Sieve Email Filtering: Body Extension
draft-ietf-sieve-body-09

Network Working Group                                      Jutta Degener
Internet Draft                                           Philip Guenther
Intended status: Standards Track                          Sendmail, Inc.
Expires: March 2008                                       September 2008
Updates: RFC-ietf-sieve-variables

                 Sieve Email Filtering: Body Extension
                      draft-ietf-sieve-body-09.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document defines a new command for the "Sieve" email
   filtering language that tests for the occurrence of one or more
   strings in the body of an email message.

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1. Introduction

   The "body" test checks for the occurrence of one
   or more strings in the body of an email 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 its syntax and semantics.

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 [SIEVE] section 1.1, including
   use of the "Usage:" label for the definition of text and tagged
   arguments syntax.

   The rules for interpreting the grammar are defined in [SIEVE]
   and inherited by this specification. In particular, readers of
   this document are reminded that according to [SIEVE] sections
   2.6.2 and 2.6.3, optional arguments such as COMPARATOR and
   MATCH-TYPE can appear in any order.

3. Capability Identifier

   The capability string associated with the extension defined in
   this document is "body".

4. Test body

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

   The body test matches content in the body of an email 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.)

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   The COMPARATOR and MATCH-TYPE keyword parameters are defined in
   [SIEVE].  As specified in section 2.7.3 of [SIEVE], the default
   COMPARATOR is "i;ascii-casemap" and the default MATCH-TYPE is
   ":is".

   The BODY-TRANSFORM is a keyword parameter that governs how a set
   of strings to be matched against are extracted from the body of
   the message.  If a message consists of a header only, not followed
   by an empty line, then that set is empty and all "body" tests
   return false, including those that test for an empty string.
   (This is similar to how the "header" test always fails when the
   named header fields aren't present.)  Otherwise, the transform
   must be followed as defined below in section 4.

   Note that the transforms defined here do *not* match against
   each line of the message independently, so the strings will
   usually contain CRLFs.  How these can be matched is governed by
   the comparator and match-type.  For example, with the default
   comparator of "i;ascii-casemap", they can be included literally
   in key string, or be matched with the "*" or "?" wildcards of
   the :matches match-type, or be skipped with :contains.

5. Body Transform

   Prior to matching content 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.

   Usage: ":raw"
        / ":content" <content-types: string-list>
        / ":text"

   The default transformation is :text.

5.1 Body Transform ":raw"

   The ":raw" transform matches against the entire, undecoded body
   of a message as a single item.

   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 treat multipart boundaries
   or the MIME headers of enclosed body parts as part of the content
   being matched against instead of MIME structures to interpret.

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   Example:

        require "body";

        # This will match a message containing the literal text
        # "MAKE MONEY FAST" in body parts (ignoring any
        # content-transfer-encodings) or MIME headers other than
        # the outermost RFC 2822 header.

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

5.2 Body Transform ":content"

   If the body transform is ":content", the MIME parts that have
   the specified content-types are matched against independently,
   each the entire part as a single string.

   If an individual content type begins or ends with a '/' (slash)
   or contains multiple slashes, it matches no content types.
   Otherwise, if it contains a slash, then 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.  All MIME parts with matching types
   are searched for the key strings.  The test returns true if any
   combination of searched MIME part and key-list argument match.

   If the :content specification matches a multipart MIME part,
   only the prologue and epilogue sections of the part will be
   searched for the key strings; the contents of nested parts are
   only searched if their respective types match the :content
   specification.

   If the :content specification matches a message/rfc822 MIME part,
   only the header of the nested message will be searched for the
   key strings; the contents of the nested message body parts are
   only searched if its content-type matches the :content specification.

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

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   Example:
        From: Whomever
        To: Someone
        Date: Whenever
        Subject: whatever
        Content-Type: multipart/mixed; boundary=outer

     &  This is a multi-part message in MIME format.
     &
        --outer
        Content-Type: multipart/alternative; boundary=inner

     &  This is a nested multi-part message in MIME format.
     &
        --inner
        Content-Type: text/plain; charset="us-ascii"

     $  Hello
     $
        --inner
        Content-Type: text/html; charset="us-ascii"

     %  <html><body>Hello</body></html>
     %
        --inner--
     &
     &  This is the end of the inner MIME multipart.
     &
        --outer
        Content-Type: message/rfc822

     !  From: Someone Else
     !  Subject: hello request

     $  Please say Hello
     $
        --outer--
     &
     &  This is the end of the outer MIME multipart.

   In the above example, the '&', '$', '%', and '!' characters at
   the start of a line are used to illustrate what portions of the
   example message are used in tests:

   - the lines starting with '&' are the ones that are tested when
     a 'body :content "multipart" :contains "MIME"'
     test is executed.

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   - the lines starting with '$' are the ones that are tested when
     a 'body :content "text/plain" :contains "Hello"' test is
     executed.

   - the lines starting with '%' are the ones that are tested when
     a 'body :content "text/html" :contains "Hello"' test is executed.

   - the lines starting with '$' or '%' are the ones that are tested
     when a 'body :content "text" :contains "Hello"' test is executed.

   - the lines starting with '!' are the ones that are tested when
     a 'body :content "message/rfc822" :contains "Hello"' test is
     executed.

