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rfc:rfc7001

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

 Message Header Field for Indicating Message Authentication Status

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/rfc7001.

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Kucherawy Standards Track [Page 1] RFC 7001 Authentication-Results Header Field September 2013

Table of Contents

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

Kucherawy Standards Track [Page 2] RFC 7001 Authentication-Results Header Field September 2013

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

1. Introduction

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

Kucherawy Standards Track [Page 3] RFC 7001 Authentication-Results Header Field September 2013

 At the time of publication of this document, the following are
 published, domain-level email authentication methods in common use:
 o  Author Domain Signing Practices ([ADSP])
 o  SMTP Service Extension for Authentication ([AUTH])
 o  DomainKeys Identified Mail Signatures ([DKIM])
 o  Sender Policy Framework ([SPF])
 o  Vouch By Reference ([VBR])
 o  reverse IP address name validation ("iprev", defined in Section 3)
 In addition, the following are non-standard methods recognized by
 this specification that are no longer common:
 o  DomainKeys ([DOMAINKEYS]) (Historic)
 o  Sender ID ([SENDERID]) (Experimental)
 This specification is not intended to be restricted to domain-based
 authentication schemes, but the existing schemes in that family have
 proven to be a good starting point for implementations.  The goal is
 to give current and future authentication schemes a common framework
 within which to deliver their results to downstream agents and
 discourage the creation of unique header fields for each.
 Although SPF defined a header field called "Received-SPF" and the
 historic DomainKeys defined one called "DomainKey-Status" for this
 purpose, those header fields are specific to the conveyance of their
 respective results only and thus are insufficient to satisfy the
 requirements enumerated below.  In addition, many SPF implementations
 have adopted the header field specified here at least as an option,
 and DomainKeys has been obsoleted by DKIM.

1.1. Purpose

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

Kucherawy Standards Track [Page 4] RFC 7001 Authentication-Results Header Field September 2013

 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.
 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.

Kucherawy Standards Track [Page 5] RFC 7001 Authentication-Results Header Field September 2013

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].

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.

Kucherawy Standards Track [Page 6] RFC 7001 Authentication-Results Header Field September 2013

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

1.5.3. Email Architecture

 o  A "border MTA" is an MTA that acts as a gateway between the
    general Internet and the users within an organizational boundary.
    (See also Section 1.2.)
 o  A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
    actually enacts delivery of a message to a user's inbox or other
    final delivery.
 o  An "intermediate MTA" is any MTA that is not a delivery MTA and is
    also not the first MTA to handle the message.
 The following diagram illustrates the flow of mail among these
 defined components.  See Internet Mail Architecture [EMAIL-ARCH] for
 further discussion on general email system architecture, which
 includes detailed descriptions of these components, and Appendix D of
 this document for discussion about the common aspects of email
 authentication in current environments.

Kucherawy Standards Track [Page 7] RFC 7001 Authentication-Results Header Field September 2013

                        +-----+   +-----+   +------------+
                        | 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
 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.

Kucherawy Standards Track [Page 8] RFC 7001 Authentication-Results Header Field September 2013

 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.

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

Kucherawy Standards Track [Page 9] RFC 7001 Authentication-Results Header Field September 2013

   authres-version = 1*DIGIT [CFWS]
           ; indicates which version of this specification is in use;
           ; this specification is version "1", and the absence of a
           ; version implies this version of the specification
   no-result = [CFWS] ";" [CFWS] "none"
             ; the special case of "none" is used to indicate that no
             ; message authentication was performed
   resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
             *( CFWS propspec )
   methodspec = [CFWS] method [CFWS] "=" [CFWS] result
              ; indicates which authentication method was evaluated
              ; and what its output was
   reasonspec = "reason" [CFWS] "=" [CFWS] value
              ; a free-form comment on the reason the given result
              ; was returned
   propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
            ; an indication of which properties of the message
            ; were evaluated by the authentication scheme being
            ; applied to yield the reported result
   method = Keyword [ [CFWS] "/" [CFWS] method-version ]
          ; a method indicates which method's result is
          ; represented by "result", and is one of the methods
          ; explicitly defined as valid in this document
          ; or is an extension method as defined below
   method-version = 1*DIGIT [CFWS]
          ; indicates which version of the method specification is
          ; in use, corresponding to the matching entry in the IANA
          ; "Email Authentication Methods" registry; a value of "1"
          ; is assumed if this version string is absent
   result = Keyword
          ; indicates the results of the attempt to authenticate
          ; the message; see below for details
   ptype = "smtp" / "header" / "body" / "policy"
         ; indicates whether the property being evaluated was
         ; a parameter to an [SMTP] command, was a value taken
         ; from a message header field, was some property of
         ; the message body, or was some other property evaluated by
         ; the receiving MTA

