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Versions: (draft-hansen-sieve-loop) 00 01 02 03 04 05 06 07 08 09 RFC 5703

Internet Engineering Task Force                                T. Hansen
Internet-Draft                                         AT&T Laboratories
Intended status: Standards Track                                C. Daboo
Expires: January 9, 2008                                  Apple Computer
                                                            July 8, 2007


  Sieve Email Filtering:  MIME part Tests, Iteration, Extraction,
                       Replacement and Enclosure
                     draft-ietf-sieve-mime-loop-03

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 9, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Sieve email filtering language has no way to examine individual
   MIME parts or any way to manipulate those individual parts.  However,
   being able to filter based on MIME content is important.  This
   document defines extensions for these needs.





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Note

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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Sieve Loops  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Changes to Sieve tests . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Test "header"  . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Test "address" . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Test "exists"  . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Action Replace . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Action Enclose . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Action extract_text  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Sieve Capability Strings . . . . . . . . . . . . . . . . . . .  9
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . . . 11
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   13. Change History (to be removed prior to publication as an
       RFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     13.1. draft-ietf-sieve-mime-02 . . . . . . . . . . . . . . . . . 14
     13.2. draft-ietf-sieve-mime-01 . . . . . . . . . . . . . . . . . 14
     13.3. draft-ietf-sieve-mime-00 . . . . . . . . . . . . . . . . . 14
     13.4. draft-hansen-sieve-loop-01 . . . . . . . . . . . . . . . . 14
     13.5. draft-hansen-sieve-loop-02 . . . . . . . . . . . . . . . . 14
     13.6. draft-hansen-sieve-loop-03 . . . . . . . . . . . . . . . . 15
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     14.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17












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1.  Introduction

   Sieve scripts are used to make decisions about the disposition of an
   email message.  The base Sieve specification,
   [I-D.ietf-sieve-3028bis], defines operators for looking at the
   message headers, such as addresses and the subject.  Other extensions
   provide access to the body of the message ([I-D.ietf-sieve-body]), or
   allow you to manipulate the header of the message
   ([I-D.ietf-sieve-editheader]).  But none of these extensions take
   into account that MIME messages ([RFC2045]) are often complex
   objects, consisting of many parts and sub-parts.  This extension
   defines mechanisms for performing tests on MIME body parts, looping
   through the MIME body parts, extracting information from a MIME body
   part, changing the contents of a MIME body part, and enclosing the
   entire message with a wrapper.


2.  Conventions Used in This Document

   Conventions for notations are as in [I-D.ietf-sieve-3028bis] section
   1.1.

   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 [RFC2119].


3.  Sieve Loops

   The base Sieve language has no looping mechanism.  Given that
   messages may contain multiple parts, in order to support filters that
   apply to any and all parts, 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 commands
   in 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".

   Usage:  for_every_part block







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   Usage:  break;

   "for_every_part" commands can be nested inside other "for_every_part"
   commands.  When this occurs, the nested "for_every_part" iterates
   over the MIME parts contained within the MIME part current being
   targeted by the nearest enclosing "for_every_part" command.  If that
   MIME part is a terminal MIME part (i.e. does not contain other MIME
   parts) then the nested "for_every_loop" is simply ignored.

   Sieve implementations MAY limit the number of nested loops that occur
   within one another, however they MUST support at least one nested
   loop inside another loop.


4.  Changes to Sieve tests

   This specification extends the base Sieve "header", "address" and
   "exists" tests to support targeting those tests at a specific MIME
   part or at all MIME parts in the enclosing scope.

4.1.  Test "header"

   The "header" test is extended with the addition of a new ":mime"
   tagged argument, which takes a number of other arguments.

   Usage:  header [:mime] [:anychild] [MIMEOPTS]
      [COMPARATOR] [MATCH-TYPE]
      <header-names: string-list> <key-list: string-list>

   Usage:  The definition of [MIMEOPTS] is:

      Syntax: ":type" / ":subtype" / ":contenttype" /
      ":param" <param-list: string-list>

   When the ":mime" tagged argument is present in the "header" test, it
   will parse the MIME header lines in a message so that tests can be
   performed on specific elements.

