Network Working Group N. Freed
Request for Comments: 5463 Sun Microsystems
Category: Standards Track March 2009
Sieve Email Filtering: Ihave Extension
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.
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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.
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.
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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.
The "ihave" extension provides a means to write scripts that make use
of extensions only when they are actually available. It 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. An
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
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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 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 run-time
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.
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.
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4. The "ihave" test accepts a list of capabilities. If any of the
specified capabilities are unavailable, the test fails and none
of the capabilities are enabled.
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
tests than necessary to find the result. Additionally,
evaluation order MUST be left to right if "ihave" is enabled.
The "ihave" extension is designed to be used with other extensions
that add tests, actions, comparators, or arguments. Implementations
MUST NOT allow it to be used with extensions that change the
underlying Sieve grammar, or extensions like encoded-character
[RFC5228], or variables [RFC5229] that change how the content of
Sieve scripts are interpreted. The test MUST fail and the extension
MUST NOT be enabled if such usage is attempted.
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
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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 5463
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.
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[RFC5235] Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
Extensions", RFC 5235, January 2008.
9. Acknowledgments
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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