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Versions: 00 01 draft-ietf-sieve-mime-loop

Internet Engineering Task Force                                T. Hansen
Internet-Draft                                         AT&T Laboratories
Expires: April 26, 2006                                         C. Daboo
                                            Cyrusoft International, Inc.
                                                        October 23, 2005


 Sieve Extensions: MIME Bodypart Iteration, MIME Tests, Replacement and
                               Enclosure
                     draft-hansen-sieve-loop-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The current Sieve language has no looping mechanism, a way to look at
   individual MIME parts, or any way to manipulate those individual
   parts.  This document defines extensions for each of these needs.

Note




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   This document is being discussed in the MTA-FILTERS mailing lists,
   ietf-mta-filters@imc.org.


1.  Introduction

   Sieve scripts are used to make decisions about the disposition of a
   mail message.  The original Sieve spec, [4], defined operators for
   looking at the message headers, such as addresses and the subject.
   Other extensions provide access to the body of the message, or allow
   you to manipulate the header of the message.  But none of these
   extensions take into account that MIME messages ([1]) are often
   complex objects, consisting of many parts and sub-parts.  This
   extension defines mechanisms for looping through the MIME body parts,
   performing tests on each body part, changing the contents of a body
   part, and enclosing the message with a wrapper.


2.  Sieve Loops

   The current Sieve language has no looping mechanism.  Given that
   messages may contain multiple attachments, in order to support
   filters that apply to any and all attachments, we introduce a new
   control command: "for.every.part", which is an iterator that walks
   though every MIME part of a message, including nested parts, and
   applies the specified block to each of them.  The iterator will start
   with the first MIME part (as its current context) and will execute a
   command block (Sieve commands enclosed by { ...}).  Upon completion
   of this command block, the iterator advances to the next MIME part
   (as its current context) and executes the same command block again.

   The iterator can be terminated prematurely by a new sieve command,
   "break".

   Syntax: for.every.part block

   Syntax: break;


3.  Test "mime"

   Syntax: mime [COMPARATOR] [MATCH-TYPE] <header-names: string-list>
      [<parameter-names: string-list>] <key-list: string-list>

   Syntax: mime [:filename] ...






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   Syntax: mime [:type] ...

   Syntax: mime [:subtype] ...

   For Sieve tests on MIME parts, a new Sieve test ("mime") is defined.
   Similar in concept to the Sieve "header" test, it will parse the MIME
   header lines so that tests can be performed on specific elements.

   If used within the context of a "for.every.part" iterator, the "mime"
   test will examine the headers associated with the current MIME part
   context.

   If used outside the context of a "for.every.part" iterator, the
   "mime" test will examine all MIME body parts and return true if any
   of them satisfies the test.

   The "mime" test has all of the options available from the header
   test, [4] section 5.7.  In addition, these options are available:

   :filename examines the "Content-Disposition:" header field for its
      "filename" parameter.  If there is no "Content-Disposition:"
      header field, then it will look at the "Content-Type:" header
      field for the "name" parameter.

   :type examines the "Content-Type:" header field type parameter.

   :subtype examines the "Content-Type:" header field subtype parameter.


4.  Action Replace

   Syntax: replace string

   A new sieve action command is defined to allow the MIME part to be
   replaced by a text message.  The "replace" command causes a MIME part
   to be removed and replaced with a text/plain part with the text
   supplied by the command.

   When used in the context of a "for.every.part" loop, the MIME part to
   be replaced is the "current" MIME part.  If the current MIME context
   is a multipart MIME part, the entire multipart MIME part is replaced.
   (Replacing a non-multipart MIME part within a "for.every.part" loop
   context does not alter the overall message structure.)

   When used outside the context of a "for.every.part" loop, the MIME
   part to be replaced is the entire message.





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5.  Action Enclose

   Syntax: enclose <:subject string> <:headers string-list> string

   A new sieve action command is defined to allow an entire message to
   be enclosed as an attachment to a new message.  This enclose action
   takes precedance over all other message modifications, such as
   "replace".  If multiple "enclose" actions are executed by a script,
   only the text specified on the last one is used when creating the
   enclosed message.  This action does not affect messages that are
   forwarded via a "redirect" action.

   Specifically, the original message becomes a multipart/mixed message
   with two parts: a text/plain portion with the string argument as its
   body, and a message/rfc822 portion with the original message
   enclosed.  The Content-Type: header field becomes multipart/mixed.
   The Subject: header is specified by the :subject argument.  Any
   headers specified by :headers are copied from the old message into
   the new message.


6.  Sieve Capability Strings

   A Sieve implementation that defines the "for.every.part" and "break"
   actions will advertise the capability string "for.every.part".

   A Sieve implementation that defines the "mime" test will advertise
   the capability string "mime".

   A Sieve implementation that defines the "replace" action will
   advertise the capability string "replace".

   A Sieve implementation that defines the "enclose" action will
   advertise the capability string "enclose".


7.  Examples

7.1.  Example 1

   A Sieve script to replace all the Windows executable attachments in a
   message would be:

   require [ "for.every.part", "mime", "replace" ];
   for.every.part {
       if ( anyof ( mime :subtype :is "exe", mime :filename :matches "*.com" ) {
           replace "Executable attachment removed by user filter";
       }



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   }

7.2.  Example 2

   A Sieve script to warn the user about executable attachment types
   would be:

   require [ "for.every.part", "mime", "enclose" ];

   for.every.part {
       if mime :filename :matches ["*.com", "*.exe", "*.vbs", "*.scr",
                                   "*.pif", "*.hta", "*.bat", "*.zip" ] {
             # these attachment types are executable
             enclose :subject "Warning" "
   WARNING! The enclosed message contains executable attachments.
   These attachments types may contain a computer virus program
   that can infect your computer and potentently damage your data

   Before clicking on these message attachments, you should verify with
   the sender that this message was sent by them and not a computer virus.
   ";
             break;
       }
   }


8.  Security Considerations

   To be provided


9.  IANA Considerations

   To be provided

10.  Normative References

   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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

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



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Authors' Addresses

   Tony Hansen
   AT&T Laboratories
   200 Laurel Ave.
   Middletown, NJ  07748
   USA

   Email: tony+sieveloop@maillennium.att.com


   Cyrus Daboo
   Cyrusoft International, Inc.
   5001 Baum Blvd.
   Suite 780
   Pittsburgh, PA  15213
   USA

   Email: daboo@cyrusoft.com
































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