol ([SMTP]) client to relay messages on
   its behalf, but they do not actually validate any other property of
   the message itself.  By contrast, DKIM is agnostic as to the routing
   of a message but uses cryptographic signatures to authenticate
   agents, assign (some) responsibility for the message (which implies
   authorization), and ensure that the listed portions of the message
   were not modified in transit.  Since the signatures are not tied to
   SMTP connections, they can be added by either the ADMD of origin,
   intermediate ADMDs (such as a mailing list server), other handling
   agents, or any combination.

   Rather than create a separate header field for each class of
   solution, this proposal groups them both into a single header field.

1.5.3.  Email Architecture

   o  A "border MTA" is an MTA that acts as a gateway between the
      general Internet and the users within an organizational boundary.
      (See also Section 1.2.)

   o  A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
      actually enacts delivery of a message to a user's inbox or other
      final delivery.

   o  An "intermediate MTA" is any MTA that is not a delivery MTA and is
      also not the first MTA to handle the message.

   The following diagram illustrates the flow of mail among these
   defined components.  See Internet Mail Architecture [EMAIL-ARCH] for
   further discussion on general email system architecture, which
   includes detailed descriptions of these components, and Appendix D of
   this document for discussion about the common aspects of email
   authentication in current environments.

                          +-----+   +-----+   +------------+
                          | MUA |-->| MSA |-->| Border MTA |
                          +-----+   +-----+   +------------+
                                               | Internet |
   +-----+   +-----+   +------------------+   +------------+
   | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA |
   +-----+   +-----+   +------------------+   +------------+

   Generally, it is assumed that the work of applying message
   authentication schemes takes place at a border MTA or a delivery MTA.
   This specification is written with that assumption in mind.  However,
   there are some sites at which the entire mail infrastructure consists
   of a single host.  In such cases, such terms as "border MTA" and
   "delivery MTA" might well apply to the same machine or even the very
   same agent.  It is also possible that some message authentication
   tests could take place on an intermediate MTA.  Although this
   document doesn't specifically describe such cases, they are not meant
   to be excluded.

1.5.4.  Other Terms

   In this document, the term "producer" refers to any component that
   adds this header field to messages it is handling, and "consumer"
   refers to any component that identifies, extracts, and parses the
   header field to use as part of a handling decision.

1.6.  Trust Environment

   This header field permits one or more message validation mechanisms
   to communicate output to one or more separate assessment mechanisms.
   These mechanisms operate within a unified trust boundary that defines
   an Administrative Management Domain (ADMD).  An ADMD contains one or
   more entities that perform validation and generate the header field
   and one or more that consume it for some type of assessment.  The
   field often contains no integrity or validation mechanism of its own,
   so its presence must be trusted implicitly.  Hence, valid use of the
   header field requires removing any occurrences of it that are present
   when the message enters the ADMD.  This ensures that later
   occurrences have been added within the trust boundary of the ADMD.

   The authserv-id token defined in Section 2.2 can be used to reference
   an entire ADMD or a specific validation engine within an ADMD.
   Although the labeling scheme is left as an operational choice, some
   guidance for selecting a token is provided in later sections of this

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 more 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

   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 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(s) 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.

   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 follows using Augmented
   Backus-Naur Form ([ABNF]):

     authres-header = "Authentication-Results:" [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 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

     method = Keyword [ [CFWS] "/" [CFWS] method-version ]
            ; a method indicates which method's result is
            ; represented by "result", and 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 = "smtp" / "header" / "body" / "policy"
           ; indicates whether the property being evaluated was
           ; a parameter to an [SMTP] command, 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

     property = special-smtp-verb / Keyword
             ; if "ptype" is "smtp", this indicates which [SMTP]
             ; command provided the value that was evaluated by the
             ; authentication scheme being applied; if "ptype" is
             ; "header", this indicates from which header field the
             ; value being evaluated was extracted; if "ptype" is
             ; "body", this indicates where in the message body
             ; a value being evaluated can be found (e.g., a specific
             ; offset into the message or a reference to a MIME part);
             ; if "ptype" is "policy", then this indicates the name
             ; of the policy that caused this header field to be
             ; added (see below)

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

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

   "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is
   defined in Section 3.2.2 of [MAIL].

   "Keyword" is defined in Section 4.1.2 of [SMTP].

   The "value" is as defined in Section 5.1 of [MIME].

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

   The "Keyword" used in "result" above is further constrained by the
   necessity of being enumerated in Section 2.6.

   See Section 2.4 for a description of the authserv-id element.

   If the value portion of a "pvalue" construction identifies something
   intended to be an e-mail 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 has a result to

   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 2.3 for details.

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

2.3.  The "policy" ptype

   A special ptype value of "policy" is 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

   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-Result field:

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

   In this case, the property is "dkim-rules", indicating 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, 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.

2.4.  Authentication 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.

   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

   For tracing and debugging purposes, the authentication 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; these 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

   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 re-assessment of the header
      field's contents.

   Examples of valid authentication identifiers are "example.com",
   "mail.example.org", "ms1.newyork.example.com", and "example-auth".

2.5.  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.

2.6.  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.

2.6.1.  DKIM and DomainKeys

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

   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

   Both DKIM and DomainKeys use the same result set, 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

   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

   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] 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 the message
   will be treated as if it had not been signed.

