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rfc:rfc5451

Network Working Group M. Kucherawy Request for Comments: 5451 Sendmail, Inc. Category: Standards Track April 2009

 Message Header Field for Indicating Message Authentication Status

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 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 in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.

Abstract

 This memo defines a new header field 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), may use this message header field to relay that information
 in a convenient way to users or to make sorting and filtering
 decisions.

Kucherawy Standards Track [Page 1] RFC 5451 Authentication-Results Header Field April 2009

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2.  Trust Boundary . . . . . . . . . . . . . . . . . . . . . .  4
   1.3.  Processing Scope . . . . . . . . . . . . . . . . . . . . .  5
   1.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
   1.5.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.5.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.5.2.  Security . . . . . . . . . . . . . . . . . . . . . . .  5
     1.5.3.  Email Architecture . . . . . . . . . . . . . . . . . .  6
   1.6.  Trust Environment  . . . . . . . . . . . . . . . . . . . .  7
 2.  Definition and Format of the Header Field  . . . . . . . . . .  8
   2.1.  General Description  . . . . . . . . . . . . . . . . . . .  8
   2.2.  Formal Definition  . . . . . . . . . . . . . . . . . . . .  8
   2.3.  Authentication Identifier Field  . . . . . . . . . . . . . 10
   2.4.  Result Values  . . . . . . . . . . . . . . . . . . . . . . 12
     2.4.1.  DKIM and DomainKeys Results  . . . . . . . . . . . . . 12
     2.4.2.  SPF and Sender-ID Results  . . . . . . . . . . . . . . 13
     2.4.3.  "iprev" Results  . . . . . . . . . . . . . . . . . . . 14
     2.4.4.  SMTP AUTH Results  . . . . . . . . . . . . . . . . . . 14
     2.4.5.  Extension Result Codes . . . . . . . . . . . . . . . . 15
   2.5.  Authentication Methods . . . . . . . . . . . . . . . . . . 15
     2.5.1.  Definition of Initial Methods  . . . . . . . . . . . . 16
     2.5.2.  Extension Methods  . . . . . . . . . . . . . . . . . . 16
 3.  The "iprev" Authentication Method  . . . . . . . . . . . . . . 17
 4.  Adding the Header Field to A Message . . . . . . . . . . . . . 18
   4.1.  Header Field Position and Interpretation . . . . . . . . . 19
   4.2.  Local Policy Enforcement . . . . . . . . . . . . . . . . . 20
 5.  Removing the Header Field  . . . . . . . . . . . . . . . . . . 20
 6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   6.1.  The Authentication-Results Header Field  . . . . . . . . . 22
   6.2.  Email Authentication Method Name Registry  . . . . . . . . 22
   6.3.  Email Authentication Result Name Registry  . . . . . . . . 24
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   7.1.  Forged Header Fields . . . . . . . . . . . . . . . . . . . 26
   7.2.  Misleading Results . . . . . . . . . . . . . . . . . . . . 27
   7.3.  Header Field Position  . . . . . . . . . . . . . . . . . . 28
   7.4.  Reverse IP Query Denial-of-Service Attacks . . . . . . . . 28
   7.5.  Mitigation of Backscatter  . . . . . . . . . . . . . . . . 28
   7.6.  Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 28
   7.7.  Attacks against Authentication Methods . . . . . . . . . . 28
   7.8.  Intentionally Malformed Header Fields  . . . . . . . . . . 29
   7.9.  Compromised Internal Hosts . . . . . . . . . . . . . . . . 29
   7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 29
   7.11. Reverse Mapping  . . . . . . . . . . . . . . . . . . . . . 29

Kucherawy Standards Track [Page 2] RFC 5451 Authentication-Results Header Field April 2009

 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   8.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
   8.2.  Informative References . . . . . . . . . . . . . . . . . . 30
 Appendix A.  Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 32
 Appendix B.  Authentication-Results Examples . . . . . . . . . . . 33
   B.1.  Trivial Case; Header Field Not Present . . . . . . . . . . 33
   B.2.  Nearly Trivial Case; Service Provided, But No
         Authentication Done  . . . . . . . . . . . . . . . . . . . 34
   B.3.  Service Provided, Authentication Done  . . . . . . . . . . 35
   B.4.  Service Provided, Several Authentications Done, Single
         MTA  . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   B.5.  Service Provided, Several Authentications Done,
         Different MTAs . . . . . . . . . . . . . . . . . . . . . . 37
   B.6.  Service Provided, Multi-Tiered Authentication Done . . . . 39
 Appendix C.  Operational Considerations about Message
              Authentication  . . . . . . . . . . . . . . . . . . . 41
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 43

1. Introduction

 This memo defines a new header field for electronic mail messages
 that presents the results of a message authentication effort in a
 machine-readable format.  The intent 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 integrity of its
 content.
 End users are not expected to be direct consumers of this header
 field.  This header field is intended for consumption by programs
 that will then use or render such data in a human-usable form.
 This memo defines both the format of this new header field and
 discusses the implications of its presence or absence.  However, it
 does not discuss how the data contained in the header field should be
 used (i.e. what filtering decisions are appropriate, or how an MUA
 might render these results) as these are local policy and/or user
 interface design questions that are not appropriate for this memo.
 At the time of publication of this memo, [AUTH], [DKIM],
 [DOMAINKEYS], [SENDERID], and [SPF] are published DNS domain-level
 email authentication methods in common use.  This proposal is not
 intended to be restricted to domain-based authentication, but this
 has proven to be a good starting point for implementations.  As
 various methods emerge, it is necessary to prepare for their
 appearance and encourage convergence in the area of interfacing
 verifiers to filters and MUAs.

