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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 RFC 5451

Individual submission                                       M. Kucherawy
Internet-Draft                                            Sendmail, Inc.
Intended status: Standards Track                      September 23, 2008
Expires: March 27, 2009


   Message Header Field for Indicating Message Authentication Status
                 draft-kucherawy-sender-auth-header-16

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on March 27, 2009.

Copyright Notice

   Copyright (C) The IETF Trust (2008).














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Abstract

   This memo defines a new message header field for use with electronic
   mail messages to indicate the results of message authentication
   efforts.  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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Definition and Format of the Header  . . . . . . . . . . . . .  7
     2.1.  General Description  . . . . . . . . . . . . . . . . . . .  7
     2.2.  Formal Definition  . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Authentication Identifier Fields . . . . . . . . . . . . . 10
     2.4.  Result Values  . . . . . . . . . . . . . . . . . . . . . . 10
       2.4.1.  DKIM and DomainKeys Results  . . . . . . . . . . . . . 10
       2.4.2.  DKIM ADSP Results  . . . . . . . . . . . . . . . . . . 11
       2.4.3.  SPF and Sender-ID Results  . . . . . . . . . . . . . . 12
       2.4.4.  iprev Results  . . . . . . . . . . . . . . . . . . . . 13
       2.4.5.  SMTP AUTH Results  . . . . . . . . . . . . . . . . . . 13
       2.4.6.  Extension Result Codes . . . . . . . . . . . . . . . . 14
     2.5.  Authentication Methods . . . . . . . . . . . . . . . . . . 15
       2.5.1.  Definition Of Initial Methods  . . . . . . . . . . . . 15
       2.5.2.  Extension Methods  . . . . . . . . . . . . . . . . . . 15
   3.  The 'iprev' Authentication Method  . . . . . . . . . . . . . . 17
   4.  Adding The Header Field To A Message . . . . . . . . . . . . . 18
     4.1.  Header Position and Interpretation . . . . . . . . . . . . 18
     4.2.  Local Policy Enforcement . . . . . . . . . . . . . . . . . 19
   5.  Removing The Header Field  . . . . . . . . . . . . . . . . . . 20
   6.  Conformance and Usage Requirements . . . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     7.1.  The Authentication-Results: header . . . . . . . . . . . . 22
     7.2.  Email Authentication Method Name Registry  . . . . . . . . 22
     7.3.  Email Authentication Result Name Registry  . . . . . . . . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     8.1.  Forged Headers . . . . . . . . . . . . . . . . . . . . . . 26
     8.2.  Misleading Results . . . . . . . . . . . . . . . . . . . . 26
     8.3.  Other Protocols  . . . . . . . . . . . . . . . . . . . . . 27
     8.4.  Header Position  . . . . . . . . . . . . . . . . . . . . . 27
     8.5.  Reverse IP Query Denial-Of-Service Attacks . . . . . . . . 27
     8.6.  Mitigation of Backscatter  . . . . . . . . . . . . . . . . 27
     8.7.  Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 28
     8.8.  Attacks Against Authentication Methods . . . . . . . . . . 28



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     8.9.  Intentionally Malformed Header Fields  . . . . . . . . . . 28
     8.10. Compromised Internal Hosts . . . . . . . . . . . . . . . . 28
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 31
   Appendix B.  Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 32
   Appendix C.  Authentication-Results Examples . . . . . . . . . . . 33
     C.1.  Trivial case; header field not present . . . . . . . . . . 33
     C.2.  Nearly-trivial case; service provided, but no
           authentication done  . . . . . . . . . . . . . . . . . . . 34
     C.3.  Service provided, authentication done  . . . . . . . . . . 34
     C.4.  Service provided, several authentications done, single
           MTA  . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     C.5.  Service provided, several authentications done,
           different MTAs . . . . . . . . . . . . . . . . . . . . . . 36
     C.6.  Service provided, multi-tiered authentication done . . . . 38
   Appendix D.  Public Discussion . . . . . . . . . . . . . . . . . . 40
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 42































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1.  Introduction

   This memo defines a new message header field for electronic mail
   messages which 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) can provide a recommendation to
   the user as to the validity of the message's origin and possibly the
   integrity of its content.

   This memo defines both the format of this new header field, and
   discusses the implications of its presence or absence.