2.6.2.  SPF and Sender ID

   SPF and Sender ID use the "spf" and "sender-id" method names,
   respectively.  The result values for SPF are defined in Section 2.5
   of [SPF], and those definitions are included here by reference:

     |    Code   | Meaning                    |
     | none      | [SPF], Section 2.5.1       |
     | pass      | [SPF], Section 2.5.3       |
     | fail      | [SPF], Section 2.5.4       |
     | softfail  | [SPF], Section 2.5.5       |
     | policy    | [RFC7001], Section 2.6.2   |
     | neutral   | [SPF], Section 2.5.2       |
     | temperror | [SPF], Section 2.5.6       |
     | permerror | [SPF], Section 2.5.7       |

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

   Similarly, the results for Sender ID are listed and described in
   Section 4.2 of [SENDERID], which in turn uses the SPF definitions.

   Note that both of those documents specify result codes that use mixed
   case, but they are typically used all lowercase in this context.

   In both cases, 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
   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 and Sender ID
   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 these mechanisms SHOULD NOT include the local-

2.6.3.  "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

   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.

   For discussion of the format of DNS replies, see "Domain Names -
   Implementation and Specification" ([DNS]).

2.6.4.  SMTP AUTH

   SMTP AUTH (defined in [AUTH]) is represented by the "auth" method,
   and 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, yet
      continued to send the message about which a result is being

   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.

   An agent making use of the data provided by this header field SHOULD
   consider "fail" and "temperror" to be synonymous in terms of message
   authentication, i.e., the client did not authenticate in either case.

2.6.5.  Other Registered Codes

   Result codes were also registered in other RFCs for Vouch By
   Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party
   Signatures (in [ATPS], represented by "dkim-atps"), and the DKIM-
   related Author Domain Signing Practices (in [ADSP], represented by

2.6.6.  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 the
   Internet Assigned Numbers Authority (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".

   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 was 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 in RFCs.
   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

2.6.7.  Extension Result Codes

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

   Extension 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.

3.  The "iprev" Authentication Method

   This section defines an additional authentication method called

   "iprev" is an attempt to verify that a client appears to be valid
   based on some DNS queries, which is to say that the IP address is
   explicitly associated with a domain name.  Upon receiving a session
   initiation of some kind from a client, the IP address of the client
   peer is queried for matching names (i.e., a number-to-name
   translation, also known as a "reverse lookup" or a "PTR" record
   query).  Once that result is acquired, a lookup of each of the names
   (i.e., a name-to-number translation, or an "A" or "AAAA" record

   query) thus retrieved is done.  The response to this second check
   will typically result in at least one mapping back to the client's IP

   Expressed as an algorithm: If the client peer's IP address is I, the
   list of names to which I maps (after a "PTR" query) is the set N, and
   the union of IP addresses to which each member of N maps (after
   corresponding "A" and "AAAA" queries) is L, then this test is
   successful if I is an element of L.

   The response to a PTR query could contain multiple names.  To prevent
   heavy DNS loads, agents performing these queries MUST be implemented
   such that the number of names evaluated by generation of
   corresponding A or AAAA queries is limited so as not to be unduly
   taxing to the DNS infrastructure, though it MAY be configurable by an
   administrator.  As an example, Section 5.5 of [SPF] chose a limit of
   10 for its implementation of this algorithm.

   "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the
   query formats for the IPv6 case.

   There is some contention regarding the wisdom and reliability of this
   test.  For example, in some regions, it can be difficult for this
   test ever to pass because the practice of arranging to match the
   forward and reverse DNS is infrequently observed.  Therefore, the
   precise implementation details of how a verifier performs an "iprev"
   test are not specified here.  The verifier MAY report a successful or
   failed "iprev" test at its discretion having done some kind of check
   of the validity of the connection's identity using DNS.  It is
   incumbent upon an agent making use of the reported "iprev" result to
   understand what exactly that particular verifier is attempting to

   Extensive discussion of reverse DNS mapping and its implications can
   be found in "Considerations for the use of DNS Reverse Mapping"
   ([DNSOP-REVERSE]).  In particular, it recommends that applications
   avoid using this test as a means of authentication or security.  Its
   presence in this document is not an endorsement but is merely
   acknowledgement that the method remains common and provides the means
   to relay the results of that test.

4.  Adding the Header Field to a Message

   This specification makes no attempt to evaluate the relative
   strengths of various message authentication methods that may become
   available.  The methods listed are an order-independent set; their
   sequence does not indicate relative strength or importance of one

   method over another.  Instead, the MUA or downstream filter consuming
   this header field is to interpret the result of each method based on
   its own knowledge of what that method evaluates.

   Each "method" MUST refer to an authentication method declared in the
   IANA registry or an extension method as described in Section 2.6.6,
   and each "result" MUST refer to a result code declared in the IANA
   registry or an extension result code as defined in Section 2.6.7.
   See Section 6 for further information about the registered methods
   and result codes.

   An MTA compliant with this specification adds this header field
   (after performing one or more message authentication tests) to
   indicate which MTA or ADMD performed the test, which test got
   applied, and what the result was.  If an MTA applies more than one
   such test, it adds this header field either once per test or once
   indicating all of the results.  An MTA MUST NOT add a result to an
   existing header field.

   An MTA MAY add this header field containing only the authentication
   identifier portion and the "none" token (see Section 2.2) to indicate
   explicitly that no message authentication schemes were applied prior
   to delivery of this message.

   An MTA adding this header field has to take steps to identify it as
   legitimate to the MUAs or downstream filters that will ultimately
   consume its content.  One process to do so is described in Section 5.
   Further measures may be necessary in some environments.  Some
   possible solutions are enumerated in Section 7.1.  This document does
   not mandate any specific solution to this issue as each environment
   has its own facilities and limitations.

   Most known message authentication methods focus on a particular
   identifier to evaluate.  SPF and Sender ID differ in that they can
   yield a result based on more than one identifier; specifically, SPF
   can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom
   parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter
   or the Purported Responsible Address (PRA) identity.  When generating
   this field to report those results, only the parameter that yielded
   the result is included.

