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rfc:rfc7601

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.

Kucherawy Standards Track [Page 1] RFC 7601 Authentication-Results Header Field August 2015

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

Kucherawy Standards Track [Page 2] RFC 7601 Authentication-Results Header Field August 2015

   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.

Kucherawy Standards Track [Page 3] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 4] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 5] RFC 7601 Authentication-Results Header Field August 2015

 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].

Kucherawy Standards Track [Page 6] RFC 7601 Authentication-Results Header Field August 2015

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.

Kucherawy Standards Track [Page 7] RFC 7601 Authentication-Results Header Field August 2015

 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

Kucherawy Standards Track [Page 8] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 9] RFC 7601 Authentication-Results Header Field August 2015

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

Kucherawy Standards Track [Page 10] RFC 7601 Authentication-Results Header Field August 2015

   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].

Kucherawy Standards Track [Page 11] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 12] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 13] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 14] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 15] RFC 7601 Authentication-Results Header Field August 2015

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

Kucherawy Standards Track [Page 16] RFC 7601 Authentication-Results Header Field August 2015

 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.

Kucherawy Standards Track [Page 17] RFC 7601 Authentication-Results Header Field August 2015

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.

Kucherawy Standards Track [Page 18] RFC 7601 Authentication-Results Header Field August 2015

 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

Kucherawy Standards Track [Page 20] RFC 7601 Authentication-Results Header Field August 2015

 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)

Kucherawy Standards Track [Page 21] RFC 7601 Authentication-Results Header Field August 2015

 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.

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
 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.

Kucherawy Standards Track [Page 22] RFC 7601 Authentication-Results Header Field August 2015

 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.

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.

Kucherawy Standards Track [Page 23] RFC 7601 Authentication-Results Header Field August 2015

 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 got
 applied, and what the result was.  If an MTA applies more than one
 such test, it adds this header field either once per test or once
 indicating all of the results.  An MTA MUST NOT add a result to an
 existing header field.
 An MTA MAY add this header field containing only the authentication
 identifier portion and the "none" token (see Section 2.2) to indicate
 explicitly that no message authentication schemes were applied prior
 to delivery of this message.
 An MTA adding this header field has to take steps to identify it as
 legitimate to the MUAs or downstream filters that will ultimately
 consume its content.  One process to do so is described in Section 5.
 Further measures may be necessary in some environments.  Some
 possible solutions are enumerated in Section 7.1.  This document does
 not mandate any specific solution to this issue as each environment
 has its own facilities and limitations.
 Most known message authentication methods focus on a particular
 identifier to evaluate.  SPF and Sender ID differ in that they can
 yield a result based on more than one identifier; specifically, SPF
 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom
 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter
 or the Purported Responsible Address (PRA) identity.  When generating
 this field to report those results, only the parameter that yielded
 the result is included.
 For MTAs that add this header field, adding header fields in order
 (at the top), per Section 3.6 of [MAIL], is particularly important.
 Moreover, this header field SHOULD be inserted above any other trace
 header fields such MTAs might prepend.  This placement allows easy
 detection of header fields that can be trusted.
 End users making direct use of this header field might inadvertently
 trust information that has not been properly vetted.  If, for
 example, a basic SPF result were to be relayed that claims an
 authenticated addr-spec, the local-part of that addr-spec has
 actually not been authenticated.  Thus, an MTA adding this header

Kucherawy Standards Track [Page 24] RFC 7601 Authentication-Results Header Field August 2015

 field SHOULD NOT include any data that has not been authenticated by
 the method(s) being applied.  Moreover, MUAs SHOULD NOT render to
 users such information if it is presented by a method known not to
 authenticate it.