Kucherawy Standards Track [Page 10] RFC 7001 Authentication-Results Header Field September 2013

   property = special-smtp-verb / Keyword
           ; if "ptype" is "smtp", this indicates which [SMTP]
           ; command provided the value that was evaluated by the
           ; authentication scheme being applied; if "ptype" is
           ; "header", this indicates from which header field the
           ; value being evaluated was extracted; if "ptype" is
           ; "body", this indicates where in the message body
           ; a value being evaluated can be found (e.g., a specific
           ; offset into the message or a reference to a MIME part);
           ; if "ptype" is "policy", then this indicates the name
           ; of the policy that caused this header field to be
           ; added (see below)
   special-smtp-verb = "mailfrom" / "rcptto"
           ; special cases of [SMTP] commands that are made up
           ; of multiple words
   pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
            [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 2.6.
 See Section 2.4 for a description of the authserv-id element.
 If the value portion of a "pvalue" construction identifies something
 intended to be an e-mail identity, then it MUST use the right hand
 portion of that ABNF definition.
 The list of commands eligible for use with the "smtp" ptype can be
 found in Section 4.1 of [SMTP].
 The "propspec" may be omitted if, for example, the method was unable
 to extract any properties to do its evaluation yet has a result to
 report.

Kucherawy Standards Track [Page 11] RFC 7001 Authentication-Results Header Field September 2013

 Where an SMTP command name is being reported as a "property", the
 agent generating the header field represents that command by
 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM"
 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).
 A "ptype" value of "policy" indicates a policy decision about the
 message not specific to a property of the message that could be
 extracted.  See Section 2.3 for details.
 Examples of complete messages using this header field can be found in
 Appendix C.

2.3. The "policy" ptype

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

Kucherawy Standards Track [Page 12] RFC 7001 Authentication-Results Header Field September 2013

2.4. Authentication Identifier Field

 Every Authentication-Results header field has an authentication
 service identifier field (authserv-id above).  Specifically, this is
 any string intended to identify the authentication service within the
 ADMD that conducted authentication checks on the message.  This
 identifier is intended to be machine-readable and not necessarily
 meaningful to users.
 Since agents consuming this field will use this identifier to
 determine whether its contents are of interest (and are safe to use),
 the uniqueness of the identifier MUST be guaranteed by the ADMD that
 generates it and MUST pertain to that ADMD.  MUAs or downstream
 filters SHOULD use this identifier to determine whether or not the
 data contained in an Authentication-Results header field ought to be
 used or ignored.
 For simplicity and scalability, the authentication service identifier
 SHOULD be a common token used throughout the ADMD.  Common practice
 is to use the DNS domain name used by or within that ADMD, sometimes
 called the "organizational domain", but this is not strictly
 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.
 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

Kucherawy Standards Track [Page 13] RFC 7001 Authentication-Results Header Field September 2013

    only the current identifier(s) in use but previous ones as well to
    account for delivery latency or later re-assessment of the header
    field's contents.
 Examples of valid authentication identifiers are "example.com",
 "mail.example.org", "ms1.newyork.example.com", and "example-auth".

2.5. Version Tokens

 The grammar above provides for the optional inclusion of versions on
 both the header field itself (attached to the authserv-id token) and
 on each of the methods being reported.  The method version refers to
 the method itself, which is specified in the documents describing
 those methods, while the authserv-id version refers to this document
 and thus the syntax of this header field.
 The purpose of including these is to avoid misinterpretation of the
 results.  That is, if a parser finds a version after an authserv-id
 that it does not explicitly know, it can immediately discontinue
 trying to parse since what follows might not be in an expected
 format.  For a method version, the parser SHOULD ignore a method
 result if the version is not supported in case the semantics of the
 result have a different meaning than what is expected.  For example,
 if a hypothetical DKIM version 2 yielded a "pass" result for
 different reasons than version 1 does, a consumer of this field might
 not want to use the altered semantics.  Allowing versions in the
 syntax is a way to indicate this and let the consumer of the header
 field decide.

