Network Working Group                                         J. Degener
Request for Comments: 5293                                   P. Guenther
Category: Standards Track                                 Sendmail, Inc.
                                                             August 2008


              Sieve Email Filtering: Editheader Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document defines two new actions for the "Sieve" email filtering
   language that add and delete email header fields.

1.  Introduction

   Email header fields are a flexible and easy-to-understand means of
   communication between email processors.  This extension enables sieve
   scripts to interact with other components that consume or produce
   header fields by allowing the script to delete and add header fields.

2.  Conventions Used in This Document

   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].

   Conventions for notations are as in Section 1.1 of [SIEVE], including
   use of the "Usage:" label for the definition of action and tagged
   arguments syntax.

   The term "header field" is used here as in [IMAIL] to mean a logical
   line of an email message header.

3.  Capability Identifier

   The capability string associated with the extension defined in this
   document is "editheader".

4.  Action addheader

   Usage: "addheader" [":last"] <field-name: string> <value: string>

   The addheader action adds a header field to the existing message
   header.  If the field-name is not a valid 7-bit US-ASCII header field
   name, as described by the [IMAIL] "field-name" nonterminal syntax
   element, the implementation MUST flag an error.  The addheader action
   does not affect Sieve's implicit keep.

   If the specified field value does not match the [IMAIL]
   "unstructured" nonterminal syntax element or exceeds a length limit
   set by the implementation, the implementation MUST either flag an
   error or encode the field using folding white space and the encodings
   described in [MIME3] or [MIMEPARAM] to be compliant with [IMAIL].

   An implementation MAY impose a length limit onto the size of the
   encoded header field; such a limit MUST NOT be less than 998
   characters, not including the terminating CRLF supplied by the
   implementation.

   By default, the header field is inserted at the beginning of the
   existing message header.  If the optional flag ":last" is specified,
   it is appended at the end.

   Example:

        /* Don't redirect if we already redirected */
        if not header :contains "X-Sieve-Filtered"
                ["<kim@job.example.com>", "<kim@home.example.com>"]
        {
                addheader "X-Sieve-Filtered" "<kim@job.example.com>";
                redirect "kim@home.example.com";
        }

5.  Action deleteheader

      Usage: "deleteheader" [":index" <fieldno: number> [":last"]]
                   [COMPARATOR] [MATCH-TYPE]
                   <field-name: string>
                   [<value-patterns: string-list>]

   By default, the deleteheader action deletes all occurrences of the
   named header field.  The deleteheader action does not affect Sieve's
   implicit keep.

   The field-name is mandatory and always matched as a case-insensitive
   US-ASCII string.  If the field-name is not a valid 7-bit header field
   name as described by the [IMAIL] "field-name" nonterminal syntax
   element, the implementation MUST flag an error.

   The value-patterns, if specified, restrict which occurrences of the
   header field are deleted to those whose values match any of the
   specified value-patterns, the matching being according to the match-
   type and comparator and performed as if by the "header" test.  In
   particular, leading and trailing whitespace in the field values is
   ignored.  If no value-patterns are specified, then the comparator and
   match-type options are silently ignored.

   If :index <fieldno> is specified, the attempts to match a value are
   limited to the <fieldno> occurrence of the named header field,
   beginning at 1, the first named header field.  If :last is specified,
   the count is backwards; 1 denotes the last named header field, 2 the
   second to last, and so on.  The counting happens before the <value-
   patterns> match, if any.  For example:

      deleteheader :index 1 :contains "Delivered-To"
                              "bob@example.com";

   deletes the first "Delivered-To" header field if it contains the
   string "bob@example.com" (not the first "Delivered-To" field that
   contains "bob@example.com").

   It is not an error if no header fields match the conditions in the
   deleteheader action or if the :index argument is greater than the
   number of named header fields.

   The implementation MUST flag an error if :last is specified without
   also specifying :index.

6.  Implementation Limitations on Changes

   As a matter of local policy, implementations MAY limit which header
   fields may be deleted and which header fields may be added.  However,
   implementations MUST NOT permit attempts to delete "Received" and
   "Auto-Submitted" header fields and MUST permit both addition and
   deletion of the "Subject" header field.

   If a script tries to make a change that isn't permitted, the attempt
   MUST be silently ignored.

7.  Interaction with Other Sieve Extensions

   Actions that generate [MDN], [DSN], or similar disposition messages
   MUST do so using the original, unmodified message header.  Similarly,
   if an error terminates processing of the script, the original message
   header MUST be used when doing the implicit keep required by Section
   2.10.6 of [SIEVE].

   All other actions that store, send, or alter the message MUST do so
   with the current set of header fields.  This includes the addheader
   and deleteheader actions themselves.  For example, the following
   leaves the message unchanged:

      addheader "X-Hello" "World";
      deleteheader :index 1 "X-Hello";

   Similarly, given a message with three or more "X-Hello" header
   fields, the following example deletes the first and third of them,
   not the first and second:

      deleteheader :index 1 "X-Hello";
      deleteheader :index 2 "X-Hello";

   Tests and actions such as "exists", "header", or "vacation"
   [VACATION] that examine header fields MUST examine the current state
   of a header as modified by any actions that have taken place so far.

