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".






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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"
               ["<[email protected]>", "<[email protected]>"]
       {
               addheader "X-Sieve-Filtered" "<[email protected]>";
               redirect "[email protected]";
       }

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.






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  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"
                             "[email protected]";

  deletes the first "Delivered-To" header field if it contains the
  string "[email protected]" (not the first "Delivered-To" field that
  contains "[email protected]").

  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.







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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.




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  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: [email protected]
  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



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  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.











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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.





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  [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: [email protected]


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

  EMail: [email protected]

































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RFC 5293      Sieve Email Filtering: Editheader Extension    August 2008


Full Copyright Statement

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  contained in BCP 78, and except as set forth therein, the authors
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