Email Authentication Status Codes
draft-ietf-appsawg-email-auth-codes-03

Network Working Group                                       M. Kucherawy
Internet-Draft                                             June 26, 2014
Updates: 7208 (if approved)
Intended status: Standards Track
Expires: December 28, 2014

                   Email Authentication Status Codes
                 draft-ietf-appsawg-email-auth-codes-03

Abstract

   There is at present no way to return a status code to an email client
   that indicates a message is being rejected or deferred specifically
   because of email authentication failures.  This document registers
   codes for this purpose.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  New Status Codes  . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  DKIM Failure Codes  . . . . . . . . . . . . . . . . . . . . 3
     3.2.  SPF Failure Codes . . . . . . . . . . . . . . . . . . . . . 4
     3.3.  Reverse DNS Failure Code  . . . . . . . . . . . . . . . . . 4
     3.4.  Multiple Authentication Failures Code . . . . . . . . . . . 5
   4.  General Considerations  . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   [RFC3463] introduced Enhanced Mail System Status Codes, and [RFC5248]
   created an IANA registry for these.

   [RFC6376] and [RFC7208] introduced, respectively, DomainKeys
   Identified Mail and Sender Policy Framework, two protocols for
   conducting email authentication.  Another common email acceptance
   test is the reverse Domain Name System check on an email client's IP
   address, as described in Section 3 of [RFC7001].

   The current set of enhanced status codes does not include any code
   for indicating that a message is being rejected or deferred due to
   local policy reasons related to any of these mechanisms.  This is
   potentially useful information to agents that need more than
   rudimentary handling information about the reason a message was
   rejected on receipt.  This document introduces enhanced status codes
   for reporting those cases to clients.

2.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

3.  New Status Codes

   The following new status codes are defined:

3.1.  DKIM Failure Codes

      Code:               X.7.20
      Sample Text:        No valid DKIM signature found
      Associated basic status code:  5
      Description:        This status code is returned when a message
                          did not contain a valid DKIM signature,
                          contrary to local policy requirements.
                          (Note that this violates the advice of
                          Section 6.1 of RFC6376.)
      Reference:          [this document]; RFC6376
      Submitter:          M. Kucherawy
      Change controller:  IESG

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      Code:               X.7.21
      Sample Text:        No valid author-aligned DKIM signature found
      Associated basic status code:  5
      Description:        This status code is returned when a message
                          did not contain a valid DKIM signature
                          matching the domain(s) found in the From
                          header field, contrary to local policy
                          requirements.  (Note that this violates the
                          advice of Section 6.1 of RFC6376.)
      Reference:          [this document]; RFC6376
      Submitter:          M. Kucherawy
      Change controller:  IESG

3.2.  SPF Failure Codes

      Code:               X.7.22
      Sample Text:        SPF validation failed
      Associated basic status code:  5
      Description:        This status code is returned when a message
                          completed an SPF check that produced a
                          "fail" result, contrary to local policy
                          requirements.  Used in place of 5.7.1 as
                          described in Section 8.4 of RFC7208.
      Reference:          [this document]; RFC7208
      Submitter:          M. Kucherawy
      Change controller:  IESG

      Code:               X.7.23
      Sample Text:        SPF validation error
      Associated basic status code:  4/5
      Description:        This status code is returned when evaluation
                          of SPF relative to an arriving message
                          resulted in an error.  Used in place of
                          4.4.3 or 5.5.2 as described in Sections
                          8.6 and 8.6 of RFC7208.
      Reference:          [this document]; RFC7208
      Submitter:          M. Kucherawy
      Change controller:  IESG

3.3.  Reverse DNS Failure Code

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      Code:               X.7.24
      Sample Text:        Reverse DNS validation failed
      Associated basic status code:  5
      Description:        This status code is returned when an SMTP
                          client's IP address failed a reverse DNS
                          validation check, contrary to local policy
                          requirements.
      Reference:          [this document]; Section 3 of RFC7001
      Submitter:          M. Kucherawy
      Change controller:  IESG

3.4.  Multiple Authentication Failures Code

      Code:               X.7.25
      Sample Text:        Multiple authentication checks failed
      Associated basic status code:  5
      Description:        This status code is returned when a message
                          failed more than one message authentication
                          check, contrary to local policy requirements.
                          The specific mechanisms that failed are not
                          specified.
      Reference:          [this document]
      Submitter:          M. Kucherawy
      Change controller:  IESG

4.  General Considerations

   By the nature of the Simple Mail Transfer Protocol (SMTP), only one
   enhanced status code can be returned for a given exchange between
   client and server.  However, an operator might decide to defer or
   reject a message for a plurality of reasons.  Clients receiving these
   codes need to consider that the failure reflected by one of these
   status codes might not reflect the only reason, or the most important
   reason, for non-acceptance of the message or command.

   It is important to note that Section 6.1 of [RFC6376] discourages
   special treatment of messages bearing no valid signature.  There are
   some operators that disregard this advice, a few of which go so far
   as to require a valid Author Domain signature (that is, one matching
   the domain(s) in the From header field) in order to accept the
   message.  Moreover, some nascent technologies built atop SPF and DKIM
   depend on such authentications.  This work does not endorse
   configurations that violate DKIM's recommendations, but rather
   acknowledges that they do exist and merely seeks to provide for
   improved interoperability with such operators.

   A specific use case is mailing list software, which processes

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   rejections in order to remove from the subscriber set those addresses
   that are no longer valid.  There is a need in that case to
   distinguish authentication failures versus indications that the
   recipient address is no longer valid.

   When multiple authentication methods fail, the SMTP server SHOULD use
   the code that indicates multiple methods failed rather than only the
   first one that failed.  It may be the case that one method is always
   expected to fail, and thus returning that method's specific code is
   not information useful to the sending agent.

   The reverse IP DNS check is defined in Section 2.6.3 of [RFC7001].

5.  Security Considerations

   Use of these codes reveals local policy with respect to email
   authentication, which can be useful information to actors attempting
   to deliver undesirable mail.  It should be noted that there is no
   specific obligation to use these codes; if an operator wishes not to
   reveal this aspect of local policy, it can continue using a generic
   result code such as 5.7.7, 5.7.1, or even 5.7.0.

6.  IANA Considerations

   Registration of new enhanced status codes, for addition to the SMTP
   Enhanced Status Codes Registry, can be found in Section 3.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, January 2003.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248, June 2008.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
              September 2011.

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, September 2013.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              April 2014.

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Appendix A.  Acknowledgments

   Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott
   Kitterman, Barry Leiba, Alexey Melnikov, and Hector Santos
   contributed to this work.

Author's Address

   Murray S. Kucherawy
   270 Upland Drive
   San Francisco, CA  94127
   USA

   EMail: superuser@gmail.com

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