   For MTAs that add this header field, adding header fields in order
   (at the top), per Section 3.6 of [MAIL], is particularly important.
   Moreover, this header field SHOULD be inserted above any other trace
   header fields such MTAs might prepend.  This placement allows easy
   detection of header fields that can be trusted.

   End users making direct use of this header field might inadvertently
   trust information that has not been properly vetted.  If, for
   example, a basic SPF result were to be relayed that claims an
   authenticated addr-spec, the local-part of that addr-spec has
   actually not been authenticated.  Thus, an MTA adding this header
   field SHOULD NOT include any data that has not been authenticated by
   the method(s) being applied.  Moreover, MUAs SHOULD NOT render to
   users such information if it is presented by a method known not to
   authenticate it.

4.1.  Header Field Position and Interpretation

   In order to ensure non-ambiguous results and avoid the impact of
   false header fields, MUAs and downstream filters SHOULD NOT interpret
   this header field unless specifically configured to do so by the user
   or administrator.  That is, this interpretation should not be "on by
   default".  Naturally then, users or administrators ought not activate
   such a feature unless they are certain the header field will be
   validly added by an agent within the ADMD that accepts the mail that
   is ultimately read by the MUA, and instances of the header field
   appearing to originate within the ADMD but are actually added by
   foreign MTAs will be removed before delivery.

   Furthermore, MUAs and downstream filters SHOULD NOT interpret this
   header field unless the authentication service identifier it bears
   appears to be one used within its own ADMD as configured by the user
   or administrator.

   MUAs and downstream filters MUST ignore any result reported using a
   "result" not specified in the IANA "Result Code" registry or a
   "ptype" not listed in the corresponding registry for such values as
   defined in Section 6.  Moreover, such agents MUST ignore a result
   indicated for any "method" they do not specifically support.

   An MUA SHOULD NOT reveal these results to end users, absent careful
   human factors design considerations and testing, for the presentation
   of trust-related materials.  For example, an attacker could register
   examp1e.com (note the digit "one") and send signed mail to intended
   victims; a verifier would detect that the signature was valid and
   report a "pass" even though it's clear the DNS domain name was
   intended to mislead.  See Section 7.2 for further discussion.

   As stated in Section 2.1, this header field MUST be treated as though
   it were a trace header field as defined in Section 3.6.7 of [MAIL]
   and hence MUST NOT be reordered and MUST be prepended to the message,
   so that there is generally some indication upon delivery of where in
   the chain of handling MTAs the message authentication was done.

   Note that there are a few message handlers that are only capable of
   appending new header fields to a message.  Strictly speaking, these
   handlers are not compliant with this specification.  They can still
   add the header field to carry authentication details, but any signal
   about where in the handling chain the work was done may be lost.
   Consumers SHOULD be designed such that this can be tolerated,
   especially from a producer known to have this limitation.

   MUAs SHOULD ignore instances of this header field discovered within
   message/rfc822 MIME attachments.

   Further discussion of these topics can be found in Section 7 below.

4.2.  Local Policy Enforcement

   Some sites have a local policy that considers any particular
   authentication policy's non-recoverable failure results (typically
   "fail" or similar) as justification for rejecting the message.  In
   such cases, the border MTA SHOULD issue an SMTP rejection response to
   the message, rather than adding this header field and allowing the
   message to proceed toward delivery.  This is more desirable than
   allowing the message to reach an internal host's MTA or spam filter,
   thus possibly generating a local rejection such as a Delivery Status
   Notification (DSN) [DSN] to a forged originator.  Such generated
   rejections are colloquially known as "backscatter".

   The same MAY also be done for local policy decisions overriding the
   results of the authentication methods (e.g., the "policy" result
   codes described in Section 2.6).

   Such rejections at the SMTP protocol level are not possible if local
   policy is enforced at the MUA and not the MTA.

5.  Removing Existing Header Fields

   For security reasons, any MTA conforming to this specification MUST
   delete any discovered instance of this header field that claims, by
   virtue of its authentication service identifier, to have been added
   within its trust boundary but that did not come directly from another
   trusted MTA.  For example, an MTA for example.com receiving a message
   MUST delete or otherwise obscure any instance of this header field
   bearing an authentication service identifier indicating that the
   header field was added within example.com prior to adding its own
   header fields.  This could mean each MTA will have to be equipped
   with a list of internal MTAs known to be compliant (and hence

   For simplicity and maximum security, a border MTA could remove all
   instances of this header field on mail crossing into its trust
   boundary.  However, this may conflict with the desire to access
   authentication results performed by trusted external service
   providers.  It may also invalidate signed messages whose signatures
   cover external instances of this header field.  A more robust border
   MTA could allow a specific list of authenticating MTAs whose
   information is to be admitted, removing the header field originating
   from all others.

   As stated in Section 1.2, a formal definition of "trust boundary" is
   deliberately not made here.  It is entirely possible that a border
   MTA for example.com will explicitly trust authentication results
   asserted by upstream host example.net even though they exist in
   completely disjoint administrative boundaries.  In that case, the
   border MTA MAY elect not to delete those results; moreover, the
   upstream host doing some authentication work could apply a signing
   technology such as [DKIM] on its own results to assure downstream
   hosts of their authenticity.  An example of this is provided in
   Appendix C.

   Similarly, in the case of messages signed using [DKIM] or other
   message-signing methods that sign header fields, this removal action
   could invalidate one or more signatures on the message if they
   covered the header field to be removed.  This behavior can be
   desirable since there's little value in validating the signature on a
   message with forged header fields.  However, signing agents MAY
   therefore elect to omit these header fields from signing to avoid
   this situation.