Kucherawy Standards Track [Page 3] RFC 5451 Authentication-Results Header Field April 2009

 Although [SPF] defined a header field called Received-SPF and
 [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.

1.1. Purpose

 The header field defined in this memo is expected to serve several
 purposes:
 1.  Convey the results of various message authentication checks being
     applied by upstream filters and Mail Transfer Agents (MTAs) to
     MUAs and downstream filters within the same "trust domain", as
     such agents may wish to render those results to end users or use
     that data to apply more or less stringent content checks based on
     authentication results;
 2.  Provide a common location within a message for this data;
 3.  Create an extensible framework for reporting new authentication
     methods as they emerge.
 In particular, the mere presence of this header field should not be
 construed as meaning that its data is valid, but rather that it is
 asserting validity based on one or more authentication schemes
 somewhere upstream.  For an MUA or downstream filter to treat the
 assertions as actually valid, there must be an assessment of the
 trust relationship between such agents and the validating MTA.

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 creator of the header field to the
 consumer must occur within a context of trust that the creator's
 information is correct.  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; "external" here includes
 all hosts that do not deliberately provide some kind of messaging
 service for the receiving ADMD's users, and "internal" includes those
 hosts that do.  By this definition, the hosts within a "trust
 boundary" may lie entirely within a receiving ADMD's direct control,

Kucherawy Standards Track [Page 4] RFC 5451 Authentication-Results Header Field April 2009

 or they can include hosts managed by another ADMD (such as an ISP or
 commercial filtering service) but that also provide services for the
 former.

1.3. Processing Scope

 This proposal is intended to address the needs of authenticating
 messages or properties of messages during their actual transport.  It
 is not meant to address the security of messages that might be
 encapsulated within other messages, such as a message/rfc822 [MIME]
 part within a message.

1.4. Requirements

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

1.5. Definitions

 This section defines various terms used throughout this document.

1.5.1. General

 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

 [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.

Kucherawy Standards Track [Page 5] RFC 5451 Authentication-Results Header Field April 2009

 o  "Authentication" is the assertion of validity of a piece of data
    about a message (such as the sender's identity) or the message in
    its entirety.
 As examples: [SPF] and [SENDERID] 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
 [SMTP] client to relay messages on its behalf but do not actually
 validate any property of the message itself.  By contrast, [DKIM] is
 agnostic as to the routing of a message but uses cryptographic
 signatures to authenticate agents claiming responsibility for the
 message (which implies authorization) and ensure it was 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), or both.
 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 an MTA that handles messages after a
    border MTA and before a delivery MTA.

Kucherawy Standards Track [Page 6] RFC 5451 Authentication-Results Header Field April 2009

 The following diagram illustrates the flow of mail among these
 defined components:
                        +-----+   +-----+   +------------+
                        | 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" may 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 from this specification.
 See [EMAIL-ARCH] for further discussion on general email system
 architecture, and Appendix C of this memo for discussion about the
 common aspects of email authentication in current environments.

1.6. Trust Environment

 This new header field permits one or more message validation
 mechanisms to communicate its 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 contains no
 integrity or validation mechanism of its own, so its presence must be
 trusted implicitly.  Hence, use of the header field depends upon
 ensuring that mail entering the ADMD has instances of the header
 field claiming to be valid within its boundaries removed, so that
 occurrences of such header fields can be used safely by consumers.

Kucherawy Standards Track [Page 7] RFC 5451 Authentication-Results Header Field April 2009

 The "authserv-id" token defined in Section 2.2 can be used to label
 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 within this proposal.

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 new header field being defined here is called "Authentication-
 Results".  It is a Structured Header Field as defined in [MAIL] and
 thus all of the related definitions in that document apply.
 This new header field SHOULD be 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 therefore should be
 treated as 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 [MAIL] comments)
 consists of an authentication identifier, an optional version, and
 then a series of "method=result" statements indicating which
 authentication method(s) were applied and their respective results,
 and then, for each applied method, an optional "reason" string plus
 optional "property=value" statements indicating which message
 properties were evaluated to reach that conclusion.
 The header field MAY appear more than once in a single message, or
 more than one result MAY be represented in a single header field, or
 a combination of these MAY be applied.

2.2. Formal Definition

 Formally, the header field is specified as follows using [ABNF]:
   authres-header = "Authentication-Results:" [CFWS] authserv-id
            [ CFWS version ]
            ( [CFWS] ";" [CFWS] "none" / 1*resinfo ) [CFWS] CRLF
          ; the special case of "none" is used to indicate that no
          ; message authentication is performed
   authserv-id = dot-atom
               ; see below for a description of this element

Kucherawy Standards Track [Page 8] RFC 5451 Authentication-Results Header Field April 2009

   version = 1*DIGIT [CFWS]
           ; indicates which version of this specification is in use;
           ; this specification is version "1"; the absence of a
           ; version implies this version of the specification
   resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
             *( CFWS propspec )
   methodspec = [CFWS] method [CFWS] "=" [CFWS] result
              ; indicates which authentication method was evaluated
   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 and would be
            ; useful to reveal to end users as authenticated
   method = dot-atom [ [CFWS] "/" [CFWS] 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
   result = dot-atom
          ; 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, or was a value taken
         ; from a message header field, or was some property of
         ; the message body, or some other property evaluated by
         ; the receiving MTA
   property = dot-atom
           ; 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 the offset into the body at which
           ; content of interest was detected; if "ptype" is "policy"
           ; then this indicates the name of the policy that caused
           ; this header field to be added (see below)