2.6. Defined Methods and Result Values

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

2.6.1. DKIM and DomainKeys

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

Kucherawy Standards Track [Page 14] RFC 7001 Authentication-Results Header Field September 2013

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

2.6.2. SPF and Sender ID

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

Kucherawy Standards Track [Page 15] RFC 7001 Authentication-Results Header Field September 2013

   +-----------+----------------------------+
   |    Code   | Meaning                    |
   +-----------+----------------------------+
   | none      | [SPF], Section 2.5.1       |
   +-----------+----------------------------+
   | pass      | [SPF], Section 2.5.3       |
   +-----------+----------------------------+
   | fail      | [SPF], Section 2.5.4       |
   +-----------+----------------------------+
   | softfail  | [SPF], Section 2.5.5       |
   +-----------+----------------------------+
   | policy    | [RFC7001], Section 2.6.2   |
   +-----------+----------------------------+
   | neutral   | [SPF], Section 2.5.2       |
   +-----------+----------------------------+
   | temperror | [SPF], Section 2.5.6       |
   +-----------+----------------------------+
   | permerror | [SPF], Section 2.5.7       |
   +-----------+----------------------------+
 These result codes are used in the context of this specification to
 reflect the result returned by the component conducting SPF
 evaluation.
 Similarly, the results for Sender ID are listed and described in
 Section 4.2 of [SENDERID], which in turn uses the SPF definitions.
 Note that both of those documents specify result codes that use mixed
 case, but they are typically used all lowercase in this context.
 In both cases, an additional result of "policy" is defined, which
 means the client was authorized to inject or relay mail on behalf of
 the sender's DNS domain according to the authentication method's
 algorithm, but local policy dictates that the result is unacceptable.
 For example, "policy" might be used if SPF returns a "pass" result,
 but a local policy check matches the sending DNS domain to one found
 in an explicit list of unacceptable DNS domains (e.g., spammers).
 If the retrieved sender policies used to evaluate SPF and Sender ID
 do not contain explicit provisions for authenticating the local-part
 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported
 along with results for these mechanisms SHOULD NOT include the local-
 part.

Kucherawy Standards Track [Page 16] RFC 7001 Authentication-Results Header Field September 2013

2.6.3. "iprev"

 The result values used by the "iprev" method, defined in Section 3,
 are as follows:
 pass:  The DNS evaluation succeeded, i.e., the "reverse" and
    "forward" lookup results were returned and were in agreement.
 fail:  The DNS evaluation failed.  In particular, the "reverse" and
    "forward" lookups each produced results, but they were not in
    agreement, or the "forward" query completed but produced no
    result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an
    RCODE of 0 (NOERROR) in a reply containing no answers, was
    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.
 For discussion of the format of DNS replies, see "Domain Names -
 Implementation and Specification" ([DNS]).

2.6.4. SMTP AUTH

 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method,
 and its result values are as follows:
 none:  SMTP authentication was not attempted.
 pass:  The SMTP client authenticated to the server reporting the
    result using the protocol described in [AUTH].

Kucherawy Standards Track [Page 17] RFC 7001 Authentication-Results Header Field September 2013

 fail:  The SMTP client attempted to authenticate to the server using
    the protocol described in [AUTH] but was not successful, yet
    continued to send the message about which a result is being
    reported.
 temperror:  The SMTP client attempted to authenticate using the
    protocol described in [AUTH] but was not able to complete the
    attempt due to some error that is likely transient in nature, such
    as a temporary directory service lookup error.  A later attempt
    may produce a final result.
 permerror:  The SMTP client attempted to authenticate using the
    protocol described in [AUTH] but was not able to complete the
    attempt due to some error that is likely not transient in nature,
    such as a permanent directory service lookup error.  A later
    attempt is not likely to produce a final result.
 An agent making use of the data provided by this header field SHOULD
 consider "fail" and "temperror" to be synonymous in terms of message
 authentication, i.e., the client did not authenticate in either case.

2.6.5. Other Registered Codes

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

2.6.6. Extension Methods

 Additional authentication method identifiers (extension methods) may
 be defined in the future by later revisions or extensions to this
 specification.  These method identifiers are registered with the
 Internet Assigned Numbers Authority (IANA) and, preferably, published
 in an RFC.  See Section 6 for further details.
 Extension methods can be defined for the following reasons:
 1.  To allow additional information from new authentication systems
     to be communicated to MUAs or downstream filters.  The names of
     such identifiers ought to reflect the name of the method being
     defined but ought not be needlessly long.
 2.  To allow the creation of "sub-identifiers" that indicate
     different levels of authentication and differentiate between
     their relative strengths, e.g., "auth1-weak" and "auth1-strong".

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 Authentication method implementers are encouraged to provide adequate
 information, via message header field comments if necessary, to allow
 an MUA developer to understand or relay ancillary details of
 authentication results.  For example, if it might be of interest to
 relay what data was used to perform an evaluation, such information
 could be relayed as a comment in the header field, such as:
      Authentication-Results: example.com;
                foo=pass bar.baz=blob (2 of 3 tests OK)
 Experimental method identifiers MUST only be used within ADMDs that
 have explicitly consented to use them.  These method identifiers and
 the parameters associated with them are not documented in RFCs.
 Therefore, they are subject to change at any time and not suitable
 for production use.  Any MTA, MUA, or downstream filter intended for
 production use SHOULD ignore or delete any Authentication-Results
 header field that includes an experimental (unknown) method
 identifier.

