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RFC 7601

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Proposed STD
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Message Header Field for Indicating Message Authentication Status

Part 1 of 3, p. 1 to 22
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Obsoletes:    7001    7410


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Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 7601                                   August 2015
Obsoletes: 7001, 7410
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.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7601.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Trust Boundary  . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Processing Scope  . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   6
     1.5.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
       1.5.1.  Key Words . . . . . . . . . . . . . . . . . . . . . .   6
       1.5.2.  Security  . . . . . . . . . . . . . . . . . . . . . .   7
       1.5.3.  Email Architecture  . . . . . . . . . . . . . . . . .   7
       1.5.4.  Other Terms . . . . . . . . . . . . . . . . . . . . .   8
     1.6.  Trust Environment . . . . . . . . . . . . . . . . . . . .   8
   2.  Definition and Format of the Header Field . . . . . . . . . .   9
     2.1.  General Description . . . . . . . . . . . . . . . . . . .   9
     2.2.  Formal Definition . . . . . . . . . . . . . . . . . . . .  10
     2.3.  Property Types (ptypes) and Properties  . . . . . . . . .  12
     2.4.  The "policy" ptype  . . . . . . . . . . . . . . . . . . .  13
     2.5.  Authentication Identifier Field . . . . . . . . . . . . .  14
     2.6.  Version Tokens  . . . . . . . . . . . . . . . . . . . . .  15
     2.7.  Defined Methods and Result Values . . . . . . . . . . . .  15
       2.7.1.  DKIM and DomainKeys . . . . . . . . . . . . . . . . .  16
       2.7.2.  SPF and Sender ID . . . . . . . . . . . . . . . . . .  18
       2.7.3.  "iprev" . . . . . . . . . . . . . . . . . . . . . . .  19
       2.7.4.  SMTP AUTH . . . . . . . . . . . . . . . . . . . . . .  20
       2.7.5.  Other Registered Codes  . . . . . . . . . . . . . . .  21
       2.7.6.  Extension Methods . . . . . . . . . . . . . . . . . .  21
       2.7.7.  Extension Result Codes  . . . . . . . . . . . . . . .  22
   3.  The "iprev" Authentication Method . . . . . . . . . . . . . .  22
   4.  Adding the Header Field to a Message  . . . . . . . . . . . .  23
     4.1.  Header Field Position and Interpretation  . . . . . . . .  25
     4.2.  Local Policy Enforcement  . . . . . . . . . . . . . . . .  26
   5.  Removing Existing Header Fields . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  The Authentication-Results Header Field . . . . . . . . .  27
     6.2.  "Email Authentication Methods" Registry Description . . .  28
     6.3.  "Email Authentication Methods" Registry Update  . . . . .  29
     6.4.  "Email Authentication Property Types" Registry  . . . . .  30
     6.5.  "Email Authentication Result Names" Description . . . . .  31
     6.6.  "Email Authentication Result Names" Update  . . . . . . .  32
     6.7.  SMTP Enhanced Status Codes  . . . . . . . . . . . . . . .  33
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
     7.1.  Forged Header Fields  . . . . . . . . . . . . . . . . . .  33
     7.2.  Misleading Results  . . . . . . . . . . . . . . . . . . .  35
     7.3.  Header Field Position . . . . . . . . . . . . . . . . . .  35
     7.4.  Reverse IP Query Denial-of-Service Attacks  . . . . . . .  35
     7.5.  Mitigation of Backscatter . . . . . . . . . . . . . . . .  36
     7.6.  Internal MTA Lists  . . . . . . . . . . . . . . . . . . .  36

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     7.7.  Attacks against Authentication Methods  . . . . . . . . .  36
     7.8.  Intentionally Malformed Header Fields . . . . . . . . . .  36
     7.9.  Compromised Internal Hosts  . . . . . . . . . . . . . . .  36
     7.10. Encapsulated Instances  . . . . . . . . . . . . . . . . .  37
     7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . .  37
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  37
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  38
   Appendix A.  Legacy MUAs  . . . . . . . . . . . . . . . . . . . .  42
   Appendix B.  Authentication-Results Examples  . . . . . . . . . .  42
     B.1.  Trivial Case; Header Field Not Present  . . . . . . . . .  42
     B.2.  Nearly Trivial Case; Service Provided, but No
           Authentication Done . . . . . . . . . . . . . . . . . . .  43
     B.3.  Service Provided, Authentication Done . . . . . . . . . .  44
     B.4.  Service Provided, Several Authentications Done, Single
           MTA . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
     B.5.  Service Provided, Several Authentications Done, Different
           MTAs  . . . . . . . . . . . . . . . . . . . . . . . . . .  46
     B.6.  Service Provided, Multi-tiered Authentication Done  . . .  48
     B.7.  Comment-Heavy Example . . . . . . . . . . . . . . . . . .  49
   Appendix C.  Operational Considerations about Message
                Authentication . . . . . . . . . . . . . . . . . . .  50
   Appendix D.  Changes since RFC 7001 . . . . . . . . . . . . . . .  51
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  53
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  53

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.

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

   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.

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   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  Author Domain Signing Practices ([ADSP]) (Historic)

   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])

   o  DomainKeys ([DOMAINKEYS]) (Historic)

   o  Sender ID ([SENDERID]) (Experimental)

   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.

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

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

1.1.  Purpose

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

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

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

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

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

1.2.  Trust Boundary

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

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

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

1.3.  Processing Scope

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

1.4.  Requirements

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

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

1.5.  Definitions

   This section defines various terms used throughout this document.

1.5.1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

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1.5.2.  Security

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

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

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

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

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

1.5.3.  Email Architecture

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

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

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

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   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.

