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

Network Working Group                                      Jutta Degener
Internet Draft                                           Philip Guenther
Intended status: Standards Track                          Sendmail, Inc.
Expires: September 2008                                       March 2008


               Sieve Email Filtering: Editheader Extension
                    draft-ietf-sieve-editheader-11.txt


Status of this memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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.


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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 [SIEVE] section 1.1, 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 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.






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   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").




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   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" 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 [SIEVE] section 2.10.6.

   With the exception of the special handling of redirect and
   "Received" header fields described above, 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.


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   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.

   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

        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.)












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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:      this RFC
    Contact Address: Jutta Degener <jutta@pobox.com>

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


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 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 and other trace 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.









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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. 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


12. 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/>.


12.1 Changes from draft-ietf-sieve-editheader-10.txt

   Update the deleteheader example to not violate the spec.

   Remove the security consideration about deleting "Received" headers.

   Add a "Capability Identifier" section to match existing RFCs.




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   Make the normative and information references subsections of a
   "References" section to match existing RFCs.


12.2 Changes from draft-ietf-sieve-editheader-09.txt

   Add section 5, completely banning <<deleteheader "Received">>
   but requiring that "Subject" changes be permitted.

   Since deletion of "Received" headers is now banned, this spec
   no longer updates the base-spec.

   Updated references to Sieves specs that have been published.


12.3 Changes from draft-ietf-sieve-editheader-08.txt

   Tighten up the permissible behaviors involving redirect and
   deleteheader "Recieved".

   Consistently quote the names of header fields, but not actions.

   For deleteheader, :last without :index is an error.  On the other
   hand, the match-type and comparator are ignored if there are no
   value-patterns.

   Clarify that addheader and deleteheader operate on the 'current'
   set of header fields and give examples demonstrating this.


12.4 Changes from draft-ietf-sieve-editheader-07.txt

   Let implementations permit redirected messages to have fewer
   "Received" header fields, but warn about the consequences.

   Updated boilerplate to match RFC 4748.

   Added "Intended-Status: Standards Track" and
   "Updates: draft-ietf-sieve-3028bis-12"

   Change the references from appendices to sections.
   Update [SIEVE], [BODY], [DSN], and [MDN] references.


12.5 Changes from draft-ietf-sieve-editheader-06.txt

   Make deleteheader match addheader on the description of invalid
   field-names.



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   Update copyright boilerplate

   Update references


12.6 Changes from draft-ietf-sieve-editheader-05.txt

   MDN and DSN references are merely informative


12.7 Changes from draft-ietf-sieve-editheader-04.txt

   Ignore leading and trailing whitespace when matching header field
   values.

   Header modifications are ignored when continuing after an error
   or generating MDNs or DSNs

   Added references for MDN and DSN

   Update IANA registration to match 3028bis

   Added [KEYWORDS] boilerplate text

   Describe an invalid field-name to addheader as an error (might
   be detected at runtime)


12.8 Changes from draft-ietf-sieve-editheader-03.txt

   Change "Syntax:" to "Usage:".

   Updated references.


12.9 Changes from draft-ietf-sieve-editheader-02.txt

   Clarify that value-patterns restrict which occurrences are deleted.

   Add informative reference to [BODY].


12.10 Changes from draft-ietf-sieve-editheader-01.txt

   Whitespace and line length tweaks noted by ID-nits.

   Clarified what is being counted by :index.

   Update the [SIEVE] reference to the I-D of the revision.


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12.11 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
       effects
     - add security consideration for combination of header changes
       and "reject"


12.12 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.


12.13 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
   Considerations.

   Added IANA Considerations section.











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13. References

13.1  Normative References

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

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


13.2.  Informative References

   [BODY]     Degener, J. and P. Guenther, "Sieve Email Filtering:
              Body Extension", draft-ietf-sieve-body-09, March 2008.

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

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

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















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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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