    Internet Engineering Task Force (IETF)                      M. Kucherawy
    Request for Comments: 8601                                      May 2019
    Obsoletes: 7601
    Category: Standards Track
    ISSN: 2070-1721

       Message Header Field for Indicating Message Authentication Status

    Abstract

       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.

       This document obsoletes RFC 7601.

    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 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8601.

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    Copyright Notice

       Copyright (c) 2019 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
       (https://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 ....................................................4
          1.1. Purpose ....................................................5
          1.2. Trust Boundary .............................................6
          1.3. Processing Scope ...........................................7
          1.4. Requirements ...............................................7
          1.5. Definitions ................................................7
               1.5.1. Key Words ...........................................7
               1.5.2. Internationalized Email .............................7
               1.5.3. Security ............................................8
               1.5.4. Email Architecture ..................................8
               1.5.5. Other Terms .........................................9
          1.6. Trust Environment .........................................10
       2. Definition and Format of the Header Field ......................10
          2.1. General Description .......................................10
          2.2. Formal Definition .........................................11
          2.3. Property Types (ptypes) and Properties ....................13
          2.4. The "policy" ptype ........................................15
          2.5. Authentication Service Identifier Field ...................15
          2.6. Version Tokens ............................................17
          2.7. Defined Methods and Result Values .........................17
               2.7.1. DKIM ...............................................17
               2.7.2. SPF ................................................19
               2.7.3. "iprev" ............................................20
               2.7.4. SMTP AUTH ..........................................21
               2.7.5. Other Registered Codes .............................22
               2.7.6. Extension Methods ..................................22
               2.7.7. Extension Result Codes .............................23
       3. The "iprev" Authentication Method ..............................23
       4. Adding the Header Field to a Message ...........................25
          4.1. Header Field Position and Interpretation ..................26
          4.2. Local Policy Enforcement ..................................27

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       5. Removing Existing Header Fields ................................28
       6. IANA Considerations ............................................29
          6.1. The Authentication-Results Header Field ...................29
          6.2. "Email Authentication Methods" Registry Description .......30
          6.3. "Email Authentication Methods" Registry Update ............31
               6.3.1. "header.a" for DKIM ................................32
               6.3.2. "header.s" for DKIM ................................32
          6.4. "Email Authentication Property Types" Registry
               Description ...............................................32
          6.5. "Email Authentication Property Types" Registry Update .....33
          6.6. "Email Authentication Result Names" Registry Description ..33
          6.7. "Email Authentication Result Names" Registry Update .......34
          6.8. SMTP Enhanced Status Codes ................................34
       7. Security Considerations ........................................35
          7.1. Forged Header Fields ......................................35
          7.2. Misleading Results ........................................36
          7.3. Header Field Position .....................................37
          7.4. Reverse IP Query Denial-of-Service Attacks ................37
          7.5. Mitigation of Backscatter .................................37
          7.6. Internal MTA Lists ........................................37
          7.7. Attacks against Authentication Methods ....................38
          7.8. Intentionally Malformed Header Fields .....................38
          7.9. Compromised Internal Hosts ................................38
          7.10. Encapsulated Instances ...................................38
          7.11. Reverse Mapping ..........................................39
       8. References .....................................................39
          8.1. Normative References ......................................39
          8.2. Informative References ....................................40
       Appendix A. Legacy MUAs ...........................................44
       Appendix B. Authentication-Results Examples .......................44
         B.1. Trivial Case: Header Field Not Present .....................44
         B.2. Nearly Trivial Case: Service Provided, but No
              Authentication Done ........................................45
         B.3. Service Provided, Authentication Done ......................46
         B.4. Service Provided, Several Authentications Done, Single
              MTA ........................................................47
         B.5. Service Provided, Several Authentications Done, Different
              MTAs .......................................................48
         B.6. Service Provided, Multi-tiered Authentication Done .........50
         B.7. Comment-Heavy Example ......................................51
       Appendix C. Operational Considerations about Message
                   Authentication ........................................52
       Appendix D. Changes since RFC 7601 ................................53
       Acknowledgments ...................................................54
       Author's Address ..................................................54

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    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
       content.