   [UPDATE PRIOR TO FINAL VERSION] At the time of publication of this
   draft, [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] and [SPF] are the
   published e-mail authentication methods in common use.  As various
   methods emerge, it is necessary to prepare for their appearance and
   encourage convergence in the area of interfacing these filters to
   MUAs.

   Although [SPF] defined a header field called Received-SPF for this
   purpose, that header field is specific to the conveyance of SPF and
   similar results only and thus is 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 to MUAs from filters and Mail Transfer Agents (MTAs) the
       results of various message authentication checks being applied;

   2.  Provide a common location for the presentation of this data;

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

   4.  Convey the results of message authentication tests to later
       filtering agents within the same "trust domain", as such agents
       might apply more or less stringent checks based on message
       authentication results.

1.2.  Requirements

   This memo establishes no new requirements on existing protocols or
   servers, as there is currently no standard place for the described
   data to be collected or presented.



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   In particular, this memo establishes no requirement on MTAs to reject
   or filter arriving messages which 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.

1.3.  Definitions

   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].

   A "border MTA" is an MTA which acts as a gateway between the general
   Internet and the users within an organizational boundary.

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

   An "intermediate MTA" is an MTA which handles messages after a border
   MTAs and before a delivery MTA.


                          +-----+   +-----+   +------------+
                          | 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 message authentication could
   take place on an intermediate MTA.  Although this document doesn't
   specifically describe such cases, they are not meant to be excluded



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   from this specification.

   See [I-D.DRAFT-CROCKER-EMAIL-ARCH] for further discussion on e-mail
   system architecture.















































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2.  Definition and Format of the Header

   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 MUST be added at the top of the message as it
   transits MTAs which 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 Header 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]:

     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-text
                 ; see below for a description of this element;
                 ; "dot-atom-text" is defined in section 3.2.4 of [MAIL]


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



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           ; 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


     method = token [ [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 = token
            ; 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











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     property = token
             ; if "ptype" is "smtp", this indicates which [SMTP]
             ; command provided the value which 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 which caused
             ; this header field to be added (see below)


     pvalue = [CFWS] ( token / addr-spec ) [CFWS]
            ; the value extracted from the message property defined
            ; by the "ptype.property" construction; if the value
            ; identifies an address, then it is an "addr-spec"
            ; as defined in section 3.4 of [MAIL]

   The "token" and "value" are as defined in section 5.1 of [MIME].

   The "token" 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 3.2.3 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.

   If the parsed "ptype.property" construction clearly identifies a



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   mailbox (in particular, smtp.mailfrom, smtp.rcpt, header.from,
   header.sender), then the "pvalue" MUST be an "addr-spec".  Other
   properties (e.g. smtp.helo) may be evaluated, but the property MUST
   still be expressed as a "token" for simplified parsing.

2.3.  Authentication Identifier Fields

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

   The authentication identifier field provides a unique identifier that
   refers to the authenticating service within a given mail
   administrative domain.  The uniqueness of the identifier is
   guaranteed by the mail administrative domain that generates it and
   must pertain to exactly that one mail administrative domain.  This
   identifier is intended to be machine-readable and not necessarily
   meaningful to users.  MUAs may use this identifier to determine
   whether or not the data contained in an Authentication-Results header
   field can be trusted.

   For tracing and debugging purposes, the authentication identifier
   SHOULD be the domain name of the MTA performing the authentication
   check whose result is being reported.

   Examples of valid authentication identifiers are mail.example.org,
   engineering.example.net and ms1.newyork.example.com.

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.
   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(s) is (were) acceptable
      to the verifier, and the signature(s) passed verification tests.






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   fail:  The message was signed and the signature(s) is (were)
      acceptable to the verifier, but it (they) failed the verification
      test(s).

   policy:  The message was signed but the signature(s) is (were) not
      acceptable to the verifier.

   neutral:  The message was signed but the signature(s) 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 which
      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 which
      is unrecoverable, such as a required header field being absent.  A
      later attempt is unlikley 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 by the domain identified in the From: header field of the
   message, thus making third-party signatures unacceptable.

2.4.2.  DKIM ADSP Results

   The result values are used by [I-D.DRAFT-IETF-DKIM-SSP] as follows:

   none:  No DKIM author domain signing practises (ADSP) record was
      published.

   pass:  A DKIM ADSP record was published which indicated the mail
      should be signed with an author signature, and this message had
      such a signature that validated.

   unknown:  No valid author signature was found on the message and
      either the published ADSP was "unknown", or no policy was
      published.

   signed:  A valid author signature was found on the message and the
      published ADSP was "unknown".