4.1. Header Field Position and Interpretation

 In order to ensure non-ambiguous results and avoid the impact of
 false header fields, MUAs and downstream filters SHOULD NOT interpret
 this header field unless specifically configured to do so by the user
 or administrator.  That is, this interpretation should not be "on by
 default".  Naturally then, users or administrators ought not activate
 such a feature unless (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 it bears
 appears to be one used within its own ADMD as configured by the user
 or administrator.
 MUAs and downstream filters MUST ignore any result reported using a
 "result" not specified in the IANA "Result Code" registry or a
 "ptype" not listed in the "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.
 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 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

Kucherawy Standards Track [Page 25] RFC 7601 Authentication-Results Header Field August 2015

 about where in the handling chain the work was done may be lost.
 Consumers SHOULD be designed such that this can be tolerated,
 especially from a producer known to have this limitation.
 MUAs SHOULD ignore instances of this header field discovered within
 message/rfc822 MIME attachments.
 Further discussion of these topics can be found in Section 7 below.

4.2. Local Policy Enforcement

 Some sites have a local policy that considers any particular
 authentication policy's non-recoverable failure results (typically
 "fail" or similar) as justification for rejecting the message.  In
 such cases, the border MTA SHOULD issue an SMTP rejection response to
 the message, rather than adding this header field and allowing the
 message to proceed toward delivery.  This is more desirable than
 allowing the message to reach an internal host's MTA or spam filter,
 thus possibly generating a local rejection such as a Delivery Status
 Notification (DSN) [DSN] to a forged originator.  Such generated
 rejections are colloquially known as "backscatter".
 The same MAY also be done for local policy decisions overriding the
 results of the authentication methods (e.g., the "policy" result
 codes described in Section 2.7).
 Such rejections at the SMTP protocol level are not possible if local
 policy is enforced at the MUA and not the MTA.

5. Removing Existing Header Fields

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

Kucherawy Standards Track [Page 26] RFC 7601 Authentication-Results Header Field August 2015

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

6. IANA Considerations

 IANA has registered the defined header field and created tables as
 described below.  These registry actions were originally defined by
 [RFC5451] and updated by [RFC6577] and [RFC7001].  The created
 registries are being further updated here to increase their
 completeness.

6.1. The Authentication-Results Header Field

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

Kucherawy Standards Track [Page 27] RFC 7601 Authentication-Results Header Field August 2015

   Header field name: Authentication-Results
   Applicable protocol: mail ([MAIL])
   Status: Standard
   Author/Change controller: IETF
   Specification document(s): RFC 7601
   Related information: none

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 is 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.

Kucherawy Standards Track [Page 28] RFC 7601 Authentication-Results Header Field August 2015

 Value:  a brief description of the value to be supplied with that
    method/ptype/property tuple.
 Status:  the status of this entry, which is either:
    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 changes have been made to this registry per this
 document:
 1.  The "Defined" field has been renamed "Definition", to be
     consistent with the other registries in this group.
 2.  The entry for the "dkim" method, "header" ptype, and "b" property
     now reference [RFC6008] as the defining document, and the
     reference has be removed from the description.
 3.  All other "dkim", "domainkeys", "iprev", "sender-id", and "spf"
     method entries have had their "Definition" fields changed to
     refer to this document, as this document contains a complete
     description of the registry and these corresponding values.
 4.  All "smime" entries have had their "Definition" fields changed to
     [SMIME-REG].
 5.  The "value" field of the "smime" entry using property "smime-
     part" has been changed to read: "The MIME body part reference
     that contains the S/MIME signature.  See Section 3.2.1 of RFC
     7281 for full syntax."

Kucherawy Standards Track [Page 29] RFC 7601 Authentication-Results Header Field August 2015

 6.  The single entry for the "auth" method was intended to reflect
     the identity indicated by the "AUTH" parameter to the SMTP "MAIL
     FROM" command verb.  However, there is also an "AUTH" command
     verb.  To clarify this ambiguity, the entry for the "auth" method
     has had its "property" field changed to "mailfrom", and its
     "Definition" field changed to this document.
 7.  The following entry has been added:
     Method:  auth
     Definition:  this document (RFC 7601)
     ptype:  smtp
     property:  auth
     Value:  identity confirmed by the AUTH command
     Status:  active
     Version:  1
 8.  The values of the "domainkeys" entries for ptype "header" have
     been updated as follows:
     from:  contents of the [MAIL] From: header field, after removing
        comments, and removing the local-part and following "@" if not
        authenticated
     sender:  contents of the [MAIL] Sender: header field, after
        removing comments, and removing the local-part and following
        "@" if not authenticated
 9.  For all entries for "dkim-adsp" and "domainkeys", their Status
     values have been changed to "deprecated", reflecting the fact
     that the corresponding specifications now have Historic status.
     Their "Definition" fields have also been modified to include a
     reference to this document.

