Network Working Group J. Degener
Request for Comments: 3894 Sendmail, Inc.
Category: Standards Track October 2004
Sieve Extension: Copying Without Side Effects
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.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
The Sieve scripting language allows users to control handling and
disposal of their incoming e-mail. By default, an e-mail message
that is processed by a Sieve script is saved in the owner's "inbox".
Actions such as "fileinto" and "redirect" cancel this default
behavior.
This document defines a new keyword parameter, ":copy", to be used
with the Sieve "fileinto" and "redirect" actions. Adding ":copy" to
an action suppresses cancellation of the default "inbox" save. It
allows users to add commands to an existing script without changing
the meaning of the rest of the script.
1. Introduction
The Sieve scripting language [SIEVE] allows users to control handling
and disposal of their incoming e-mail. Two frequently used Sieve
commands are "fileinto" (saving into a local message store, such as
an IMAP server) and "redirect" (forwarding to another e-mail
address). Both of these cancel the Sieve default behavior of saving
into the user's "inbox".
But some users have the notion of forwarding an extra copy of a
message for safekeeping to another e-mail address, or of saving a
copy in a folder - in addition to the regular message delivery, which
shouldn't be affected by the copy.
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If saving an extra copy is all the user wanted to do,
fileinto "unfiltered";
keep;
would do the job. The "keep" command does explicitly what the
cancelled default behavior did. But the explicit "keep" is a poor
substitute for the implicit "keep" when more processing follows:
fileinto "unfiltered";
keep;
if header "Subject" "MAKE MONEY FAST!!!"
{
discard;
}
In this example, the "discard" is ineffective against the explicit
"keep"; the discarded message still ends up in the user's inbox.
It is possible to generate Sieve code that perfectly expresses a
user's wishes, but such code quickly grows unwieldy because it needs
to keep track of the state that the implicit "keep" would have had
without the "fileinto" or "redirect" command.
This extension tries to make life easier for user interface designers
and script writers by allowing them to express the "copy" semantics
directly.
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 "copy".
3. ":copy" extension to the "fileinto" and "redirect" commands
Syntax:
"fileinto" [":copy"] <folder: string>
"redirect" [":copy"] <address: string>
If the optional ":copy" keyword is specified with "fileinto" or
"redirect", the tagged command does not cancel the implicit "keep".
Instead, it merely files or redirects a copy in addition to whatever
else is happening to the message.
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Example:
require ["copy", "fileinto"];
fileinto :copy "incoming";
# ... more processing follows ...
4. Security Considerations
The "copy" extension makes it easier to eavesdrop on a user's message
stream without the user noticing. This was technically possible
before if an attacker gained read/write access to a user's Sieve
scripts, but now an attacker no longer needs to parse a script in
order to modify it. Write access to Sieve scripts must be protected
as strongly as read/write access to e-mail, for example by using
secure directory protocols such as correctly parameterized LDAP over
TLS [LDAP].
Organizations that wish to monitor their users' e-mail traffic must
familiarize themselves with local data protection laws before
creating stores of old e-mail traffic without control, or perhaps
even knowledge, of the sender or intended recipients.
Organizations that legally use "redirect :copy" to eavesdrop on
correspondence (for example, by keeping a log to answer questions
about insider trading at a later time) can avoid future problems by
setting users' privacy expectations correctly.
5. IANA Considerations
The following template specifies the IANA registration of the "copy"
Sieve extension specified in this document.
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: copy
Capability keyword: copy
Capability arguments: N/A
Standards Track: RFC 3894
Person and email address to contact for further information:
Jutta Degener
Sendmail, Inc.
6425 Christie Ave, 4th Floor
Emeryville, CA 94608
Email: jutta@sendmail.com
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This information has been added to the list of Sieve extensions given
on http://www.iana.org/assignments/sieve-extensions.
6. Acknowledgments
Thanks to Eric Allman, Ned Freed, Will Lee, Nigel Swinson, and Rand
Wacker for corrections and comments.
7. References
7.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SIEVE] Showalter, T., "Sieve: A Mail Filtering Language", RFC
3028, January 2001.
7.2. Informative References
[LDAP] Wahl, M., Alvestrand, H., Hodges, J., and R. Morgan,
"Authentication Methods for LDAP", RFC 2829, May 2000.
Author's Address
Jutta Degener
Sendmail, Inc.
6425 Christie Ave, 4th Floor
Emeryville, CA 94608
EMail: jutta@sendmail.com
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