       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 email authentication methods:

       o  SMTP Service Extension for Authentication [AUTH]

       o  DomainKeys Identified Mail Signatures [DKIM]

       o  Domain-based Message Authentication, Reporting, and Conformance
          [DMARC]

       o  Sender Policy Framework [SPF]

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

       o  Require-Recipient-Valid-Since Header Field and SMTP Service
          Extension [RRVS]

       o  S/MIME Signature Verification [SMIME-REG]

       o  Vouch By Reference [VBR]

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       The following Historic specifications were previously supported by
       this framework but have since become obsolete:

       o  Author Domain Signing Practices [ADSP] (Historic)

       o  DomainKeys [DOMAINKEYS] (Historic)

       Note that at the time of publication of this document the Sender ID
       specification [SENDERID] (Experimental) is no longer supported by
       this framework.  Discussion regarding moving it to Historic status is
       underway.

       There exist registries for tokens used within this header field that
       refer to the specifications listed above.  Section 6 describes the
       registries and their contents and specifies the process by which
       entries are added or updated.  It also updates the existing contents
       to match the current states of these specifications.

       The goal of this work 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 such data.

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

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       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 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, the paths
       between them, 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
       possible.

       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, if found within one of those media
       types, this field is typically ignored.

       Note that an MUA could be configured to retrieve messages from a
       receiver yet not be within the receiver's ADMD.  In this case, for
       the purposes of this work, that MUA is considered to be within the
       receiver's ADMD if it is configured to identify and ascribe value to
       authentication results recorded by that ADMD.

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    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,
       insofar as a non-participating service will continue to interoperate
       with the deployed messaging infrastructure.

       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.

       A participating ADMD does undertake some filtering and message
       modification obligations as described in Section 5.

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

    1.5.2.  Internationalized Email

       In this document, there are references to messages formatted to
       support Email Address Internationalization (EAI).  Reference material
       for this can be found in [RFC6530], [RFC6531], and [RFC6532].
       Generally speaking, these documents allow UTF-8 in most places that
       free-form text can be found and U-labels where domain names can be
       used, and this document extends Authentication-Results accordingly.

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    1.5.3.  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 is an authorization mechanism in that it expresses a
       result that shows whether the ADMD that apparently sent the message
       has explicitly authorized the connecting Simple Mail Transfer
       Protocol (SMTP) client [SMTP] to relay messages on its behalf, but it
       does 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 the ADMD of origin, intermediate ADMDs (such as a
       mailing list server), other handling agents, or any combination of
       these.

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

    1.5.4.  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.

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

       o  A Message Submission Agent (MSA) is an agent that accepts a
          message from an MUA, introducing it to the mail-handling stream.

       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 C of
       this document for discussion about the common aspects of email
       authentication in current environments.

                              +-----+   +-----+   +------------+
                              | MUA |-->| MSA |-->| Border MTA |
                              +-----+   +-----+   +------------+
                                                        |
                                                        |
                                                        V
                                                   +----------+
                                                   | Internet |
                                                   +----------+
                                                        |
                                                        |
                                                        V
       +-----+   +-----+   +------------------+   +------------+
       | 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.

    1.5.5.  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.

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    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 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 claim to be associated with the
       ADMD 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
       document.

    2.  Definition and Format of the Header Field

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

    2.1.  General Description

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

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

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

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       The header field can appear more than once in a single message, more
       than one result can be represented in a single header field, or a
       combination of these can be applied.

    2.2.  Formal Definition

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

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

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

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

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

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

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

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

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

         propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
                  ; an indication of which properties of the message
                  ; were evaluated by the authentication scheme being
                  ; applied to yield the reported result

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         method = Keyword [ [CFWS] "/" [CFWS] method-version ]
                ; a method indicates which method's result is
                ; represented by "result"; it is one of the methods
                ; explicitly defined as valid in this document
                ; or is an extension method as defined below

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

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

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

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

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

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

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

       "CFWS" is defined in Section 3.2.2 of [MAIL].

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

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

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

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

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

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

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

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

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

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

    2.3.  Property Types (ptypes) and Properties

       The "ptype" in the ABNF above indicates the general type of property
       being described by the result being reported, upon which the reported
       result was based.  Coupled with the "property", which is more
       specific, it indicates from where the reported "pvalue" was
       extracted.  This can include a particular part of the message header
       or body, some part of the SMTP session, a secondary output of an
       authentication method (apart from its pure result), or some other
       aspect of the message's handling.