6.4. "Email Authentication Property Types" Registry

 [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:

Kucherawy Standards Track [Page 30] RFC 7601 Authentication-Results Header Field August 2015

 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.
 Description:  A brief description of what sort of information this
    "ptype" is meant to cover.
 For new entries, the Designated Expert needs to assure 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.  References to
 [RFC7001] and [RFC7410] have been removed.

6.5. "Email Authentication Result Names" 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.  A registry was
 created by [RFC5451] for this purpose.  [RFC6577] added the "status"
 column and [RFC7001] updated the rules governing that registry.
 New entries are assigned only for values that have received Expert
 Review, per [IANA-CONSIDERATIONS].  The designated expert shall be
 appointed by the IESG.  The designated expert has discretion to
 request that a publication be referenced if a clear, concise
 definition of the authentication result cannot be provided such that
 interoperability is assured.  Registrations should otherwise be
 permitted.  The designated expert can also handle requests to mark
 any current registration as "deprecated".
 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.

Kucherawy Standards Track [Page 31] RFC 7601 Authentication-Results Header Field August 2015

 Specification:  either free form text explaining the meaning of this
    method-code combination, or a reference to such a definition.
 Status:  the status of this entry, which is either:
    active:  The entry is in current use.
    deprecated:  The entry is no longer in current use.

6.6. "Email Authentication Result Names" Update

 This document includes a complete description of the registry,
 obsoleting [RFC7001].  Accordingly, the following changes have been
 made to this registry per this document:
 o  The "Defined" field has been removed.
 o  The "Meaning" field has been renamed "Specification", as described
    above.
 o  The "Auth Method" field now appears before the "Code" field.
 o  For easier searching, the table has been arranged such that it is
    sorted first by Auth Method, then by Code within each Auth Method
    grouping.
 o  All entries for the "dkim", "domainkeys", "spf", "sender-id",
    "auth", and "iprev" methods have had their "Specification" fields
    replaced as follows:
    dkim:  Section 2.7.1 of this document (RFC 7601)
    domainkeys:  Section 2.7.1 of this document (RFC 7601)
    spf:  for "hardfail", Section 2.4.2 of [RFC5451]; for all others,
       Section 2.7.2 of this document (RFC 7601)
    sender-id:  for "hardfail", Section 2.4.2 of [RFC5451]; for all
       others, Section 2.7.2 of this document (RFC 7601)
    auth:  Section 2.7.4 of this document (RFC 7601)
    iprev:  Section 2.7.3 of this document (RFC 7601)
 o  All entries for "dkim-adsp" that were missing an explicit
    reference to a defining document now reference [ADSP] in their
    "Specification" fields.

Kucherawy Standards Track [Page 32] RFC 7601 Authentication-Results Header Field August 2015

 o  All entries for "dmarc" have had their "Specification" fields
    changed to reference Section 11.2 of [DMARC].
 o  All entries for "dkim-adsp" and "domainkeys" have had their Status
    values changed to "deprecated", reflecting the fact that the
    corresponding specifications now have Historic status.  Their
    "Specification" fields have also been modified to include a
    reference to this document.

6.7. SMTP Enhanced Status Codes

 The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of
 the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
 Registry" has been updated to refer to this document instead of
 [RFC7001].