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   fail:  No valid author signature was found on the message and the
      published ASDP record indicated an "all" policy.

   discard:  No valid author signature was found on the message and the
      published ADSP record indicated a "discardable" policy.

   nxdomain:  Evaluating the ADSP for the author's domain indicated that
      the author's domain does not exist.

   temperror:  A DKIM policy could not be retrieved due to some error
      which is likely transient in nature, such as a temporary DNS
      error.  A later attempt may produce a final result.

   permerror:  A DKIM policy could not be retrieved due to some error
      which is likely not transient in nature, such as a permanent DNS
      error.  A later attempt is unlikely to produce a final result.

2.4.3.  SPF and Sender-ID Results

   The result values are used by [SPF] and [SENDERID] as follows:

   none:  No policy records were published by the sender's domain.

   neutral:  The sender's domain has asserted that it cannot or does not
      want to assert whether or not the sending IP address is authorized
      to send mail on behalf of the sender's domain.

   pass:  The client is authorized to inject or relay mail on behalf of
      the sender's domain.

   policy:  The client is authorized to inject or relay mail on behalf
      of the sender's 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 on behalf of the sender's domain.

   softfail:  The sender's domain believes the client was not authorized
      to inject or relay mail on its behalf but is unwilling to make a
      strong assertion to that effect.

   temperror:  The message could not be verified due to some error which
      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.





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   permerror:  The message could not be verified due to some error which
      is unrecoverable, such as a required header field being absent.  A
      later attempt is unlikley 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 domain
   to one found in an explicit list of unacceptable domains (e.g.
   spammers).

2.4.4.  iprev Results

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

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

   hardfail:  The reverse DNS evaluation failed.  In particular, the
      "reverse" and "forward" lookups each produced results but they
      were not in agreement.

   softfail:  The reverse DNS evaluation failed.  In particular, one or
      both of the "reverse" and forward lookups returned no data (i.e. a
      DNS reply code of NODATA).

   temperror:  The reverse DNS evaluation could not be completed due to
      some error which is likely transient in nature, such as a
      temporary DNS error.  A later attempt may produce a final result.

   permerror:  The reverse DNS evaluation could not be completed due to
      some error which is unrecoverable (e.g. a DNS reply code of NODATA
      or NXDOMAIN).  A later attempt is unlikely to produce a final
      result.

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

2.4.5.  SMTP AUTH Results

   The result values are used by the [AUTH] method are as follows:







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   none:  SMTP authentication was not attempted.

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

   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 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 which 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.

2.4.6.  Extension Result Codes

   Additional result codes (extension results) may 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 7 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 trust domains 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 or MUA intended for production use SHOULD ignore or delete
   any Authentication-Results header field that includes an extension
   result.




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2.5.  Authentication Methods

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

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", "senderid" and "spf" respectively are hereby
   defined for MTAs applying those specifications for e-mail message
   authentication.

   Furthermore, method "iprev" is defined in Section 3.

   Finally, as its publication is imminent, this document also defines
   "dkim-adsp" per [I-D.DRAFT-IETF-DKIM-SSP].

   See Section 7 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 7 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.  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" which 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



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   information, via [MAIL] comments if necessary, to allow an MUA
   developer to understand or relay ancilliary 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: mx.example.com;
                  foo=pass bar.baz=blob (2 of 3 tests OK)

   Experimental method identifiers MUST only be used within trust
   domains 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 or MUA intended
   for production use SHOULD ignore or delete any Authentication-Results
   header field that includes an experimental method identifier.



































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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" 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 A, the list
   of names to which A maps (after a "PTR" query) is the set N, and the
   union of IP addresses to which each member of N maps (after an "A"
   query) is L, then this test is successful if A is an element of L.

   Section 5.5 of [SPF] contains more detail about this process as well
   as some discussion of possible denial-of-service attacks.  [DNS-IP6]
   discusses the format of this query for the IPv6 case.

   A successful test using this algorithm constitutes a result of "pass"
   since the domain in which the client's PTR claims it belongs has
   confirmed that claim.  A failure to match constitutes a "hardfail".
   There is no case in which "softfail" or "neutral" can be returned.
   The remaining "temperror" and "permerror" cases refer respectively to
   temporary and permanent DNS query errors.

