    Internet Engineering Task Force (IETF)                        H. Fontana
    Request for Comments: 6591                                    April 2012
    Category: Standards Track
    ISSN: 2070-1721

       Authentication Failure Reporting Using the Abuse Reporting Format

    Abstract

       This memo registers an extension report type for the Abuse Reporting
       Format (ARF), affecting multiple registries, for use in generating
       receipt-time reports about messages that fail one or more email
       message authentication checks.

    Status of This Memo

       This is an Internet Standards Track document.

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

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6591.

    Copyright Notice

       Copyright (c) 2012 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

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

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    Table of Contents

       1. Introduction ....................................................2
       2. Definitions .....................................................3
          2.1. Key Words ..................................................3
          2.2. Email Architecture .........................................3
          2.3. Base64 .....................................................3
          2.4. Technologies ...............................................3
       3. ARF Extension for Authentication Failure Reporting ..............3
          3.1. New ARF Feedback Type ......................................4
          3.2. New ARF Header Field Names .................................5
               3.2.1. Required for All Reports ............................5
               3.2.2. Optional for All Reports ............................5
               3.2.3. Required for DKIM Reports ...........................5
               3.2.4. Optional for DKIM Reports ...........................6
               3.2.5. Required for ADSP Reports ...........................6
               3.2.6. Required for SPF Reports ............................6
          3.3. Authentication Failure Types ...............................6
       4. Syntax for Added ARF Header Fields ..............................7
       5. IANA Considerations .............................................8
          5.1. Updates to ARF Feedback Types ..............................8
          5.2. Updates to ARF Header Field Names ..........................8
       6. Security Considerations ........................................10
          6.1. Inherited Considerations ..................................10
          6.2. Forgeries .................................................10
          6.3. Automatic Generation ......................................11
          6.4. Envelope Sender Selection .................................11
          6.5. Reporting Multiple Incidents ..............................11
          6.6. Redaction of Data in DKIM Reports .........................12
       7. References .....................................................12
          7.1. Normative References ......................................12
          7.2. Informative References ....................................13
       Appendix A. Acknowledgements ......................................14
       Appendix B. Example ...............................................14
         B.1. Example Use of ARF Extension Headers .......................14

    1.  Introduction

       The Abuse Reporting Format [ARF] defines a message format for sending
       reports of abuse in the messaging infrastructure, with an eye towards
       automating both the generation and consumption of those reports.
       There is now also a desire to extend the ARF to include the reporting
       of messages that fail to authenticate using known message
       authentication methods, such as DomainKeys Identified Mail [DKIM] and
       Sender Policy Framework [SPF], as these are sometimes evidence of
       abuse that can be detected and reported through automated means.  The
       same mechanism can be used to convey forensic information about the

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       specific reason the authentication method failed.  Thus, this memo
       presents such extensions to ARF that allow for detailed reporting of
       message authentication method failures.

    2.  Definitions

    2.1.  Key Words

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

    2.2.  Email Architecture

       This memo uses some terms whose definitions and descriptions can be
       found in [EMAIL-ARCH].

    2.3.  Base64

       Base64 is defined in Section 4 of [BASE64].

       The values that are base64 encodings MAY contain folding whitespace
       (FWS) for formatting purposes as per the usual header field wrapping
       defined in [MAIL].  During decoding, any characters not in the base64
       alphabet are ignored so that such line wrapping does not harm the
       value.  The ABNF token "FWS" is defined in [DKIM].  No other
       extensions to the valid base64 character set are permitted.

    2.4.  Technologies

       There are technologies in email security that provide authentication
       services and some that do authorization.  These are often conflated.
       A discussion that is useful for establishing context can be found in
       Section 1.5.2 of [AUTH-RESULTS].

    3.  ARF Extension for Authentication Failure Reporting

       The current report format defined in [ARF] lacks some specific
       features required to do effective email authentication failure
       reporting.  This section defines extensions to ARF to accommodate
       this requirement.

       A single report describes a single email authentication failure.
       Multiple reports MAY be used to report multiple failures for a single
       message.

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    3.1.  New ARF Feedback Type

       A new feedback type, "auth-failure", is defined in this document as
       an extension, per Section 7.3 of [ARF].

