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rfc:rfc5183

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.

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Freed Standards Track [Page 10]

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