Network Working Group                                       Jutta Degener
Internet Draft                                             Sendmail, Inc.
Expires: October 2003                                          April 2003


                      Sieve -- "editheader" extension
                  <draft-degener-sieve-editheader-00.txt>


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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Abstract

   This document defines three new actions for the "sieve"
   language that add, delete, and change 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, modify, and add header fields itself.


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


3. Action addheader

   Syntax:
        "addheader" <name: string> <value: string>

   The addheader action appends 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.

   The header field MUST be added at the end of the existing
   header.

   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.

   Example:
        /* 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

   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 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-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").


5. Action replaceheader

   Syntax:
        "replaceheader"
                [":index" <fieldno: number> [":last"]]
                [":newname" <newname: string>]
                [":newvalue" <newvalue: string>]
                [COMPARATOR] [MATCH-TYPE] <field-name: string>
                [<value-patterns: string-list>]

   The replaceheader action replaces all or parts of a header
   field, in some or all occurrences of that header field.

   If :index <fieldno> is specified, the attempts to match
   a value are limited to the named 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.  As with "deleteheader",
   the counting happens before the <value-patterns> match, if any.

   The header field names in the field-name and in the ":newname"
   argument, if specified, MUST be valid 7-bit header field names
   as described by the RFC 2822 "field-name" nonterminal.

   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 no value-patterns are specified, they
   always match.)

   If :newname <newname> is specified, the action changes the
   name of all matching header fields to <newname>.

   If :newvalue <newvalue> is specified, the action changes the
   value of all matching header fields to to <newvalue>.
   If the [VARIABLES] extension is present and the MATCH-TYPE
   is a type that interacts with the [VARIABLES] extension,
   variable references to ${1}...${N} in the replacement string
   following ":newvalue" are evaluated according to the matched
   substring in the header field that is being replaced.

   For example,

        replaceheader :newvalue "[ADV] ${1}" :matches "Subject" "*";

   would insert an "[ADV]" tag before the value of the
   "Subject" header field, changing

        Subject: Make Money Fast!!
  to    Subject: [ADV] Make Money Fast!!

   If neither :newname nor :newvalue are specified, the operation
   still matches its argument and, in the presence of the
   [VARIABLES] extension and a match type that interacts with it,
   sets the ${1}...${N} variables to the last header matched.


6. Interaction with Other Sieve Extensions

   Tests and actions such as "exist" or "header" that examine
   headers MUST examine the current state of a header as modified
   by any actions that have taken place so far.

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


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 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
   material.

   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.


8. Acknowledgments

   Thanks to Eric Allman, Philip Guenther, Will Lee, Chris Markle,
   Randall Schwartz, and Rand Wacker for extensive corrections
   and suggestions.


9. Author's Address

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

   Email: jutta@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/>.


Appendices

Appendix A.  References

   [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.

   [VARIABLES] Homme, K.T., "Sieve -- Variables Extension",
               draft-homme-sieve-variables-01.txt, April 2003.


Appendix B. Full Copyright Statement

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