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       Combinations of ptypes and properties are registered and described in
       the "Email Authentication Methods" registry, coupled with the
       authentication methods with which they are used.  This is further
       described in Section 6.

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

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

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

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

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

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

       Entries in the "Email Authentication Methods" registry can define
       properties that deviate from these definitions when appropriate.
       Such deviations need to be clear in the registry and/or in the
       defining document.  See Section 2.7.1 for an example.

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    2.4.  The "policy" ptype

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

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

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

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

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

    2.5.  Authentication Service Identifier Field

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

       Note that in an EAI-formatted message, this identifier may be
       expressed in UTF-8.

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       Since agents consuming this field will use this identifier to
       determine whether its contents are of interest (and are safe to use),
       the uniqueness of the identifier MUST be guaranteed by the ADMD that
       generates it and MUST pertain to that ADMD.  MUAs or downstream
       filters SHOULD use this identifier to determine whether or not the
       data contained in an Authentication-Results header field ought to be
       used or ignored.

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

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

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

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

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

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

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    2.6.  Version Tokens

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

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

    2.7.  Defined Methods and Result Values

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

    2.7.1.  DKIM

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

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

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       The DKIM result set is as follows:

       none:  The message was not signed.

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

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

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

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

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

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

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

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

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

       The ability to report different DKIM results for a message with
       multiple signatures is described in [RFC6008].

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       [DKIM] advises that if a message fails verification, it is to be
       treated as an unsigned message.  A report of "fail" here permits the
       receiver of the report to decide how to handle the failure.  A report
       of "neutral" or "none" preempts that choice, ensuring that the
       message will be treated as if it had not been signed.

    2.7.2.  SPF

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

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

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

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

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

       For this method, an additional result of "policy" is defined, which
       means the client was authorized to inject or relay mail on behalf of
       the sender's DNS domain according to the authentication method's

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       algorithm, but local policy dictates that the result is unacceptable.
       For example, "policy" might be used if SPF returns a "pass" result,
       but a local policy check matches the sending DNS domain to one found
       in an explicit list of unacceptable DNS domains (e.g., spammers).

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

    2.7.3.  "iprev"

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

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

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

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

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

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

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

       For discussion of the format of DNS replies, see "Domain names -
       implementation and specification" [DNS].

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    2.7.4.  SMTP AUTH

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

       none:  SMTP authentication was not attempted.

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

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

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

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

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

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

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

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

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

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

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

         ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example

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       Note that in all cases other than "pass", the message was sent by an
       unauthenticated client.  All non-"pass" cases SHOULD thus be treated
       as equivalent with respect to this method.

    2.7.5.  Other Registered Codes

       Result codes were also registered in other RFCs as follows:

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

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

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

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

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

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

    2.7.6.  Extension Methods

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

       Extension methods can be defined for the following reasons:

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

       2.  To allow the creation of "sub-identifiers" that indicate
           different levels of authentication and differentiate between
           their relative strengths, e.g., "auth1-weak" and "auth1-strong".

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       Authentication method implementers are encouraged to provide adequate
       information, via message header field comments if necessary, to allow
       an MUA developer to understand or relay ancillary details of
       authentication results.  For example, if it might be of interest to
       relay what data were used to perform an evaluation, such information
       could be relayed as a comment in the header field, such as:

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

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

    2.7.7.  Extension Result Codes

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

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

    3.  The "iprev" Authentication Method

       This section defines an additional authentication method called
       "iprev".

       "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

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       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
       address.

       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.

       Often an MTA receiving a connection that fails this test will simply
       reject the connection using the enhanced status code defined in
       [AUTH-ESC].  If an operator instead wishes to make this information
       available to downstream agents as a factor in handling decisions, it
       records a result in accordance with Section 2.7.3.

       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 4.6.4 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
       report.

       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
       acknowledgment that the method remains common and provides the means
       to relay the results of that test.

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    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 2.7.6,
       and each "result" MUST refer to a result code declared in the IANA
       registry or an extension result code as defined in Section 2.7.7.
       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 was
       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
       service 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 differs in that it can yield a result
       based on more than one identifier; specifically, SPF can evaluate the
       RFC5321.HELO parameter or the RFC5321.MailFrom parameter.  When
       generating this field to report those results, only the parameter
       that yielded the result is included.

