Internet Engineering Task Force T. Hansen
Internet-Draft AT&T Laboratories
Expires: August 31, 2006 C. Daboo
Cyrusoft International, Inc.
February 26, 2006
Sieve Extensions: MIME Tests, MIME Bodypart Iteration, Replacement and
Enclosure
draft-ietf-sieve-mime-loop-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The current Sieve language has no way to look at individual MIME
parts, looping mechanism, or any way to manipulate those individual
parts. This document defines extensions for each of these needs.
Note
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This document is being discussed in the MTA-FILTERS mailing lists,
ietf-mta-filters@imc.org.
1. Introduction
Sieve scripts are used to make decisions about the disposition of a
mail message. The original Sieve spec, [5], defined operators for
looking at the message headers, such as addresses and the subject.
Other extensions provide access to the body of the message, or allow
you to manipulate the header of the message. But none of these
extensions take into account that MIME messages ([1]) 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, changing the contents of a MIME
body part, and enclosing the message with a wrapper.
2. Sieve Loops
The current Sieve language has no looping mechanism. Given that
messages may contain multiple attachments, in order to support
filters that apply to any and all attachments, 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 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.
NOTE: Need to deal with this comment
(http://www.imc.org/ietf-mta-filters/mail-archive/msg02707.html):
What will this do: for.every.part { if (some test here) {
for.every.part { ... } } } i.e. will the internal "for" be anchored
at the focus established by the enclosing "for" ? I'd hope so, but
this should be stated.
NOTE: Need to deal with this comment
(http://www.imc.org/ietf-mta-filters/mail-archive/msg02707.html):
BTW, shouldn't there be a "mime" shorthand for testing the type/
subtype, without requiring if allof (mime :type "multipart", mime
:subtype "alternative") ? Especially for any recursive version of
"mime" (since the elements inside of "allof" can be matching
different mime parts).
The iterator can be terminated prematurely by a new sieve command,
"break".
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Usage: for_every_part block
Usage: break;
3. Test "mime"
Usage: mime [:anychild] [COMPARATOR] [MATCH-TYPE] <header-names:
string-list> [<parameter-names: string-list>] <key-list: string-
list>
Usage: mime [:filename] ...
Usage: mime [:type] ...
Usage: mime [:subtype] ...
For Sieve tests on MIME parts, a new Sieve test ("mime") is defined.
Similar in concept to the Sieve "header" test, it will parse the MIME
header lines so that tests can be performed on specific elements.
If :anychild is NOT specified:
If used within the context of a "for_every_part" iterator, the
"mime" test will examine the headers associated with the current
MIME part context.
If used outside the context of a "for_every_part" iterator, the
"mime" test will examine only the outer headers of the message.
If :anychild IS specified, the "mime" test will examine all MIME body
parts and return true if any of them satisfies the test.
The "mime" test has all of the options available from the header
test, [5] section 5.7. In addition, these options are available:
:filename examines the "Content-Disposition:" header field for its
"filename" parameter. If there is no "Content-Disposition:"
header field, then it will look at the "Content-Type:" header
field for the "name" parameter.
:type examines the "Content-Type:" header field type parameter.
:subtype examines the "Content-Type:" header field subtype parameter.
NOTE: We need to add some way to look at parameter lists, ala
http://www.imc.org/ietf-mta-filters/mail-archive/msg01655.html and
related messages: Content-Type: text/plain; charset="foo"
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4. Action Replace
Usage: replace ["mime"] [":subject" string] [":from" string]
<replacement: string>
A new sieve action command is defined to allow the MIME part to be
replaced by a text message. The "replace" command causes a MIME part
to be removed and replaced with the text supplied by the command.
When used in the context of a "for_every_part" loop, 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.)
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 [1] 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 [2] 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 [4]
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
specified in a ":from" parameter; it is suggested that values which
fail such a validity check simply be ignored rather than causing the
vacation action to fail. Implementations MUST preserve the previous
From header as an Original-From header.
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5. 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. This enclose action
takes precedance over all other message modifications, such as
"replace". 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.
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.
6. 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" test 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".
7. Examples
7.1. 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 ( mime :subtype :is "exe", mime :filename :matches "*.com" ) {
replace "Executable attachment removed by user filter";
}
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}
7.2. Example 2
A Sieve script to warn the user about executable attachment types
would be:
require [ "for_every_part", "mime", "enclose" ];
for_every_part {
if mime :filename :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;
}
}
8. Acknowledgements
Comments from members of the MTA Filters Working Group, in particular
Ned Freed, Nigel Swinson and Mark Mallett, are gratefully
acknowledged.
9. Security Considerations
To be provided
10. IANA Considerations
To be provided
11. Change History
11.1. draft-ietf-sieve-mime-00
Changed title to emphasize MIME Tests.
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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.
11.2. draft-hansen-sieve-loop-01
Merged with draft-daboo-sieve-mime-00.txt.
12. Normative References
[1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[2] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[5] Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028,
January 2001.
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Authors' Addresses
Tony Hansen
AT&T Laboratories
200 Laurel Ave.
Middletown, NJ 07748
USA
Email: tony+sieveloop@maillennium.att.com
Cyrus Daboo
Cyrusoft International, Inc.
5001 Baum Blvd.
Suite 780
Pittsburgh, PA 15213
USA
Email: daboo@cyrusoft.com
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