       A message that uses this feedback type has the following modified
       header field requirements for the second (machine-parseable) [MIME]
       part of the report:

       Authentication-Results:  Syntax as specified in [AUTH-RESULTS].
          Furthermore, [ARF] specifies this field is OPTIONAL and appears at
          most once; for this extension, this field MUST be present, but it
          MUST reflect only a single authentication method's result.

       Original-Envelope-Id:  Syntax as specified in [ARF].  Furthermore,
          [ARF] specifies this field is OPTIONAL and appears at most once;
          for this extension, this field's inclusion is RECOMMENDED, where
          that value is available, to aid in diagnosing the authentication
          failure.

       Original-Mail-From:  Syntax as specified in [ARF].  Furthermore,
          [ARF] specifies this field is OPTIONAL and appears at most once;
          for this extension, this field's inclusion is RECOMMENDED, where
          that value is available, to aid in diagnosing the authentication
          failure.

       Source-IP:  Syntax as specified in [ARF].  Furthermore, [ARF]
          specifies this field is OPTIONAL and appears at most once; for
          this extension, this field's inclusion is RECOMMENDED, where that
          value is available, to aid in diagnosing the authentication
          failure.

       Reported-Domain:  Syntax as specified in [ARF].  Furthermore, [ARF]
          specifies this field is OPTIONAL and appears at most once; for
          this extension, this field MUST be present if such a value is
          available.

       Delivery-Result:  As specified in Section 3.2.2.  This field is
          OPTIONAL, but it MUST NOT appear more than once.  If present, it
          SHOULD indicate the outcome of the message in some meaningful way,
          but it MAY be set to "other" for local policy reasons.

       The third MIME part of the message is either of type "message/rfc822"
       (as defined in [MIME-TYPES]) or of type "text/rfc822-headers" (as
       defined in [REPORT]) and contains a copy of the entire header block
       from the original message.  This part MUST be included (contrary to
       [REPORT], which makes it optional).

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       For privacy reasons, report generators might need to redact portions
       of a reported message, such as an identifier or address associated
       with the end user whose complaint action resulted in the report.  A
       discussion of relevant issues and a suggested method for doing so can
       be found in [RFC6590].

    3.2.  New ARF Header Field Names

       The following new ARF field names are defined as extensions to
       Section 3.1 of [ARF].

    3.2.1.  Required for All Reports

       Auth-Failure:  Indicates the failure from an email authentication
          method that is being reported.  The list of valid values is
          enumerated in Section 3.3.

    3.2.2.  Optional for All Reports

       Delivery-Result:  The final message disposition that was enacted by
          the ADministrative Management Domain (ADMD) generating the report.
          It MUST NOT appear more than once.  Possible values are as
          follows:

          delivered:  The message was delivered (not specific as to where).

          spam:  The message was delivered to the recipient's spam folder
             (or equivalent).

          policy:  The message was not delivered to the intended inbox due
             to a failure from an email authentication method.  The specific
             action taken is not specified.

          reject:  The message was rejected.

          other:  The message had a final disposition not covered by one of
             the above values.

    3.2.3.  Required for DKIM Reports

       DKIM-Domain:  The domain that signed the message, taken from the "d="
          tag of the signature.

       DKIM-Identity:  The identity of the signature that failed
          verification, taken from the "i=" tag of the signature.

       DKIM-Selector:  The selector of the signature that failed
          verification, taken from the "s=" tag of the signature.

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    3.2.4.  Optional for DKIM Reports

       DKIM-Canonicalized-Header:  A base64 encoding of the canonicalized
          header of the message as generated by the verifier.

       DKIM-Canonicalized-Body:  A base64 encoding of the canonicalized body
          of the message as generated by the verifier.  The encoded content
          MUST be limited to those octets that contribute to the DKIM body
          hash (i.e., the value of the "l=" tag; see Section 3.7 of [DKIM]).