2.6.7. Extension Result Codes

 Additional result codes (extension results) might be defined in the
 future by later revisions or extensions to this specification.
 Result codes MUST be registered with the Internet Assigned Numbers
 Authority (IANA) and preferably published in an RFC.  See Section 6
 for further details.
 Extension results MUST only be used within ADMDs that have explicitly
 consented to use them.  These results and the parameters associated
 with them are not formally documented.  Therefore, they are subject
 to change at any time and not suitable for production use.  Any MTA,
 MUA, or downstream filter intended for production use SHOULD ignore
 or delete any Authentication-Results header field that includes an
 extension result.

3. The "iprev" Authentication Method

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

Kucherawy Standards Track [Page 19] RFC 7001 Authentication-Results Header Field September 2013

 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.
 The response to a PTR query could contain multiple names.  To prevent
 heavy DNS loads, agents performing these queries MUST be implemented
 such that the number of names evaluated by generation of
 corresponding A or AAAA queries is limited so as not to be unduly
 taxing to the DNS infrastructure, though it MAY be configurable by an
 administrator.  As an example, Section 5.5 of [SPF] chose a limit of
 10 for its implementation of this algorithm.
 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the
 query formats for the IPv6 case.
 There is some contention regarding the wisdom and reliability of this
 test.  For example, in some regions, it can be difficult for this
 test ever to pass because the practice of arranging to match the
 forward and reverse DNS is infrequently observed.  Therefore, the
 precise implementation details of how a verifier performs an "iprev"
 test are not specified here.  The verifier MAY report a successful or
 failed "iprev" test at its discretion having done some kind of check
 of the validity of the connection's identity using DNS.  It is
 incumbent upon an agent making use of the reported "iprev" result to
 understand what exactly that particular verifier is attempting to
 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
 acknowledgement that the method remains common and provides the means
 to relay the results of that test.

4. Adding the Header Field to a Message

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

Kucherawy Standards Track [Page 20] RFC 7001 Authentication-Results Header Field September 2013

 method over another.  Instead, the MUA or downstream filter consuming
 this header field is to interpret the result of each method based on
 its own knowledge of what that method evaluates.
 Each "method" MUST refer to an authentication method declared in the
 IANA registry or an extension method as described in Section 2.6.6,
 and each "result" MUST refer to a result code declared in the IANA
 registry or an extension result code as defined in Section 2.6.7.
 See Section 6 for further information about the registered methods
 and result codes.
 An MTA compliant with this specification adds this header field
 (after performing one or more message authentication tests) to
 indicate which MTA or ADMD performed the test, which test got
 applied, and what the result was.  If an MTA applies more than one
 such test, it adds this header field either once per test or once
 indicating all of the results.  An MTA MUST NOT add a result to an
 existing header field.
 An MTA MAY add this header field containing only the authentication
 identifier portion and the "none" token (see Section 2.2) to indicate
 explicitly that no message authentication schemes were applied prior
 to delivery of this message.
 An MTA adding this header field has to take steps to identify it as
 legitimate to the MUAs or downstream filters that will ultimately
 consume its content.  One process to do so is described in Section 5.
 Further measures may be necessary in some environments.  Some
 possible solutions are enumerated in Section 7.1.  This document does
 not mandate any specific solution to this issue as each environment
 has its own facilities and limitations.
 Most known message authentication methods focus on a particular
 identifier to evaluate.  SPF and Sender ID differ in that they can
 yield a result based on more than one identifier; specifically, SPF
 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom
 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter
 or the Purported Responsible Address (PRA) identity.  When generating
 this field to report those results, only the parameter that yielded
 the result is included.
 For MTAs that add this header field, adding header fields in order
 (at the top), per Section 3.6 of [MAIL], is particularly important.
 Moreover, this header field SHOULD be inserted above any other trace
 header fields such MTAs might prepend.  This placement allows easy
 detection of header fields that can be trusted.