Kucherawy Standards Track [Page 9] RFC 5451 Authentication-Results Header Field April 2009

   pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
            [CFWS]
          ; the value extracted from the message property defined
          ; by the "ptype.property" construction; if the value
          ; identifies something intended to be an e-mail identity,
          ; then it MUST use the right hand portion of this ABNF
          ; definition
 The "local-part" is as defined in Section 3.4.1, and "dot-atom" is
 defined in Section 3.2.3, of [MAIL].
 The "value" is as defined in Section 5.1 of [MIME].
 The "domain-name" is as defined in Section 3.5 of [DKIM].
 The "dot-atom" used in a "result" above is further constrained by the
 necessity of being enumerated in Section 2.4 or an amendment to it.
 See Section 2.3 for a description of the "authserv-id" element.
 The list of commands eligible for use with the "smtp" ptype can be
 found in [SMTP] and subsequent amendments.
 "CFWS" is as defined in Section 3.2.2 of [MAIL].
 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.
 The "ptype" and "property" values used by each authentication method
 should be defined in the specification for that method (or its
 amendments).
 The "ptype" and "property" are case-insensitive.
 A "ptype" value of "policy" indicates a policy decision about the
 message not specific to a property of the message that could be
 extracted.  For example, if a method would normally report a
 "ptype.property" of "header.From" and no From: header field was
 present, the method can use "policy" to indicate that no conclusion
 about the authenticity of the message could be reached.

2.3. Authentication Identifier Field

 Every Authentication-Results header field has an authentication
 identifier field ("authserv-id" above).  This is similar in syntax to
 a fully-qualified domain name.

Kucherawy Standards Track [Page 10] RFC 5451 Authentication-Results Header Field April 2009

 The authentication identifier field provides a unique identifier that
 refers to the authenticating service within a given ADMD.  The
 uniqueness of the identifier MUST be guaranteed by the ADMD that
 generates it and must pertain to exactly that one ADMD.  This
 identifier is intended to be machine-readable and not necessarily
 meaningful to users.  MUAs or downstream filters SHOULD use this
 identifier to determine whether or not the data contained in an
 Authentication-Results header field should be used.
 For simplicity and scalability, the authentication identifier SHOULD
 be a common token used throughout the ADMD, such as the DNS domain
 name used by or within that ADMD.
 For tracing and debugging purposes, the authentication identifier MAY
 instead be the hostname of the MTA performing the authentication
 check whose result is being reported.  This is also useful for
 another purpose, as described in Section 4.  Moreover, some
 implementations have considered appending a delimiter such as "/" and
 following it with useful transport tracing data such as the [SMTP]
 queue ID or a timestamp.
 It should be noted, however, that using a local, relative identifier
 like a single 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
 must use the identifier to determine whether to use the contents of
 the header field:
 o  Changes to the identifier impose a large, centralized
    administrative burden.
 o  Ongoing administrative changes require constantly updating this
    centralized table, making it difficult to ensure that an MUA or
    downstream filter will have access to accurate information for
    assessing the usability of the header field's content.  In
    particular, consumers of the header field will need to know not
    only the current identifier(s) in use, but previous ones as well
    to account for delivery latency or later 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".

Kucherawy Standards Track [Page 11] RFC 5451 Authentication-Results Header Field April 2009

2.4. Result Values

 Each individual authentication method returns one of a set of
 specific result values.  The subsections below define these results
 for the authentication methods specifically supported by this memo,
 and verifiers SHOULD use these values as described below.  New
 methods not specified in this document intended to be supported by
 the header field defined in this memo MUST include a similar result
 table either in its defining memo or in a supplementary one.

2.4.1. DKIM and DomainKeys Results

 The result values used by [DKIM] and [DOMAINKEYS] are as follows:
 none:  The message was not signed.
 pass:  The message was signed, the signature or signatures were
    acceptable to the verifier, and the signature(s) passed
    verification tests.
 fail:  The message was signed and the signature or signatures were
    acceptable to the verifier, but they failed the verification
    test(s).
 policy:  The message was signed but the signature or signatures were
    not acceptable to the verifier.
 neutral:  The message was signed but the signature or signatures
    contained syntax errors or were not otherwise able to be
    processed.  This result SHOULD also be 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.
 A signature is "acceptable to the verifier" if it passes local policy
 checks (or there are no specific local policy checks).  For example,
 a verifier 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.

Kucherawy Standards Track [Page 12] RFC 5451 Authentication-Results Header Field April 2009

 [DKIM] advises that if a message fails verification, it should 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.