   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 practise 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.










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4.  Adding The Header Field To A Message

   This specification makes no attempt to evaluate the relative
   strengths of various message authentication methods that may become
   available.  As such, the order of the presented authentication
   methods and results MUST NOT be used to determine the importance or
   strength of any given method over another.  Instead, the MUA must
   interpret the result of each method based on its 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 2.4.6.
   See Section 7 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 at which host which the test was done, which test got
   applied and what the result was.  If an MTA applies more than one
   such test, it MUST either add this header field once per test, or one
   header field indicating all of the results.  An MTA MUST NOT add a
   result to an existing header.

   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.

4.1.  Header Position and Interpretation

   In order to ensure non-ambiguous results and avoid the impact of
   false header fields, an MUA 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
   trust domain but actually added by foreign MTAs will be removed
   before delivery.

   Furthermore, an MUA SHOULD NOT interpret this header field unless the
   authentication identifier it bears appears to be one within its own
   trust domain as configured by the user or administrator.

   An MUA MUST ignore any result reported using a "result" not specified



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   in the result code registry, or a "ptype" not listed in the
   corresponding registry for such values as defined in Section 7.
   Moreover, an MUA MUST ignore a result indicated for any "method" it
   does not support.

   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 message that was authenticated.  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
   domain name was intended to mislead.  See Section 8.2 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 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.

   Further discussion of this can be found in Section 8 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 or "discard" for
   DKIM-ADSP) 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 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 4.2.

   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.









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5.  Removing The Header Field

   For security reasons, any MTA conforming to this specification MUST
   delete any discovered instance of this header field which claims to
   have been added within its trust boundary and 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
   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).

   Furthermore, a border MTA MAY elect simply to remove all instances of
   this header field on mail crossing into its trust boundary.

   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 C.

   Similarly, in the case of messages signed using [DKIM] or other
   message signing methods that sign headers, this may invalidate one or
   more signature on the message if they included the header field to be
   removed at the time of signing.  This behaviour 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 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
   from not a trusted internal MTA.













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6.  Conformance and Usage Requirements

   An MTA or gateway conforms to this specification if it applies one or
   more message authentication mechanisms and inserts a header field
   corresponding to this specification after doing so and prior to
   delivery (per Section 4) and removes apparently improper headers (per
   Section 5).

   MTAs that are relaying mail rather than delivering it, i.e. are not
   part of an intended recipient's trust boundary, MAY perform message
   authentication or even take actions based on the results found, but
   SHOULD NOT add an "Authentication-Results" header field if relaying
   (rather than rejecting or discarding at the gateway).  Conversely, an
   MTA doing local delivery and some form of message authentication MUST
   add this header field prior to delivering the message in order to be
   compliant.  An exception to the former case is described in
   Section 5.

   A minimal implementation that does at least one message
   authentication check will add the header field defined by this memo
   prior to invoking local delivery procedures.






























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7.  IANA Considerations

   IANA is requested to register a new header field and to create a new
   table as described below.

7.1.  The Authentication-Results: header

   Per [IANA-HEADERS], the "Authentication-Results:" header field is
   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): [TBD]
     Related information:
       Requesting review of any proposed changes and additions to
       this field is recommended.

7.2.  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 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, and
   which "property" value(s) should be reported by that method.

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


















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+------------+----------+--------+----------------+--------------------+
|   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  |
+------------+----------+--------+----------------+--------------------+
| dkim-adsp  |  [TBD]   | header | from           | value of From      |
|            |          |        |                | header field       |
|            |          |        |                | w/comments removed |
+------------+----------+--------+----------------+--------------------+
| domainkeys | RFC4870  | header | from           | value of From      |
|            |          |        |                | header field       |
|            |          |        |                | w/comments removed |
|            |          |        +----------------+--------------------+
|            |          |        | sender         | value of Sender    |
|            |          |        |                | header field       |
|            |          |        |                | w/comments removed |
+------------+----------+--------+----------------+--------------------+
|    iprev   | this     | policy | iprev          | client IP address  |
|            | document |        |                |                    |
+------------+----------+--------+----------------+--------------------+
|  senderid  | RFC4406  | header | name of header | value of header    |
|            |          |        | field used by  | field used by PRA  |
|            |          |        | PRA            | w/comments removed |
+------------+----------+--------+----------------+--------------------+
|     spf    | RFC4408  | smtp   | mailfrom       | envelope sender    |
|            |          +--------+----------------+--------------------+
|            |          | smtp   | helo           | HELO/EHLO value    |
+------------+----------+--------+----------------+--------------------+

7.3.  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 2.4.6.