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       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 have 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 (1) 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 (2) instances of the header field
       that appear 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 of the
       header field is used within the 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 "Email Authentication Property Types"
       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.  The exception to this is experimental methods
       as discussed in Section 2.7.6.

       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 "1" (one)) and send signed
       mail to intended victims; a verifier would detect that the signature

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       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.  They are likely to contain the
       results of authentication checks done in the past, possibly long ago,
       and have no contemporary value.  Due caution therefore needs to be
       taken when choosing to consume them.

       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 2.7).

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

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    5.  Removing Existing Header Fields

       To mitigate the impact of forged header fields, 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 internal MTA will need to be configured with a list of other
       known, trusted MTAs that are thus expected to be using that same
       identifier.

       In the case of EAI-formatted messages, this test is done after
       converting A-labels into U-labels.

       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 B.

       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.

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       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 [SMTP]
       relaying the message is not from a trusted internal MTA.  (As
       discussed above, this too can result in invalidation of signatures.)
       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 registries
       as described below.  These registry actions were originally defined
       by [RFC5451] and updated by [RFC6577] and [RFC7001].  The created
       registries were further updated in [RFC7601] to make them more
       complete.

       Each registry has two related sections below.  The first describes
       the registry and its update procedures, which are unchanged from
       [RFC7601].  The second enumerates changes to entries that are
       relevant to this document.

    6.1.  The Authentication-Results Header Field

       The Authentication-Results header field was added 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 8601
          Related information: none

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    6.2.  "Email Authentication Methods" Registry Description

       Names of message authentication methods supported by this
       specification have been registered with IANA, with the exception of
       experimental names as described in Section 2.7.6.  Along with each
       method are recorded the properties that accompany the method's
       result.

       The "Email Authentication Parameters" group, and within it the "Email
       Authentication Methods" registry, were created by [RFC5451] for this
       purpose.  [RFC6577] added a "Status" field for each entry.  [RFC7001]
       amended the rules governing that registry and also added a "Version"
       field to the registry.

       The reference for that registry has been updated to reference this
       document.

       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".

       No two entries can have the same combination of method, ptype, and
       property.

       An entry in this registry contains the following:

       Method:  the name of the method.

       Definition:  a reference to the document that created this entry, if
          any (see below).

       ptype:  a "ptype" value appropriate for use with that method.

       Property:  a "property" value matching that "ptype" also appropriate
          for use with that method.

       Value:  a brief description of the value to be supplied with that
          method/ptype/property tuple.

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       Status:  the status of this entry, which is one of the following:

          active:  The entry is in current use.

          deprecated:  The entry is no longer in current use.

       Version:  a version number associated with the method (preferably
          starting at "1").

       The "Definition" field will typically refer to a permanent document,
       or at least some descriptive text, where additional information about
       the entry being added can be found.  This might in turn reference the
       document where the method is defined so that all of the semantics
       around creating or interpreting an Authentication-Results header
       field using this method, ptype, and property can be understood.

    6.3.  "Email Authentication Methods" Registry Update

       The following entries in this registry have been updated to replace
       [RFC7601] with this document:

         +------------+--------+----------------------------------+
         |   Method   | ptype  | Property                         |
         +------------+--------+----------------------------------+
         |    auth    |  smtp  | auth                             |
         +------------+--------+----------------------------------+
         |    auth    |  smtp  | mailfrom                         |
         +------------+--------+----------------------------------+
         |    dkim    | header | d                                |
         +------------+--------+----------------------------------+
         |    dkim    | header | i                                |
         +------------+--------+----------------------------------+
         |   iprev    | policy | iprev                            |
         +------------+--------+----------------------------------+
         |    spf     |  smtp  | mailfrom                         |
         +------------+--------+----------------------------------+
         |    spf     |  smtp  | helo                             |
         +------------+--------+----------------------------------+

       Notably, the DomainKeys and Sender ID entries are not updated to
       refer to this revised specification, as they are considered obsolete.
       Accordingly, IANA has changed the "Status" field of the "sender-id"
       entry in this table to "deprecated".