1.5.4.  Other Terms

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

1.6.  Trust Environment

   This header field permits one or more message validation mechanisms
   to communicate output to one or more separate assessment mechanisms.
   These mechanisms operate within a unified trust boundary that defines
   an Administrative Management Domain (ADMD).  An ADMD contains one or
   more entities that perform validation and generate the header field

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   and one or more that consume it for some type of assessment.  The
   field often contains no integrity or validation mechanism of its own,
   so its presence must be trusted implicitly.  Hence, valid use of the
   header field requires removing any occurrences of it that are present
   when the message enters the ADMD.  This ensures that later
   occurrences have been added within the trust boundary of the ADMD.

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

2.1.  General Description

   The header field specified here is called Authentication-Results.  It
   is a Structured Header Field as defined in Internet Message Format
   ([MAIL]), and thus all of the related definitions in that document
   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 identifier, an optional version, and then a series
   of statements and supporting data.  The statements are of the form
   "method=result" and indicate which authentication method(s) were
   applied and their respective results.  For each such statement, the
   supporting data can include a "reason" string and one or more
   "property=value" statements indicating which message properties were
   evaluated to reach that conclusion.

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

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2.2.  Formal Definition

   Formally, the header field is specified as follows using Augmented
   Backus-Naur Form ([ABNF]):

     authres-header = "Authentication-Results:" [CFWS] authserv-id
              [ CFWS authres-version ]
              ( no-result / 1*resinfo ) [CFWS] CRLF

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

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

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

     resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
               *( CFWS propspec )

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

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

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

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

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

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

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

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

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

   The "Keyword" used in "result" above is further constrained by the
   necessity of being enumerated in Section 2.7.

   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].

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   The "propspec" may be omitted if, for example, the method was unable
   to extract any properties to do its evaluation yet has a result to
   report.

   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, they indicate from which particular part of the message the
   reported data were extracted.

   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.

   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.

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   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.

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-Result field:

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

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

   This ptype existed in the original specification for this header
   field, but without a complete description or example of intended use.

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   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 Identifier Field

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

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

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

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

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

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

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

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

2.6.  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.

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2.7.1.  DKIM and DomainKeys

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

   Section 3.8 of [DOMAINKEYS] enumerates some possible results of a
   DomainKeys evaluation.  Those results are not used when generating
   this header field; rather, the results returned are listed below.

   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.

   Both DKIM and DomainKeys use the same result set, as follows:

   none:  The message was not signed.

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

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   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".

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

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

   Section 3.1 of [DOMAINKEYS] describes a process by which the sending
   address of the message is determined.  DomainKeys results are thus
   reported along with the signing domain name, the sending address of
   the message, and the name of the header field from which the latter
   was extracted.  This means that a DomainKeys result includes a ptype-
   property combination of "header.d", plus one of "header.from" and
   "header.sender".  The sending address extracted from the header is
   included with any [MAIL]-style comments removed; moreover, the local-
   part of the address and the "@" character are removed if it has not
   been authenticated in some way.

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2.7.2.  SPF and Sender ID

   SPF and Sender ID use the "spf" and "sender-id" method names,
   respectively.  The result values for SPF are defined in Section 2.6
   of [SPF], and those definitions are included here by reference:

     +-----------+--------------------------------+
     |    Code   | Meaning                        |
     +-----------+--------------------------------+
     | none      | [RFC7208], Section 2.6.1       |
     +-----------+--------------------------------+
     | pass      | [RFC7208], Section 2.6.3       |
     +-----------+--------------------------------+
     | fail      | [RFC7208], Section 2.6.4       |
     +-----------+--------------------------------+
     | softfail  | [RFC7208], Section 2.6.5       |
     +-----------+--------------------------------+
     | policy    | RFC 7601, Section 2.4          |
     +-----------+--------------------------------+
     | neutral   | [RFC7208], Section 2.6.2       |
     +-----------+--------------------------------+
     | temperror | [RFC7208], Section 2.6.6       |
     +-----------+--------------------------------+
     | permerror | [RFC7208], 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".)

   The "sender-id" method is described in [SENDERID].  For this method,
   the ptype used is "header" and the property will be the name of the
   header field from which the Purported Responsible Address (see [PRA])
   was extracted -- namely, one of "Resent-Sender", "Resent-From",
   "Sender", or "From".

   The results for Sender ID are listed and described in Section 4.2 of
   [SENDERID], but for the purposes of this specification, the SPF
   definitions enumerated above are used instead.  Also, [SENDERID]
   specifies result codes that use mixed case, but they are typically
   used all lowercase in this context.

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

   If the retrieved sender policies used to evaluate SPF and Sender ID
   do not contain explicit provisions for authenticating the local-part
   (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported
   along with results for these mechanisms SHOULD NOT include the local-
   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".

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

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.

   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").

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 the
   Internet Assigned Numbers Authority (IANA) and, preferably, published
   in an RFC.  See Section 6 for further details.

   Extension methods can be defined for the following reasons:

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

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

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

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

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   Experimental method identifiers MUST only be used within ADMDs that
   have explicitly consented to use them.  These method identifiers and
   the parameters associated with them are not documented in RFCs.
   Therefore, they are subject to change at any time and not suitable
   for production use.  Any MTA, MUA, or downstream filter intended for
   production use SHOULD ignore or delete any Authentication-Results
   header field that includes an experimental (unknown) method
   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.
   Result codes MUST be registered with the Internet Assigned Numbers
   Authority (IANA) and preferably published in an RFC.  See Section 6
   for further details.

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



(page 22 continued on part 2)

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