   New entries are assigned only for result codes that have been
   documented in a published RFC that has IETF Review, per
   [IANA-CONSIDERATIONS].  Each code must register a name, the document
   which establishes the registration, the authentication method(s)
   which uses it, and either a definition of the semantics of its use or



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   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     |                              |
|           |          +----------------+------------------------------+
|           |          | dkim-adsp      | section 2.4.2                |
|           |          +----------------+------------------------------+
|           |          | spf            | section 2.4.3                |
|           |          | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | auth           | section 2.4.5                |
+-----------+----------+----------------+------------------------------+
| pass      | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
|           |          +----------------+------------------------------+
|           |          | dkim-adsp      | section 2.4.2                |
|           |          +----------------+------------------------------+
|           |          | spf            | section 2.4.3                |
|           |          | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | iprev          | section 2.4.4                |
|           |          +----------------+------------------------------+
|           |          | auth           | section 2.4.5                |
+-----------+----------+----------------+------------------------------+
| fail      | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
|           |          +----------------+------------------------------+
|           |          | dkim-adsp      | section 2.4.2                |
|           |          +----------------+------------------------------+
|           |          | auth           | section 2.4.5                |
+-----------+----------+----------------+------------------------------+
| policy    | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
+-----------+----------+----------------+------------------------------+
| neutral   | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
|           |          +----------------+------------------------------+
|           |          | spf            | section 2.4.3                |
|           |          | sender-id      |                              |
+-----------+----------+----------------+------------------------------+
| temperror | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
|           |          +----------------+------------------------------+



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|           |          | dkim-adsp      | section 2.4.2                |
|           |          +----------------+------------------------------+
|           |          | spf            | section 2.4.3                |
|           |          | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | iprev          | section 2.4.4                |
|           |          +----------------+------------------------------+
|           |          | auth           | section 2.4.5                |
+-----------+----------+----------------+------------------------------+
| permerror | this     | dkim           | section 2.4.1                |
|           | document | domainkeys     |                              |
|           |          +----------------+------------------------------+
|           |          | dkim-adsp      | section 2.4.2                |
|           |          +----------------+------------------------------+
|           |          | spf            | section 2.4.3                |
|           |          | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | iprev          | section 2.4.4                |
|           |          +----------------+------------------------------+
|           |          | auth           | section 2.4.5                |
+-----------+----------+----------------+------------------------------+
| nxdomain  | this     | dkim-adsp      | section 2.4.2                |
|           | document |                |                              |
+-----------+----------+----------------+------------------------------+
| signed    | this     | dkim-adsp      | section 2.4.2                |
|           | document |                |                              |
+-----------+----------+----------------+------------------------------+
| unknown   | this     | dkim-adsp      | section 2.4.2                |
|           | document |                |                              |
+-----------+----------+----------------+------------------------------+
| discard   | this     | dkim-adsp      | section 2.4.2                |
|           | document |                |                              |
+-----------+----------+----------------+------------------------------+
| hardfail  | this     | spf            | section 2.4.3                |
|           | document | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | iprev          | section 2.4.4                |
+-----------+----------+----------------+------------------------------+
| softfail  | this     | spf            | section 2.4.3                |
|           | document | sender-id      |                              |
|           |          +----------------+------------------------------+
|           |          | iprev          | section 2.4.4                |
+-----------+----------+----------------+------------------------------+








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8.  Security Considerations

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

8.1.  Forged Headers

   An MUA 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 destination MX for a receiving domain as the hostname token in
   the value of the header, 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.  Possibly
   the simplest is a digital signature on the header field that can be
   verified by a posted public key.  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.  In either
   case, a mechanism needs to exist 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.

8.2.  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 removal discussed above.

   Hence, MUAs must take some care with use of this header even after



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   possibly malicious headers are scrubbed.

8.3.  Other Protocols

   Mitigation of the forged header 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.

8.4.  Header 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.

8.5.  Reverse IP Query Denial-Of-Service Attacks

   Section 5.5 of [SPF] describes a DNS-based denial-of-service attack
   for verifiers that attempt to 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.