   Comparisons are performed on octets.  Implementations decode
   the content-transfer-encoding and convert text to [UTF-8] as
   input to the comparator.  MIME parts that cannot be decoded and
   converted MAY be treated as plain US-ASCII, omitted, or processed
   according to local conventions.  A NUL octet (character zero)
   SHOULD NOT cause early termination of the content being compared
   against.  Implementations MUST support the "quoted-printable",
   "base64", "7bit", "8bit", and "binary" content transfer encodings.
   Implementations MUST be capable of converting to UTF-8 the
   US-ASCII, ISO-8859-1, and the US-ASCII subset of
   ISO-8859-* character sets.

   Each matched part is matched against independently: search
   expressions MUST NOT match across MIME part boundaries.
   MIME headers of the containing part MUST NOT be included in the
   data.

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

        # Save any message with any text MIME part that contains the
        # words "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";
        }

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5.3 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.

   It is unspecified whether this transformation results in a single
   string or multiple strings being matched against.  All the text
   extracted from a given non-container MIME part MUST be in the
   same string 

   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.)

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

        # Save messages mentioning the project schedule in the
        # project/schedule folder.
        if body :text :contains "project schedule" {
                fileinto "project/schedule";
        }

6. 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".

   Wildcard expressions used with "body" are exempt from the side
   effects described in [VARIABLES].  That is, they MUST NOT set
   match variables (${1}, ${2}...) to the input values corresponding
   to wildcard sequences in the matched pattern.  However, if the
   extension is present, variable references in the key strings or
   content type strings are evaluated as described in the draft.

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7.  IANA Considerations

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

    To: iana@iana.org
    Subject: Registration of new Sieve extension

    Capability name: body
    Description:     Provides a test for matching against the
                     the body of the message being processed
    RFC number:      this RFC
    Contact Address: Jutta Degener <jutta@pobox.com>

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

8. 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 email
   message may be more general than intended.  Text matches are no
   replacement for a spam, virus, or other security related
   filtering system.

9. 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, Bob Johannessen, Simon Josefsson,
   Mark E. Mallett, Chris Markle, Alexey Melnikov, Ken Murchison,
   Greg Shapiro, Tim Showalter, Nigel Swinson, Dowson Tong, and
   Christian Vogt for reviews and suggestions.

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10. Authors' Addresses

   Jutta Degener
   5245 College Ave, Suite #127
   Oakland, CA 94618

   Email: jutta@pobox.com

   Philip Guenther
   Sendmail, Inc.
   6425 Christie Ave, 4th Floor
   Emeryville, CA 94608

   Email: guenther@sendmail.com

11. 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/>.

11.1 Changes from draft-ietf-sieve-body-08.txt

   Add a "Capability Identifier" section to match existing RFCs.

   Make the normative and information references subsections of a
   "References" section to match existing RFCs.

   Tweak description field of the IANA registration.

   Change "wild card" to "wildcard".

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11.2 Changes from draft-ietf-sieve-body-07.txt

   Clarify how transforms generate one or more strings to match against.

   Reiterate the default COMPARATOR and MATCH-TYPE from the base spec.

   [SIEVE] and [VARIABLES] have been published.

11.3 Changes from draft-ietf-sieve-body-06.txt

   Changed "matched text" to "matched content".  Drop the word
   "proposed".

11.4 Changes from draft-ietf-sieve-body-05.txt

   Updated boilerplate to match RFC 4748.

   Added "Intended-Status: Standards Track" and
   "Updates: draft-ietf-sieve-variables-08"

   Change the references from appendices to sections.
   Update [SIEVE] reference.

11.5 Changes from draft-ietf-sieve-body-04.txt

   Changed 'reject' to 'discard' in the example.

   Removed reference to regex draft.

   Update copyright boilerplate.

11.6 Changes from draft-ietf-sieve-body-03.txt

   Update IANA registration to match 3028bis.

   Added direct boilerplate for [KEYWORDS].

11.7 Changes from draft-ietf-sieve-body-02.txt

   Updated charset conversion to match draft-ietf-sieve-3028bis-06.txt.

   Change "Syntax:" to "Usage:".

   Updated references.

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11.8 Changes from draft-ietf-sieve-body-01.txt

   Updated charset conversion requirements to match those in
   draft-ietf-sieve-3028bis-03.txt for headers.

11.9 Changes from draft-ietf-sieve-body-00.txt

   Updated IPR boilerplate to RFC 3978/3979.

   Many prose corrections in response to WGLC comments.  Of particular
   note:
     - made clear that :raw treats MIME boundaries and headers as
       text to be matched against
     - corrected description in comment of :raw example
     - clarified the interpretation of invalid content-types in
       :content
     - gave precise description of what gets matched when :content
       is used with message/rfc822 or any multipart type, as well
       as a comprehensive example
     - include an example of :text
     - tightened wording of interaction with [VARIABLES]
     - added informative reference to [REGEX]

11.10 Changes from draft-degener-sieve-body-04.txt

   Renamed to draft-ietf-sieve-body-00.txt; tweaked the title and
   abstract.

   Added Philip Guenther as co-author.

   Split references into normative and informative.  Updated [UTF-8]
   and [VARIABLES] references.

   Updated IPR boilerplate.

11.11 Changes from draft-degener-sieve-body-03.txt

   Made "body" exempt from variable-setting side effects in the
   presence of the "variables" extension and wildcards.  It's too
   hard to implement.

   Removed :binary.  It's uglier and less useful than it needs to be
   to bother.

   Added IANA section.

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12. References

12.1. Normative References

   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 15, 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]      Guenther, P. and T. Showalter, "Sieve: An Email
                Filtering Language", RFC 5228, January 2008.

   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3629, November 2003.

12.2. Informative References

   [VARIABLES] Homme, K., "Sieve Email Filtering: Variables Extension",
               RFC 5229, January 2008.

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