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       Finally, two new entries have been added to this registry, as
       follows:

    6.3.1.  "header.a" for DKIM

       Method:  dkim

       Definition:  RFC 8601

       ptype:  header

       Property:  a

       Value:  value of signature "a" tag

       Status:  active

       Version:  1

    6.3.2.  "header.s" for DKIM

       Method:  dkim

       Definition:  RFC 8601

       ptype:  header

       Property:  s

       Value:  value of signature "s" tag

       Status:  active

       Version:  1

    6.4.  "Email Authentication Property Types" Registry Description

       [RFC7410] created the "Email Authentication Property Types" registry.

       Entries in this registry are subject to the Expert Review rules as
       described in [IANA-CONSIDERATIONS].  Each entry in the registry
       requires the following values:

       ptype:  the name of the ptype being registered, which must fit within
          the ABNF described in Section 2.2.

       Definition:  an optional reference to a defining specification.

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       Description:  a brief description of what sort of information this
          "ptype" is meant to cover.

       For new entries, the designated expert needs to ensure that the
       description provided for the new entry adequately describes the
       intended use.  An example would be helpful to include in the entry's
       defining document (if any), although entries in the "Email
       Authentication Methods" registry or the "Email Authentication Result
       Names" registry might also serve as examples of intended use.

       As this is a complete restatement of the definition and rules for
       this registry, IANA has updated this registry to show Section 2.3 of
       this document as the current definitions for the "body", "header",
       "policy", and "smtp" entries of that registry.

    6.5.  "Email Authentication Property Types" Registry Update

       All current entries in this registry have been updated to replace
       [RFC7601] with this document.

    6.6.  "Email Authentication Result Names" Registry Description

       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 2.7.7.

       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".

       No two entries can have the same combination of method and code.

       An entry in this registry contains the following:

       Auth Method:  an authentication method for which results are being
          returned using the header field defined in this document.

       Code:  a result code that can be returned for this authentication
          method.

       Specification:  either free-form text explaining the meaning of this
          method-code combination or a reference to such a definition.

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       Status:  the status of this entry, which is one of the following:

          active:  The entry is in current use.

          deprecated:  The entry is no longer in current use.

    6.7.  "Email Authentication Result Names" Registry Update

       For the following entries in this registry, the new "Specification"
       field has been set as follows:

       o  All "auth" method result codes ("fail", "none", "pass",
          "permerror", and "temperror") are now specified in Section 2.7.4
          of this document.

       o  All "dkim" method result names ("fail", "neutral", "none", "pass",
          "permerror", "policy", and "temperror") are now specified in
          Section 2.7.1 of this document.

       o  All "iprev" method result names ("fail", "pass", "permerror", and
          "temperror") are now specified in Section 2.7.3 of this document.

       o  The "spf" method result names "fail", "neutral", "none", "pass",
          "permerror", "policy", "softfail", and "temperror" are now
          specified in Section 2.7.2 of this document.  The registration for
          result name "hardfail" is not updated.

       The following entries in this registry have been updated with a new
       "Status" field set to "deprecated", and with no change to the
       "Specification" field as they reference historic protocols:

       o  All "domainkeys" method result names ("fail", "neutral", "none",
          "pass", "permerror", "policy", and "temperror").

       o  All "sender-id" method result names ("fail", "neutral", "none",
          "pass", "permerror", "policy", "softfail", and "temperror").

    6.8.  SMTP Enhanced Status Codes

       The entry for X.7.25 in the "Enumerated Status Codes" subregistry of
       the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
       Registry" has been updated to refer only to Section 3.3 of
       [AUTH-ESC], as that is where that registration was done.

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    7.  Security Considerations

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

    7.1.  Forged Header Fields

       An MTA not applying the filtering discussed in Section 5 exposes MUAs
       to 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.

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       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, this header field 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 [SMTP] 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 [IMAP] 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

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       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
       acknowledgment 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 4.6.4 of [SPF] observes that limits are necessary on
       recursive evaluations of SPF records in order to avoid abuse of or
       attacks on the DNS when verifying 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
       [DSN] (or equivalent) to addresses that did not send the messages
       being rejected.

    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
       attack.

