Network Working Group N. Freed
Internet-Draft Sun Microsystems
Expires: August 26, 2008 February 23, 2008
Sieve Email Filtering: Ihave Extension
draft-freed-sieve-ihave-01
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Abstract
This document describes the "ihave" extension to the Sieve email
filtering language. The "ihave" extension provides a means to write
scripts that can take advantage of optional Sieve features but can
still run when those optional features are not available. The
extension also defines a new error control command intended to be
used to report situations where no combination of available
extensions satifies the needs of the script.
Change History (to be removed prior to publication as an RFC
Changed the comparator used in the ihave test from "i;ascii-casemap"
to "i;octet".
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Updated the IANA registration template.
Simplified the semantics of ihave to be independent of block
structure.
Moved the environment extension to a separate document so the
standards status of the two extensions can be different.
Added error action.
1. Introduction
Sieve [RFC5228] is a language for filtering email messages at or
around the time of final delivery. It is designed to be
implementable on either a mail client or mail server. It is suitable
for running on a mail server where users may not be allowed to
execute arbitrary programs, such as on black box Internet Message
Access Protocol [RFC3501] servers, as it has no user-controlled loops
or the ability to run external programs.
Various sieve extensions have already been defined, e.g., [RFC5229]
[RFC5230] [RFC5231] [RFC5232] [RFC5233] [RFC5235], and many more are
sure to be created over time. Sieve's require clause is used to
specify the extensions a particular sieve needs; an error results if
the script's require clause calls for an extension that isn't
available. This mechanism is sufficient in most situations.
However, there can be cases where a script may be able to take
advantage of an extension if it is available but can still function
if it is not, possibly with some degradation of capabilities. Cases
can also arise where a script would prefer one extension but can
employ a different one if the first one is not available.
The "ihave" extension provides a means to write scripts that make use
of extensions only when they are actually available. Ihave defines a
new ihave test that takes a list of capability names as an argument
and succeeds if and only if all of the those capabilities are
present. Additionally, specification of the "ihave" extension in the
require clause disables parse time checking of extension use in
scripts; run-time checking must be used instead.
The "ihave" extension also defines a new error control command.
Error causes script execution to terminate with the error message
given as the argument to the error control.
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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 RFC 2119 [RFC2119].
The terms used to describe the various components of the Sieve
language are taken from [RFC5228] section 1.1.
3. Capability Identifiers
The capability string associated with the extension defined in this
document is "ihave".
4. Ihave Test
Usage: ihave <capabilities: string-list>
The ihave test provides a means for Sieve scripts to test for the
existence of a given extension prior to actually using it. The
capabilities argument to ihave is the same as the similarly-named
argument to the require control statement: It specifies the names of
one or more Sieve extensions or comparators. The ihave test succeeds
if all the extensions specified in the capabilities list are
available to the script.
Unlike most Sieve tests, ihave accepts no match or comparator
arguments. The type of match for ihave is always ":is" and the
comparator is always "i;octet".
The strings in the capabilities list are constant strings in context
of Sieve variables [RFC5229]. It is an error to pass a non-constant
string as an argument to ihave.
The Sieve base specification demands that that all Sieve extensions
used in a given script be specified in the initial require control
statement. It is an error for a script to call for extensions the
interpreter doesn't support or to attempt to use extensions that have
not been listed in the script's require clause. Using ihave changes
Sieve interpreter behavior and the underlying requirements in the
following ways:
1. Use of a given extension is allowed subsequent to the successful
evaluation of an ihave test on that extension. The extension
then operates just as if it had been specified in the script's
require clause. The extension cannot be used prior to the
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evaluation of such a test and a runtime error MUST be generated
if such usage is attempted.
2. Sieve interpreters normally have the option of checking extension
use at either parse time or execution time. The specification of
"ihave" in a script's require clause changes this behavior:
Scripts MUST either defer extension checking to run time or else
take the presence of ihave tests into account at parse time.
Note that since ihave can be used inside of anyof, allof, and not
tests full parse time checking of ihave may be very difficult to
implement.
3. Although it makes little sense to do so, an extension can be
specified in both the require control statement and in an ihave
test. If this is done and the extension has been implemented the
extension can be used anywhere in the script and an ihave test of
that extension will always return true.
4. Ihave accepts a list of capabilities. The ihave test fails and
none of the capabilities are enabled if any of the specified
capabilities are unavailable.
Ihave is designed to be used with extensions that add tests, actions,
or comparators. It MUST NOT be used with extensions that change how
the content of Sieve scripts are interpreted such as the variables
extension [RFC5229].
5. Error Control
Usage: error <message: string>
The error control causes script execution to terminate with a run-
time error. The message argument provides a text description of the
error condition that SHOULD be included in any generated report
regarding the error. Section 2.10.6 of [RFC5228] describes how run-
time errors are handled in Sieve.
The error control is included as part of the ihave extension so that
it is unconditionally available to scripts using ihave.
6. Security Considerations
A potential security issue with Sieve scripts is that when a script
fails to run due to the lack of some extension it may fail to block
dangerous email. The ihave extension makes it possible to improve
script portability and generality, which may improve the overall
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security provided by Sieve.
Script robustness aside, ihave is essentially a more flexible variant
of Sieve's existing require mechanism. As such, it does not add any
additional capabilities to a Sieve implementation that could create
security issues. Of course all of the security considerations given
in the base Sieve specification and in any extensions that are
employed are still relevant.
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: ihave
Description: The "ihave" extension provides a means to write
scripts that make use of other extensions only
when they are actually available.
RFC number: RFC XXXX
Contact address: Sieve discussion list <ietf-mta-filters@imc.org>
8. References
8.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
8.2. Informative references
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
[RFC5230] Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", RFC 5230, January 2008.
[RFC5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering:
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Relational Extension", RFC 5231, January 2008.
[RFC5232] Melnikov, A., "Sieve Email Filtering: Imap4flags
Extension", RFC 5232, January 2008.
[RFC5233] Murchison, K., "Sieve Email Filtering: Subaddress
Extension", RFC 5233, January 2008.
[RFC5235] Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
Extensions", RFC 5235, January 2008.
Author's Address
Ned Freed
Sun Microsystems
3401 Centrelake Drive, Suite 410
Ontario, CA 92761-1205
USA
Phone: +1 909 457 4293
Email: ned.freed@mrochek.com
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