


Individual submission                                       M. Kucherawy
Internet-Draft                                            Sendmail, Inc.
Intended status: Standards Track                           July 10, 2008
Expires: January 11, 2009


                Reporting of DKIM Verification Failures
                   draft-kucherawy-dkim-reporting-04

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Copyright Notice

   Copyright (C) The IETF Trust (2008).












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Abstract

   This memo presents an extension to the DomainKeys Identified Mail
   (DKIM) specifications to allow public keys for verification to
   include a reporting address to be used to report message verification
   issues, and extends an Internet Message reporting format to be
   followed when generating such reports.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Imported Definitions . . . . . . . . . . . . . . . . . . .  4
   3.  Optional Key Reporting Address for DKIM  . . . . . . . . . . .  5
   4.  Optional Key Reporting Address for DKIM-ADSP . . . . . . . . .  7
   5.  Requested Reports  . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Requested Reports for DKIM Failures  . . . . . . . . . . .  9
     5.2.  Requested Reports for DKIM ADSP Failures . . . . . . . . .  9
   6.  Reporting Formats  . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Extension ARF Fields for DKIM Reporting  . . . . . . . . . . . 11
     7.1.  New ARF Feedback Type  . . . . . . . . . . . . . . . . . . 11
     7.2.  New ARF Header Names . . . . . . . . . . . . . . . . . . . 11
     7.3.  DKIM Failure Types . . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  DKIM Key Tag Registration  . . . . . . . . . . . . . . . . 13
     8.2.  DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 13
     8.3.  Updates to ARF Feedback Types  . . . . . . . . . . . . . . 13
     8.4.  Updates to ARF Header Names  . . . . . . . . . . . . . . . 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     9.1.  Forgeries  . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.2.  Automatic Generation . . . . . . . . . . . . . . . . . . . 15
     9.3.  Envelope Sender Selection  . . . . . . . . . . . . . . . . 15
     9.4.  Reporting Multiple Incidents . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 18
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 19
     B.1.  Example Use of DKIM Key Extension Tags . . . . . . . . . . 19
     B.2.  Example Use of DKIM ADSP Extension Tags  . . . . . . . . . 19
     B.3.  Example Use of ARF Extension Headers . . . . . . . . . . . 20
   Appendix C.  Public Discussion . . . . . . . . . . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23





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1.  Introduction

   [DKIM] introduced a standard for digital signing of messages for the
   purposes of sender authentication.  There exist cases in which a
   domain name owner might want to receive reports from verifiers that
   determine DKIM-signed mail apparently from its domain is failing to
   verify according to [DKIM] or is "Suspicious" according to
   [I-D.DRAFT-IETF-DKIM-SSP].

   This document extends [DKIM] and [I-D.DRAFT-IETF-DKIM-SSP] to add an
   optional reporting address to selector records, an optional means of
   specifying a desired report format, and furthermore extends
   [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT] to add features required for
   DKIM reporting.

   This memo presumes those specifications thus modified will issue as
   RFCs without these modifications.  If the modifications are adopted
   prior to their publicatons, clearly those sections of this memo can
   be removed.
































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2.  Definitions

2.1.  Keywords

   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.  Imported Definitions

   The ABNF token "qp-section" is imported from [MIME].

   base64 is defined in [MIME].






































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3.  Optional Key Reporting Address for DKIM

   There exist cases in which a domain name owner employing [DKIM] for
   e-mail signing and authentication may want to know when signatures in
   use by specific keys are not successfully verifying.  Currently there
   is no such mechanism defined.

   This document adds the following two optional "tags" (as defined in
   [DKIM]) to the DKIM key records, using the form defined in that
   specification:

   r= Reporting Address (plain-text; OPTIONAL; no default).  The value
      MUST be a dkim-quoted-printable string containing the local-part
      of an e-mail address to which a report SHOULD be sent when mail
      signed with this key fails verification because either (a) the
      signature verification itself failed, or (b) the body hash test
      failed.  The format of this reply is selected by the value of the
      "rf=" tag, defined below.  To generate a complete address to which
      the report is sent, the verifier simply appends to this value an
      "@" followed by the domain found in the "d=" tag of the signature
      whose validation failed.