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    7.7.  Attacks against Authentication Methods

       If an attack against an authentication method becomes known, 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

       As with any other header field found in the message, 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 [SMTP] 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 simple guidance to MUA developers is
       appropriate here.

       Since MTAs are generally unlikely to strip Authentication-Results
       header fields during mailbox delivery, normative language exists in
       Section 4.1 cautioning MUAs to ignore such instances within MIME
       attachments, as might be included when a message is forwarded.
       Moreover, when extracting a message digest to separate mail store

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       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.

       There can be cases where these header fields included as part of
       encapsulated messages might actually be of value, such as when they
       are taken from messages within the same ADMD where they will be
       consumed.  Caution must be taken to ensure that the consumer fully
       understands the semantics of what the header field is indicating and
       the message's handling history before ascribing any value, positive
       or negative, to such data.

    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 specification 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
       "iprev".

    8.  References

    8.1.  Normative References

       [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <https://www.rfc-editor.org/info/rfc5234>.

       [IANA-HEADERS]
                  Klyne, G., Nottingham, M., and J. Mogul, "Registration
                  Procedures for Message Header Fields", BCP 90, RFC 3864,
                  DOI 10.17487/RFC3864, September 2004,
                  <https://www.rfc-editor.org/info/rfc3864>.

       [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
                  DOI 10.17487/RFC5322, October 2008,
                  <https://www.rfc-editor.org/info/rfc5322>.

       [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part One: Format of Internet Message
                  Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
                  <https://www.rfc-editor.org/info/rfc2045>.

    Kucherawy                    Standards Track                   [Page 39]

    RFC 8601           Authentication-Results Header Field          May 2019

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
                  Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
                  February 2012, <https://www.rfc-editor.org/info/rfc6530>.

       [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
                  Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
                  <https://www.rfc-editor.org/info/rfc6531>.

       [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
                  Email Headers", RFC 6532, DOI 10.17487/RFC6532,
                  February 2012, <https://www.rfc-editor.org/info/rfc6532>.

       [RFC7601]  Kucherawy, M., "Message Header Field for Indicating
                  Message Authentication Status", RFC 7601,
                  DOI 10.17487/RFC7601, August 2015,
                  <https://www.rfc-editor.org/info/rfc7601>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
                  RFC 2119 Key Words", BCP 14, RFC 8174,
                  DOI 10.17487/RFC8174, May 2017,
                  <https://www.rfc-editor.org/info/rfc8174>.

       [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                  DOI 10.17487/RFC5321, October 2008,
                  <https://www.rfc-editor.org/info/rfc5321>.

    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, DOI 10.17487/RFC5617,
                  August 2009, <https://www.rfc-editor.org/info/rfc5617>.

       [AR-VBR]   Kucherawy, M., "Authentication-Results Registration for
                  Vouch by Reference Results", RFC 6212,
                  DOI 10.17487/RFC6212, April 2011,
                  <https://www.rfc-editor.org/info/rfc6212>.

       [ATPS]     Kucherawy, M., "DomainKeys Identified Mail (DKIM)
                  Authorized Third-Party Signatures", RFC 6541,
                  DOI 10.17487/RFC6541, February 2012,
                  <https://www.rfc-editor.org/info/rfc6541>.

    Kucherawy                    Standards Track                   [Page 40]

    RFC 8601           Authentication-Results Header Field          May 2019

       [AUTH]     Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
                  Extension for Authentication", RFC 4954,
                  DOI 10.17487/RFC4954, July 2007,
                  <https://www.rfc-editor.org/info/rfc4954>.

       [AUTH-ESC] Kucherawy, M., "Email Authentication Status Codes",
                  RFC 7372, DOI 10.17487/RFC7372, September 2014,
                  <https://www.rfc-editor.org/info/rfc7372>.

       [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
                  "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
                  RFC 6376, DOI 10.17487/RFC6376, September 2011,
                  <https://www.rfc-editor.org/info/rfc6376>.

       [DMARC]    Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
                  Message Authentication, Reporting, and Conformance
                  (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
                  <https://www.rfc-editor.org/info/rfc7489>.

       [DNS]      Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
                  November 1987, <https://www.rfc-editor.org/info/rfc1035>.

       [DNS-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
                  "DNS Extensions to Support IP Version 6", STD 88,
                  RFC 3596, DOI 10.17487/RFC3596, October 2003,
                  <https://www.rfc-editor.org/info/rfc3596>.

