Sieve body: Sieve Email Filtering: Body Extension

Network Working Group                                         J. Degener
Request for Comments: 5173                                   P. Guenther
Updates: 5229                                             Sendmail, Inc.
Category: Standards Track                                     April 2008



                 Sieve Email Filtering: Body 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 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.

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
   the use of the "Usage:" label for the definition of text and tagged
   argument 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.)

   The COMPARATOR and MATCH-TYPE keyword parameters are defined in
   [SIEVE].  As specified in Sections 2.7.1 and 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 5.

   Note that the transformations 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 the key
   strings, 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.

   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.

   If an individual content type begins or ends with a '/' (slash) or
   contains multiple slashes, then 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 a 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, treating the entire prologue and the entire epilogue as
   separate 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, treating the header as a single string; the contents of the
   nested message body parts are only searched if their content type
   matches the :content specification.

   For other MIME types, the entire part will be searched as a single
   string.

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

   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.

   - 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";
        }

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 this document.

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
                    body of the message being processed
   RFC number:      RFC 5173
   Contact Address: The Sieve discussion list
                    <ietf-mta-filters@imc.org>

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.

10.  References

10.1.  Normative References

   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, 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., Ed., and T. Showalter, Ed., "Sieve: An
                Email Filtering Language", RFC 5228, January 2008.

   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.

10.2.  Informative References

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

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

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.