      ABNF:

      key-r-tag = %x72 *WSP "=" *WSP qp-section

   rf=  Reporting Format (plain-text; OPTIONAL; default is "arf").  The
      value MUST be a colon-separated list of tokens representing
      desired reporting formats in order of preference.  Each element of
      the list MUST be a token which is taken from the registered list
      of DKIM report formats.  See Section 8 for a description of the
      registry and Section 6 for a description of recognized formats.
      The verifier generating reports MUST generate a report using the
      first token in the list which represents a report format it is
      capable of generating.

      ABNF:

      rep-format = ( "arf" / "smtp" )

      key-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP
      rep-format )

   ri=  Requested Report Interval (plain-text; OPTIONAL; default is
      "0").  The value is an integer which specifies an interval during
      which no more than one report about a given type of incident
      should be generated.  A value of "0" requests a report for every
      incident.  Where the requested interval is not zero, the agent



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      generating a report SHOULD include an "Incidents:" header in the
      generated report so the receiving agent has some indication of how
      many reports were suppressed.

      ABNF:

      key-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT

   ro=  Requested Reports (plain-text; OPTIONAL; default is "all").  The
      value MUST be a colon-separated list of tokens representing those
      conditions under which a report is desired.  See Section 5.1 for a
      list of valid tags.

      ABNF:

      key-ro-type = ( "all" / "s" / "v" / "x" )

      key-ro-tag = %x72 %x6f *WSP "=" *WSP key-ro-type *WSP 0* ( ":"
      *WSP key-ro-type )

   rs=  Requested SMTP Error String (plain-text; OPTIONAL; no default).
      The value is a string the signing domain requests be included in
      SMTP error strings when messages are rejected.

      ABNF:

      key-rs-tag = %x72 %x73 *WSP "=" qp-section
























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4.  Optional Key Reporting Address for DKIM-ADSP

   There also exist cases in which a domain name owner employing
   [I-D.DRAFT-IETF-DKIM-SSP] for announcing signing practises with DKIM
   may want to know when messages are received unsigned or signed with
   third-party signatures contrary to the sender's published policy.
   Currently there is no such mechanism defined.

   This document adds the following two optional "tags" (as defined in
   [I-D.DRAFT-IETF-DKIM-SSP]) to the DKIM ADSP records, using the form
   defined in that specification:

   r= Reporting Address (plain-text; OPTIONAL; no default).  The value
      MUST be a dkim-quoted-printable string containing the local-part
      of an e-mail address to which a report SHOULD be sent when mail
      claiming to be from this domain failed the verification algorithm
      described in [I-D.DRAFT-IETF-DKIM-SSP], in particular because a
      message arrived unsigned or signed by a third-party signature in
      violation of the sender's published policy.  The format of this
      reply MUST be in the format specified by the "rf=" tag defined
      below.  To generate a complete address to which the report is
      sent, the verifier simply appends to this value an "@" followed by
      the domain whose policy was queried in order to evaluate the
      sender's ADSP.

      ABNF:

      adsp-r-tag = %x72 *WSP "=" qp-section

      ABNF:

      key-r-tag = %x72 *WSP "=" qp-section

   rf=  Reporting Format (plain-text; OPTIONAL; default is "arf").  The
      value MUST be a colon-separated list of tokens representing
      desired reporting formats in decreasing order of preference.  Each
      element of the list MUST be a token which is taken from the
      registered list of DKIM report formats.  See Section 8 for a
      description of the registry and Section 6 for a description of
      recognized formats.  The verifier generating reports MUST generate
      a report using the first token in the list which represents a
      report format it is capable of generating.