   If the ":anychild" tagged argument is NOT specified:

   o  If used within the context of a "for_every_part" iterator, the
      "header" test will examine the headers associated with the current
      MIME part context from the loop.

   o  If used outside the context of a "for_every_part" iterator, the
      "header" test will examine only the outer, top-level, headers of
      the message.




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   If the ":anychild" tagged argument IS specified, the "header" test
   will examine all MIME body parts and return true if any of them
   satisfies the test.

   The "header" test with the ":mime" tagged argument can test various
   aspects of certain structed MIME headers.  These options are
   available:

   :type  parses the header assuming it has the format of a "Content-
      Type:" MIME header field, and tests the value of the MIME type
      specified in the header.

   :subtype  parses the header assuming it has the format of a "Content-
      Type:" MIME header field, and tests the value of the MIME subtype
      specified in the header.

   :contenttype  parses the header assuming it has the format of a
      "Content-Type:" MIME header field, and tests the combined value of
      the MIME type and subtype specified in the header.

   :param  parses the header looking for MIME parameters in the header.
      The supplied string-list lists the names of any parameters to be
      tested.  If any one named parameter value matches the test string
      value, the test will return true.

   Example:

   require ["mime", "fileinto"];

   if header :mime :type "Content-Type" "image"
   {
       fileinto "INBOX.images";
   }

   In this example, any message that contains a MIME image type part at
   the top-level is saved to the mailbox "INBOX.images".

   Example:

   require ["mime", "fileinto"];

   if header :mime :anychild :contenttype :comparator
             "Content-Type" "text/html"
   {
       fileinto "INBOX.html";
   }

   In this example, any message that contains any MIME part with a



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   content-type of "text/html" is saved to the mailbox "INBOX.html".

   Example:

   require ["mime", "for_every_part", "fileinto"];

   for_every_part
   {
       if header :mime :param "filename" :comparator
          "Content-Disposition" "important"
       {
           fileinto "INBOX.important";
           break;
       }
   }

   In this example, any message that contains any MIME part with a
   content-disposition with a filename parameter containing the text
   "important" is saved to the mailbox "INBOX.important".

4.2.  Test "address"

   The "address" test is extended with the addition of a new ":mime"
   tagged argument, which takes a number of other arguments.

   Usage:  address [:mime] [:anychild] [COMPARATOR]
      [ADDRESS-PART] [MATCH-TYPE]
      <header-list: string-list> <key-list: string-list>

   When the ":mime" tagged argument is present in the "address" test, it
   will parse the MIME header lines as if they were standard address
   header lines in a message so that tests can be performed on specific
   elements.

   The behavior of the ":anychild" tagged argument and the interaction
   with the "for_every_part" iterator is the same as for the extended
   "header" test Section 4.1.

   Example:

   require ["mime", "fileinto"];

   if address :mime :is :all "content-from" "tim@example.com"
   {
       fileinto "INBOX.part-from-tim";
   }

   In this example, any message that contains a MIME Content-From header



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   at the top-level matching the text "tim@example.com" is saved to the
   mailbox "INBOX.part-from-time".

4.3.  Test "exists"

   The "exists" test is extended with the addition of a new ":mime"
   tagged argument, which takes one other argument.

   Usage:  exists [:mime] [:anychild] <header-names: string-list>

   When the ":mime" tagged argument is present in the "exists" test, the
   test is extended to check for the existence of MIME headers in MIME
   parts.

   The behavior of the ":anychild" tagged argument and the interaction
   with the "for_every_part" iterator is the same as for the extended
   "header" test Section 4.1.

   Example:

   require ["mime", "fileinto"];

   if exists :mime :anychild "content-md5"
   {
       fileinto "INBOX.md5";
   }

   In this example, any message that contains a MIME Content-MD5 header
   in any MIME part is saved to the mailbox "INBOX.md5".


5.  Action Replace

   Usage:  replace [:mime] [:subject string] [:from string]
      <replacement: string>

   The "replace" command is defined to allow a MIME part to be replaced
   with the text supplied in the command.

   When used in the context of a "for_every_part" iterator, 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, which would alter the MIME structure of the message by
   eliminating all of the children of the multipart part.  (Replacing a
   non-multipart MIME part within a "for_every_part" loop context does
   not alter the overall message structure.)  If the MIME structure is
   altered, the change takes effect immediately: the "for_every_part"
   iterator that is executing does not go into the no-longer existing



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   body parts, and subsequent "for_every_part" iterators would use the
   new message structure.