       If DKIM-Canonicalized-Header and DKIM-Canonicalized-Body encode
       redacted data, they MUST NOT be included.  Otherwise, they SHOULD be
       included.  The data presented there have to be exactly the
       canonicalized header and body as defined by [DKIM] and computed at
       the verifier.  This is because these fields are intended to aid in
       identifying message alterations that invalidate DKIM signatures in
       transit.  Including redacted data in them renders the data unusable.
       (See also Sections 3.1 and 6.6 for further discussion.)

    3.2.5.  Required for ADSP Reports

       DKIM-ADSP-DNS:  Includes the Author Domain Signing Practices (ADSP)
          policy used to obtain the verifier's ADSP result.  This MUST be
          formatted per Section 4.2.1 of [ADSP].

    3.2.6.  Required for SPF Reports

       SPF-DNS:  This field MUST appear once for every SPF record [SPF] used
          to obtain the SPF result.  It MUST include the DNS RRTYPE used,
          the DNS domain from which the record was retrieved, and the
          content of that record.  The syntax is defined in Section 4.

    3.3.  Authentication Failure Types

       The list of defined email authentication failure types used in the
       "Auth-Failure:" header field (defined above), is as follows:

       adsp:  The message did not conform to the author domain's published
          [ADSP] signing practices.  The DKIM-ADSP-DNS field MUST be
          included in the report.

       bodyhash:  The body hash in the signature and the body hash computed
          by the verifier did not match.  The DKIM-Canonicalized-Body field
          SHOULD be included in the report (see Section 3.2.4).

       revoked:  The DKIM key referenced by the signature on the message has
          been revoked.  The DKIM-Domain and DKIM-Selector fields MUST be
          included in the report.

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       signature:  The DKIM signature on the message did not successfully
          verify against the header hash and public key.  The DKIM-Domain
          and DKIM-Selector fields MUST be included in the report, and the
          DKIM-Canonicalized-Header field SHOULD be included in the report
          (see Section 3.2.4).

       spf:  The evaluation of the author domain's SPF record produced a
          "none", "fail", "softfail", "temperror", or "permerror" result.
          ("none" is not strictly a failure per [SPF], but a service that
          demands successful SPF evaluations of clients could treat it like
          a failure.)

       Supplementary data MAY be included in the form of comments compliant
       with [MAIL].  For example, "Auth-Failure: adsp" could be augmented by
       a comment to indicate that the failed message was rejected because it
       was not signed when it should have been.  See Appendix B for an
       example.

    4.  Syntax for Added ARF Header Fields

       The [ABNF] definitions for the new fields are as follows:

         auth-failure = "Auth-Failure:" [CFWS]
                        ( "adsp" / "bodyhash" / "revoked" /
                          "signature" / "spf" ) [CFWS] CRLF
                      ; "CFWS" is defined in [MAIL]

         delivery-result = "Delivery-Result:" [CFWS]
                           ( "delivered" / "spam" / "policy" /
                             "reject" / "other" ) [CFWS] CRLF

         dkim-header = "DKIM-Canonicalized-Header:" [CFWS]
                       base64string CRLF
                     ; "base64string" is defined in [DKIM]

         dkim-sig-domain = "DKIM-Domain:" [CFWS] domain-name [CFWS]
                           CRLF
                         ; "domain-name" is defined in [DKIM]

         dkim-identity = "DKIM-Identity:" [CFWS] [ local-part ] "@"
                         domain-name [CFWS] CRLF
                       ; "local-part" is defined in [MAIL]

         dkim-selector = "DKIM-Selector:" [CFWS] selector [CFWS] CRLF
                       ; "selector" is defined in [DKIM]

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         dkim-adsp-dns = "DKIM-ADSP-DNS:" [CFWS]
                         quoted-string [CFWS] CRLF
                       ; "quoted-string" is defined in [MAIL]

         dkim-body = "DKIM-Canonicalized-Body:" [CFWS]
                     base64string CRLF

         dkim-selector-dns = "DKIM-Selector-DNS:" [CFWS]
                             quoted-string [CFWS] CRLF

         spf-dns = "SPF-DNS:" [CFWS] ( "txt" / "spf" ) [CFWS] ":" [CFWS]
                   domain [CFWS] ":" [CFWS] quoted-string [CFWS] CRLF

    5.  IANA Considerations

       As required by [IANA], this section contains registry information for
       the extension to [ARF].