       [DNSOP-REVERSE]
                  Senie, D. and A. Sullivan, "Considerations for the use
                  of DNS Reverse Mapping", Work in Progress,
                  draft-ietf-dnsop-reverse-mapping-considerations-06,
                  March 2008.

       [DOMAINKEYS]
                  Delany, M., "Domain-Based Email Authentication Using
                  Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
                  DOI 10.17487/RFC4870, May 2007,
                  <https://www.rfc-editor.org/info/rfc4870>.

       [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
                  for Delivery Status Notifications", RFC 3464,
                  DOI 10.17487/RFC3464, January 2003,
                  <https://www.rfc-editor.org/info/rfc3464>.

    Kucherawy                    Standards Track                   [Page 41]

    RFC 8601           Authentication-Results Header Field          May 2019

       [EMAIL-ARCH]
                  Crocker, D., "Internet Mail Architecture", RFC 5598,
                  DOI 10.17487/RFC5598, July 2009,
                  <https://www.rfc-editor.org/info/rfc5598>.

       [IANA-CONSIDERATIONS]
                  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

       [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
                  4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
                  <https://www.rfc-editor.org/info/rfc3501>.

       [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
                  STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
                  <https://www.rfc-editor.org/info/rfc1939>.

       [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
                  Message Authentication Status", RFC 5451,
                  DOI 10.17487/RFC5451, April 2009,
                  <https://www.rfc-editor.org/info/rfc5451>.

       [RFC6008]  Kucherawy, M., "Authentication-Results Registration for
                  Differentiating among Cryptographic Results", RFC 6008,
                  DOI 10.17487/RFC6008, September 2010,
                  <https://www.rfc-editor.org/info/rfc6008>.

       [RFC6577]  Kucherawy, M., "Authentication-Results Registration Update
                  for Sender Policy Framework (SPF) Results", RFC 6577,
                  DOI 10.17487/RFC6577, March 2012,
                  <https://www.rfc-editor.org/info/rfc6577>.

       [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
                  Message Authentication Status", RFC 7001,
                  DOI 10.17487/RFC7001, September 2013,
                  <https://www.rfc-editor.org/info/rfc7001>.

       [RFC7410]  Kucherawy, M., "A Property Types Registry for the
                  Authentication-Results Header Field", RFC 7410,
                  DOI 10.17487/RFC7410, December 2014,
                  <https://www.rfc-editor.org/info/rfc7410>.

       [RFC8301]  Kitterman, S., "Cryptographic Algorithm and Key Usage
                  Update to DomainKeys Identified Mail (DKIM)", RFC 8301,
                  DOI 10.17487/RFC8301, January 2018,
                  <https://www.rfc-editor.org/info/rfc8301>.

    Kucherawy                    Standards Track                   [Page 42]

    RFC 8601           Authentication-Results Header Field          May 2019

       [RRVS]     Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
                  Since Header Field and SMTP Service Extension", RFC 7293,
                  DOI 10.17487/RFC7293, July 2014,
                  <https://www.rfc-editor.org/info/rfc7293>.

       [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
                  Text on Security Considerations", BCP 72, RFC 3552,
                  DOI 10.17487/RFC3552, July 2003,
                  <https://www.rfc-editor.org/info/rfc3552>.

       [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
                  RFC 4406, DOI 10.17487/RFC4406, April 2006,
                  <https://www.rfc-editor.org/info/rfc4406>.

       [SMIME-REG]
                  Melnikov, A., "Authentication-Results Registration for
                  S/MIME Signature Verification", RFC 7281,
                  DOI 10.17487/RFC7281, June 2014,
                  <https://www.rfc-editor.org/info/rfc7281>.

       [SPF]      Kitterman, S., "Sender Policy Framework (SPF) for
                  Authorizing Use of Domains in Email, Version 1", RFC 7208,
                  DOI 10.17487/RFC7208, April 2014,
                  <https://www.rfc-editor.org/info/rfc7208>.

       [VBR]      Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
                  Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
                  <https://www.rfc-editor.org/info/rfc5518>.

