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Network Working Group N. Freed
Request for Comments: 5183 Sun Microsystems
Category: Standards Track May 2008
Sieve Email Filtering: Environment 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.
Abstract
This document describes the "environment" extension to the Sieve
email filtering language. The "environment" extension gives a Sieve
script access to information about the Sieve interpreter itself,
where it is running, and about any transport connection currently
involved in transferring the message.
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.
Although Sieve is intended to be independent of access protocol, mail
architecture, and operating system, in some cases it is useful to
allow scripts to access information about their execution context.
The "environment" extension provides a new environment test that can
be used to implement scripts that behave differently when moved from
one system to another, when messages arrive from different remote
sources or when otherwise operated in different contexts.
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].
Freed Standards Track [Page 1]
RFC 5183 Sieve Environment Extension May 2008 The terms used to describe the various components of the Sieve language are taken from Section 1.1 of [RFC5228]. This document refers to the ABNF productions IPv4-address-literal, IPv6-address-literal, and General-address-literal defined in Section 4.1.3 of [RFC2821]. The location item makes use of standard terms for email service components. Additional information and background on these terms can be found in [EMAIL-ARCH]. 3. Capability Identifiers The capability string associated with the extension defined in this document is "environment". 4. Environment Test Usage: environment [COMPARATOR] [MATCH-TYPE] <name: string> <key-list: string-list> The environment test retrieves the item of environment information specified by the name string and matches it to the values specified in the key-list argument. The test succeeds if a match occurs. The type of match defaults to ":is" and the default comparator is "i;ascii-casemap". The current message is not a direct source of information for the environment test; the item of information specified by the name string is extracted from the script's operating environment and the key-list argument comes from the script. The environment test MUST fail unconditionally if the specified information item does not exist. A script MUST NOT fail with an error if the item does not exist. This allows scripts to be written that handle nonexistent items gracefully. In particular, the test: if environment :contains "item" "" { ... } only succeeds if "item" is known to the implementation, and always succeeds if it is. The "relational" extension [RFC5231] adds a match type called ":count". The count of an environment test is 0 if the environment information returned is the empty string, or 1 otherwise. Freed Standards Track [Page 2]
RFC 5183 Sieve Environment Extension May 2008 Environment items can be standardized or vendor-defined. An IANA registry is defined for both types of items. Extensions designed for interoperable use SHOULD be defined in standards track or experimental RFCs. 4.1. Initial Standard Environment Items The initial set of standardized environment items is as follows: "domain" => The primary DNS domain associated with the Sieve execution context, usually but not always a proper suffix of the host name. "host" => The fully-qualified domain name of the host where the Sieve script is executing. "location" => Sieve evaluation can be performed at various different points as messages are processed. This item provides additional information about the type of service that is evaluating the script. Possible values are "MTA", meaning the Sieve is being evaluated by a Message Transfer Agent, "MDA", meaning evaluation is being performed by a Mail Delivery Agent, "MUA", meaning evaluation is being performed by a Mail User Agent, and "MS", meaning evaluation is being performed by a Message Store. Additional information and background on these terms can be found in [EMAIL-ARCH]. "name" => The product name associated with the Sieve interpreter. "phase" => The point relative to final delivery where the Sieve script is being evaluated. Possible values are "pre", "during", and "post", referring respectively to processing before, during, and after final delivery has taken place. "remote-host" => Host name of remote SMTP/LMTP/Submission client expressed as a Fully Qualified Domain Name (FQDN), if applicable and available. The empty string will be returned if for some reason this information cannot be obtained for the current client. Freed Standards Track [Page 3]