Kucherawy Standards Track [Page 21] RFC 7001 Authentication-Results Header Field September 2013

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

4.1. Header Field Position and Interpretation

 In order to ensure non-ambiguous results and avoid the impact of
 false header fields, MUAs and downstream filters SHOULD NOT interpret
 this header field unless specifically configured to do so by the user
 or administrator.  That is, this interpretation should not be "on by
 default".  Naturally then, users or administrators ought not activate
 such a feature unless they are certain the header field will be
 validly added by an agent within the ADMD that accepts the mail that
 is ultimately read by the MUA, and instances of the header field
 appearing to originate within the ADMD but are actually added by
 foreign MTAs will be removed before delivery.
 Furthermore, MUAs and downstream filters SHOULD NOT interpret this
 header field unless the authentication service identifier it bears
 appears to be one used within its own ADMD as configured by the user
 or administrator.
 MUAs and downstream filters MUST ignore any result reported using a
 "result" not specified in the IANA "Result Code" registry or a
 "ptype" not listed in the corresponding registry for such values as
 defined in Section 6.  Moreover, such agents MUST ignore a result
 indicated for any "method" they do not specifically support.
 An MUA SHOULD NOT reveal these results to end users, absent careful
 human factors design considerations and testing, for the presentation
 of trust-related materials.  For example, an attacker could register
 examp1e.com (note the digit "one") and send signed mail to intended
 victims; a verifier would detect that the signature was valid and
 report a "pass" even though it's clear the DNS domain name was
 intended to mislead.  See Section 7.2 for further discussion.
 As stated in Section 2.1, this header field MUST be treated as though
 it were a trace header field as defined in Section 3.6.7 of [MAIL]
 and hence MUST NOT be reordered and MUST be prepended to the message,
 so that there is generally some indication upon delivery of where in
 the chain of handling MTAs the message authentication was done.

Kucherawy Standards Track [Page 22] RFC 7001 Authentication-Results Header Field September 2013

 Note that there are a few message handlers that are only capable of
 appending new header fields to a message.  Strictly speaking, these
 handlers are not compliant with this specification.  They can still
 add the header field to carry authentication details, but any signal
 about where in the handling chain the work was done may be lost.
 Consumers SHOULD be designed such that this can be tolerated,
 especially from a producer known to have this limitation.
 MUAs SHOULD ignore instances of this header field discovered within
 message/rfc822 MIME attachments.
 Further discussion of these topics can be found in Section 7 below.

4.2. Local Policy Enforcement

 Some sites have a local policy that considers any particular
 authentication policy's non-recoverable failure results (typically
 "fail" or similar) as justification for rejecting the message.  In
 such cases, the border MTA SHOULD issue an SMTP rejection response to
 the message, rather than adding this header field and allowing the
 message to proceed toward delivery.  This is more desirable than
 allowing the message to reach an internal host's MTA or spam filter,
 thus possibly generating a local rejection such as a Delivery Status
 Notification (DSN) [DSN] to a forged originator.  Such generated
 rejections are colloquially known as "backscatter".
 The same MAY also be done for local policy decisions overriding the
 results of the authentication methods (e.g., the "policy" result
 codes described in Section 2.6).
 Such rejections at the SMTP protocol level are not possible if local
 policy is enforced at the MUA and not the MTA.

5. Removing Existing Header Fields

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

Kucherawy Standards Track [Page 23] RFC 7001 Authentication-Results Header Field September 2013

 For simplicity and maximum security, a border MTA could remove all
 instances of this header field on mail crossing into its trust
 boundary.  However, this may conflict with the desire to access
 authentication results performed by trusted external service
 providers.  It may also invalidate signed messages whose signatures
 cover external instances of this header field.  A more robust border
 MTA could allow a specific list of authenticating MTAs whose
 information is to be admitted, removing the header field originating
 from all others.
 As stated in Section 1.2, a formal definition of "trust boundary" is
 deliberately not made here.  It is entirely possible that a border
 MTA for example.com will explicitly trust authentication results
 asserted by upstream host example.net even though they exist in
 completely disjoint administrative boundaries.  In that case, the
 border MTA MAY elect not to delete those results; moreover, the
 upstream host doing some authentication work could apply a signing
 technology such as [DKIM] on its own results to assure downstream
 hosts of their authenticity.  An example of this is provided in
 Appendix C.
 Similarly, in the case of messages signed using [DKIM] or other
 message-signing methods that sign header fields, this removal action
 could invalidate one or more signatures on the message if they
 covered the header field to be removed.  This behavior can be
 desirable since there's little value in validating the signature on a
 message with forged header fields.  However, signing agents MAY
 therefore elect to omit these header fields from signing to avoid
 this situation.
 An MTA SHOULD remove any instance of this header field bearing a
 version (express or implied) that it does not support.  However, an
 MTA MUST remove such a header field if the [SMTP] connection relaying
 the message is not from a trusted internal MTA.  This means the MTA
 needs to be able to understand versions of this header field at least
 as late as the ones understood by the MUAs or other consumers within
 its ADMD.