    Kucherawy                    Standards Track                   [Page 43]

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    Appendix A.  Legacy MUAs

       Implementers of this specification should be aware that many MUAs are
       unlikely to be retrofitted to support the Authentication-Results
       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 (i.e., while this
       specification and its successors continue to be adopted), other
       interim means of conveying authentication results may be necessary.

    Appendix B.  Authentication-Results Examples

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

    B.1.  Trivial Case: Header Field Not Present

       The trivial case:

            Received: from mail-router.example.com
                          (mail-router.example.com [192.0.2.1])
                      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: Header Field Not Present

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       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 (1) the message authentication services
       were not available at the time of delivery, (2) no service is
       provided, or (3) the MTA is not in compliance with this
       specification.

    B.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 [192.0.2.1])
                      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 or 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.

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    B.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 [192.0.2.200])
                      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.

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    B.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
       checks:

            Authentication-Results: example.com;
                      auth=pass (cram-md5) smtp.auth=sender@example.net;
                      spf=pass smtp.mailfrom=example.net
            Authentication-Results: example.com; iprev=pass
                      policy.iprev=192.0.2.200
            Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6)
                          (dialup-1-2-3-4.example.net [192.0.2.200])
                      by mail-router.example.com (8.11.6/8.11.6)
                          with ESMTPA 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 themselves to the MTA via a method specified
       in [AUTH], and both SPF and "iprev" checks were done and passed.  The
       MUA could extract and relay this extra information if desired.

       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
       dial-up connection (example.net) sends mail to a border MTA
       (example.com) using SMTP authentication to prove identity.  The
       dial-up provider has been explicitly authorized to relay mail as
       example.net, producing a "pass" result from the SPF check.

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    B.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
       checks:

            Authentication-Results: example.com;
                      dkim=pass (good signature) header.d=example.com
            Received: from mail-router.example.com
                          (mail-router.example.com [192.0.2.1])
                      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:
                      Message-Id:Authentication-Results;
                      bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70;
                      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 [192.0.2.200])
                      by mail-router.example.com (8.11.6/8.11.6)
                          with ESMTPA 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.

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       The second MTA, auth-checker.example.com, reports that it did a DKIM
       test (which passed).  Again, additional data about one of the tests
       are 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 a message into
       example.com from a user on a dial-up connection example.net.  The
       user appears to be legitimate, as they had a valid password allowing
       authentication at the border MTA using SMTP AUTH.  The SPF test
       failed, since example.com has not granted example.net's dial-up
       network authority to relay mail on its behalf.  The DKIM test passed
       because the sending user had a private key matching one of
       example.com's published public keys and mail-router.example.com used
       it to sign the message.

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    B.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"
                    header.i=@mail-router.example.net;
                  dkim=fail reason="bad signature"
                    header.i=@newyork.example.com
            Received: from mail-router.example.net
                      (mail-router.example.net [192.0.2.250])
                  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;
                  h=From:Date:To:Message-Id:Subject:Authentication-Results;
                  bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=;
                  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 [192.0.2.220])
                  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;
                  d=newyork.example.com;
                  t=1188964191; c=simple/simple;
                  h=From:Date:To:Message-Id:Subject;
                  bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
                  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.

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       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.

    B.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

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    Appendix C.  Operational Considerations about Message Authentication

       Implementation of the Authentication-Results header field 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
       header field and its deployment to date are based on that model.  The
       assumption satisfies several common ADMD requirements:

       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 inside of an ADMD 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 cryptographic
           verification (e.g., public/private key work), do have a cost 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
           vector.

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       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, 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 elsewhere 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 D.  Changes since RFC 7601

       o  Added IANA registration for DKIM "a" and "s" properties.

       o  Included EAI guidance.

       o  Adjusted some ABNF tokens and names for easier inclusion by other
          documents.

       o  Made minor editorial adjustments.

       o  Deprecated entries from RFCs that are now Historic.

       o  Erratum 4671 resolved.

       o  Erratum 5435 resolved.

    Kucherawy                    Standards Track                   [Page 53]

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    Acknowledgments

       The author wishes to acknowledge the following individuals for their
       review and constructive criticism of this document: Kurt Andersen,
       Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro
       Vesely.

    Author's Address

       Murray S. Kucherawy
       270 Upland Drive
       San Francisco, CA  94127
       United States of America

       Email: superuser@gmail.com

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