Network Working Group                                       Jutta Degener
Internet Draft                                             Sendmail, Inc.
Expires: July 2004                                           January 2004

                      Sieve -- "editheader" extension

Status of this memo

   This document is an Internet-Draft and is subject to all
   provisions of Section 10 of RFC2026.

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   This document defines two new actions for the "sieve"
   language that add and delete e-mail header fields.

1. Introduction

   Email headers 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 "Syntax:" label for the definition of action
   and tagged arguments syntax.

   The term "header field" is used here as in [RFC-2822] to mean a
   logical line of an e-mail message header.

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

3. Action addheader

        "addheader" [":last"] <name: string> <value: string>

   The addheader action adds a header field to the existing
   message header.  The name MUST be a valid 7-bit US-ASCII header
   field name as described by [RFC-2822] "field-name" nonterminal.

   If the specified field value does not match the RFC 2822
   "unstructured" nonterminal 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 RFC 2047 or RFC 2231 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 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.tld>", "<kim@home.tld>"]
                addheader "X-Sieve-Filtered" "<kim@job.tld>";
                redirect "kim@home.tld";

4. Action 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 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 [RFC-2822] "field-name" nonterminal.

   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;

        deleteheader :index 2 :contains "Received" "via carrier-pidgeon"

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

5. Interaction with Other Sieve Extensions

   Tests and actions such as "exist" or "header" 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 "Hi";

   Actions that create messages in storage or in transport to
   MTAs MUST store and send messages with the current set of
   header fields.

   A message modified by addheader or deleteheader MUST NOT
   be considered the same as the original message unless it
   matches the original message exactly.

   For example, the following code fragment

        addheader "X-Flavor: vanilla"

   files two messages, not one.

   The "implicit keep" is thought to be executed at the end of
   the script, after the headers have been modified.

6. 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 headers may unwisely destroy
   evidence about the path a header has taken.

   Any change in a message content may interfere with digital
   signature mechanisms that include the header in the signed

   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.

7. Acknowledgments

   Thanks to Eric Allman, Ned Freed, Philip Guenther, Simon Josefsson,
   Will Lee, Chris Markle, Randall Schwartz, Kjetil Torgrim Homme,
   and Rand Wacker for extensive corrections and suggestions.

8. Author's Address

   Jutta Degener
   Sendmail, Inc.
   6425 Christie Ave, 4th Floor
   Emeryville, CA 94608


9. 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 <>.  Subscription  requests can
   be sent to <> (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 <>.

9.1 Changes from the previous version

   Removed "replaceheader" action and reference to variables.
   Expanded discussion of interactions with existing sieve primitives.
   Referenced RFC 2822's definition of "header field".
   Changed insertion position default to beginning of header,
   and added :last parameter to restore the original default.


Appendix A.  References

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

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

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

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