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Network Working Group                                           N. FreedRequest for Comments: 5463                              Sun MicrosystemsCategory: Standards Track                                     March 2009Sieve Email Filtering:  Ihave ExtensionStatus 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 toBCP 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 [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.Freed                       Standards Track                     [Page 1]

RFC 5463                 Sieve Ihave Extension                March 2009   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 fromSection 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-namedFreed                       Standards Track                     [Page 2]

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 [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.Freed                       Standards Track                     [Page 3]

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   [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 createFreed                       Standards Track                     [Page 4]

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.  References8.1.  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.Freed                       Standards Track                     [Page 5]

RFC 5463                 Sieve Ihave Extension                March 2009   [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.comFreed                       Standards Track                     [Page 6]