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Versions: (draft-kucherawy-dkim-reporting) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6651

MARF Working Group                                          M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Standards Track                      September 10, 2011
Expires: March 13, 2012


                Extensions to DKIM for Failure Reporting
                   draft-ietf-marf-dkim-reporting-03

Abstract

   This memo presents extensions to the DomainKeys Identified Mail
   (DKIM) specification to allow for detailed reporting of message
   authentication failures in an on-demand fashion.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 13, 2012.

Copyright Notice

   Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Imported Definitions . . . . . . . . . . . . . . . . . . .  4
   3.  Optional Reporting Address for DKIM  . . . . . . . . . . . . .  5
   4.  Optional 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.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  DKIM Key Tag Registration  . . . . . . . . . . . . . . . . 11
     7.2.  DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     8.1.  Inherited Considerations . . . . . . . . . . . . . . . . . 12
     8.2.  Forgeries  . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.3.  Automatic Generation . . . . . . . . . . . . . . . . . . . 12
     8.4.  Envelope Sender Selection  . . . . . . . . . . . . . . . . 13
     8.5.  Reporting Multiple Incidents . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 16
     B.1.  Example Use of DKIM Key Extension Tags . . . . . . . . . . 16
     B.2.  Example Use of DKIM ADSP Extension Tags  . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18






















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1.  Introduction

   [DKIM] introduced a mechanism for message sender authentication.  It
   uses digital signing to associate a domain name with a message in a
   reliable (i.e. not forgeable) manner.  The output is a verified
   domain name that can then be subjected to some sort of evaluation
   process (e.g., advertised sender policy, comparison to a known-good
   list, submission to a reputation service, etc.).

   Deployers of message sender authentication technologies are
   increasingly seeking visibility into DKIM verification failures and
   conformance failures involving the published signing practices (e.g.,
   [ADSP]) of an Administrative Mail Domain (ADMD; see [EMAIL-ARCH]).

   This document extends [DKIM] and [ADSP] to add an optional reporting
   address, and an optional means of specifying a desired report format
   and other parameters.  Similar documents extend [ARF] and SPF
   (RFC4408) to improve the reporting capabilities of those standards as
   well, and are intended to be deployed together.  The full suite of
   documents is:

   o  RFCxxxx: Abuse Report Format (ARF) Extensions for Authentication
      Failure Reporting

   o  RFCxxxx+1: DKIM and ADSP Extensions for Authentication Failure
      Reporting (this memo)

   o  RFCxxxx+2: SPF Extensions for Authentication Failure Reporting























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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].

   The base64 encoding method is defined in [MIME].






































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3.  Optional Reporting Address for DKIM

   A domain name owner employing [DKIM] for e-mail signing and
   authentication might 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 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 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.  The value MUST be interpreted as a local-part
      only.  To construct the actual 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 that is taken from the registered list of
      report formats.  See Section 7 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 that 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 unsigned 32-bit integer that specifies an
      interval during which the report generator SHOULD NOT issue more
      than one report about a given incident type.  A value of "0"



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      requests a report for every incident; a value of "1" requests a
      report for every other incident; etc.  Where the requested
      interval is not zero, the agent generating a report SHOULD include
      an "Incidents:" field 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

   In the absence of an "r=" tag, all other tags listed above MUST be
   ignored.



















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4.  Optional Reporting Address for DKIM-ADSP

   There also exist cases in which a domain name owner employing [ADSP]
   for announcing signing practises with DKIM may want to know when
   messages are received without valid author domain signatures.
   Currently there is no such mechanism defined.

   This document adds the following optional "tags" (as defined in
   [ADSP]) 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 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
      [ADSP], in particular because a message arrived without a
      signature that validates, which contradicts what the ADSP record
      claims, the format of this reply MUST be in the format specified
      by the "rf=" tag, defined below.  The value MUST be interpreted as
      a local-part only.  To construct the actual 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, i.e., the one taken from the
      RFC5322.From domain of the message under evaluation.

      ABNF:

      adsp-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 that is taken from the
      registered list of DKIM report formats.  See Section 7 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 that represents a
      report format it is capable of generating.

      ABNF:

      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 unsigned 32-bit integer that specifies an
      interval during which the report generator SHOULD NOT issue more
      than one report about a given type of incident should be



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      generated.  A value of "0" requests a report for every incident; a
      value of "1" requests a report for every other incident; etc.
      Where the requested interval is not zero, the agent generating a
      report SHOULD include an "Incidents:" field 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

   In the absence of an "r=" tag, all other tags listed above MUST be
   ignored.


















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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 that do
   not match a requested report, and MUST ignore requests for reports
   not included in this these lists.

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 ADSP records:

   all  All reports are requested.

   s  Reports are requested for messages that have a valid [DKIM]
      signature but do not match the published [ADSP] policy.

   u  Reports are requested for messages that have no valid [DKIM]
      signature but do not match the published [ADSP] policy.


















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6.  Reporting Formats

   This section lists reporting formats supported by this reporting
   mechanism.  Initially, two formats are supported:

   arf:  Abuse Reporting Format, as defined in [ARF] and extended in
      RFCxxxx.

   smtp:  An [SMTP] error with a string descriptive of the problem that
      caused the sender authentication to fail.  This explicitly
      requests evaluation of sender authentication 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.  IANA Considerations

   As required by [IANA-CONSIDERATIONS], this section contains registry
   information for the new [DKIM] key tags, and the new [ADSP] tags.

7.1.  DKIM Key Tag Registration

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

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

7.2.  DKIM ADSP Tag Registration

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

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

















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8.  Security Considerations

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

8.1.  Inherited Considerations

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

8.2.  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 authentication 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 may
   produce reports that aren't trusted or even accepted by their
   intended recipients.

8.3.  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 sender authentication failures for whatever reason.

   Limiting the rate of generation of these messages may 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



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   discovered data in the DNS.

8.4.  Envelope Sender Selection

   In the case of transmitted 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 report SHOULD be chosen to ensure that
   no delivery status reports will be issued in response to the report
   itself, and MUST be chosen so that these reports will not generate
   mail loops.  Whenever an [SMTP] transaction is used to send a report,
   the MAIL FROM command MUST use a NULL return address, i.e.  "MAIL
   FROM:<>".

8.5.  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 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 [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 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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9.  References

9.1.  Normative References

   [ADSP]     Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
              Sender Signing Practises", RFC 5617, August 2009.

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

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

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture", RFC 5598,
              October 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.

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

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

9.2.  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 authors wish to acknowledge the following for their review and
   constructive criticism of this proposal: Monica Chew, Dave Crocker,
   Tim Draegen and JD Falk.














































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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.


















































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Author's Address

   Murray S. Kucherawy
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com









































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