7. Security Considerations

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

7.1. Forged Header Fields

 An MUA or filter that accesses a mailbox whose messages are handled
 by a non-conformant MTA, and understands Authentication-Results
 header fields, could potentially make false conclusions based on
 forged header fields.  A malicious user or agent could forge a header
 field using the DNS domain of a receiving ADMD as the authserv-id
 token in the value of the header field and, with the rest of the
 value, claim that the message was properly authenticated.  The non-
 conformant MTA would fail to strip the forged header field, and the
 MUA could inappropriately trust it.
 For this reason, it is best not to have processing of the
 Authentication-Results header field enabled by default; instead, it
 should be ignored, at least for the purposes of enacting filtering
 decisions, unless specifically enabled by the user or administrator
 after verifying that the border MTA is compliant.  It is acceptable
 to have an MUA aware of this specification but have an explicit list
 of hostnames whose Authentication-Results header fields are
 trustworthy; however, this list should initially be empty.
 Proposed alternative solutions to this problem were made some time
 ago and are listed below.  To date, they have not been developed due
 to lack of demand but are documented here should the information be
 useful at some point in the future:

Kucherawy Standards Track [Page 33] RFC 7601 Authentication-Results Header Field August 2015

 1.  Possibly the simplest is a digital signature protecting the
     header field, such as using [DKIM], that can be verified by an
     MUA by using a posted public key.  Although one of the main
     purposes of this document is to relieve the burden of doing
     message authentication work at the MUA, this only requires that
     the MUA learn a single authentication scheme even if a number of
     them are in use at the border MTA.  Note that [DKIM] requires
     that the From header field be signed, although in this
     application, the signing agent (a trusted MTA) likely cannot
     authenticate that value, so the fact that it is signed should be
     ignored.  Where the authserv-id is the ADMD's domain name, the
     authserv-id matching this valid internal signature's "d=" DKIM
     value is sufficient.
 2.  Another would be a means to interrogate the MTA that added the
     header field to see if it is actually providing any message
     authentication services and saw the message in question, but this
     isn't especially palatable given the work required to craft and
     implement such a scheme.
 3.  Yet another might be a method to interrogate the internal MTAs
     that apparently handled the message (based on Received header
     fields) to determine whether any of them conform to Section 5 of
     this memo.  This, too, has potentially high barriers to entry.
 4.  Extensions to [IMAP], [SMTP], and [POP3] could be defined to
     allow an MUA or filtering agent to acquire the authserv-id in use
     within an ADMD, thus allowing it to identify which
     Authentication-Results header fields it can trust.
 5.  On the presumption that internal MTAs are fully compliant with
     Section 3.6 of [MAIL] and the compliant internal MTAs are using
     their own hostnames or the ADMD's DNS domain name as the
     authserv-id token, the header field proposed here should always
     appear above a Received header added by a trusted MTA.  This can
     be used as a test for header field validity.
 Support for some of these is being considered for future work.
 In any case, a mechanism needs to exist for an MUA or filter to
 verify that the host that appears to have added the header field (a)
 actually did so and (b) is legitimately adding that header field for
 this delivery.  Given the variety of messaging environments deployed
 today, consensus appears to be that specifying a particular mechanism
 for doing so is not appropriate for this document.

Kucherawy Standards Track [Page 34] RFC 7601 Authentication-Results Header Field August 2015

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

7.2. Misleading Results

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

7.3. Header Field Position

 Despite the requirements of [MAIL], header fields can sometimes be
 reordered en route by intermediate MTAs.  The goal of requiring
 header field addition only at the top of a message is an
 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] describes a DNS-based denial-of-service attack
 for verifiers that attempt DNS-based identity verification of
 arriving client connections.  A verifier wishing to do this check and
 report this information needs to take care not to go to unbounded
 lengths to resolve "A" and "PTR" queries.  MUAs or other filters
 making use of an "iprev" result specified by this document need to be
 aware of the algorithm used by the verifier reporting the result and,
 especially, its limitations.