2.4.2. SPF and Sender-ID Results

 The result values are used by [SPF] and [SENDERID] as follows:
 none:  No policy records were published at the sender's DNS domain.
 neutral:  The sender's ADMD has asserted that it cannot or does not
    want to assert whether or not the sending IP address is authorized
    to send mail using the sender's DNS domain.
 pass:  The client is authorized by the sender's ADMD to inject or
    relay mail on behalf of the sender's DNS domain.
 policy:  The client is 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.
 hardfail:  This client is explicitly not authorized to inject or
    relay mail using the sender's DNS domain.
 softfail:  The sender's ADMD believes the client was not authorized
    to inject or relay mail using the sender's DNS domain, but is
    unwilling to make a strong assertion to that effect.
 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 policy record from DNS.  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 or
    a syntax error in a retrieved DNS TXT record.  A later attempt is
    unlikely to produce a final result.
 The distinction between and interpretation of "none" and "neutral"
 under these methods is discussed further in [SPF].
 The "policy" result would be returned if, for example, [SPF] returned
 as "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).

Kucherawy Standards Track [Page 13] RFC 5451 Authentication-Results Header Field April 2009

 If the retrieved sender policies used to evaluate [SPF] and
 [SENDERID] 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.

2.4.3. "iprev" Results

 The result values are 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.
 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 [DNS].

2.4.4. SMTP AUTH Results

 The result values are used by the [AUTH] method are as follows:
 none:  SMTP authentication was not attempted.
 pass:  The SMTP client had authenticated to the server reporting the
    result using the protocol described in [AUTH].

Kucherawy Standards Track [Page 14] RFC 5451 Authentication-Results Header Field April 2009

 fail:  The SMTP client had 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 which is likely transient in nature,
    such as a temporary Lightweight Directory Access Protocol (LDAP)
    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 LDAP lookup error.  A later attempt is not
    likely produce a final result.
 Note that an agent making use of the data provided by this header
 field SHOULD consider "fail" and "temperror" to be the synonymous in
 terms of message authentication, i.e., the client did not
 authenticate.

2.4.5. Extension Result Codes

 Additional result codes (extension results) might be defined in the
 future by later revisions or extensions to this specification.
 Extension results beginning with "x-" will never be defined as
 standard fields; such names are reserved for experimental use.
 Result codes not beginning with "x-" MUST be registered with the
 Internet Assigned Numbers Authority (IANA) and published in an RFC.
 See Section 6 for further details.
 Implementations reporting new result codes MUST use the "x-" prefix
 until such time as the new method is registered by IANA.
 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 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
 extension result.

2.5. Authentication Methods

 This section defines the supported authentication methods and
 discusses the proper means for applying experimental and other
 extension methods.

Kucherawy Standards Track [Page 15] RFC 5451 Authentication-Results Header Field April 2009

2.5.1. Definition of Initial Methods

 As they are currently existing specifications for message
 authentication, it is appropriate to define an authentication method
 identifier for each of [AUTH], [DKIM], [DOMAINKEYS], [SENDERID], and
 [SPF].  Therefore, the authentication method identifiers "auth",
 "dkim", "domainkeys", "sender-id", and "spf", respectively are hereby
 defined for MTAs applying those specifications for email message
 authentication.
 Furthermore, method "iprev" is defined in Section 3.
 See Section 6 for details.

2.5.2. Extension Methods

 Additional authentication method identifiers (extension methods) may
 be defined in the future by later revisions or extensions to this
 specification.  Extension methods beginning with "x-" will never be
 defined as standard fields; such names are reserved for experimental
 use.  Method identifiers not beginning with "x-" MUST be registered
 with the Internet Assigned Numbers Authority (IANA) and published in
 an RFC.  See Section 6 for further details.
 Extension methods may 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 should reflect the name of the method being
     defined, but should 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".
 Implementations of new methods MUST use the "x-" prefix until such
 time as the new method is registered by IANA.
 Authentication method implementors are encouraged to provide adequate
 information, via [MAIL] comments if necessary, to allow an MUA
 developer to understand or relay ancillary details of authentication
 results.  For example, if it might be of interest to relay what data
 was used to perform an evaluation, such information could be relayed
 as a comment in the header field, such as:
      Authentication-Results: example.com;
                foo=pass bar.baz=blob (2 of 3 tests OK)

Kucherawy Standards Track [Page 16] RFC 5451 Authentication-Results Header Field April 2009

 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 method identifier.

3. The "iprev" Authentication Method

 This section defines an additional authentication method called
 "iprev".
 In general, "iprev" is an attempt to verify that a client appears to
 be valid based on some DNS queries.  Upon receiving a session
 initiation of some kind from a client, the IP address of the client
 peer is queried for matching names (i.e., a number-to-name
 translation, also known as a "reverse lookup" or a "PTR" record
 query).  Once that result is acquired, a lookup of each of the names
 (i.e., a name-to-number translation, or an "A" or "AAAA" record
 query) thus retrieved is done.  The response to this second check
 should result in at least one mapping back to the client's IP
 address.
 More algorithmically: 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 finite, 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-IP6] discusses the query formats for the IPv6 case.
 A successful test using this algorithm constitutes a result of "pass"
 since the ADMD in which the client's PTR claims it belongs has
 confirmed that claim by including corresponding data in its DNS
 domain.  A failure to match constitutes a "fail".  There is no case
 in which a "neutral" result can be returned.  The remaining
 "temperror" and "permerror" cases refer, respectively, to temporary
 and permanent DNS query errors.