   An MTA SHOULD remove any instance of this header field bearing a
   version (express or implied) that it does not support.  However, an
   MTA MUST remove such a header field if the [SMTP] connection relaying
   the message is not from a trusted internal MTA.  This means the MTA
   needs to be able to understand versions of this header field at least
   as late as the ones understood by the MUAs or other consumers within
   its ADMD.

6.  IANA Considerations

   IANA has registered the defined header field and created two tables
   as described below.  These registry actions were originally defined
   by [RFC5451] and are repeated here to provide a single, current

6.1.  The Authentication-Results Header Field

   [RFC5451] added the Authentication-Results header field to the IANA
   "Permanent Message Header Field Names" registry, per the procedure
   found in [IANA-HEADERS].  That entry has been updated to reference
   this document.  The following is the registration template:

     Header field name: Authentication-Results
     Applicable protocol: mail ([MAIL])
     Status: Standard
     Author/Change controller: IETF
     Specification document(s): RFC 7001
     Related information:
       Requesting review of any proposed changes and additions to
       this field is recommended.

6.2.  "Email Authentication Methods" Registry

   Names of message authentication methods supported by this
   specification are to be registered with IANA, with the exception of
   experimental names as described in Section 2.6.6.  A registry was
   created by [RFC5451] for this purpose.  This document changes the
   rules governing that registry.

   New entries are assigned only for values that have received Expert
   Review, per [IANA-CONSIDERATIONS].  The designated expert shall be
   appointed by the IESG.  The designated expert has discretion to
   request that a publication be referenced if a clear, concise
   definition of the authentication method cannot be provided such that
   interoperability is assured.  Registrations should otherwise be
   permitted.  The designated expert can also handle requests to mark
   any current registration as "deprecated".

   Each method must register a name, the specification that defines it,
   a version number associated with the method being registered
   (preferably starting at "1"), zero or more "ptype" values appropriate
   for use with that method, which "property" value(s) should be
   reported by that method, and a description of the "value" to be used
   with each.

   All existing registry entries that reference [RFC5451] have been
   updated to reference this document, except where entries have already
   been deprecated.

   IANA has also added a "version" field to all existing registry
   entries.  All current methods are recorded as version "1".

6.3.  "Email Authentication Result Names" Registry

   Names of message authentication result codes supported by this
   specification must be registered with IANA, with the exception of
   experimental codes as described in Section 2.6.7.  A registry was
   created by [RFC5451] for this purpose.  This document changes the
   rules governing that registry.

   New entries are assigned only for values that have received Expert
   Review, per [IANA-CONSIDERATIONS].  The designated expert shall be
   appointed by the IESG.  The designated expert has discretion to
   request that a publication be referenced if a clear, concise
   definition of the authentication result cannot be provided such that
   interoperability is assured.  Registrations should otherwise be
   permitted.  The designated expert can also handle requests to mark
   any current registration as "deprecated".

   All existing registry entries that reference [RFC5451] have been
   updated to reference this document.

   The definitions for the SPF and Sender ID authentication methods are
   updated using the references found in Section 2.6.2.

7.  Security Considerations

   The following security considerations apply when adding or processing
   the Authentication-Results header field:

7.1.  Forged Header Fields

   An MUA or filter that accesses a mailbox whose messages are handled
   by a non-conformant MTA, and understands Authentication-Results
   header fields, could potentially make false conclusions based on
   forged header fields.  A malicious user or agent could forge a header
   field using the DNS domain of a receiving ADMD as the authserv-id
   token in the value of the header field and, with the rest of the
   value, claim that the message was properly authenticated.  The non-
   conformant MTA would fail to strip the forged header field, and the
   MUA could inappropriately trust it.

   For this reason, it is best not to have processing of the
   Authentication-Results header field enabled by default; instead, it
   should be ignored, at least for the purposes of enacting filtering
   decisions, unless specifically enabled by the user or administrator
   after verifying that the border MTA is compliant.  It is acceptable
   to have an MUA aware of this specification but have an explicit list
   of hostnames whose Authentication-Results header fields are
   trustworthy; however, this list should initially be empty.

   Proposed alternative solutions to this problem were made some time
   ago and are listed below.  To date, they have not been developed due
   to lack of demand but are documented here should the information be
   useful at some point in the future:

   1.  Possibly the simplest is a digital signature protecting the
       header field, such as using [DKIM], that can be verified by an
       MUA by using a posted public key.  Although one of the main
       purposes of this document is to relieve the burden of doing
       message authentication work at the MUA, this only requires that
       the MUA learn a single authentication scheme even if a number of
       them are in use at the border MTA.  Note that [DKIM] requires
       that the From header field be signed, although in this
       application, the signing agent (a trusted MTA) likely cannot
       authenticate that value, so the fact that it is signed should be
       ignored.  Where the authserv-id is the ADMD's domain name, the
       authserv-id matching this valid internal signature's "d=" DKIM
       value is sufficient.

   2.  Another would be a means to interrogate the MTA that added the
       header field to see if it is actually providing any message
       authentication services and saw the message in question, but this
       isn't especially palatable given the work required to craft and
       implement such a scheme.

   3.  Yet another might be a method to interrogate the internal MTAs
       that apparently handled the message (based on Received header
       fields) to determine whether any of them conform to Section 5 of
       this memo.  This, too, has potentially high barriers to entry.

   4.  Extensions to [IMAP], [SMTP], and [POP3] could be defined to
       allow an MUA or filtering agent to acquire the authserv-id in use
       within an ADMD, thus allowing it to identify which
       Authentication-Results header fields it can trust.