Kucherawy Standards Track [Page 35] RFC 7601 Authentication-Results Header Field August 2015

7.5. Mitigation of Backscatter

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

7.7. Attacks against Authentication Methods

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

7.8. Intentionally Malformed Header Fields

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

7.9. Compromised Internal Hosts

 An internal MUA or MTA that has been compromised could generate mail
 with a forged From header field and a forged Authentication-Results
 header field that endorses it.  Although it is clearly a larger
 concern to have compromised internal machines than it is to prove the
 value of this header field, this risk can be mitigated by arranging
 that internal MTAs will remove this header field if it claims to have
 been added by a trusted border MTA (as described above), yet the
 [SMTP] connection is not coming from an internal machine known to be
 running an authorized MTA.  However, in such a configuration,
 legitimate MTAs will have to add this header field when legitimate
 internal-only messages are generated.  This is also covered in
 Section 5.

Kucherawy Standards Track [Page 36] RFC 7601 Authentication-Results Header Field August 2015

7.10. Encapsulated Instances

 MIME messages can contain attachments of type "message/rfc822", which
 contain other messages.  Such an encapsulated message can also
 contain an Authentication-Results header field.  Although the
 processing of these is outside of the intended scope of this document
 (see Section 1.3), some early guidance to MUA developers is
 appropriate here.
 Since MTAs are unlikely to strip Authentication-Results header fields
 after mailbox delivery, MUAs are advised in Section 4.1 to ignore
 such instances within MIME attachments.  Moreover, when extracting a
 message digest to separate mail store messages or other media, such
 header fields should be removed so that they will never be
 interpreted improperly by MUAs that might later consume them.

7.11. Reverse Mapping

 Although Section 3 of this memo includes explicit support for the
 "iprev" method, its value as an authentication mechanism is limited.
 Implementers of both this proposal and agents that use the data it
 relays are encouraged to become familiar with the issues raised by
 [DNSOP-REVERSE] when deciding whether or not to include support for
 "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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc3864>.
 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
            DOI 10.17487/RFC5322, October 2008,
            <http://www.rfc-editor.org/info/rfc5322>.

Kucherawy Standards Track [Page 37] RFC 7601 Authentication-Results Header Field August 2015

 [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,
            <http://www.rfc-editor.org/info/rfc2045>.
 [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <http://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, <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc6541>.
 [AUTH]     Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
            Extension for Authentication", RFC 4954,
            DOI 10.17487/RFC4954, July 2007,
            <http://www.rfc-editor.org/info/rfc4954>.
 [AUTH-ESC]
            Kucherawy, M., "Email Authentication Status Codes",
            RFC 7372, DOI 10.17487/RFC7372, September 2014,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc7489>.

Kucherawy Standards Track [Page 38] RFC 7601 Authentication-Results Header Field August 2015

 [DNS]      Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [DNS-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
            "DNS Extensions to Support IP Version 6", RFC 3596,
            DOI 10.17487/RFC3596, October 2003,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc3464>.
 [EMAIL-ARCH]
            Crocker, D., "Internet Mail Architecture", RFC 5598,
            DOI 10.17487/RFC5598, July 2009,
            <http://www.rfc-editor.org/info/rfc5598>.
 [IANA-CONSIDERATIONS]
            Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
            <http://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,
            <http://www.rfc-editor.org/info/rfc1939>.
 [PRA]      Lyon, J., "Purported Responsible Address in E-Mail
            Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006,
            <http://www.rfc-editor.org/info/rfc4407>.

Kucherawy Standards Track [Page 39] RFC 7601 Authentication-Results Header Field August 2015

 [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
            Message Authentication Status", RFC 5451,
            DOI 10.17487/RFC5451, April 2009,
            <http://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,
            <http://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,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc7410>.
 [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,
            <http://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,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc7281>.

Kucherawy Standards Track [Page 40] RFC 7601 Authentication-Results Header Field August 2015

 [SPF]      Kitterman, S., "Sender Policy Framework (SPF) for
            Authorizing Use of Domains in Email, Version 1", RFC 7208,
            DOI 10.17487/RFC7208, April 2014,
            <http://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,
            <http://www.rfc-editor.org/info/rfc5518>.