Kucherawy Standards Track [Page 17] RFC 5451 Authentication-Results Header Field April 2009

 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 actual
 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 [DNSOP-REVERSE].  In particular, it recommends that
 applications avoid using this test as a means of authentication or
 security.  Its presence in this memo 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.  As such, the order of the presented authentication
 methods and results MUST NOT be used either to imply or infer the
 importance or strength of any given method over another.  Instead,
 the MUA or downstream filter consuming this header field must
 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 defined in Section 2.5.2,
 and each "result" MUST refer to a result code declared in the IANA
 registry, or an extension result code as defined in Section 2.4.5.
 See Section 6 for further information about the registered methods
 and result codes.
 An MTA compliant with this specification MUST add 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 MUST add 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 to indicate explicitly that no message
 authentication schemes were applied prior to delivery of this
 message.

Kucherawy Standards Track [Page 18] RFC 5451 Authentication-Results Header Field April 2009

 An MTA adding this header field must take steps to identify it as
 legitimate to the MUAs or downstream filters that will ultimately
 consume its content.  One required process to do so is described in
 Section 5.  Further measures may be required in some environments.
 Some possible solutions are enumerated in Section 7.1.  This memo
 does not mandate any specific solution to this issue as each
 environment has its own facilities and limitations.
 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 allows easy detection of
 header fields that can be trusted.
 End users making direct use of this header field may 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 instructed to do so by the user
 or administrator.  That is, this interpretation should not be "on by
 default".  Naturally then, users or administrators should not
 activate such a feature unless they are certain the header field will
 be added by the border MTA that accepts the mail that is ultimately
 read by the MUA, and instances of the header field appearing to be
 from within the ADMD but actually added by foreign MTAs will be
 removed before delivery.
 Furthermore, MUAs and downstream filters SHOULD NOT interpret this
 header field unless the authentication 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 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.

Kucherawy Standards Track [Page 19] RFC 5451 Authentication-Results Header Field April 2009

 An MUA SHOULD NOT reveal these results to end users unless the
 results are accompanied by, at a minimum, some associated reputation
 data about the authenticated origin identifiers within the message.
 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 SHOULD 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.
 MUAs SHOULD ignore instances of this header field discovered within
 message/rfc822 [MIME] attachments.
 Further discussion of this can be found in Section 7 below.

4.2. Local Policy Enforcement

 If a site's local policy is to consider a non-recoverable failure
 result (e.g., "fail" for DKIM, "hardfail" for SPF) for any particular
 authentication method as justification to reject the message
 completely, the border MTA SHOULD issue an [SMTP] rejection response
 to the message rather than adding this header field with the failure
 result and allowing it 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
 [DSN] to a forged originator.
 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 2.4).
 Such rejections at the SMTP protocol level are not possible if local
 policy is enforced at the MUA and not the MTA.  Unfortunately, this
 may be a common scenario.

5. Removing the Header Field

 For security reasons, any MTA conforming to this specification MUST
 delete any discovered instance of this header field that claims to
 have been added within its trust boundary and that did not come from
 another trusted MTA.  For example, an MTA (border or otherwise) for
 example.com receiving a message MUST delete any instance of this

Kucherawy Standards Track [Page 20] RFC 5451 Authentication-Results Header Field April 2009

 header field bearing an authentication identifier indicating the
 header field was added within example.com prior to adding its own
 header fields.  This may mean each MTA will have to be equipped with
 a list of internal MTAs known to be compliant (and hence
 trustworthy).
 For simplicity and maximum security, a border MTA MAY 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 should be let in, removing 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 might explicitly trust authentication results
 asserted by upstream host example.net even though they exist in
 completely disjoint administrative boundaries.  In that case, the
 border MTA MAY elect not to delete those results; moreover, the
 upstream host doing some authentication work could apply a signing
 technology such as [DKIM] on its own results to assure downstream
 hosts of their authenticity.  An example of this is provided in
 Appendix B.
 Similarly, in the case of messages signed using [DKIM] or other
 message signing methods that sign header fields, this may invalidate
 one or more signatures on the message if they covered the header
 field to be removed at the time of signing.  This behavior can be
 desirable since there's little value in validating the signature on a
 message with forged headers.  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 if the [SMTP] connection relaying the
 message is not from a trusted internal MTA.

Kucherawy Standards Track [Page 21] RFC 5451 Authentication-Results Header Field April 2009

6. IANA Considerations

 IANA has registered a new header field and created two new tables as
 described below.

6.1. The Authentication-Results Header Field

 Per [IANA-HEADERS], the "Authentication-Results" header field has
 been added to the IANA Permanent Message Header Field Registry.  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 5451
   Related information:
     Requesting review of any proposed changes and additions to
     this field is recommended.

6.2. Email Authentication Method Name Registry

 Names of message authentication methods supported by this
 specification must be registered with IANA, with the exception of
 experimental names as described in Section 2.5.2.
 New entries are assigned only for values that have been documented in
 a published RFC that has had IETF Review, per [IANA-CONSIDERATIONS].
 Each method must register a name, the specification that defines it,
 one 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.

