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Versions: 00 01 02 03 draft-ietf-sieve-body

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


                      Sieve -- "body" extension
                  <draft-degener-sieve-body-02.txt>


Status of this memo

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

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document defines a new primitive for the "sieve" language
   that tests for the occurrence of one or more strings in the body
   of an e-mail message.


1. Introduction

   The proposed "body" test checks for the occurrence of one
   or more strings in the body of an e-mail message.
   Such a test was initially discussed for the [SIEVE] base
   document, but was subsequently removed because it was
   thought to be too costly to implement.

   Nevertheless, several server vendors have implemented
   some form of the "body" test.

   This document reintroduces the "body" test as an extension,
   and specifies it syntax and semantics.


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 "body".


3. Test body

   Syntax:
        "body" [COMPARATOR] [MATCH-TYPE] [BODY-TRANSFORM]
                <key-list: string-list>

   The body test matches text in the body of an e-mail message,
   that is, anything following the first empty line after the header.
   (The empty line itself, if present, is not considered to be part
   of the body.)

   The COMPARATOR and MATCH-TYPE keyword parameters are defined
   in [SIEVE].  The BODY-TRANSFORM is a keyword parameter
   discussed in section 4, below.

   If a message consists of a header only, not followed by an empty
   line, all "body" tests fail, including that for an empty string.

   If a message consists of a header followed only by an empty
   line with no body lines following it, the message is considered
   to have an empty string as a body.


4. Body Transform

   Prior to matching text in a message body, "transformations"
   can be applied that filter and decode certain parts of the body.
   These transformations are selected by a "BODY-TRANSFORM"
   keyword parameter.

   Syntax:
          ":raw"
        / ":content" <content-types: string-list>
        / ":binary" <content-types: string-list> [:offset <start: number>]
        / ":text"

   The default transformation is :text.


4.1 Body Transform ":raw"

   The ":raw" transform is intended to match against the undecoded
   body of a message.

   If the specified body-transform is ":raw", the [MIME] structure of
   the body is irrelevant.  The implementation MUST NOT remove any
   transfer encoding from the message, MUST NOT refuse to filter
   messages with syntactic errors (unless the environment it is part
   of rejects them outright), and MUST NOT interpret or skip MIME
   headers of enclosed body parts.

   Example:

        require "body";

        # This will match a message containing the words "MAKE MONEY FAST"
        # in body or MIME headers other than the outermost RFC 822 header,
        # but will not match a message containing the words in a
        # content-transfer-encoded body.

        if body :raw :contains "MAKE MONEY FAST" {
                reject;
        }



4.2 Body Transform ":content"

   If the body transform is ":content", only MIME parts that have
   the specified content-types are selected for matching.

   If an individual content type contains a '/' (slash), it
   specifies a full <type>/<subtype> pair, and matches only
   that specific content type.  If it is the empty string, all
   MIME content types are matched.  Otherwise, it specifies a
   <type> only, and any subtype of that type matches it.

   The search for MIME parts matching the :content specification is
   recursive and automatically descends into multipart and
   message/rfc822 MIME parts.  Once a MIME part has been identified
   as suitable for searching, only its direct contents are searched
   for the key strings.

   For example, a document with "multipart" major content type only
   directly contains the text in its epilogue and prologue section;
   all the user-visible data inside it is directly contained in
   documents with MIME types other than multipart.

   (Nevertheless, matches against container types with an empty
   match string can be useful as tests for the existence of such
   document parts.)

   MIME parts encoded in "quoted-printable" or "base64" content
   transfer encodings MUST be decoded to prior to the match.
   MIME parts in other transfer encodings MAY be decoded, omitted
   from the test, or processed as raw data.

   MIME parts identified as using charsets other than UTF-8 as
   defined in [UTF-8] SHOULD be converted to UTF-8 prior to the match.
   A conversion from US-ASCII to UTF-8 MUST be supported.
   If an implementation does not support conversion of a given
   charset to  UTF-8, it MAY compare against the US-ASCII subset
   of the transfer-decoded character data instead.  Characters from
   documents tagged with charsets that the local implementation
   cannot convert to UTF-8 and text from mistagged documents MAY
   be omitted or processed according to local conventions.