      ABNF:







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      adsp-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP
      rep-format )

   ri=  Requested Report Interval (plain-text; OPTIONAL; default is
      "0").  The value is an integer which specifies an interval during
      which no more than one report about a given type of incident
      should be generated.  A value of "0" requests a report for every
      incident.  Where the requested interval is not zero, the agent
      generating a report SHOULD include an "Incidents:" header in the
      generated report so the receiving agent has some indication of how
      many reports were suppressed.

      ABNF:

      adsp-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT

   ro=  Requested Reports (plain-text; OPTIONAL; default is "all").  The
      value MUST be a colon-separated list of tokens representing those
      conditions under which a report is desired.  See Section 5.2 for a
      list of valid tags.

      ABNF:

      adsp-ro-type = ( "all" / "s" / "u" )

      adsp-ro-tag = %x72 %x6f *WSP "=" *WSP adsp-ro-type *WSP 0* ( ":"
      *WSP adsp-ro-type )

   rs=  Requested SMTP Error String (plain-text; OPTIONAL; no default).
      The value is a string the signing domain requests be included in
      SMTP error strings when messages are rejected.

      ABNF:

      adsp-rs-tag = %x72 %x73 *WSP "=" qp-section
















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5.  Requested Reports

   This memo also includes, as the "ro" tags defined above, the means by
   which the sender can request reports for specific circumstances of
   interest.  Verifiers MUST NOT generate reports for incidents which do
   not match a requested report.

5.1.  Requested Reports for DKIM Failures

   The following report requests are defined for DKIM keys:

   all  All reports are requested.

   s  Reports are requested for signature or key syntax errors.

   v  Reports are requested for signature verification failures or body
      hash mismatches.

   x  Reports are requested for signatures rejected by the verifier
      because the expiration time has passed.

5.2.  Requested Reports for DKIM ADSP Failures

   The following report requests are defined for DKIM keys:

   all  All reports are requested.

   s  Reports are requested for messages for which the ADSP algorithm
      determines the message is both signed and Suspicious.

   u  Reports are requested for messages for which the ADSP algorithm
      determines the message is both unsigned and Suspicious.



















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6.  Reporting Formats

   This section lists reporting formats supported by this DKIM reporting
   mechanism.  Currently only two formats are supported:

   arf:  Abuse Reporting Format, as defined in
      [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT], and as extended in
      Section 7.

   smtp:  An SMTP error with a string descriptive of the problem which
      caused the DKIM verification to fail.  This explicitly requests
      evaluation of DKIM concurrent with the SMTP session, and rejection
      (if appropriate) whenever possible rather than acceptance of the
      message and later generation of a feedback report of some kind
      (e.g. "arf" above) when verification fails.  The presence of an
      "rs" tag (see Section 3 and Section 4) further requests a specific
      substring be included in the reply to ease automatic handling of
      such errors by sending or relaying MTAs.

































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7.  Extension ARF Fields for DKIM Reporting

   The current ARF format defined in
   [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT] lacks some specific features
   required to do effective DKIM reporting.  This section describes the
   extensions required to do so and thus required to conform to this
   specification.

7.1.  New ARF Feedback Type

   A new feedback type of "dkim" is defined as an extension to Section
   8.2 of [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT].  See Section 8.3 for
   details.

   The header names listed in that draft which may appear for this new
   feedback type include all shown in the draft except "Relevant-URI"
   and "Removal-Recipient" as they have no semantics relating to DKIM.

7.2.  New ARF Header Names

   The following new ARF header names are defined as extensions to
   Section 8.1 of [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT]:

   DKIM-Canonicalized-Body:  A base64 encoding of the canonicalized body
      of the message as generated by the verifier.  This header and
      value MUST be present for reports using feedback type "dkim" when
      reporting a "bodyhash" failure.

   DKIM-Canonicalized-Headers:  A base64 encoding of the canonicalized
      header of the message as generated by the verifier.  This header
      and value MUST be present for reports using feedback type "dkim".

