Internet Engineering Task Force T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Standards Track C. Daboo
Expires: August 26, 2008 Apple Inc.
February 23, 2008
Sieve Email Filtering: MIME part Tests, Iteration, Extraction,
Replacement and Enclosure
draft-ietf-sieve-mime-loop-04
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document defines extensions to the Sieve email filtering
language to permit analysis and manipulation of the MIME body parts
of an email message.
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Note
This document is being discussed on the MTA-FILTERS mailing list,
ietf-mta-filters@imc.org.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Sieve Loops . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Changes to Sieve tests . . . . . . . . . . . . . . . . . . . . 4
4.1. Test "header" . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Test "address" . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Test "exists" . . . . . . . . . . . . . . . . . . . . . . 7
5. Action Replace . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Action Enclose . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Action extract_text . . . . . . . . . . . . . . . . . . . . . 9
8. Sieve Capability Strings . . . . . . . . . . . . . . . . . . . 9
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 10
9.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 10
9.3. Example 3 . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 12
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
12.1. for_every_part capability . . . . . . . . . . . . . . . . 13
12.2. mime capability . . . . . . . . . . . . . . . . . . . . . 13
12.3. replace capability . . . . . . . . . . . . . . . . . . . . 14
12.4. enclose capability . . . . . . . . . . . . . . . . . . . . 14
12.5. extract_text capability . . . . . . . . . . . . . . . . . 14
13. Change History (to be removed prior to publication as an
RFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
13.1. draft-ietf-sieve-mime-02 . . . . . . . . . . . . . . . . . 15
13.2. draft-ietf-sieve-mime-01 . . . . . . . . . . . . . . . . . 15
13.3. draft-ietf-sieve-mime-00 . . . . . . . . . . . . . . . . . 15
13.4. draft-hansen-sieve-loop-01 . . . . . . . . . . . . . . . . 16
13.5. draft-hansen-sieve-loop-02 . . . . . . . . . . . . . . . . 16
13.6. draft-hansen-sieve-loop-03 . . . . . . . . . . . . . . . . 16
13.7. draft-sieve-mime-loop-04 . . . . . . . . . . . . . . . . . 16
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
MIME messages ([RFC2045]) 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, extracting information from a MIME body part, changing the
contents of a MIME body part, and enclosing the entire message within
a wrapper.
2. Conventions Used in This Document
Conventions for notations are as in [RFC5228] section 1.1.
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 [RFC2119].
3. Sieve Loops
The base Sieve language has no looping mechanism. Given that
messages may contain multiple parts, in order to support filters that
apply to any and all parts, 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, depth first, and applies
the commands in 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.
The iterator can be terminated prematurely by a new Sieve command,
"break".
Usage: for_every_part block
Usage: break;
"for_every_part" commands can be nested inside other "for_every_part"
commands. When this occurs, the nested "for_every_part" iterates
over the MIME parts contained within the MIME part currently being
targeted by the nearest enclosing "for_every_part" command. If that
MIME part is a terminal MIME part (i.e. does not contain other MIME
parts) then the nested "for_every_loop" is simply ignored.
Sieve implementations MAY limit the number of nested loops that occur
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within one another, however they MUST support at least one nested
loop inside another loop.
4. Changes to Sieve tests
This specification extends the base Sieve "header", "address" and
"exists" tests to support targeting those tests at a specific MIME
part or at all MIME parts in the enclosing scope.
4.1. Test "header"
The "header" test is extended with the addition of a new ":mime"
tagged argument and its associated options.
Usage: header [:mime] [:anychild] [MIMEOPTS]
[COMPARATOR] [MATCH-TYPE]
<header-names: string-list> <key-list: string-list>
Usage: The definition of [MIMEOPTS] is:
Syntax: ":type" / ":subtype" / ":contenttype" /
":param" <param-list: string-list>
When the ":mime" tagged argument is present in the "header" test, it
will parse the MIME header lines in the message so that tests can be
performed on specific elements.
When used outside the context of a "for_every_part" iterator, and
without an ":anychild" tagged argument, the "header" test will
examine only the outer top-level RFC2822 headers of the message.