   5.  On the presumption that internal MTAs are fully compliant with
       Section 3.6 of [MAIL] and the compliant internal MTAs are using
       their own hostnames or the ADMD's DNS domain name as the
       authserv-id token, the header field proposed here should always
       appear above a Received header added by a trusted MTA.  This can
       be used as a test for header field validity.

   Support for some of these is being considered for future work.

   In any case, a mechanism needs to exist for an MUA or filter to
   verify that the host that appears to have added the header field (a)
   actually did so and (b) is legitimately adding that header field for

   this delivery.  Given the variety of messaging environments deployed
   today, consensus appears to be that specifying a particular mechanism
   for doing so is not appropriate for this document.

   Mitigation of the forged header field attack can also be accomplished
   by moving the authentication results data into metadata associated
   with the message.  In particular, an [SMTP] extension could be
   established to communicate authentication results from the border MTA
   to intermediate and delivery MTAs; the latter of these could arrange
   to store the authentication results as metadata retrieved and
   rendered along with the message by an [IMAP] client aware of a
   similar extension in that protocol.  The delivery MTA would be told
   to trust data via this extension only from MTAs it trusts, and border
   MTAs would not accept data via this extension from any source.  There
   is no vector in such an arrangement for forgery of authentication
   data by an outside agent.

7.2.  Misleading Results

   Until some form of service for querying the reputation of a sending
   agent is widely deployed, the existence of this header field
   indicating a "pass" does not render the message trustworthy.  It is
   possible for an arriving piece of spam or other undesirable mail to
   pass checks by several of the methods enumerated above (e.g., a piece
   of spam signed using [DKIM] by the originator of the spam, which
   might be a spammer or a compromised system).  In particular, this
   issue is not resolved by forged header field removal discussed above.

   Hence, MUAs and downstream filters must take some care with use of
   this header even after possibly malicious headers are scrubbed.

7.3.  Header Field Position

   Despite the requirements of [MAIL], header fields can sometimes be
   reordered en route by intermediate MTAs.  The goal of requiring
   header field addition only at the top of a message is an
   acknowledgement that some MTAs do reorder header fields, but most do
   not.  Thus, in the general case, there will be some indication of
   which MTAs (if any) handled the message after the addition of the
   header field defined here.

7.4.  Reverse IP Query Denial-of-Service Attacks

   Section 5.5 of [SPF] describes a DNS-based denial-of-service attack
   for verifiers that attempt DNS-based identity verification of
   arriving client connections.  A verifier wishing to do this check and
   report this information needs to take care not to go to unbounded
   lengths to resolve "A" and "PTR" queries.  MUAs or other filters

   making use of an "iprev" result specified by this document need to be
   aware of the algorithm used by the verifier reporting the result and,
   especially, its limitations.

7.5.  Mitigation of Backscatter

   Failing to follow the instructions of Section 4.2 can result in a
   denial-of-service attack caused by the generation of [DSN] messages
   (or equivalent) to addresses that did not send the messages being

7.6.  Internal MTA Lists

   Section 5 describes a procedure for scrubbing header fields that may
   contain forged authentication results about a message.  A compliant
   installation will have to include, at each MTA, a list of other MTAs
   known to be compliant and trustworthy.  Failing to keep this list
   current as internal infrastructure changes may expose an ADMD to

7.7.  Attacks against Authentication Methods

   If an attack becomes known against an authentication method, clearly
   then the agent verifying that method can be fooled into thinking an
   inauthentic message is authentic, and thus the value of this header
   field can be misleading.  It follows that any attack against the
   authentication methods supported by this document is also a security
   consideration here.

7.8.  Intentionally Malformed Header Fields

   It is possible for an attacker to add an Authentication-Results
   header field that is extraordinarily large or otherwise malformed in
   an attempt to discover or exploit weaknesses in header field parsing
   code.  Implementers must thoroughly verify all such header fields
   received from MTAs and be robust against intentionally as well as
   unintentionally malformed header fields.

7.9.  Compromised Internal Hosts

   An internal MUA or MTA that has been compromised could generate mail
   with a forged From header field and a forged Authentication-Results
   header field that endorses it.  Although it is clearly a larger
   concern to have compromised internal machines than it is to prove the
   value of this header field, this risk can be mitigated by arranging
   that internal MTAs will remove this header field if it claims to have
   been added by a trusted border MTA (as described above), yet the
   [SMTP] connection is not coming from an internal machine known to be

   running an authorized MTA.  However, in such a configuration,
   legitimate MTAs will have to add this header field when legitimate
   internal-only messages are generated.  This is also covered in
   Section 5.

7.10.  Encapsulated Instances

   MIME messages can contain attachments of type "message/rfc822", which
   contain other messages.  Such an encapsulated message can also
   contain an Authentication-Results header field.  Although the
   processing of these is outside of the intended scope of this document
   (see Section 1.3), some early guidance to MUA developers is
   appropriate here.

   Since MTAs are unlikely to strip Authentication-Results header fields
   after mailbox delivery, MUAs are advised in Section 4.1 to ignore
   such instances within MIME attachments.  Moreover, when extracting a
   message digest to separate mail store messages or other media, such
   header fields should be removed so that they will never be
   interpreted improperly by MUAs that might later consume them.

7.11.  Reverse Mapping

   Although Section 3 of this memo includes explicit support for the
   "iprev" method, its value as an authentication mechanism is limited.
   Implementers of both this proposal and agents that use the data it
   relays are encouraged to become familiar with the issues raised by
   [DNSOP-REVERSE] when deciding whether or not to include support for

8.  References

8.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

              Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

8.2.  Informative References

   [ADSP]     Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, August 2009.

   [AR-VBR]   Kucherawy, M., "Authentication-Results Registration for
              Vouch by Reference Results", RFC 6212, April 2011.