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

   If the :mime parameter is not specified, the replacement string is a
   text/plain part.

   If the :mime parameter is specified, then the replacement string is,
   in fact, a MIME entity as defined in [RFC2045] section 2.4, including
   both MIME headers and content.  If the optional :mime parameter is
   not supplied, the reason string is considered to be a UTF-8 string.

   If the entire message is being replaced, a ":subject" parameter
   specifies a subject line to attach to the message that is generated.
   UTF-8 characters can be used in the string argument; implementations
   MUST convert the string to [RFC2047] encoded words if and only if
   non-ASCII characters are present.  Implementations MUST preserve the
   previous Subject header as an Original-Subject header.

   If the entire message is being replaced, a ":from" parameter may be
   used to specify an alternate address to use in the From field of the
   message that is generated.  The string must specify a valid [RFC2822]
   mailbox-list.  Implementations SHOULD check the syntax and generate
   an error when a syntactically invalid ":from" parameter is specified.
   Implementations MAY also impose restrictions on what addresses can be
   specified in a ":from" parameter; it is suggested that values that
   fail such a validity check simply be ignored rather than causing the
   replace action to fail.  Implementations MUST preserve the previous
   From header as an Original-From header.


6.  Action Enclose

   Usage:  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.  After enclosure,
   subsequent actions affecting the message header or content use the
   newly create message instead of the original message; this means that
   any use of a "replace" action or other similar actions should be
   executed before the "enclose" action.

   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.



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   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.  If not specified by :headers, Date: and From:
   headers should be synthesized to reflect the current date and the
   user running the Sieve action.


7.  Action extract_text

   Usage:  extract_text [MODIFIER] [":first" number] <varname: string>

   The extract_text action may be used within the context of a
   "for_every_part" loop.  It stores at most :first bytes of the current
   MIME body part in the variable identified by varname.  If the :first
   parameter is not present, the whole content of the current MIME body
   part is stored.  In either case the actually stored data MAY be
   truncated to conform to implementation specific limit on variable
   length and/or on MIME body part length.  QUESTION: What do we do if
   the Content-Transfer-Encoding is anything other than 7bit?

   If extract_text is used outside the context of a "for_every_part"
   loop, the action will set the variable identified by varname to the
   empty string.

   Modifiers are applied on the extracted text before it is stored in
   the variable.  See [I-D.ietf-sieve-variables] for details.


8.  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" tagged arguments to
   the "header", "address" and "exists" commands 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".

   A Sieve implementation that defines the "extract_text" action will



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   advertise the capability string "extract_text".  Note that to be
   useful, the "extract_text" action also requires the "variables"
   [I-D.ietf-sieve-variables] and "mime" capabilities.


9.  Examples

9.1.  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 (
         header :mime :contenttype :is "Content-Type" "application/exe",
         header :mime :param "filename"
           ["Content-Type", "Content-Disposition"] :matches "*.com" )
  {
    replace "Executable attachment removed by user filter";
  }
}

9.2.  Example 2

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























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   require [ "for_every_part", "mime", "enclose" ];

   for_every_part
   {
     if header :mime :param "filename"
        ["Content-Type", "Content-Disposition"] :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;
     }
   }

9.3.  Example 3

   A Sieve script to extract subject and text out of messages from the
   boss
























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   require ["mime", "variables", "extract_text"];

   if header :contains "from" "boss@example.org"
   {
     # :matches is used to get the value of the Subject header
     if header :matches "Subject" "*"
     {
       set "subject" "${1}";
     }

     # extract the first 100 bytes of the first text/* part
     for_every_part
     {
       if header :mime :type :is "Content-Type" "text"
       {
         extract_text :first 100 "msgcontent";
         break;
       }
     }

     # if it's not a 'for your information' message
     if not header :contains "subject" "FYI:"
     {
       # do something using ${subject} and ${msgcontent}
       # such as sending a notification using a notification extion
     }
   }


10.  Acknowledgements

   Comments from members of the MTA Filters Working Group, in particular
   Ned Freed, Nigel Swinson, Mark Mallett and Alexey Melnikov, are
   gratefully acknowledged.


