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
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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.
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