Kucherawy Standards Track [Page 41] RFC 7601 Authentication-Results Header Field August 2015

Appendix A. Legacy MUAs

 Implementers of this protocol should be aware that many MUAs are
 unlikely to be retrofitted to support the new header field and its
 semantics.  In the interests of convenience and quicker adoption, a
 delivery MTA might want to consider adding things that are processed
 by existing MUAs in addition to the Authentication-Results header
 field.  One suggestion is to include a Priority header field, on
 messages that don't already have such a header field, containing a
 value that reflects the strength of the authentication that was
 accomplished, e.g., "low" for weak or no authentication, "normal" or
 "high" for good or strong authentication.
 Some modern MUAs can already filter based on the content of this
 header field.  However, there is keen interest in having MUAs make
 some kind of graphical representation of this header field's meaning
 to end users.  Until this capability is added (i.e., while this
 proposal and its successors are being 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: Trivial Case

Kucherawy Standards Track [Page 42] RFC 7601 Authentication-Results Header Field August 2015

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

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.

Kucherawy Standards Track [Page 43] RFC 7601 Authentication-Results Header Field August 2015

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.

Kucherawy Standards Track [Page 44] RFC 7601 Authentication-Results Header Field August 2015

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;
                sender-id=pass header.from=example.net
      Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6)
                    (dialup-1-2-3-4.example.net [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 herself/himself to the MTA via a method
 specified in [AUTH], and both SPF and Sender ID checks were done and
 passed.  The MUA could extract and relay this extra information if
 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.com,
 producing "pass" results from the SPF and Sender ID checks.

Kucherawy Standards Track [Page 45] RFC 7601 Authentication-Results Header Field August 2015

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;
                sender-id=fail header.from=example.com;
                dkim=pass (good signature) header.d=example.com
      Received: from mail-router.example.com
                    (mail-router.example.com [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.

Kucherawy Standards Track [Page 46] RFC 7601 Authentication-Results Header Field August 2015

 The second MTA, auth-checker.example.com, reports that it did a
 Sender ID test (which failed) and a DKIM test (which passed).  Again,
 additional data about one of the tests is provided as a comment,
 which the MUA may choose to render.  Also noteworthy here is the fact
 that there is a DKIM signature added by example.com that assured the
 integrity of the lower Authentication-Results field.
 Since different hosts did the two sets of authentication checks, the
 header fields cannot be consolidated in this example.
 This example illustrates more typical transmission of mail into
 example.com from a user on a dial-up connection example.net.  The
 user appears to be legitimate as he/she had a valid password allowing
 authentication at the border MTA using SMTP AUTH.  The SPF and Sender
 ID tests failed since example.com has not granted example.net
 authority to relay mail on its behalf.  However, the DKIM test passed
 because the sending user had a private key matching one of
 example.com's published public keys and used it to sign the message.

Kucherawy Standards Track [Page 47] RFC 7601 Authentication-Results Header Field August 2015

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.

Kucherawy Standards Track [Page 48] RFC 7601 Authentication-Results Header Field August 2015

 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

Kucherawy Standards Track [Page 49] RFC 7601 Authentication-Results Header Field August 2015

Appendix C. Operational Considerations about Message Authentication

 This protocol is predicated on the idea that authentication (and
 presumably in the future, reputation) work is typically done by
 border MTAs rather than MUAs or intermediate MTAs; the latter merely
 make use of the results determined by the former.  Certainly this is
 not mandatory for participation in electronic mail or message
 authentication, but this protocol and its deployment to date are
 based on that model.  The assumption satisfies several common ADMD
 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 are more likely to be heavily firewalled.  Thus, some MUAs
     might not even be able to complete the task of performing
     authentication or reputation evaluations without complex proxy
     configurations or similar burdens.
 3.  MUAs rely upon the upstream MTAs within their trust boundaries to
     make correct (as much as is possible) evaluations about the
     message's envelope, header, and content.  Thus, MUAs don't need
     to know how to do the work that upstream MTAs do; they only need
     the results of that work.
 4.  Evaluations about the quality of a message, from simple token
     matching (e.g., a list of preferred DNS domains) to cryptanalysis
     (e.g., public/private key work), 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.