6. IANA Considerations

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

Kucherawy Standards Track [Page 24] RFC 7001 Authentication-Results Header Field September 2013

6.1. The Authentication-Results Header Field

 [RFC5451] added the Authentication-Results header field to the IANA
 "Permanent Message Header Field Names" registry, per the procedure
 found in [IANA-HEADERS].  That entry has been updated to reference
 this document.  The following is the registration template:
   Header field name: Authentication-Results
   Applicable protocol: mail ([MAIL])
   Status: Standard
   Author/Change controller: IETF
   Specification document(s): RFC 7001
   Related information:
     Requesting review of any proposed changes and additions to
     this field is recommended.

6.2. "Email Authentication Methods" Registry

 Names of message authentication methods supported by this
 specification are to be registered with IANA, with the exception of
 experimental names as described in Section 2.6.6.  A registry was
 created by [RFC5451] for this purpose.  This document changes the
 rules governing that registry.
 New entries are assigned only for values that have received Expert
 Review, per [IANA-CONSIDERATIONS].  The designated expert shall be
 appointed by the IESG.  The designated expert has discretion to
 request that a publication be referenced if a clear, concise
 definition of the authentication method cannot be provided such that
 interoperability is assured.  Registrations should otherwise be
 permitted.  The designated expert can also handle requests to mark
 any current registration as "deprecated".
 Each method must register a name, the specification that defines it,
 a version number associated with the method being registered
 (preferably starting at "1"), zero or more "ptype" values appropriate
 for use with that method, which "property" value(s) should be
 reported by that method, and a description of the "value" to be used
 with each.
 All existing registry entries that reference [RFC5451] have been
 updated to reference this document, except where entries have already
 been deprecated.
 IANA has also added a "version" field to all existing registry
 entries.  All current methods are recorded as version "1".

Kucherawy Standards Track [Page 25] RFC 7001 Authentication-Results Header Field September 2013

6.3. "Email Authentication Result Names" Registry

 Names of message authentication result codes supported by this
 specification must be registered with IANA, with the exception of
 experimental codes as described in Section 2.6.7.  A registry was
 created by [RFC5451] for this purpose.  This document changes the
 rules governing that registry.
 New entries are assigned only for values that have received Expert
 Review, per [IANA-CONSIDERATIONS].  The designated expert shall be
 appointed by the IESG.  The designated expert has discretion to
 request that a publication be referenced if a clear, concise
 definition of the authentication result cannot be provided such that
 interoperability is assured.  Registrations should otherwise be
 permitted.  The designated expert can also handle requests to mark
 any current registration as "deprecated".
 All existing registry entries that reference [RFC5451] have been
 updated to reference this document.
 The definitions for the SPF and Sender ID authentication methods are
 updated using the references found in Section 2.6.2.

7. Security Considerations

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

7.1. Forged Header Fields

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

Kucherawy Standards Track [Page 26] RFC 7001 Authentication-Results Header Field September 2013

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

Kucherawy Standards Track [Page 27] RFC 7001 Authentication-Results Header Field September 2013

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

7.2. Misleading Results

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

7.3. Header Field Position

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

7.4. Reverse IP Query Denial-of-Service Attacks

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

Kucherawy Standards Track [Page 28] RFC 7001 Authentication-Results Header Field September 2013

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

7.5. Mitigation of Backscatter

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

Kucherawy Standards Track [Page 29] RFC 7001 Authentication-Results Header Field September 2013

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

7.10. Encapsulated Instances

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

7.11. Reverse Mapping

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

8. References

8.1. Normative References

 [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [IANA-HEADERS]
            Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004.
 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
            October 2008.

Kucherawy Standards Track [Page 30] RFC 7001 Authentication-Results Header Field September 2013

 [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            October 2008.

8.2. Informative References

 [ADSP]     Allman, E., Fenton, J., Delany, M., and J. Levine,
            "DomainKeys Identified Mail (DKIM) Author Domain Signing
            Practices (ADSP)", RFC 5617, August 2009.
 [AR-VBR]   Kucherawy, M., "Authentication-Results Registration for
            Vouch by Reference Results", RFC 6212, April 2011.
 [ATPS]     Kucherawy, M., "DomainKeys Identified Mail (DKIM)
            Authorized Third-Party Signatures", RFC 6541,
            February 2012.
 [AUTH]     Siemborski, R. and A. Melnikov, "SMTP Service Extension
            for Authentication", RFC 4954, July 2007.
 [DKIM]     Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
            Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
            September 2011.
 [DNS]      Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [DNS-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
            "DNS Extensions to Support IP Version 6", RFC 3596,
            October 2003.
 [DNSOP-REVERSE]
            Senie, D. and A. Sullivan, "Considerations for the use of
            DNS Reverse Mapping", Work in Progress, March 2008.
 [DOMAINKEYS]
            Delany, M., "Domain-Based Email Authentication Using
            Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
            May 2007.
 [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
            for Delivery Status Notifications", RFC 3464,
            January 2003.