11.  Security Considerations

   The "enclose" action creates an entirely new message, as compared to
   just redirecting or forwarding the existing message.  Therefore, any
   site policies applicable to message submission should be enforced
   here.

   The looping specification specified here provides easier access to
   information about the message contents, which may also be achieved
   through other sieve tests.  This is not believed to raise any
   additional security issues beyond those for the Sieve "envelope" and
   "body" tests.



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   The system MUST be sized and restricted in such a manner that even
   malicious use of mime part matching does not deny service to other
   users of the host system.

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

   All of the security considerations given in the base Sieve
   specification also apply to these extensions.


12.  IANA Considerations

   The Original-Subject: and Original-From: headers are to be registered
   in the Permanent Message Header Fields table.

   The following template specifies the IANA registration of the Sieve
   extensions specified in this document:

   To: iana@iana.org Subject: Registration of new Sieve extensions
   Capability name: for_every_part
   Description: adds the "for_every_part" and "break" actions for
   iterating through MIME parts of a message.
   Capability name: mime
   Description: adds ":mime" tagged arguments to the "header", "address"
   and "exists" commands.
   Capability name: replace
   Description: adds the "replace" action for replacing a MIME body part
   of a message.
   Capability name: enclose
   Description: adds the "enclose" action for enclosing a message with a
   wrapper.
   Capability name: extract_text
   Description: adds the "extract_text" action for extracting text from
   a MIME body part.
   RFC number: RFC XXXX
   Contact address: The Sieve discussion list
   <ietf-mta-filters@imc.org>.

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


13.  Change History (to be removed prior to publication as an RFC)







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13.1.  draft-ietf-sieve-mime-02

   minor syntax glitches in examples

   Add clarification on "replace" affecting subsequent for_every_part
   loops?

   Add IANA considerations for Original-Subject: and Original-From:.

   Add note on "enclose" creating From: and Date: headers.

13.2.  draft-ietf-sieve-mime-01

   what happens when nested for_every_loop's

   a "mime" shorthand for testing the type/subtype, without requiring

   interactions with variables
   notifications
   notifications to calendar service
   address tests, exists tests
   mimeheader, mimeparameter tests

13.3.  draft-ietf-sieve-mime-00

   Changed title and text to emphasize MIME Tests.

   Changed for.every.part to for_every_part.

   Added :anychild to mime test.  Default is to use the current context
   or outer envelope; specifying :anychild will look at all children.

   Added clarifications to replacing parts affecting the structure.

   Added :mime option to replace, ala draft-ietf-sieve-vacation-06.

   Various other minor nit fixes.

13.4.  draft-hansen-sieve-loop-01

   Merged with draft-daboo-sieve-mime-00.txt.

13.5.  draft-hansen-sieve-loop-02

      Update to 3028bis reference.

      Added 2119 conventions section.




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      Terminology/title tweaks.

      Added informative references to body and editheader extensions.

      Added description of nested loops.

      Replaced mime test by extensions to header, address and exists
      tests.

13.6.  draft-hansen-sieve-loop-03

      after enclosure, subsequent actions affect newly created message

      synthesis of Date/From headers by the enclose action is no longer
      controversial

      Filled in Security Considerations

      Picked up extract_text action from draft-ietf-sieve-notify

      Expanded the IANA considerations section


14.  References

14.1.  Normative References

   [I-D.ietf-sieve-3028bis]
              Showalter, T. and P. Guenther, "Sieve: An Email Filtering
              Language", draft-ietf-sieve-3028bis-12 (work in progress),
              February 2007.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.






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14.2.  Informative References

   [I-D.ietf-sieve-body]
              Guenther, P. and J. Degener, "Sieve Email Filtering: Body
              Extension", draft-ietf-sieve-body-06 (work in progress),
              February 2007.

   [I-D.ietf-sieve-editheader]
              Guenther, P. and J. Degener, "Sieve Email Filtering:
              Editheader Extension", draft-ietf-sieve-editheader-08
              (work in progress), March 2007.

   [I-D.ietf-sieve-variables]
              Homme, K., "Sieve Extension: Variables",
              draft-ietf-sieve-variables-08 (work in progress),
              December 2005.


Authors' Addresses

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

   Email: tony+sieveloop@maillennium.att.com


   Cyrus Daboo
   Apple Computer, Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Email: cyrus@daboo.name
   URI:   http://www.apple.com/














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