   As an example, the "header" test in the following fragment will
   always evaluate to true, regardless of whether or not the incoming
   message contained an "X-Hello" header field:

      addheader "X-Hello" "World";
      if header :contains "X-Hello" "World"
      {
              fileinto "international";
      }

   However, if the presence or value of a header field affects how the
   implementation parses or decodes other parts of the message, then,
   for the purposes of that parsing or decoding, the implementation MAY
   ignore some or all changes made to those header fields.  For example,
   in an implementation that supports the [BODY] extension, "body" tests
   may be unaffected by deleting or adding "Content-Type" or "Content-
   Transfer-Encoding" header fields.  This does not rescind the
   requirement that changes to those header fields affect direct tests;
   only the semantic side effects of changes to the fields may be
   ignored.

   For the purpose of weeding out duplicates, a message modified by
   addheader or deleteheader MUST be considered the same as the original
   message.  For example, in an implementation that obeys the constraint
   in Section 2.10.3 of [SIEVE] and does not deliver the same message to
   a folder more than once, the following code fragment

      keep;
      addheader "X-Flavor" "vanilla";
      keep;

   MUST only file one message.  It is up to the implementation to pick
   which of the redundant "fileinto" or "keep" actions is executed, and
   which ones are ignored.

   The "implicit keep" is thought to be executed at the end of the
   script, after the headers have been modified.  (However, a canceled
   "implicit keep" remains canceled.)

8.  IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:

   To: iana@iana.org
   Subject: Registration of new Sieve extension

   Capability name: editheader
   Description:     Adds actions 'addheader' and 'deleteheader' that
                    modify the header of the message being processed
   RFC number:      RFC 5293
   Contact Address: The Sieve discussion list <ietf-mta-filters&imc.org>

9.  Security Considerations

   Someone with write access to a user's script storage may use this
   extension to generate headers that a user would otherwise be shielded
   from (e.g., by a gateway Mail Transport Agent (MTA) that removes
   them).

   This is the first Sieve extension to be standardized that allows
   alteration of messages being processed by Sieve engines.  A Sieve
   script that uses Sieve tests defined in [SIEVE], the editheader
   extension, and the redirect action back to the same user can keep
   some state between different invocations of the same script for the
   same message. But note that it would not be possible to introduce an
   infinite loop using any such script, because each iteration adds a
   new Received header field, so email loop prevention described in
   [SMTP] will eventually non deliver the message, and because the

   editheader extension is explicitly prohibited to alter or delete
   Received header fields (i.e., it can't interfere with loop
   prevention).

   A sieve filter that removes header fields may unwisely destroy
   evidence about the path a message has taken.

   Any change in message content may interfere with digital signature
   mechanisms that include the header in the signed material.  For
   example, changes to (or deletion/addition of) header fields included
   in the "SHOULD be included in the signature" list in Section 5.5 of
   [DKIM] can invalidate DKIM signatures.  This also includes DKIM
   signatures that guarantee a header field absence.

   The editheader extension doesn't directly affect [IMAIL] header field
   signatures generated using [SMIME] or [OPENPGP], because these
   signature schemes include a separate copy of the header fields inside
   the signed message/rfc822 body part.  However, software written to
   detect differences between the inner (signed) copy of header fields
   and the outer (modified by editheader) header fields might be
   affected by changes made by editheader.

   Since normal message delivery adds "Received" header fields and other
   trace fields to the beginning of a message, many such digital
   signature mechanisms are impervious to headers prefixed to a message,
   and will work with "addheader" unless :last is used.

   Any decision mechanism in a user's filter that is based on headers is
   vulnerable to header spoofing.  For example, if the user adds an
   APPROVED header or tag, a malicious sender may add that tag or header
   themselves.  One way to guard against this is to delete or rename any
   such headers or stamps prior to processing the message.

10.  Acknowledgments

   Thanks to Eric Allman, Cyrus Daboo, Matthew Elvey, Ned Freed, Arnt
   Gulbrandsen, Kjetil Torgrim Homme, Simon Josefsson, Will Lee, William
   Leibzon, Mark E. Mallett, Chris Markle, Alexey Melnikov, Randall
   Schwartz, Aaron Stone, Nigel Swinson, and Rand Wacker for extensive
   corrections and suggestions.

11.  References

11.1.  Normative References

   [IMAIL]      Resnick, P., Ed., "Internet Message Format", RFC 2822,
                April 2001.

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

   [MIME3]      Moore, K., "MIME (Multipurpose Internet Mail Extensions)
                Part Three: Message Header Extensions for Non-ASCII
                Text", RFC 2047, November 1996.

   [MIMEPARAM]  Freed, N. and K. Moore, "MIME Parameter Value and
                Encoded Word Extensions: Character Sets, Languages, and
                Continuations", RFC 2231, November 1997.

   [SIEVE]      Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An
                Email Filtering Language", RFC 5228, January 2008.

11.2.  Informative References

   [BODY]       Degener, J. and P. Guenther, "Sieve Email Filtering:
                Body Extension", RFC 5173, April 2008.

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

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

   [MDN]        Hansen, T., Ed., and G. Vaudreuil, Ed., "Message
                Disposition Notification", RFC 3798, May 2004.

   [OPENPGP]    Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
                "MIME Security with OpenPGP", RFC 3156, August 2001.

   [SMIME]      Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
                Extensions (S/MIME) Version 3.1 Message Specification",
                RFC 3851, July 2004.

   [SMTP]       Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
                2821, April 2001.

   [VACATION]   Showalter, T. and N. Freed, Ed., "Sieve Email Filtering:
                Vacation Extension", RFC 5230, January 2008.

Authors' Addresses

   Jutta Degener
   5245 College Ave, Suite #127
   Oakland, CA 94618

   EMail: jutta@pobox.com


   Philip Guenther
   Sendmail, Inc.
   6475 Christie Ave., Ste 350
   Emeryville, CA 94608

   EMail: guenther@sendmail.com

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