   DKIM-Failure:  Indicates the type of DKIM failure that is being
      reported.  The list of valid values is enumerated below.  This
      header and value MUST be present for reports using feedback type
      "dkim".

   DKIM-Identity:  The identity of the signature which failed
      verification, taken from the "i=" tag of the signature.  This
      header and value MUST be present for reports using feedback type
      "dkim" when reporting anything other than an "asp" failure.

   DKIM-Selector:  The selector of the signature which failed
      verification, taken from the "s=" tag of the signature.  This
      header and value MUST be present for reports using feedback type
      "dkim" when reporting anything other than an "asp" failure.





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   Incidents:  A count of the number of times the cited incident
      occurred.  This permits a limited form of aggregate reporting
      rather than sending one report message per incident.  The absence
      of this field implies the report refers to a single incident.

   The values which are base64 encodings may contain 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].

7.3.  DKIM Failure Types

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

   asp:  The sender's published signing practises determined the message
      is suspicious.

   bodyhash:  The body hash in the signature and the body hash computed
      by the verifier did not match.

   granularity:  The key referenced by the signature on the message was
      not authorized for use by the sender.

   revoked:  The key referenced by the signature on the message has been
      revoked.

   signature:  The signature on the message did not successfully verify
      against the header hash and public key.

   Supplementary data may be included in the form of [MAIL]-compliant
   comments.  For example, "Failure: asp" 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
   examples.















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8.  IANA Considerations

   As required by [IANA-CONSIDERATIONS], this section contains registry
   information for the new [DKIM] key tag, the new
   [I-D.DRAFT-IETF-DKIM-SSP] tag, and the extensions to
   [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT].

8.1.  DKIM Key Tag Registration

   IANA is requested to update the DKIM Key Tag Specification Registry
   to include the following new items:

                           +------+-----------------+
                           | TYPE | REFERENCE       |
                           +------+-----------------+
                           | r    | (this document) |
                           | rf   | (this document) |
                           | ri   | (this document) |
                           | ro   | (this document) |
                           | rs   | (this document) |
                           +------+-----------------+

8.2.  DKIM ADSP Tag Registration

   IANA is requested to update the DKIM ADSP Tag Specification Registry
   to include the following new items:

                           +------+-----------------+
                           | TYPE | REFERENCE       |
                           +------+-----------------+
                           | r    | (this document) |
                           | rf   | (this document) |
                           | ri   | (this document) |
                           | ro   | (this document) |
                           | rs   | (this document) |
                           +------+-----------------+

8.3.  Updates to ARF Feedback Types

   The following feedback type is added to the Feedback Report Feedback
   Type Registry:

       Feedback Type: dkim
       Description: DKIM failure report
       Registration: (this document)






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8.4.  Updates to ARF Header Names

   The following headers are added to the Feedback Report Header Names
   Registry:

       Field Name: DKIM-Canonicalized-Body
       Description: Canonicalized body, per DKIM
       Multiple Appearances: No
       Related "Feedback-Type": dkim


       Field Name: DKIM-Canonicalized-Headers
       Description: Canonicalized headers, per DKIM
       Multiple Appearances: No
       Related "Feedback-Type": dkim


       Field Name: DKIM-Failure
       Description: Type of DKIM failure
       Multiple Appearances: No
       Related "Feedback-Type": dkim


       Field Name: DKIM-Identity
       Description: Identity from DKIM signature
       Multiple Appearances: No
       Related "Feedback-Type": dkim


       Field Name: DKIM-Selector
       Description: Selector from DKIM signature
       Multiple Appearances: No
       Related "Feedback-Type": dkim


       Field Name: Incidents
       Description: Incident count, for aggregate reporting
       Multiple Appearances: No
       Related "Feedback-Type": any












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9.  Security Considerations

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

9.1.  Forgeries

   These reports may 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
   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 DKIM 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 DKIM reports should themselves be signed.  On the other hand, if
   there's a problem with the DKIM infrastructure at the verifier,
   signing DKIM failure reports may produce reports that aren't trusted
   or even accepted by their intended recipients.