    5.1.  Updates to ARF Feedback Types

       The following feedback type has been added to the Feedback Report
       Type Values registry:

           Feedback Type: auth-failure
           Description: email authentication failure report
           Published in: [RFC6591]
           Status: current

    5.2.  Updates to ARF Header Field Names

       The following headers are added to the Feedback Report Header Fields
       registry:

           Field Name: Auth-Failure
           Description: Type of email authentication method failure
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: Delivery-Result
           Description: Final disposition of the subject message
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

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           Field Name: DKIM-ADSP-DNS
           Description: Retrieved DKIM ADSP record
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Canonicalized-Body
           Description: Canonicalized body, per DKIM
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Canonicalized-Header
           Description: Canonicalized header, per DKIM
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Domain
           Description: DKIM signing domain from "d=" tag
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Identity
           Description: Identity from DKIM signature
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Selector
           Description: Selector from DKIM signature
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

           Field Name: DKIM-Selector-DNS
           Description: Retrieved DKIM key record
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

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           Field Name: SPF-DNS
           Description: Retrieved SPF record
           Multiple Appearances: No
           Related "Feedback-Type": auth-failure
           Published in: [RFC6591]
           Status: current

    6.  Security Considerations

       Security issues with respect to these reports are similar to those
       found in [DSN].

    6.1.  Inherited Considerations

       Implementers are advised to consider the Security Considerations
       sections of [DKIM], [ADSP], [SPF], and [ARF].

    6.2.  Forgeries

       These reports can be forged as easily as ordinary Internet electronic
       mail.  User agents and automatic mail-handling facilities (such as
       mail distribution list exploders) that wish to make automatic use of
       Delivery Status Notifications (DSNs) of any kind should take
       appropriate precautions to minimize the potential damage from denial-
       of-service attacks.

       Security threats related to forged DSNs include the sending of

       a.  A falsified email authentication method failure notification when
           the message was in fact delivered to the indicated recipient;

       b.  Falsified signature information, such as selector, domain, etc.

       Perhaps the simplest means of mitigating this threat is to assert
       that these reports should themselves be signed with something like
       DKIM.  On the other hand, if there's a problem with the DKIM
       infrastructure at the verifier, signing DKIM failure reports might
       produce reports that aren't trusted or even accepted by their
       intended recipients.

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    6.3.  Automatic Generation

       Automatic generation of these reports by verifying agents can cause a
       denial-of-service attack when a large volume of email is sent that
       causes email authentication failures for whatever reason.

       Limiting the rate of generation of these messages might be
       appropriate but threatens to inhibit the distribution of important
       and possibly time-sensitive information.

       In general ARF feedback loop terms, it is suggested that report
       generators only create these (or any) ARF reports after an out-of-
       band arrangement has been made between two parties.  This mechanism
       then becomes a way to adjust parameters of an authorized abuse report
       feedback loop that is configured and activated by private agreement
       rather than starting to send them automatically based solely on
       discovered data in the DNS.

    6.4.  Envelope Sender Selection

       In the case of transmitted reports in the form of a new message, it
       is necessary to consider the construction and transmission of the
       message so as to avoid amplification attacks, deliberate or
       otherwise.  See Section 5 of [ARF] for further information.

    6.5.  Reporting Multiple Incidents

       If it is known that a particular host generates abuse reports upon
       certain incidents, an attacker could forge a high volume of messages
       that will trigger such a report.  The recipient of the report could
       then be inundated with reports.  This could easily be extended to a
       distributed denial-of-service attack by finding a number of report-
       generating servers.

       The incident count referenced in [ARF] provides a limited form of
       mitigation.  The host generating reports may elect to send reports
       only periodically, with each report representing a number of
       identical or near-identical incidents.  One might even do something
       inverse-exponentially, sending reports for each of the first ten
       incidents, then every tenth incident up to 100, then every 100th
       incident up to 1000, etc., until some period of relative quiet after
       which the limitation resets.