Kucherawy Standards Track [Page 22] RFC 5451 Authentication-Results Header Field April 2009

 The initial set of entries in this registry is as follows:

+————+———-+——–+—————-+——————–+

Method Defined ptype property value

+————+———-+——–+—————-+——————–+

auth RFC4954 smtp auth AUTH parameter of
the SMTP MAIL
command

+————+———-+——–+—————-+——————–+

dkim RFC4871 header d value of
signature "d" tag
i value of
signature "i" tag

+————+———-+——–+—————-+——————–+

domainkeys RFC4870 header d value of
signature "d" tag
from value of From
header field after
removing comments
and local-part if
not authenticated
sender value of Sender
header field after
removing comments
and local-part if
not authenticated

+————+———-+——–+—————-+——————–+

iprev this policy iprev client IP address
document

+————+———-+——–+—————-+——————–+

sender-id RFC4406 header name of header value of header
field used by field used by PRA
the Purported after removing
Responsible comments and parts
Address (PRA) not authenticated

+————+———-+——–+—————-+——————–+

spf RFC4408 smtp mailfrom envelope sender
after removing
parts not
authenticated
smtp helo HELO/EHLO value

+————+———-+——–+—————-+——————–+

Kucherawy Standards Track [Page 23] RFC 5451 Authentication-Results Header Field April 2009

6.3. Email Authentication Result Name 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 2.4.5.
 New entries are assigned only for result codes that have been
 documented in a published RFC that has had IETF Review, per
 [IANA-CONSIDERATIONS].  Each code must register a name, the document
 that establishes the registration, the authentication method(s) that
 uses it, and either a definition of the semantics of its use or a
 reference to the place where those semantics are defined.
 The initial set of entries in this registry is as follows:

+———–+———-+—————-+——————————+

Code Defined Auth Method(s) Meaning

+———–+———-+—————-+——————————+

none this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id
auth section 2.4.4

+———–+———-+—————-+——————————+

pass this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id
iprev section 2.4.3
auth section 2.4.4

+———–+———-+—————-+——————————+

fail this dkim section 2.4.1
document domainkeys
iprev section 2.4.3
auth section 2.4.4

+———–+———-+—————-+——————————+

Kucherawy Standards Track [Page 24] RFC 5451 Authentication-Results Header Field April 2009

policy this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id

+———–+———-+—————-+——————————+

neutral this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id

+———–+———-+—————-+——————————+

temperror this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id
iprev section 2.4.3
auth section 2.4.4

+———–+———-+—————-+——————————+

permerror this dkim section 2.4.1
document domainkeys
spf section 2.4.2
sender-id
iprev section 2.4.3
auth section 2.4.4

+———–+———-+—————-+——————————+

hardfail this spf section 2.4.2
document sender-id

+———–+———-+—————-+——————————+

softfail this spf section 2.4.2
document sender-id

+———–+———-+—————-+——————————+

Kucherawy Standards Track [Page 25] RFC 5451 Authentication-Results Header Field April 2009

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

Kucherawy Standards Track [Page 26] RFC 5451 Authentication-Results Header Field April 2009

     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 host names 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 planned for future work.
 In any case, a mechanism needs to exist for an MUA or filter to
 verify that the host that appears to have added the header field (a)
 actually did so, and (b) is legitimately adding that header field for
 this delivery.  Given the variety of messaging environments deployed
 today, consensus appears to be that specifying a particular mechanism
 for doing so is not appropriate for this memo.
 Mitigation of the forged header field attack can also be accomplished
 by moving the authentication results data into meta-data associated
 with the message.  In particular, an [SMTP] extension could be
 established which is used 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 meta-data
 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.

Kucherawy Standards Track [Page 27] RFC 5451 Authentication-Results Header Field April 2009

 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 enroute 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 SHOULD take care not to go to unbounded
 lengths to resolve "A" and "PTR" queries.  MUAs or other filters
 making use of an "iprev" result specified by this memo SHOULD be
 aware of the algorithm used by the verifier reporting the result and
 thus be aware of 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 headers 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 (and later
 amendments to it) is also a security consideration here.

Kucherawy Standards Track [Page 28] RFC 5451 Authentication-Results Header Field April 2009

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.  Implementors must thoroughly verify all such header fields
 received from MTAs and be robust against intentionally as well as
 unintentionally malformed header fields.

7.9. Compromised Internal Hosts

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

7.10. Encapsulated Instances

 [MIME] messages may contain attachments of type "message/rfc822",
 which contain other [MAIL] messages.  Such an encapsulated message
 may 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.
 Implementors 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".

Kucherawy Standards Track [Page 29] RFC 5451 Authentication-Results Header Field April 2009

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

8.2. Informative References

 [AUTH]                 Siemborski, R. and A. Melnikov, "SMTP Service
                        Extension for Authentication", RFC 4954,
                        July 2007.
 [DKIM]                 Allman, E., Callas, J., Delany, M., Libbey,
                        M., Fenton, J., and M. Thomas, "DomainKeys
                        Identified Mail (DKIM) Signatures", RFC 4871,
                        May 2007.
 [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.

Kucherawy Standards Track [Page 30] RFC 5451 Authentication-Results Header Field April 2009

 [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.
 [EMAIL-ARCH]           Crocker, D., "Internet Mail Architecture",
                        Work in Progress, October 2008.
 [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.
 [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.
 [SMTP]                 Klensin, J., "Simple Mail Transfer Protocol",
                        RFC 5321, October 2008.
 [SPF]                  Wong, M. and W. Schlitt, "Sender Policy
                        Framework (SPF) for Authorizing Use of Domains
                        in E-Mail, Version 1", RFC 4408, April 2006.

Kucherawy Standards Track [Page 31] RFC 5451 Authentication-Results Header Field April 2009

Appendix A. Legacy MUAs

 Implementors of this proposal 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.