   [ATPS]     Kucherawy, M., "DomainKeys Identified Mail (DKIM)
              Authorized Third-Party Signatures", RFC 6541,
              February 2012.

   [AUTH]     Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [DKIM]     Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
              September 2011.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [DNS-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003.

              Senie, D. and A. Sullivan, "Considerations for the use of
              DNS Reverse Mapping", Work in Progress, March 2008.

              Delany, M., "Domain-Based Email Authentication Using
              Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
              May 2007.

   [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

              Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

              4rev1", RFC 3501, March 2003.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

   [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, April 2006.

   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [VBR]      Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, April 2009.

Appendix A.  Acknowledgements

   The author wishes to acknowledge the following individuals for their
   review and constructive criticism of this document: Dave Cridland,
   Dave Crocker, Bjoern Hoehrmann, Scott Kitterman, John Levine, Alexey
   Melnikov, S. Moonesamy, and Alessandro Vesely.

Appendix B.  Legacy MUAs

   Implementers of this protocol should be aware that many MUAs are
   unlikely to be retrofitted to support the new header field and its
   semantics.  In the interests of convenience and quicker adoption, a
   delivery MTA might want to consider adding things that are processed
   by existing MUAs in addition to the Authentication-Results header
   field.  One suggestion is to include a Priority header field, on
   messages that don't already have such a header field, containing a
   value that reflects the strength of the authentication that was
   accomplished, e.g., "low" for weak or no authentication, "normal" or
   "high" for good or strong authentication.

   Some modern MUAs can already filter based on the content of this
   header field.  However, there is keen interest in having MUAs make
   some kind of graphical representation of this header field's meaning
   to end users.  Until this capability is added, other interim means of
   conveying authentication results may be necessary while this proposal
   and its successors are adopted.

Appendix C.  Authentication-Results Examples

   This section presents some examples of the use of this header field
   to indicate authentication results.

C.1.  Trivial Case; Header Field Not Present

   The trivial case:

        Received: from mail-router.example.com
                      (mail-router.example.com [])
                  by server.example.org (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Message-Id: <12345.abc@example.com>
        Subject: here's a sample

        Hello!  Goodbye!

                          Example 1: Trivial Case

   The Authentication-Results header field is completely absent.  The
   MUA may make no conclusion about the validity of the message.  This
   could be the case because the message authentication services were
   not available at the time of delivery, or no service is provided, or
   the MTA is not in compliance with this specification.

C.2.  Nearly Trivial Case; Service Provided, but No Authentication Done

   A message that was delivered by an MTA that conforms to this
   specification but provides no actual message authentication service:

        Authentication-Results: example.org 1; none
        Received: from mail-router.example.com
                      (mail-router.example.com [])
                  by server.example.org (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Message-Id: <12345.abc@example.com>
        Subject: here's a sample

        Hello!  Goodbye!

           Example 2: Header Present but No Authentication Done

   The Authentication-Results header field is present, showing that the
   delivering MTA conforms to this specification.  It used its DNS
   domain name as the authserv-id.  The presence of "none" (and the
   absence of any method and result tokens) indicates that no message
   authentication was done.  The version number of the specification to
   which the field's content conforms is explicitly provided.

C.3.  Service Provided, Authentication Done

   A message that was delivered by an MTA that conforms to this
   specification and applied some message authentication:

        Authentication-Results: example.com;
                  spf=pass smtp.mailfrom=example.net
        Received: from dialup-1-2-3-4.example.net
                      (dialup-1-2-3-4.example.net [])
                  by mail-router.example.com (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        From: sender@example.net
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.com
        Message-Id: <12345.abc@example.net>
        Subject: here's a sample

        Hello!  Goodbye!

                    Example 3: Header Reporting Results

   The Authentication-Results header field is present, indicating that
   the border MTA conforms to this specification.  The authserv-id is
   once again the DNS domain name.  Furthermore, the message was
   authenticated by that MTA via the method specified in [SPF].  Note
   that since that method cannot authenticate the local-part, it has
   been omitted from the result's value.  The MUA could extract and
   relay this extra information if desired.

C.4.  Service Provided, Several Authentications Done, Single MTA

   A message that was relayed inbound via a single MTA that conforms to
   this specification and applied three different message authentication

        Authentication-Results: example.com;
                  auth=pass (cram-md5) smtp.auth=sender@example.net;
                  spf=pass smtp.mailfrom=example.net
        Authentication-Results: example.com;
                  sender-id=pass header.from=example.net
        Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6)
                      (dialup-1-2-3-4.example.net [])
                  by mail-router.example.com (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.com
        From: sender@example.net
        Message-Id: <12345.abc@example.net>
        Subject: here's a sample

        Hello!  Goodbye!

             Example 4: Headers Reporting Results from One MTA

   The Authentication-Results header field is present, indicating that
   the delivering MTA conforms to this specification.  Once again, the
   receiving DNS domain name is used as the authserv-id.  Furthermore,
   the sender authenticated herself/himself to the MTA via a method
   specified in [AUTH], and both SPF and Sender ID checks were done and
   passed.  The MUA could extract and relay this extra information if

   Two Authentication-Results header fields are not required since the
   same host did all of the checking.  The authenticating agent could
   have consolidated all the results into one header field.

   This example illustrates a scenario in which a remote user on a
   dialup connection (example.net) sends mail to a border MTA
   (example.com) using SMTP authentication to prove identity.  The
   dialup provider has been explicitly authorized to relay mail as
   example.com resulting in passes by the SPF and Sender ID checks.