       The use of this technique for "near-identical" incidents in
       particular causes a degradation in reporting quality, however.  If,
       for example, a large number of pieces of spam arrive from one
       attacker, a reporting agent might decide only to send a report about

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       a fraction of those messages.  While this averts a flood of reports
       to a system administrator, the precise details of each incident are
       similarly not sent.

    6.6.  Redaction of Data in DKIM Reports

       This memo requires that the canonicalized header and body be returned
       without being subject to redaction when a DKIM failure is being
       reported.  This is necessary to ensure that the returned
       canonicalized forms are useful for debugging, as they must be
       compared to the equivalent form at the signer.  If a message is
       altered in transit, and the returned data are also redacted, the
       redacted portion and the altered portion may overlap, rendering the
       comparison results meaningless.  However, unredacted data can leak
       information the reporting entity considers to be private.  It is for
       this reason the return of the canonicalized forms is not required.

    7.  References

    7.1.  Normative References

       [ABNF]     Crocker, D., Ed., and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", STD 68, RFC 5234,
                  January 2008.

       [ADSP]     Allman, E., Fenton, J., Delany, M., and J. Levine,
                  "DomainKeys Identified Mail (DKIM) Author Domain Signing
                  Practices (ADSP)", RFC 5617, August 2009.

       [ARF]      Shafranovich, Y., Levine, J., and M. Kucherawy, "An
                  Extensible Format for Email Feedback Reports", RFC 5965,
                  August 2010.

       [AUTH-RESULTS]
                  Kucherawy, M., "Message Header Field for Indicating
                  Message Authentication Status", RFC 5451, April 2009.

       [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
                  Encodings", RFC 4648, October 2006.

       [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
                  "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
                  September 2011.

       [IANA]     Narten, T. and H. Alvestrand, "Guidelines for Writing an
                  IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                  May 2008.

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       [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.

       [MIME-TYPES]
                  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part Two: Media Types", RFC 2046,
                  November 1996.

       [REPORT]   Kucherawy, M., Ed., "The Multipart/Report Media Type for
                  the Reporting of Mail System Administrative Messages",
                  STD 73, RFC 6522, January 2012.

       [RFC6590]  Falk, J., Ed., and M. Kucherawy, Ed., "Redaction of
                  Potentially Sensitive Data from Mail Abuse Reports",
                  RFC 6590, April 2012.

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

    7.2.  Informative References

       [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", RFC 5598,
                  July 2009.

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    Appendix A.  Acknowledgements

       The author wishes to acknowledge the following for their review and
       constructive criticism of this proposal: Frank Ellermann, J.D. Falk,
       Scott Kitterman, John Levine, Mike Markley, Kelly Wanser, Murray
       Kucherawy, and Alessandro Vesely.

    Appendix B.  Example

       This section contains an example of the use of the extension defined
       by this memo.

    B.1.  Example Use of ARF Extension Headers

       An ARF-formatted report using the proposed ARF extension fields:

       Message-ID: <433689.81121.example@mta.mail.receiver.example>
       From: "SomeISP Antispam Feedback" <feedback@mail.receiver.example>
       To: arf-failure@sender.example
       Subject: FW: You have a new bill from your bank
       Date: Sat, 8 Oct 2011 15:15:59 -0500 (CDT)
       MIME-Version: 1.0
       Content-Type: multipart/report;
         boundary="------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg";
         report-type=feedback-report
       Content-Transfer-Encoding: 7bit

       --------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
       Content-Type: text/plain; charset="us-ascii"
       Content-Disposition: inline
       Content-Transfer-Encoding: 7bit

       This is an authentication failure report for an email message
       received from a.sender.example on 8 Oct 2011 20:15:58 +0000 (GMT).
       For more information about this format, please see [RFC6591].