Kucherawy Standards Track [Page 31] RFC 7001 Authentication-Results Header Field September 2013

 [EMAIL-ARCH]
            Crocker, D., "Internet Mail Architecture", RFC 5598,
            July 2009.
 [IANA-CONSIDERATIONS]
            Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, March 2003.
 [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, May 1996.
 [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
            Message Authentication Status", RFC 5451, April 2009.
 [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
            Text on Security Considerations", BCP 72, RFC 3552,
            July 2003.
 [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
            RFC 4406, April 2006.
 [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
            for Authorizing Use of Domains in E-Mail, Version 1",
            RFC 4408, April 2006.
 [VBR]      Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
            Reference", RFC 5518, April 2009.

Kucherawy Standards Track [Page 32] RFC 7001 Authentication-Results Header Field September 2013

Appendix A. Acknowledgements

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

Appendix B. Legacy MUAs

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

Appendix C. Authentication-Results Examples

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

Kucherawy Standards Track [Page 33] RFC 7001 Authentication-Results Header Field September 2013

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

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

 A message that was delivered by an MTA that conforms to this
 specification but provides no actual message authentication service:
      Authentication-Results: example.org 1; none
      Received: from mail-router.example.com
                    (mail-router.example.com [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

Kucherawy Standards Track [Page 34] RFC 7001 Authentication-Results Header Field September 2013

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

C.3. Service Provided, Authentication Done

 A message that was delivered by an MTA that conforms to this
 specification and applied some message authentication:
      Authentication-Results: example.com;
                spf=pass smtp.mailfrom=example.net
      Received: from dialup-1-2-3-4.example.net
                    (dialup-1-2-3-4.example.net [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 35] RFC 7001 Authentication-Results Header Field September 2013

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

 A message that was relayed inbound via a single MTA that conforms to
 this specification and applied three different message authentication
 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 ESMTP id g1G0r1kA003489;
                Fri, Feb 15 2002 17:19:07 -0800
      Date: Fri, Feb 15 2002 16:54:30 -0800
      To: receiver@example.com
      From: sender@example.net
      Message-Id: <12345.abc@example.net>
      Subject: here's a sample
      Hello!  Goodbye!
           Example 4: Headers Reporting Results from One MTA
 The Authentication-Results header field is present, indicating that
 the delivering MTA conforms to this specification.  Once again, the
 receiving DNS domain name is used as the authserv-id.  Furthermore,
 the sender authenticated herself/himself to the MTA via a method
 specified in [AUTH], and both SPF and Sender ID checks were done and
 passed.  The MUA could extract and relay this extra information if
 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
 dialup connection (example.net) sends mail to a border MTA
 (example.com) using SMTP authentication to prove identity.  The
 dialup provider has been explicitly authorized to relay mail as
 example.com resulting in passes by the SPF and Sender ID checks.

Kucherawy Standards Track [Page 36] RFC 7001 Authentication-Results Header Field September 2013

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

 A message that was relayed inbound by two different MTAs that conform
 to this specification and applied multiple message authentication
 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 ESMTP id g1G0r1kA003489;
                Fri, Feb 15 2002 17:19:07 -0800
      From: sender@example.com
      Date: Fri, Feb 15 2002 16:54:30 -0800
      To: receiver@example.com
      Message-Id: <12345.abc@example.com>
      Subject: here's a sample
      Hello!  Goodbye!
        Example 5: Headers Reporting Results from Multiple MTAs
 The Authentication-Results header field is present, indicating
 conformance to this specification.  Once again, the authserv-id used
 is the recipient's DNS domain name.  The header field is present
 twice because two different MTAs in the chain of delivery did
 authentication tests.  The first MTA, mail-router.example.com,
 reports that SMTP AUTH and SPF were both used and that the former
 passed while the latter failed.  In the SMTP AUTH case, additional
 information is provided in the comment field, which the MUA can
 choose to render if desired.

Kucherawy Standards Track [Page 37] RFC 7001 Authentication-Results Header Field September 2013

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

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

 A message that had authentication done at various stages, one of
 which was outside the receiving ADMD:
      Authentication-Results: example.com;
            dkim=pass reason="good signature"
              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;

Kucherawy Standards Track [Page 38] RFC 7001 Authentication-Results Header Field September 2013

            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.
 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.

