Network Working Group N. Freed
Internet-Draft Sun Microsystems
Intended status: Standards Track October 9, 2008
Expires: April 12, 2009
Sieve Email Filtering: Ihave Extension
draft-freed-sieve-ihave-03
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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 satisfies 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.
Added some text to make the portability advantages of ihave clearer.
Added a note about the possibility that the argument to error uses
UTF-8 characters.
(from WGLC) Various editorial fixups.
(from WGLC) Incorporated the same short-circuit, left to right
requirements the variables extension imposes, because without it
invocation of the variables extension could potentially change the
meaning of ihave constructs in anyof or allof clauses.
(from WGLC) Added a resriction that ihave MUST NOT be used with any
extension that changes the underlying Sieve grammar. Hopefully there
won't be any such extensions, but better safe than sorry.
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 operate if
it is not, possibly with some degradation of functionality. Cases
can also arise where a script would prefer one extension but can
employ a different one if the first one is not available.
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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 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. This makes it possible to write
portable scripts that can operate in multiple environments making
effective use of whatever extensions are available even though
differing sets of extensions are provided in different places.
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.
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 [RFC2119].
The terms used to describe the various components of the Sieve
language are taken from Section 1.1 of [RFC5228].
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".
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The strings in the capabilities list are constant strings in the
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 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 all the way to the
end of the script, even outside the block enclosed by the ihave
test. In other words, subsequent to a successful ihave things
operate just as if the extension had been specified in the
script's require clause. The extension cannot be used prior to
the evaluation of such a test and a runtime error MUST be
generated if such usage is attempted. However, subsequent use of
that extension may still need to be conditionally handled via an
ihave test to deal with the case where it is not supported. The
extension cannot be used prior to the 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.
5. The Sieve base specification does not require that interpreters
evaluate arguments in any particular order or that test
evaluation be short-circuited. If ihave is enabled the
interpreter MUST short-circuit tests, i.e., not perform more
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tests than necessary to find the result. Additionally,
evaluation order MUST be left to right if ihave is enabled.
Ihave is designed to be used with extensions that add tests, actions,
comparators, or arguments. It MUST NOT be used with extensions that
change the underlying Sieve grammer or extensions like variables
[RFC5229] that change how the content of Sieve scripts are
interpreted.
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.
Note that the message argument, like all Sieve strings, employs the
UTF-8 charset and can contain non-US-ASCII characters. This must be
taken into consideration when reporting script errors.
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
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.
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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:
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.
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Appendix A. Acknowledgements
Stephan Bosch, Cyrus Daboo, Arnt Gulbrandsen, Andrew McKeon, and
Alexey Melnikov provided helpful suggestions and corrections.
Author's Address
Ned Freed
Sun Microsystems
800 Royal Oaks
Monrovia, CA 91016-6347
USA
Phone: +1 909 457 4293
Email: ned.freed@mrochek.com
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