9.2.  Automatic Generation

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

   It is unclear what a good solution for this issue is.  Limiting the
   rate of generation of these messages may be apropos but threatens to
   inhibit the distribution of important and possibly time-sensitive
   information.

9.3.  Envelope Sender Selection

   In the case of transmitted DKIM reports in the form of a new message,
   it is necessary to construct the message so as to avoid amplification
   attacks, deliberate or otherwise.  Thus, per Section 2 of [DSN], the
   envelope sender address of the DKIM report SHOULD be chosen to ensure
   that no delivery status reports will be issued in response to the
   DKIM report itself, and MUST be chosen so that these reports will not
   generate mail loops.  Whenever an SMTP transaction is used to send a
   DKIM report, the MAIL FROM command MUST use a NULL return address,
   i.e.  "MAIL FROM:<>".



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9.4.  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
   which will trigger such a report.  The recipient of the report could
   then be innundated 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 Section 7.2 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 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
   may decide only to send a report about 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.



























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10.  References

10.1.  Normative References

   [DKIM]     Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [I-D.DRAFT-IETF-DKIM-SSP]
              Allman, E., Delany, M., and J. Fenton, "DKIM Sender
              Signing Practises", I-D DRAFT-IETF-DKIM-SSP-03.

   [I-D.DRAFT-SHAFRANOVICH-FEEDBACK-REPORT]
              Shafranovich, Y., Levine, J., Hoffman, P., and M.
              Kucherawy, "An Extensible Format for Email Feedback
              Reports", I-D DRAFT-SHAFRANOVICH-FEEDBACK-REPORT-04,
              March 2008.

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

   [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.

10.2.  Informative References

   [DSN]  Moore, K. and G. Vaudreuil, "An Extensible Message Format for
          Delivery Status Notifications", RFC 3464, January 2003.














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Appendix A.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this proposal: (add names here)















































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Appendix B.  Examples

   This section contains examples of the use of each of the extensions
   defined by this memo.

B.1.  Example Use of DKIM Key Extension Tags

   A DKIM key record including use of the extensions defined by this
   memo:

       v=DKIM1; k=rsa; t=y; r=dkim-errors; rf=arf; ro=v:x; p=MIGfMA0GCS
       qGSIb3DQEBAQUAA4GNADCBiQKBgQDh2vbhJTijCs2qbyJcwRCa8WqDTxI+PisFJo
       faPtoDJy0Qn41uNayCajfKADVcLqc87sXQS6GxfchPfzx7Vh9crYdxRbN/o/URCu
       ZsKmym1i1IPTwRLcXSnuKS0XDs1eRW2WQHGYlXksUDqSHWOS3ZO1W5t/FLcZHpIl
       l/80xs4QIDAQAB

   Example 1: DKIM key record using these extensions

   This example DKIM key record contains the following data in addition
   to the basic DKIM key data:

   o  Reports about signature evaluation failures should be send to the
      address "dkim-errors" at the sender's domain;

   o  The sender's domain requests reports in the "arf" format;

   o  Only reports about signature verification failures and expired
      signatures should be generated.

B.2.  Example Use of DKIM ADSP Extension Tags

   A DKIM ADSP record including use of the extensions defined by this
   memo:

       dkim=all; r=dkim-adsp-errors; rf=arf; ro=u

   Example 2: DKIM ADSP record using these extensions

   This example ADSP record makes the following assertions:

   o  The sending domain (i.e. the one that is advertising this policy)
      signs all mail it sends;

   o  Reports about ADSP evaluation failures should be send to the
      address "dkim-adsp-errors" at the sender's domain;

   o  The sender's domain requests reports in the "arf" format;




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   o  Only reports about unsigned messages should be generated.

B.3.  Example Use of ARF Extension Headers
















































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Appendix C.  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

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