       --------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
       Content-Type: message/feedback-report
       Content-Transfer-Encoding: 7bit

       Feedback-Type: auth-failure
       User-Agent: Someisp!Mail-Feedback/1.0
       Version: 1
       Original-Mail-From: anexample.reply@a.sender.example
       Original-Envelope-Id: o3F52gxO029144
       Authentication-Results: mta1011.mail.tp2.receiver.example;
        dkim=fail (bodyhash) header.d=sender.example
       Auth-Failure: bodyhash

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       DKIM-Canonicalized-Body: VGhpcyBpcyBhIG1lc3NhZ2UgYm9keSB0
         aGF0IGdvdCBtb2RpZmllZCBpbiB0cmFuc2l0LgoKQXQgdGhlIHNhbWU
         gdGltZSB0aGF0IHRoZSBib2R5aGFzaCBmYWlscyB0byB2ZXJpZnksIH
         RoZQptZXNzYWdlIGNvbnRlbnQgaXMgY2xlYXJseSBhYnVzaXZlIG9yI
         HBoaXNoeSwgYXMgdGhlClN1YmplY3QgYWxyZWFkeSBoaW50cy4gIElu
         ZGVlZCwgdGhpcyBib2R5IGFsc28gY29udGFpbnMKdGhlIGZvbGxvd2l
         uZyB0ZXh0OgoKICAgUGxlYXNlIGVudGVyIHlvdXIgZnVsbCBiYW5rIG
         NyZWRlbnRpYWxzIGF0CiAgIGh0dHA6Ly93d3cuc2VuZGVyLmV4YW1wb
         GUvCgpXZSBhcmUgaW1wbHlpbmcgdGhhdCwgYWx0aG91Z2ggbXVsdGlw
         bGUgZmFpbHVyZXMKcmVxdWlyZSBtdWx0aXBsZSByZXBvcnRzLCBhIHN
         pbmdsZSBmYWlsdXJlIGNhbiBiZQpyZXBvcnRlZCBhbG9uZyB3aXRoIH
         BoaXNoaW5nIGluIGEgc2luZ2xlIHJlcG9ydC4K
       DKIM-Domain: sender.example
       DKIM-Identity: @sender.example
       DKIM-Selector: testkey
       Arrival-Date: 8 Oct 2011 20:15:58 +0000 (GMT)
       Source-IP: 192.0.2.1
       Reported-Domain: a.sender.example
       Reported-URI: http://www.sender.example/

       --------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
       Content-Type: text/rfc822-headers
       Content-Transfer-Encoding: 7bit

       Authentication-Results: mta1011.mail.tp2.receiver.example;
        dkim=fail (bodyhash) header.d=sender.example;
        spf=pass smtp.mailfrom=anexample.reply@a.sender.example
       Received: from smtp-out.sender.example
        by mta1011.mail.tp2.receiver.example
        with SMTP id oB85W8xV000169;
        Sat, 08 Oct 2011 13:15:58 -0700 (PDT)
       DKIM-Signature: v=1; c=relaxed/simple; a=rsa-sha256;
        s=testkey; d=sender.example; h=From:To:Subject:Date;
        bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
        b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
        4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
        KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
        4bmp/YzhwvcubU4=
       Received: from mail.sender.example
        by smtp-out.sender.example
        with SMTP id o3F52gxO029144;
        Sat, 08 Oct 2011 13:15:31 -0700 (PDT)
       Received: from internal-client-001.sender.example
        by mail.sender.example
        with SMTP id o3F3BwdY028431;
        Sat, 08 Oct 2011 13:15:24 -0700 (PDT)
       Date: Sat, 8 Oct 2011 16:15:24 -0400 (EDT)
       Reply-To: anexample.reply@a.sender.example

    Fontana                      Standards Track                   [Page 15]

    RFC 6591                 Auth Failure Reporting               April 2012

       From: anexample@a.sender.example
       To: someuser@receiver.example
       Subject: You have a new bill from your bank
       Message-ID: <87913910.1318094604546@out.sender.example>

       --------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg--

       Example 1: Example ARF Report Using These Extensions

       This example ARF message is making the following assertion:

       o  DKIM verification of the signature added within "sender.example"
          failed.

       o  The cause of the verification failure was a mismatch between the
          body contents observed at the verifier and the body hash contained
          in the signature.

    Author's Address

       Hilda L. Fontana
       3579 E. Foothill Blvd., Suite 282
       Pasadena, CA  91107
       US

       Phone: +1 626 676 8852
       EMail: hilda@hfontana.com

    Fontana                      Standards Track                   [Page 16]