A Property Types Registry for the Authentication-Results Header Field

Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 7410                                 December 2014
Updates: 7001
Category: Standards Track
ISSN: 2070-1721


 A Property Types Registry for the Authentication-Results Header Field

Abstract

   This document updates RFC 7001 by creating a registry for property
   types in the Authentication-Results header field, used in email
   authentication work, rather than limiting participants to using the
   original, small set of fixed values.

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/rfc7410.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updated "ptype" Definition  . . . . . . . . . . . . . . . . .   2
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC7001] defines the email Authentication-Results header field that
   presents the results of an authentication effort in a machine-
   readable format.  The header field creates a place to collect the
   output from authentication processes that are disjoint from later
   processes that might use the output, such as analysis, filtering, or
   sorting mechanisms.

   The specification in that document enumerated a small set of types of
   properties that can be reported using this mechanism.  There has
   emerged a desire to report types of properties about a message
   through this mechanism.  Accordingly, this document updates the
   specification to allow for additional property types ("ptypes")
   beyond the original set and creates a registry where new ones can be
   listed and their defining documents referenced.

2.  Updated "ptype" Definition

   Advanced Backus Naur Form (ABNF) is defined in [RFC5234].

   The ABNF in Section 2.2 of [RFC7001] is updated as follows:

       ptype = Keyword
             ; indicates whether the property being evaluated was
             ; a parameter to an [SMTP] command, was a value taken
             ; from a message header field, was some property of
             ; the message body, or was some other property evaluated by
             ; the receiving Message Transfer Agent (MTA)

   The ABNF token "Keyword" is defined in Section 4.1.2 of [RFC5321].

   Legal values of "ptype" are as defined in the IANA "Email
   Authentication Property Types" registry (see Section 3).  The initial
   values are as follows, matching those defined in [RFC7001]:

   body:  Indicates information that was extracted from the body of the
      message.  This might be an arbitrary string of bytes, a hash of a
      string of bytes, a Uniform Resource Identifier, or some other
      content of interest.

   header:  Indicates information that was extracted from the header of
      the message.  This might be the value of a header field or some
      portion of a header field.

   policy:  A local policy mechanism was applied that augments or
      overrides the result returned by the authentication mechanism.
      See Section 2.3 of [RFC7001].

   smtp:  Indicates information that was extracted from an SMTP command
      that was used to relay the message.

   When a consumer of this header field encounters a "ptype" that it
   does not understand, it ignores the result reported with that
   "ptype".

3.  IANA Considerations

   IANA has created the "Email Authentication Property Types" sub-
   registry within the existing "Email Authentication Parameters"
   registry.  Entries in this registry are subject to the Expert Review
   rules as described in [RFC5226].  Each entry in the registry requires
   the following values:

   o  The "ptype" token to be registered, which must fit within the ABNF
      described in Section 2.

   o  A brief description of what sort of information this "ptype" is
      meant to cover.

   o  An optional reference to the defining document.  This is
      recommended, but not required.

   The initial entries in this table are as follows, taken from
   [RFC7001]:

       +--------+-------------+----------------------------------------+
       | ptype  | Definition  | Description                            |
       +--------+-------------+----------------------------------------+
       | body   | RFC 7001    | The property being reported was found  |
       |        | Section 2.2 | in the body of the message.            |
       +--------+-------------+----------------------------------------+
       | header | RFC 7001    | The property being reported was found  |
       |        | Section 2.2 | in a header field of the message.      |
       +--------+-------------+----------------------------------------+
       | policy | RFC 7001    | The property being reported relates to |
       |        | Section 2.3 | a locally defined policy.              |
       +--------+-------------+----------------------------------------+
       | smtp   | RFC 7001    | The property being reported is a       |
       |        | Section 2.2 | parameter to an SMTP command used to   |
       |        |             | relay the message.                     |
       +--------+-------------+----------------------------------------+

   For new entries, the Designated Expert needs to assure that the
   description provided for the new entry adequately describes the
   intended use.  An example would be helpful to include in the entry's
   defining document, if any, although entries in the "Email
   Authentication Methods" registry or the "Email Authentication Result
   Names" registry might also serve as examples of intended use.

4.  Security Considerations

   It is unknown how legacy code, which expects one of a fixed set of
   "ptype" tokens, will handle new tokens as they begin to appear.
   There are typically two options: prevent delivery of the message, or
   ignore those portions of the field that use unknown "ptype" tokens
   and allow processing of the message to continue.

   The choice comes down to whether the consumer considers it a threat
   when there are unknown "ptypes" present.  The semantics of the report
   are unknown; the report might be indicating the message is authentic,
   fraudulent, or that a test failed to complete.  The report itself is
   not actionable because it cannot be understood, and only its presence
   is certain.

   Generally, the advice in this situation is to ignore unknown
   "ptypes".  It is anticipated that a new property type evaluated by
   earlier handling agents would also result in the filtering of
   messages by those agents until consumers can be updated to interpret
   them.

5.  Normative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008, <http://www.rfc-editor.org/info/rfc5321>.

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, September 2013,
              <http://www.rfc-editor.org/info/rfc7001>.

Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this update: Dave Crocker, Tim Draegen,
   Scott Kitterman, and Franck Martin.

Author's Address

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

   EMail: superuser@gmail.com