Kucherawy Standards Track [Page 32] RFC 5451 Authentication-Results Header Field April 2009

Appendix B. Authentication-Results Examples

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

B.1. Trivial Case; Header Field Not Present

 The trivial case:
      Received: from mail-router.example.com
                    (mail-router.example.com [192.0.2.1])
                by server.example.org (8.11.6/8.11.6)
                    with ESMTP id g1G0r1kA003489;
                Fri, Feb 15 2002 17:19:07 -0800
      From: sender@example.com
      Date: Fri, Feb 15 2002 16:54:30 -0800
      To: receiver@example.org
      Message-Id: <12345.abc@example.com>
      Subject: here's a sample
      Hello!  Goodbye!
 Example 1: Trivial case
 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.

Kucherawy Standards Track [Page 33] RFC 5451 Authentication-Results Header Field April 2009

B.2. Nearly Trivial Case; Service Provided, But No Authentication Done

 A message that was delivered by an MTA that conforms to this
 specification but provides no actual message authentication service:
      Authentication-Results: example.org; none
      Received: from mail-router.example.com
                    (mail-router.example.com [192.0.2.1])
                by server.example.org (8.11.6/8.11.6)
                    with ESMTP id g1G0r1kA003489;
                Fri, Feb 15 2002 17:19:07 -0800
      From: sender@example.com
      Date: Fri, Feb 15 2002 16:54:30 -0800
      To: receiver@example.org
      Message-Id: <12345.abc@example.com>
      Subject: here's a sample
      Hello!  Goodbye!
 Example 2: Header present but no authentication done
 The "Authentication-Results" header field is present, showing that
 the delivering MTA conforms to this specification.  It used its DNS
 domain name as the authserv-id.  The presence of "none" (and the
 absence of any method and result tokens) indicates that no message
 authentication was done.

Kucherawy Standards Track [Page 34] RFC 5451 Authentication-Results Header Field April 2009

B.3. Service Provided, Authentication Done

 A message that was delivered by an MTA that conforms to this
 specification and applied some message authentication:
      Authentication-Results: example.com;
                spf=pass smtp.mailfrom=example.net
      Received: from dialup-1-2-3-4.example.net
                    (dialup-1-2-3-4.example.net [192.0.2.200])
                by mail-router.example.com (8.11.6/8.11.6)
                    with ESMTP id g1G0r1kA003489;
                Fri, Feb 15 2002 17:19:07 -0800
      From: sender@example.net
      Date: Fri, Feb 15 2002 16:54:30 -0800
      To: receiver@example.com
      Message-Id: <12345.abc@example.net>
      Subject: here's a sample
      Hello!  Goodbye!
 Example 3: Header reporting results
 The "Authentication-Results" header field is present, indicating that
 the border MTA conforms to this specification.  The authserv-id is
 once again the DNS domain name.  Furthermore, the message was
 authenticated by that MTA via the method specified in [SPF].  Note
 that since that method cannot authenticate the local-part, it has
 been omitted from the result's value.  The MUA could extract and
 relay this extra information if desired.

Kucherawy Standards Track [Page 35] RFC 5451 Authentication-Results Header Field April 2009

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

 A message that was relayed inbound via a single MTA that conforms to
 this specification and applied three different message authentication
 checks:
      Authentication-Results: example.com;
                auth=pass (cram-md5) smtp.auth=sender@example.com;
                spf=pass smtp.mailfrom=example.com
      Authentication-Results: example.com;
                sender-id=pass header.from=example.com
      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.net
      From: sender@example.com
      Message-Id: <12345.abc@example.com>
      Subject: here's a sample
      Hello!  Goodbye!
 Example 4: Headers reporting results from one MTA
 The "Authentication-Results" header field is present, indicating 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 [SENDERID] 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 SenderID checks.

Kucherawy Standards Track [Page 36] RFC 5451 Authentication-Results Header Field April 2009

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

 A message that was relayed inbound by two different MTAs that conform
 to this specification and applied multiple message authentication
 checks:
      Authentication-Results: example.com;
                sender-id=hardfail header.from=example.com;
                dkim=pass (good signature) header.i=sender@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
      Authentication-Results: example.com;
                auth=pass (cram-md5) smtp.auth=sender@example.com;
                spf=hardfail 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
      DKIM-Signature:  v=1; a=rsa-sha256; s=gatsby; d=example.com;
                i=sender@example.com; t=1188964191; c=simple/simple;
                h=From:Date:To:Message-Id:Subject;
                bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70;
                b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
      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, "mail-router.example.com" reports
 that [AUTH] and [SPF] were both used, and [AUTH] passed but [SPF]
 failed.  In the [AUTH] case, additional data is provided in the
 comment field, which the MUA can choose to render if desired.

Kucherawy Standards Track [Page 37] RFC 5451 Authentication-Results Header Field April 2009

 The second MTA, "auth-checker.example.com", reports that it did a
 [SENDERID] 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.
 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 [AUTH].  The [SPF] and
 [SENDERID] tests failed since "example.com" has not granted
 "example.net" authority to relay mail on its behalf.  However, the
 [DKIM] test passed because the sending user had a private key
 matching one of "example.com"'s published public keys and used it to
 sign the message.