Kucherawy Standards Track [Page 50] RFC 7601 Authentication-Results Header Field August 2015

 5.  Minimizing change is good.  As new authentication and reputation
     methods emerge, the list of methods supported by this header
     field would presumably be extended.  If MUAs simply consume the
     contents of this header field rather than actually attempt to do
     authentication and/or reputation work, then MUAs only need to
     learn to parse this header field once; emergence of new methods
     requires only a configuration change at the MUAs and software
     changes at the MTAs (which are presumably fewer in number).  When
     choosing to implement these functions in MTAs vs. MUAs, the
     issues of individual flexibility, infrastructure inertia, and
     scale of effort must be considered.  It is typically easier to
     change a single MUA than an MTA because the modification affects
     fewer users and can be pursued with less care.  However, changing
     many MUAs is more effort than changing a smaller number of MTAs.
 6.  For decisions affecting message delivery and display, assessment
     based on authentication and reputation is best performed close to
     the time of message transit, as a message makes its journey
     toward a user's inbox, not afterwards.  DKIM keys and IP address
     reputations, etc., can change over time or even become invalid,
     and users can take a long time to read a message once delivered.
     The value of this work thus degrades, perhaps quickly, once the
     delivery process has completed.  This seriously diminishes the
     value of this work when done 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 7001

 o  Applied RFC 7410.
 o  Updated all references to RFC 4408 with RFC 7208.
 o  Added section explaining "property" values.  (Addressed Erratum
    #4201.)
 o  Did some minor text reorganization.
 o  Gave registry history -- enough that this is now the authoritative
    registry definition.
 o  Added text explaining each of the method-ptype-property tuples
    registered by this document.

Kucherawy Standards Track [Page 51] RFC 7601 Authentication-Results Header Field August 2015

 o  Changed the meaning of the "Defined" column of the methods
    registry to be the place where each entry was created and
    described; it is expected that this will then refer to the
    method's defining document.  Provided IANA with corresponding
    update instructions.
 o  Cleaned up registry structure and content, and replaced all
    references to RFC 7001 with pointers to this document.
 o  Added references: [DMARC], [PRA], [RFC6008], [RFC6577], [RRVS],
    [SMIME-REG].
 o  Added description of values that can be extracted from SMTP AUTH
    sessions and an example.
 o  Provided much more complete descriptions of reporting DomainKeys
    results.
 o  Added more detail about Sender ID.
 o  Marked all ADSP and DomainKeys entries as deprecated since their
    defining documents are as well.
 o  Reworked some text around ignoring unknown ptypes.
 o  Completely described the ptypes registry.
 o  Mentioned that EHLO is mapped to HELO for SPF.
 o  RFC 7208 uses all-lowercase result strings now, so adjusted prose
    accordingly.
 o  Updated list of supported methods, and mentioned the registries
    immediately below.
 o  Mentioned that when a local-part is removed, the "@" goes with it.
 o  Referred to RFC 7328 in the "iprev" definition.
 o  Corrected the "smime-part" prose.
 o  Updated examples that use SMTP AUTH to claim "with ESMTPA" in the
    Received fields.
 o  Made minor editorial adjustments.

Kucherawy Standards Track [Page 52] RFC 7601 Authentication-Results Header Field August 2015

Acknowledgments

 The author wishes to acknowledge the following individuals for their
 review and constructive criticism of this document: Stephane
 Bortzmeyer, Scott Kitterman, John Levine, Tom Petch, and Pete
 Resnick.

Author's Address

 Murray S. Kucherawy
 270 Upland Drive
 San Francisco, CA  94127
 United States
 Email: superuser@gmail.com

Kucherawy Standards Track [Page 53]

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