When used inside the context of a "for_every_part" iterator, and
without an ":anychild" tagged argument, the "header" test will
examine the headers associated with the current MIME part context
from the loop.
When used outside the context of a "for_every_part" iterator, and
with an ":anychild" tagged argument, the "header" test will examine
all MIME body parts and return true if any of them satisfies the
test.
When used inside the context of a "for_every_part" iterator, and with
an ":anychild" tagged argument, the "header" test will examine the
current MIME part context and all it's nested MIME body parts,
returning true if any of them satisfies the test.
The "header" test with the ":mime" tagged argument can test various
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aspects of certain structured MIME headers. These options are
available:
:type parses the header assuming it has the format of a "Content-
Type:" MIME header field, and tests the value of the MIME type
specified in the header.
:subtype parses the header assuming it has the format of a "Content-
Type:" MIME header field, and tests the value of the MIME subtype
specified in the header.
:contenttype parses the header assuming it has the format of a
"Content-Type:" MIME header field, and tests the combined value of
the MIME type and subtype specified in the header.
:param parses the header looking for MIME parameters in the header.
The supplied string-list lists the names of any parameters to be
tested. If any one named parameter value matches the test string
value, the test will return true.
Example:
require ["mime", "fileinto"];
if header :mime :type "Content-Type" "image"
{
fileinto "INBOX.images";
}
In this example, any message that contains a MIME image type part at
the top-level is saved to the mailbox "INBOX.images".
Example:
require ["mime", "fileinto"];
if header :mime :anychild :contenttype
"Content-Type" "text/html"
{
fileinto "INBOX.html";
}
In this example, any message that contains any MIME part with a
content-type of "text/html" is saved to the mailbox "INBOX.html".
Example:
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require ["mime", "for_every_part", "fileinto"];
for_every_part
{
if allof (
header :mime :param "filename" :contains
"Content-Disposition" "important",
header :mime :subtype "Content-Type" "pdf",
size :over "100K")
{
fileinto "INBOX.important";
break;
}
}
In this example, any message that contains a MIME part that has a
content-disposition with a filename parameter containing the text
"important", has a content-subtype of "pdf" and is bigger than 100 Kb
is saved to the mailbox "INBOX.important".
4.2. Test "address"
The "address" test is extended with the addition of a new ":mime"
tagged argument, which takes a number of other arguments.
Usage: address [:mime] [:anychild] [COMPARATOR]
[ADDRESS-PART] [MATCH-TYPE]
<header-list: string-list> <key-list: string-list>
When the ":mime" tagged argument is present in the "address" test, it
will parse the MIME header lines as if they were standard address
header lines in a message so that tests can be performed on specific
elements.
The behavior of the ":anychild" tagged argument and the interaction
with the "for_every_part" iterator is the same as for the extended
"header" test Section 4.1.
Example:
require ["mime", "fileinto"];
if address :mime :is :all "content-from" "tim@example.com"
{
fileinto "INBOX.part-from-tim";
}
In this example, any message that contains a MIME Content-From header
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at the top-level matching the text "tim@example.com" is saved to the
mailbox "INBOX.part-from-time".
4.3. Test "exists"
The "exists" test is extended with the addition of a new ":mime"
tagged argument, which takes one other argument.
Usage: exists [:mime] [:anychild] <header-names: string-list>
When the ":mime" tagged argument is present in the "exists" test, the
test is extended to check for the existence of MIME headers in MIME
parts.
The behavior of the ":anychild" tagged argument and the interaction
with the "for_every_part" iterator is the same as for the extended
"header" test Section 4.1.
Example:
require ["mime", "fileinto"];
if exists :mime :anychild "content-md5"
{
fileinto "INBOX.md5";
}
In this example, any message that contains a MIME Content-MD5 header
in any MIME part is saved to the mailbox "INBOX.md5".
5. Action Replace
Usage: replace [:mime] [:subject string] [:from string]
<replacement: string>
The "replace" command is defined to allow a MIME part to be replaced
with the text supplied in the command.