Kucherawy Standards Track [Page 38] RFC 5451 Authentication-Results Header Field April 2009

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

 A message that had authentication done at various stages, one of
 which was outside the receiving ADMD:
   Authentication-Results: example.com;
         dkim=pass (good signature) header.i=@mail-router.example.net;
         dkim=fail (bad signature) header.i=@newyork.example.com
   Received: from mail-router.example.net
             (mail-router.example.net [192.0.2.250])
         by chicago.example.com (8.11.6/8.11.6)
             for <recipient@chicago.example.com>
             with ESMTP id i7PK0sH7021929;
         Fri, Feb 15 2002 17:19:22 -0800
   DKIM-Signature: v=1; a=rsa-sha256; s=furble;
         d=mail-router.example.net; t=1188964198; c=relaxed/simple;
         h=From:Date:To:Message-Id:Subject:Authentication-Results;
         bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=;
         b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
   Authentication-Results: example.net;
         dkim=pass (good signature) header.i=@newyork.example.com
   Received: from smtp.newyork.example.com
             (smtp.newyork.example.com [192.0.2.220])
         by mail-router.example.net (8.11.6/8.11.6)
             with ESMTP id g1G0r1kA003489;
         Fri, Feb 15 2002 17:19:07 -0800
   DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=newyork.example.com;
         t=1188964191; c=simple/simple;
         h=From:Date:To:Message-Id:Subject;
         bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
         b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
   From: sender@newyork.example.com
   Date: Fri, Feb 15 2002 16:54:30 -0800
   To: meetings@example.net
   Message-Id: <12345.abc@newyork.example.com>
   Subject: here's a sample
 Example 6: Headers reporting results from multiple MTAs in different
 ADMDs
 In this example we see multi-tiered authentication with an extended
 trust boundary.
 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].

Kucherawy Standards Track [Page 39] RFC 5451 Authentication-Results Header Field April 2009

 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
 that 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 40] RFC 5451 Authentication-Results Header Field April 2009

Appendix C. Operational Considerations about Message Authentication

 This proposal 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 the work of this proposal and its deployment to
 date is 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, rejections of messages at the SMTP level
     rather than generating a DSN internally.  Thus, doing any of the
     authentication or reputation work exclusively at the MUA or
     intermediate MTA renders this desire unattainable.
 2.  Border MTAs are more likely to have direct access to external
     sources of authentication or reputation information since modern
     MUAs are more likely to be heavily firewalled.  Thus, some MUAs
     might not even be able to complete the task of performing
     authentication or reputation evaluations without complex proxy
     configurations or similar burdens.
 3.  MUAs rely upon the upstream MTAs within their trust boundaries to
     make correct (as much as that 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 should be minimized.  To that end, performing
     those tests at the border MTA is far preferred to doing that work
     at each MUA that handles a message.  If an ADMD's environment
     adheres to common messaging protocols, a reputation query or an
     authentication check performed by a border MTA would return the
     same result as the same query performed by an MUA.  By contrast,
     in an environment where the MUA does the work, a message arriving
     for multiple recipients would thus cause authentication or
     reputation evaluation to be done more than once for the same
     message (i.e., at each MUA) causing needless amplification of
     resource use and creating a possible denial-of-service attack
     vector.

Kucherawy Standards Track [Page 41] RFC 5451 Authentication-Results Header Field April 2009

 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 attempting to
     do authentication and/or reputation work, then MUAs only need to
     learn to parse this header field once; emergence of new methods
     requires only a configuration change at the MUAs and software
     changes at the MTAs (which are presumably fewer in number).  When
     choosing to implement these functions in MTAs vs MUAs, the issues
     of individual flexibility, infrastructure inertia and scale of
     effort must be considered.  It is typically easier to change a
     single MUA than an MTA because the modification affects fewer
     users and can be pursued with less care.  However, changing many
     MUAs is more effort than changing a smaller number of MTAs.
 6.  For decisions affecting message delivery and display, assessment
     based on authentication and reputation is best performed close to
     the time of message transit, as a message makes its journey
     toward a user's inbox, not afterwards.  DKIM keys and IP address
     reputations, etc., can change over time or even become invalid,
     and users can take a long time to read a message once delivered.
     The value of this work thus degrades, perhaps quickly, once the
     delivery process has completed.  This seriously diminishes the
     value of this work when done other than at MTAs.
 Many operational choices are possible within an ADMD, including the
 venue for performing authentication and/or reputation assessment.
 The current specification does not dictate any of those choices.
 Rather, it facilitates those cases in which information produced by
 one stage of analysis needs to be transported with the message to the
 next stage.

Kucherawy Standards Track [Page 42] RFC 5451 Authentication-Results Header Field April 2009

Acknowledgements

 The author wishes to acknowledge the following for their review and
 constructive criticism of this proposal: Eric Allman, Mark Delany,
 Victor Duchovni, Frank Ellermann, Jim Fenton, Philip Guenther, Tony
 Hansen, Paul Hoffman, Scott Kitterman, Eliot Lear, John Levine, Miles
 Libbey, Charles Lindsey, Alexey Melnikov, Douglas Otis, Juan Altmayer
 Pizzorno, Michael Thomas, and Kazu Yamamoto.
 Special thanks to Dave Crocker and S. Moonesamy for their logistical
 support, and feedback on and contributions to the numerous proposed
 edits throughout the lifetime of this work.

Author's Address

 Murray S. Kucherawy
 Sendmail, Inc.
 6475 Christie Ave., Suite 350
 Emeryville, CA  94608
 US
 Phone: +1 510 594 5400
 EMail: msk+ietf@sendmail.com

Kucherawy Standards Track [Page 43]

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