RFC 5183 Sieve Environment Extension May 2008 "remote-ip" => IP address of remote SMTP/LMTP/Submission client, if applicable and available. IPv4, IPv6, and other types of addresses are respectively represented in the formats defined by the IPv4-address-literal, IPv6-address-literal, and General-address-literal productions defined in Section 4.1.3 of [RFC2821]. "version" => The product version associated with the Sieve interpreter. The meaning of the product version string is product-specific and should always be considered in the context of the product name given by the "name" item. Implementations SHOULD support as many of the items on this initial list as possible. Additional standardized items can only be defined in standards-track or experimental RFCs. 4.2. Vendor-defined Environment Items Environment item names beginning with "vnd." represent vendor-defined extensions. Such extensions are not defined by Internet standards or RFCs, but are still registered with IANA in order to prevent conflicts. 4.3. IANA Registration of Environment Items A registry of environment items is provided by IANA. Item names may be registered on a first-come, first-served basis. Groups of items defined in a standards track or experimental RFC MAY choose to use a common name prefix of the form "name.", where "name" is a string that identifies the group of related items. Items not defined in a standards track or experimental RFC MUST have a name that begins with the "vnd." prefix, and this prefix is followed by the name of the vendor or product, such as "vnd.acme.rocket-sled-status". Freed Standards Track [Page 4]
RFC 5183 Sieve Environment Extension May 2008 4.3.1. Template for Environment Registrations The following template is to be used for registering new Sieve environment item names with IANA. To: iana@iana.org Subject: Registration of new Sieve environment item Item name: [the string for use in the 'environment' test] Description: [a brief description of the semantics of the value the item returns] RFC number: [for extensions published as RFCs] Contact address: [email and/or physical address to contact for additional information] Multiple items and descriptions MAY be specified in a single registration request. Both standardized and vendor-defined items use this form. 5. Security Considerations The environment extension may be used to obtain information about the system the Sieve implementation is running on. This information in turn may reveal details about service provider or enterprise infrastructure. An implementation can use any technique to determine the remote-host environment item defined in this specification, and the trustworthiness of the result will vary. One common method will be to perform a PTR DNS lookup on the client IP address. This information may come from an untrusted source. For example, the test: if environment :matches "remote-host" "*.example.com" { ... } is not a good way to test whether the message came from "outside" because anyone who can create a PTR record can create one that refers to whatever domain they choose. All of the security considerations given in the base Sieve specification also apply to this extension. Freed Standards Track [Page 5]
RFC 5183 Sieve Environment Extension May 2008 6. 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: environment Description: The "environment" extension provides a new environment test that can be used to implement scripts that behave differently when moved from one system to another or otherwise operated in different contexts. RFC number: RFC 5183 Contact address: Sieve discussion list <ietf-mta-filters@imc.org> This specification also defines a new IANA registry for Sieve environment item names. The specifics of this registry are given in Section 4.3. The initial contents of the registry are given in the following section. Freed Standards Track [Page 6]
RFC 5183 Sieve Environment Extension May 2008 6.1. Initial Environment Item Registrations The following template specifies the initial IANA registrations for the environment items defined in this document: To: iana@iana.org Subject: Registration of new Sieve environment items Capability name: domain Description: The primary DNS domain associated with the Sieve execution context, usually but not always a proper suffix of the host name. Capability name: host Description: The fully-qualified domain name of the host where the Sieve script is executing. Capability name: location Description: Type of service executing the Sieve script. Capability name: name Description: The product name associated with the Sieve interpreter. Capability name: phase Description: Point relative to final delivery at which the Sieve script is being evaluated. Capability name: remote-host Description: Host name of remote SMTP client, if applicable and available. Capability name: remote-ip Description: IP address of remote SMTP client, if applicable and available. Capability name: version Description: The product version associated with the Sieve interpreter. RFC number: RFC 5183 Contact address: Sieve discussion list <ietf-mta-filters@imc.org> Freed Standards Track [Page 7]
RFC 5183 Sieve Environment Extension May 2008 7. References 7.1. Normative references [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering Language", RFC 5228, January 2008. [RFC5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering: Relational Extension", RFC 5231, January 2008. 7.2. Informative references [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", Work in Progress, February 2008. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. Freed Standards Track [Page 8]
RFC 5183 Sieve Environment Extension May 2008 Appendix A. Acknowledgements Brian Carpenter, Dave Crocker, Cyrus Daboo, Philip Guenther, Kjetil Torgrim Homme, John Klensin, Mark Mallett, Alexey Melnikov, and Dilyan Palauzo provided helpful suggestions and corrections. Author's Address Ned Freed Sun Microsystems 3401 Centrelake Drive, Suite 410 Ontario, CA 92761-1205 USA Phone: +1 909 457 4293 EMail: ned.freed@mrochek.com Freed Standards Track [Page 9]
RFC 5183 Sieve Environment Extension May 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Freed Standards Track [Page 10]