Kucherawy Standards Track [Page 39] RFC 7001 Authentication-Results Header Field September 2013

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

C.7. Comment-Heavy Example

 The formal syntax permits comments within the content in a number of
 places.  For the sake of illustration, this example is also legal:
     Authentication-Results: foo.example.net (foobar) 1 (baz);
         dkim (Because I like it) / 1 (One yay) = (wait for it) fail
           policy (A dot can go here) . (like that) expired
           (this surprised me) = (as I wasn't expecting it) 1362471462
      Example 7: A Very Comment-Heavy but Perfectly Legal Example

Appendix D. Operational Considerations about Message Authentication

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

Kucherawy Standards Track [Page 40] RFC 7001 Authentication-Results Header Field September 2013

 3.  MUAs rely upon the upstream MTAs within their trust boundaries to
     make correct (as much as is possible) evaluations about the
     message's envelope, header, and content.  Thus, MUAs don't need
     to know how to do the work that upstream MTAs do; they only need
     the results of that work.
 4.  Evaluations about the quality of a message, from simple token
     matching (e.g., a list of preferred DNS domains) to cryptanalysis
     (e.g., public/private key work), are at least a little bit
     expensive and thus need to be minimized.  To that end, performing
     those tests at the border MTA is far preferred to doing that work
     at each MUA that handles a message.  If an ADMD's environment
     adheres to common messaging protocols, a reputation query or an
     authentication check performed by a border MTA would return the
     same result as the same query performed by an MUA.  By contrast,
     in an environment where the MUA does the work, a message arriving
     for multiple recipients would thus cause authentication or
     reputation evaluation to be done more than once for the same
     message (i.e., at each MUA), causing needless amplification of
     resource use and creating a possible denial-of-service attack
     vector.
 5.  Minimizing change is good.  As new authentication and reputation
     methods emerge, the list of methods supported by this header
     field would presumably be extended.  If MUAs simply consume the
     contents of this header field rather than actually attempt to do
     authentication and/or reputation work, then MUAs only need to
     learn to parse this header field once; emergence of new methods
     requires only a configuration change at the MUAs and software
     changes at the MTAs (which are presumably fewer in number).  When
     choosing to implement these functions in MTAs vs. MUAs, the
     issues of individual flexibility, infrastructure inertia, and
     scale of effort must be considered.  It is typically easier to
     change a single MUA than an MTA because the modification affects
     fewer users and can be pursued with less care.  However, changing
     many MUAs is more effort than changing a smaller number of MTAs.
 6.  For decisions affecting message delivery and display, assessment
     based on authentication and reputation is best performed close to
     the time of message transit, as a message makes its journey
     toward a user's inbox, not afterwards.  DKIM keys and IP address
     reputations, etc., can change over time or even become invalid,
     and users can take a long time to read a message once delivered.
     The value of this work thus degrades, perhaps quickly, once the
     delivery process has completed.  This seriously diminishes the
     value of this work when done other than at MTAs.

Kucherawy Standards Track [Page 41] RFC 7001 Authentication-Results Header Field September 2013

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

Appendix E. Changes since RFC 5451

 o  Erratum #2617 was addressed in RFC 6577 and was incorporated here.
 o  Requested Internet Standard status.
 o  Changed IANA rules from "IETF Review" to "designated expert".
 o  Updated existing IANA registries from the old RFC to this one.
 o  Added references to ADSP, ATPS, and VBR.
 o  Removed all the "X-" stuff, per BCP 178.
 o  Adjusted language to indicate that this header field was already
    defined and that we're just refreshing and revising.
 o  In a few places, RFC 2119 language had been used in lowercase
    terms; fixed here.
 o  Erratum #2818 addressed.
 o  Erratum #3195 addressed.
 o  Performed some minor wordsmithing and removed odd prose.
 o  ABNF: changed "dot-atom" to "Keyword" since "dot-atom" allows "=",
    which leads to ambiguous productions.
 o  ABNF: the authserv-id can be a "value", not a "dot-atom".
 o  ABNF: separated the spec version from the method version; they're
    syntactically the same but semantically different.  Added a
    section discussing them.
 o  Called out the SMTP verb exceptions ("mailfrom" and "rcptto"); the
    previous RFC didn't do this, leading to interoperability problems.
 o  Rather than deleting suspect header fields, they could also be
    renamed to something harmless; there is at least one
    implementation of this.

Kucherawy Standards Track [Page 42] RFC 7001 Authentication-Results Header Field September 2013

 o  Updated IANA "Email Authentication Methods" registry to include
    version numbers.
 o  Rather than repeating what RFC 4408 says the SPF results are, just
    referred to those documents.
 o  To avoid confusing consumers, constrained inclusion of unnecessary
    properties.
 o  Reviewed usage of "should" vs. "SHOULD".
 o  Updated prose around authserv-id (Section 2.4).

Author's Address

 Murray S. Kucherawy
 270 Upland Drive
 San Francisco, CA  94127
 US
 EMail: superuser@gmail.com

Kucherawy Standards Track [Page 43]

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