Sieve Email Filtering: Ihave Extension

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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.

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 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.

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 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.
   
   [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