Sieve Email Filtering: Environment Extension
RFC 5183

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].

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   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.

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   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.

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    "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".

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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.

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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.

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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>

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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.

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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

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