The RFC Archive
 The RFC Archive   RFC 5463   « Jump to any RFC number directly 
 RFC Home
Full RFC Index
Recent RFCs
RFC Standards
Best Current Practice
RFC Errata
1 April RFC



IETF RFC 5463

Sieve Email Filtering: Ihave Extension

Last modified on Tuesday, March 10th, 2009

Permanent link to RFC 5463
Search GitHub Wiki for RFC 5463
Show other RFCs mentioning RFC 5463







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.

 Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

 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 [RFC 5228] 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 [RFC 3501] servers, as it has no user-controlled loops
   or the ability to run external programs.






Freed                       Standards Track                  PAGE 1 top


RFC 5463 Sieve Ihave Extension March 2009 Various sieve extensions have already been defined, e.g., [RFC 5229], [RFC 5230], [RFC 5231], [RFC 5232], [RFC 5233], [RFC 5235], 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 [RFC 2119]. The terms used to describe the various components of the Sieve language are taken from Section 1.1 of [RFC 5228]. 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 Freed Standards Track PAGE 2 top

RFC 5463 Sieve Ihave Extension March 2009 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 [RFC 5229]. 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. Freed Standards Track PAGE 3 top

RFC 5463 Sieve Ihave Extension March 2009 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 [RFC 5228], or variables [RFC 5229] 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 [RFC 5228] 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 Freed Standards Track PAGE 4 top

RFC 5463 Sieve Ihave Extension March 2009 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 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering Language", RFC 5228, January 2008. 8.2. Informative References [RFC 3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [RFC 5229] Homme, K., "Sieve Email Filtering: Variables Extension", RFC 5229, January 2008. [RFC 5230] Showalter, T. and N. Freed, "Sieve Email Filtering: Vacation Extension", RFC 5230, January 2008. [RFC 5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering: Relational Extension", RFC 5231, January 2008. [RFC 5232] Melnikov, A., "Sieve Email Filtering: Imap4flags Extension", RFC 5232, January 2008. [RFC 5233] Murchison, K., "Sieve Email Filtering: Subaddress Extension", RFC 5233, January 2008. Freed Standards Track PAGE 5 top

RFC 5463 Sieve Ihave Extension March 2009 [RFC 5235] 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 Freed Standards Track PAGE 6 top

Sieve Email Filtering: Ihave Extension RFC TOTAL SIZE: 12481 bytes PUBLICATION DATE: Tuesday, March 10th, 2009 LEGAL RIGHTS: The IETF Trust (see BCP 78)


RFC-ARCHIVE.ORG

© RFC 5463: The IETF Trust, Tuesday, March 10th, 2009
© the RFC Archive, 2024, RFC-Archive.org
Maintainer: J. Tunnissen

Privacy Statement