When used in the context of a "for_every_part" iterator, 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.) If the MIME structure is
altered, the change takes effect immediately: the "for_every_part"
iterator that is executing does not go into the no-longer existing
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body parts, and subsequent "for_every_part" iterators would use the
new 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 in UTF-8.
If the :mime parameter is specified, then the replacement string is,
in fact, a MIME entity as defined in [RFC2045] section 2.4, including
both MIME headers and content.
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 [RFC2047] 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 [RFC2822]
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 be
specified in a ":from" parameter; it is suggested that values that
fail such a validity check simply be ignored rather than causing the
replace action to fail. Implementations MUST preserve the previous
From header as an Original-From header.
6. 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. After enclosure,
subsequent actions affecting the message header or content use the
newly created message instead of the original message; this means
that any use of a "replace" action or other similar actions should be
executed before the "enclose" action.
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.
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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. If not specified by :headers, Date: and From:
headers should be synthesized to reflect the current date and the
user running the Sieve action.
7. Action extract_text
Usage: extract_text [MODIFIER] [":first" number] <varname: string>
The extract_text action may be used within the context of a
"for_every_part" loop. Servers MUST support transcoding of any
textual body part into UTF-8 for use with this action. This requires
decoding any transfer encoding as well as transcoding from the
indicated character set into UTF-8. It stores at most :first
characters of the transcoded content of the current MIME body part in
the variable identified by varname. If the :first parameter is not
present, the whole content of the current MIME body part is stored.
In either case the actually stored data MAY be truncated to conform
to implementation-specific limit on variable length and/or on MIME
body part length. If the transfer encoding or character set is
unrecognized by the implementation or recognized but invalid, an
empty string will result.
If extract_text is used outside the context of a "for_every_part"
loop, the action will set the variable identified by varname to the
empty string.
Modifiers are applied on the extracted text before it is stored in
the variable. See [RFC5229] for details.
8. 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" and ":anychild"
tagged arguments to the "header", "address" and "exists" tests will
advertise the capability string "mime".
A Sieve implementation that defines the "replace" action will
advertise the capability string "replace".
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A Sieve implementation that defines the "enclose" action will
advertise the capability string "enclose".
A Sieve implementation that defines the "extract_text" action will
advertise the capability string "extract_text". Note that to be
useful, the "extract_text" action also requires the "variables"
[RFC5229] and "for_every_part" capabilities.
9. Examples
9.1. 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 (
header :mime :contenttype :is "Content-Type" "application/exe",
header :mime :param "filename"
["Content-Type", "Content-Disposition"] :matches "*.com" )
{
replace "Executable attachment removed by user filter";
}
}
9.2. Example 2
A Sieve script to warn the user about executable attachment types
would be:
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require [ "for_every_part", "mime", "enclose" ];
for_every_part
{
if header :mime :param "filename"
["Content-Type", "Content-Disposition"] :matches
["*.com", "*.exe", "*.vbs", "*.scr",
"*.pif", "*.hta", "*.bat", "*.zip" ]
{
# these attachment types are executable
enclose :subject "Warning" :text
WARNING! The enclosed message contains executable attachments.
These attachments types may contain a computer virus program
that can infect your computer and potentially 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;
}
}
9.3. Example 3
A Sieve script to extract subject and text out of messages from the
boss:
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require ["mime", "variables", "extract_text"];
if header :contains "from" "boss@example.org"
{
# :matches is used to get the value of the Subject header
if header :matches "Subject" "*"
{
set "subject" "${1}";
}
# extract the first 100 characters of the first text/* part
for_every_part
{
if header :mime :type :is "Content-Type" "text"
{
extract_text :first 100 "msgcontent";
break;
}
}
# if it's not a 'for your information' message
if not header :contains "subject" "FYI:"
{
# do something using ${subject} and ${msgcontent}
# such as sending a notification using a
# notification extensions
}
}
10. Acknowledgements
Comments from members of the MTA Filters Working Group, in particular
Ned Freed, Kjetil Torgrim Homme, Mark Mallett, Alexey Melnikov, Aaron
Stone and Nigel Swinson are gratefully acknowledged.
11. Security Considerations
The "enclose" action creates an entirely new message, as compared to
just redirecting or forwarding the existing message. Therefore, any
site policies applicable to message submission should be enforced.