C.5.  Service Provided, Several Authentications Done, Different MTAs

   A message that was relayed inbound by two different MTAs that conform
   to this specification and applied multiple message authentication

        Authentication-Results: example.com;
                  sender-id=fail header.from=example.com;
                  dkim=pass (good signature) header.d=example.com
        Received: from mail-router.example.com
                      (mail-router.example.com [])
                  by auth-checker.example.com (8.11.6/8.11.6)
                      with ESMTP id i7PK0sH7021929;
                  Fri, Feb 15 2002 17:19:22 -0800
        DKIM-Signature:  v=1; a=rsa-sha256; s=gatsby; d=example.com;
                  t=1188964191; c=simple/simple; h=From:Date:To:Subject:
                  b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
        Authentication-Results: example.com;
                  auth=pass (cram-md5) smtp.auth=sender@example.com;
                  spf=fail smtp.mailfrom=example.com
        Received: from dialup-1-2-3-4.example.net
                      (dialup-1-2-3-4.example.net [])
                  by mail-router.example.com (8.11.6/8.11.6)
                      with ESMTP id g1G0r1kA003489;
                  Fri, Feb 15 2002 17:19:07 -0800
        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.com
        Message-Id: <12345.abc@example.com>
        Subject: here's a sample

        Hello!  Goodbye!

          Example 5: Headers Reporting Results from Multiple MTAs

   The Authentication-Results header field is present, indicating
   conformance to this specification.  Once again, the authserv-id used
   is the recipient's DNS domain name.  The header field is present
   twice because two different MTAs in the chain of delivery did
   authentication tests.  The first MTA, mail-router.example.com,
   reports that SMTP AUTH and SPF were both used and that the former
   passed while the latter failed.  In the SMTP AUTH case, additional
   information is provided in the comment field, which the MUA can
   choose to render if desired.

   The second MTA, auth-checker.example.com, reports that it did a
   Sender ID test (which failed) and a DKIM test (which passed).  Again,
   additional data about one of the tests is provided as a comment,
   which the MUA may choose to render.  Also noteworthy here is the fact
   that there is a DKIM signature added by example.com that assured the
   integrity of the lower Authentication-Results field.

   Since different hosts did the two sets of authentication checks, the
   header fields cannot be consolidated in this example.

   This example illustrates more typical transmission of mail into
   example.com from a user on a dialup connection example.net.  The user
   appears to be legitimate as he/she had a valid password allowing
   authentication at the border MTA using SMTP AUTH.  The SPF and Sender
   ID tests failed since example.com has not granted example.net
   authority to relay mail on its behalf.  However, the DKIM test passed
   because the sending user had a private key matching one of
   example.com's published public keys and used it to sign the message.

C.6.  Service Provided, Multi-Tiered Authentication Done

   A message that had authentication done at various stages, one of
   which was outside the receiving ADMD:

        Authentication-Results: example.com;
              dkim=pass reason="good signature"
              dkim=fail reason="bad signature"
        Received: from mail-router.example.net
                  (mail-router.example.net [])
              by chicago.example.com (8.11.6/8.11.6)
                  for <recipient@chicago.example.com>
                  with ESMTP id i7PK0sH7021929;
              Fri, Feb 15 2002 17:19:22 -0800
        DKIM-Signature: v=1; a=rsa-sha256; s=furble;
              d=mail-router.example.net; t=1188964198; c=relaxed/simple;
              b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
        Authentication-Results: example.net;
              dkim=pass (good signature) header.i=@newyork.example.com
        Received: from smtp.newyork.example.com
                  (smtp.newyork.example.com [])
              by mail-router.example.net (8.11.6/8.11.6)
                  with ESMTP id g1G0r1kA003489;
              Fri, Feb 15 2002 17:19:07 -0800
        DKIM-Signature: v=1; a=rsa-sha256; s=gatsby;

              t=1188964191; c=simple/simple;
              b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
        From: sender@newyork.example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: meetings@example.net
        Message-Id: <12345.abc@newyork.example.com>
        Subject: here's a sample

          Example 6: Headers Reporting Results from Multiple MTAs
                            in Different ADMDs

   In this example, we see multi-tiered authentication with an extended
   trust boundary.

   The message was sent from someone at example.com's New York office
   (newyork.example.com) to a mailing list managed at an intermediary.
   The message was signed at the origin using DKIM.

   The message was sent to a mailing list service provider called
   example.net, which is used by example.com.  There,
   meetings@example.net is expanded to a long list of recipients, one of
   whom is at the Chicago office.  In this example, we will assume that
   the trust boundary for chicago.example.com includes the mailing list
   server at example.net.

   The mailing list server there first authenticated the message and
   affixed an Authentication-Results header field indicating such using
   its DNS domain name for the authserv-id.  It then altered the message
   by affixing some footer text to the body, including some
   administrivia such as unsubscription instructions.  Finally, the
   mailing list server affixes a second DKIM signature and begins
   distribution of the message.

   The border MTA for chicago.example.com explicitly trusts results from
   mail-router.example.net, so that header field is not removed.  It
   performs evaluation of both signatures and determines that the first
   (most recent) is a "pass" but, because of the aforementioned
   modifications, the second is a "fail".  However, the first signature
   included the Authentication-Results header added at mail-
   router.example.net that validated the second signature.  Thus,
   indirectly, it can be determined that the authentications claimed by
   both signatures are indeed valid.

   Note that two styles of presenting metadata about the result are in
   use here.  In one case, the "reason=" clause is present, which is
   intended for easy extraction by parsers; in the other case, the CFWS
   production of the ABNF is used to include such data as a header field
   comment.  The latter can be harder for parsers to extract given the
   varied supported syntaxes of mail header fields.

