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Versions: (draft-degener-sieve-editheader) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5293

Internet Draft                                            Philip Guenther
Expires: November 2005                                     Sendmail, Inc.
                                                                 May 2005

               Sieve Email Filtering: Editheader Extension

Status of this memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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.

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS] and the "Syntax:" 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.

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

3. Action addheader

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

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

   If the specified field value does not match the RFC 2822
   "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 [RFC2047] or [RFC2231] to be compliant
   with RFC 2822.

   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.

        /* 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";

4. Action deleteheader

   Syntax: "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.  The value-patterns,
   if specified, are matched according to the match type and
   comparator.  If none are specified, all values match.

   The field-name MUST be a valid 7-bit header field name as described
   by the [IMAIL] "field-name" nonterminal syntax element.

   If :index <fieldno> is specified, the attempts to match
   a value are limited to the header field <fieldno> (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 2 :contains "Received" "via carrier-pigeon"

   deletes the second "Received:" header field if it contains
   the string "via carrier-pigeon" (not the second Received: field
   that contains "via carrier-pigeon").

   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.

5. Interaction with Other Sieve Extensions

   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 the incoming
   message contained an "X-Hello" header field or not:

        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.

   Actions that store or send the message MUST do so with the current
   set of header fields.

   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 [SIEVE] section 2.10.3 and does not deliver
   the same message to a folder more than once, the following
   code fragment

        addheader "X-Flavor" "vanilla";

   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.)

6.  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
    Capability keyword: editheader
    Capability arguments: N/A
    Standards Track/IESG-approved experimental RFC number: this RFC
    Person and email address to contact for further information:

    Jutta Degener

    This information should be added to the list of sieve extensions
    given on http://www.iana.org/assignments/sieve-extensions.

7. 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 (by a gateway MTA that removes them).

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

   Any change in a message content may interfere with digital
   signature mechanisms that include the header in the signed
   material.  Since normal message delivery adds "Received:"
   header fields to the beginning of a message, many such schemas
   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.

   Modifying the header of a message and then using the "reject"
   action may let a sender 'probe' the logic of the sieve filter.

8. Acknowledgments

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

9. Authors' Addresses

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

   Email: jutta@pobox.com

   Philip Guenther
   Sendmail, Inc.
   6425 Christie Ave, 4th Floor
   Emeryville, CA 94608

   Email: guenther@sendmail.com

10. Discussion

   This section will be removed when this document leaves the
   Internet-Draft stage.

   This draft is intended as an extension to the Sieve mail filtering
   language.  Sieve extensions are discussed on the MTA Filters mailing
   list at <ietf-mta-filters@imc.org>.  Subscription  requests can
   be sent to <ietf-mta-filters-request@imc.org> (send an email
   message with the word "subscribe" in the body).

   More information on the mailing list along with a WWW archive of
   back messages is available at <http://www.imc.org/ietf-mta-filters/>.

10.1 Changes from draft-ietf-sieve-editheader-00.txt

   Updated IPR boilerplate to RFC 3978/3979.

   Many corrections in response to WGLC comments.  Of particular note:
     - correct a number of spelling and grammar errors
     - document that neither addheader nor deleteheader affects the
       implicit keep
     - add normative references to RFC 2047 and RFC 2231
     - it is not an error for deleteheader to affect nothing
     - change "foo.tld" to "foo.example.com"
     - add an informative reference to [VACATION], citing it as an
       example of an action that examines header fields
     - add weasel words about changes to fields that have secondary
     - add security consideration for combination of header changes
       and "reject"

10.2 Changes from draft-degener-sieve-editheader-03.txt

   Renamed to draft-ietf-sieve-editheader-00.txt;
   tweaked the title and abstract.

   Added Philip Guenther as co-author.

   Updated IPR boilerplate.

10.3 Changes from draft-degener-sieve-editheader-02.txt

   Changed the duplicate restrictions from "messages with different
   headers MUST be considered different" to their direct opposite,
   "messages with different headers MUST be considered the same,"
   as requested by workgroup members on the mailing list.

   Expanded mention of header signature schemes to Security

   Added IANA Considerations section.


Appendix A.  Normative References

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

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

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail
              Extensions) Part Three: Message Header Extensions for
              Non-ASCII Text", RFC 2047, November 1996.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and
              Encoded Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, November 1997.

   [SIEVE]    Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028,
              January 2001.

Appendix B.  Informative References

   [VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering:
              Vacation Extension", draft-ietf-sieve-vacation-02,
              April 2005

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