The looping specification specified here provides easier access to
information about the message contents, which may also be achieved
through other sieve tests. This is not believed to raise any
additional security issues beyond those for the Sieve "envelope" and
"body" [I-D.ietf-sieve-body] tests.
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The system MUST be sized and restricted in such a manner that even
malicious use of mime part matching does not deny service to other
users of the host system.
Any change in a message content may interfere with digital signature
mechanisms that include the body in the signed material.
All of the security considerations given in the base Sieve
specification also apply to these extensions.
12. IANA Considerations
The Original-Subject: and Original-From: headers are to be registered
in the Permanent Message Header Fields table.
The following templates specify the IANA registrations of the Sieve
extensions specified in this document. This information should be
added to the list of sieve extensions given on
http://www.iana.org/assignments/sieve-extensions.
12.1. for_every_part capability
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: for_every_part
Description: adds the "for_every_part" and "break" actions for
iterating through MIME parts of a message.
RFC number: This RFC
Contact address: The Sieve discussion list
<ietf-mta-filters@imc.org>.
12.2. mime capability
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: mime
Description: adds the ":mime" and ":anychild" tagged arguments to the
"header", "address" and "exists" tests.
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RFC number: This RFC
Contact address: The Sieve discussion list
<ietf-mta-filters@imc.org>.
12.3. replace capability
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: mime
Description: adds the "replace" action for replacing a MIME body part
of a message.
RFC number: This RFC
Contact address: The Sieve discussion list
<ietf-mta-filters@imc.org>.
12.4. enclose capability
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: mime
Description: adds the "enclose" action for enclosing a message with a
wrapper.
RFC number: This RFC
Contact address: The Sieve discussion list
<ietf-mta-filters@imc.org>.
12.5. extract_text capability
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: mime
Description: adds the "extract_text" action for extracting text from
a MIME body part.
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RFC number: This RFC
Contact address: The Sieve discussion list
<ietf-mta-filters@imc.org>.
13. Change History (to be removed prior to publication as an RFC)
13.1. draft-ietf-sieve-mime-02
minor syntax glitches in examples
Add clarification on "replace" affecting subsequent for_every_part
loops?
Add IANA considerations for Original-Subject: and Original-From:.
Add note on "enclose" creating From: and Date: headers.
13.2. draft-ietf-sieve-mime-01
what happens when nested for_every_loop's
a "mime" shorthand for testing the type/subtype, without requiring
interactions with variables
notifications
notifications to calendar service
address tests, exists tests
mimeheader, mimeparameter tests
13.3. draft-ietf-sieve-mime-00
Changed title and text to emphasize MIME Tests.
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.
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13.4. draft-hansen-sieve-loop-01
Merged with draft-daboo-sieve-mime-00.txt.
13.5. draft-hansen-sieve-loop-02
Update to 3028bis reference.
Added 2119 conventions section.
Terminology/title tweaks.
Added informative references to body and editheader extensions.
Added description of nested loops.
Replaced mime test by extensions to header, address and exists
tests.
13.6. draft-hansen-sieve-loop-03
after enclosure, subsequent actions affect newly created message
synthesis of Date/From headers by the enclose action is no longer
controversial
Filled in Security Considerations
Picked up extract_text action from draft-ietf-sieve-notify
Expanded the IANA considerations section
13.7. draft-sieve-mime-loop-04
update reference for recent published rfcs
extract-text now required to do decode transfer encoding and
transcode to UTF-8
removed editheader reference since its not actually used
several text changes as suggested by Nigel Swinson, including re-
writes to abstract and introduction
tweaked IANA registrations
14. References
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14.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
14.2. Informative References
[I-D.ietf-sieve-body]
Guenther, P. and J. Degener, "Sieve Email Filtering: Body
Extension", draft-ietf-sieve-body-07 (work in progress),
December 2007.
[RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
Authors' Addresses
Tony Hansen
AT&T Laboratories
200 Laurel Ave.
Middletown, NJ 07748
USA
Email: tony+sieveloop@maillennium.att.com
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Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Email: cyrus@daboo.name
URI: http://www.apple.com/
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