8.6.  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 which did not send the messages being
   rejected.







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8.7.  Internal MTA Lists

   Section 5 describes a procedure for scrubbing headers which 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 a domain to
   attack.

8.8.  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.

8.9.  Intentionally Malformed Header Fields

   It is possible for an attacker to add an Authentication-Results:
   header field which 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.

8.10.  Compromised Internal Hosts

   An internal MUA or MTA which has been compromised could generate mail
   with a forged From: header and a forged Authentication-Results:
   header which endorses it.  Although it is clearly a larger concern to
   have compromised internal machines than it is to prove the value of
   this header, 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.











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9.  References

9.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.

   [IANA-HEADERS]
              Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", RFC 3864,
              September 2004.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [MAIL]     Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

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

9.2.  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-IP6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003.

   [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.

   [I-D.DRAFT-CROCKER-EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture",
              I-D draft-crocker-email-arch, May 2007.



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   [I-D.DRAFT-IETF-DKIM-SSP]
              Allman, E., Delany, M., and J. Fenton, "DKIM Author
              Signing Practices", I-D draft-ietf-dkim-ssp, January 2008.

   [IANA-CONSIDERATIONS]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              October 1998.

   [IMAP]     Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

   [SENDERID]
              Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, April 2006.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.





























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Appendix A.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this proposal: Eric Allman, Dave Crocker,
   Mark Delany, Frank Ellermann, Jim Fenton, Philip Guenther, Tony
   Hansen, Paul Hoffman, Eliot Lear, John Levine, Miles Libbey, Charles
   Lindsey, S. Moonesamy, Juan Altmayer Pizzorno, Michael Thomas.












































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Appendix B.  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 adaptation, 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.
































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Appendix C.  Authentication-Results Examples

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

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.sendmail.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@sendmail.com
        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.






















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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: mail-router.example.com; none
        Received: from mail-router.example.com
                      (mail-router.example.com [192.0.2.1])
                  by server.sendmail.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@sendmail.com
        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, indicating that
   the delivering MTA (which is named in the value of the header field)
   conforms to this specification.  The presence of "none" (and the
   absence of any method and result tokens) indicates that no message
   authentication was done.

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: mail-router.example.com;
                  spf=pass smtp.mailfrom=sender@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



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   The "Authentication-Results" header field is present, indicating that
   the border MTA (which is identified in the value of the header field)
   conforms to this specification.  Furthermore, the message was
   authenticated by that MTA via the method specified in [SPF].  The MUA
   could extract and relay this extra information if desired.

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: mail-router.example.com;
                  auth=pass (cram-md5) smtp.auth=sender@example.com;
                  spf=pass smtp.mailfrom=sender@example.com
        Authentication-Results: mail-router.example.com;
                  sender-id=pass header.from=sender@example.com
        Received: from mail-router.example.com
                      (mail-router.example.com [192.0.2.1])
                  by dialup-1-2-3-4.example.net (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 (which is identified in the value of the header field)
   conforms to this specification.  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.



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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: auth-checker.example.com;
                  sender-id=hardfail header.from=sender@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: mail-router.example.com;
                  auth=pass (cram-md5) smtp.auth=sender@example.com;
                  spf=hardfail smtp.mailfrom=sender@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;
                  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@sendmail.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.  It 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.

   The second MTA, identifying itself as "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



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   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.





































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C.6.  Service provided, multi-tiered authentication done

   A message that had authentication done at various stages, one of
   which was outside the receiving domain:

     Authentication-Results: chicago.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: mail-router.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
   domains

   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



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   example.net which is used by example.com.  There meetings@example.net
   is expanded to a long list of recipients, one of which 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.  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 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 "hardfail".  However, the first signature included
   the Authentication-Results: header added at mail-router.example.net
   which validated the second signature.  Thus, indirectly, it can be
   determined that the authentication claimed by both signatures are
   indeed valid.





























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Appendix D.  Public Discussion

   [REMOVE BEFORE PUBLICATION]

   Public discussion of this proposed specification is handled via the
   mail-vet-discuss@mipassoc.org mailing list.  The list is open.
   Access to subscription forms and to list archives can be found at
   http://mipassoc.org/mailman/listinfo/mail-vet-discuss.











































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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









































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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   The IETF invites any interested party to bring to its attention any
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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