C.7.  Comment-Heavy Example

   The formal syntax permits comments within the content in a number of
   places.  For the sake of illustration, this example is also legal:

       Authentication-Results: foo.example.net (foobar) 1 (baz);
           dkim (Because I like it) / 1 (One yay) = (wait for it) fail
             policy (A dot can go here) . (like that) expired
             (this surprised me) = (as I wasn't expecting it) 1362471462

        Example 7: A Very Comment-Heavy but Perfectly Legal Example

Appendix D.  Operational Considerations about Message Authentication

   This protocol is predicated on the idea that authentication (and
   presumably in the future, reputation) work is typically done by
   border MTAs rather than MUAs or intermediate MTAs; the latter merely
   make use of the results determined by the former.  Certainly this is
   not mandatory for participation in electronic mail or message
   authentication, but this protocol and its deployment to date are
   based on that model.  The assumption satisfies several common ADMD

   1.  Service operators prefer to resolve the handling of problem
       messages as close to the border of the ADMD as possible.  This
       enables, for example, rejection of messages at the SMTP level
       rather than generating a DSN internally.  Thus, doing any of the
       authentication or reputation work exclusively at the MUA or
       intermediate MTA renders this desire unattainable.

   2.  Border MTAs are more likely to have direct access to external
       sources of authentication or reputation information since modern
       MUAs are more likely to be heavily firewalled.  Thus, some MUAs
       might not even be able to complete the task of performing
       authentication or reputation evaluations without complex proxy
       configurations or similar burdens.

   3.  MUAs rely upon the upstream MTAs within their trust boundaries to
       make correct (as much as is possible) evaluations about the
       message's envelope, header, and content.  Thus, MUAs don't need
       to know how to do the work that upstream MTAs do; they only need
       the results of that work.

   4.  Evaluations about the quality of a message, from simple token
       matching (e.g., a list of preferred DNS domains) to cryptanalysis
       (e.g., public/private key work), are at least a little bit
       expensive and thus need to be minimized.  To that end, performing
       those tests at the border MTA is far preferred to doing that work
       at each MUA that handles a message.  If an ADMD's environment
       adheres to common messaging protocols, a reputation query or an
       authentication check performed by a border MTA would return the
       same result as the same query performed by an MUA.  By contrast,
       in an environment where the MUA does the work, a message arriving
       for multiple recipients would thus cause authentication or
       reputation evaluation to be done more than once for the same
       message (i.e., at each MUA), causing needless amplification of
       resource use and creating a possible denial-of-service attack

   5.  Minimizing change is good.  As new authentication and reputation
       methods emerge, the list of methods supported by this header
       field would presumably be extended.  If MUAs simply consume the
       contents of this header field rather than actually attempt to do
       authentication and/or reputation work, then MUAs only need to
       learn to parse this header field once; emergence of new methods
       requires only a configuration change at the MUAs and software
       changes at the MTAs (which are presumably fewer in number).  When
       choosing to implement these functions in MTAs vs. MUAs, the
       issues of individual flexibility, infrastructure inertia, and
       scale of effort must be considered.  It is typically easier to
       change a single MUA than an MTA because the modification affects
       fewer users and can be pursued with less care.  However, changing
       many MUAs is more effort than changing a smaller number of MTAs.

   6.  For decisions affecting message delivery and display, assessment
       based on authentication and reputation is best performed close to
       the time of message transit, as a message makes its journey
       toward a user's inbox, not afterwards.  DKIM keys and IP address
       reputations, etc., can change over time or even become invalid,
       and users can take a long time to read a message once delivered.
       The value of this work thus degrades, perhaps quickly, once the
       delivery process has completed.  This seriously diminishes the
       value of this work when done other than at MTAs.

   Many operational choices are possible within an ADMD, including the
   venue for performing authentication and/or reputation assessment.
   The current specification does not dictate any of those choices.
   Rather, it facilitates those cases in which information produced by
   one stage of analysis needs to be transported with the message to the
   next stage.

Appendix E.  Changes since RFC 5451

   o  Erratum #2617 was addressed in RFC 6577 and was incorporated here.

   o  Requested Internet Standard status.

   o  Changed IANA rules from "IETF Review" to "designated expert".

   o  Updated existing IANA registries from the old RFC to this one.

   o  Added references to ADSP, ATPS, and VBR.

   o  Removed all the "X-" stuff, per BCP 178.

   o  Adjusted language to indicate that this header field was already
      defined and that we're just refreshing and revising.

   o  In a few places, RFC 2119 language had been used in lowercase
      terms; fixed here.

   o  Erratum #2818 addressed.

   o  Erratum #3195 addressed.

   o  Performed some minor wordsmithing and removed odd prose.

   o  ABNF: changed "dot-atom" to "Keyword" since "dot-atom" allows "=",
      which leads to ambiguous productions.

   o  ABNF: the authserv-id can be a "value", not a "dot-atom".

   o  ABNF: separated the spec version from the method version; they're
      syntactically the same but semantically different.  Added a
      section discussing them.

   o  Called out the SMTP verb exceptions ("mailfrom" and "rcptto"); the
      previous RFC didn't do this, leading to interoperability problems.

   o  Rather than deleting suspect header fields, they could also be
      renamed to something harmless; there is at least one
      implementation of this.

   o  Updated IANA "Email Authentication Methods" registry to include
      version numbers.

   o  Rather than repeating what RFC 4408 says the SPF results are, just
      referred to those documents.

   o  To avoid confusing consumers, constrained inclusion of unnecessary

   o  Reviewed usage of "should" vs. "SHOULD".

   o  Updated prose around authserv-id (Section 2.4).

Author's Address

   Murray S. Kucherawy
   270 Upland Drive
   San Francisco, CA  94127

   EMail: superuser@gmail.com


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