   Search expressions MUST NOT match across MIME part boundaries.
   MIME headers of the containing text MUST NOT be included in the
   data.

   Example:
        require ["body", "fileinto"];

        # Save any message with any text MIME part that contains the
        # worlds "missile" or "coordinates" in the "secrets" folder.

        if body :content "text" :contains ["missile", "coordinates"] {
                fileinto "secrets";
        }

        # Save any message with an audio/mp3 MIME part in
        # the "jukebox" folder.

        if body :content "audio/mp3" :contains "" {
                fileinto "jukebox";
        }


4.3 Body Transform ":text"

   The ":text" body transform matches against the results of
   an implementation's best effort at extracting UTF-8 encoded
   text from a message.

   In simple implementations, :text MAY be treated the same
   as :content "text".

   Sophisticated implementations MAY strip mark-up from the text
   prior to matching, and MAY convert media types other than text
   to text prior to matching.

   (For example, they may be able to convert proprietary text
   editor formats to text or apply optical character recognition
   algorithms to image data.)


4.4 Body Transform ":binary"

   If the body transform is ":binary", the rules for selecting MIME
   body parts for matching are the same as with the ":content"
   body transform.

   MIME parts encoded in "quoted-printable" or "base64" content
   transfer encodings MUST be decoded prior to the match.
   MIME parts in other transfer encodings MAY be decoded, omitted
   from the test, or processed as raw data.

   Unlike in :content, the charset of the :binary MIME content is
   disregarded.   Instead, the match against the keys provided in
   the "body" statement proceeds as if the file's content data had
   been translated into space-separated hex bytes of the form
   [0-9a-f][0-9a-f] prior to matching.

   Search expressions MUST NOT match across MIME part boundaries.
   MIME headers of the containing text MUST NOT be included in the
   data.

   If the optional ":offset <start: number>" is provided, the
   binary match is executed after skipping <number> octets of
   the binary data.  (Note that the offset counts bytes of the
   internal data, not characters of the hexadecimal representation.)

   Example:
        require ["body", "fileinto"];

        # Save any message with any application MIME part that contains
        # an ascii C string representation of "Hello, World!" into the
        # "helloworld" folder.

        if body :binary ["application"]
                :contains "48 65 6c 6c 6f 2c 20 57 6f 72 6c 64 21 00"
        {
                fileinto "helloworld";
                stop;
        }

        # Check if the bytes at offsets 1000...1003 match some fictitious
        # signature 44, 3c, 0, 1; if yes, reject the message.

        if body :binary ["application"] :offset 1000 :matches "44 3c 00 01 *"
        {
                reject "example virus detected";
                stop;
        }


5. Interaction with Other Sieve Extensions

   Any extension that extends the grammar for the COMPARATOR or
   MATCH-TYPE nonterminals will also affect the implementation of
   "body".

   The [REGEX] extension can place a considerable load on a system
   when applied to whole bodies of messages, especially when
   implemented naively or used maliciously.


6. Security Considerations

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

   Filters relying on string matches in the raw body of an e-mail
   message may be more general than intended.  Text matches are no
   replacement for a virus or spam filtering system.


7. Acknowledgments

   This document has been revised in part based on comments and
   discussions that took place on and off the SIEVE mailing list.
   Thanks to Cyrus Daboo, Ned Freed, Simon Josefsson, Chris Markle,
   Greg Shapiro, Tim Showalter, Nigel Swinson, and Dowson Tong
   for reviews and suggestions.


8. Author's Address

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

   Email: jutta@sendmail.com


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 <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/>.


9.1 Changes from the previous version

   Added body transform ":binary".
   Define the meaning of an empty content type string.


Appendices

Appendix A.  References

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

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

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

   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of Unicode
                and ISO 10646", RFC 2044, October 1996.


Appendix B. Full Copyright Statement

    Copyright (C) The Internet Society 2002,2003. All Rights Reserved.

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    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
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    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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