Sieve Email Filtering: Environment Extension

Sieve 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 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.

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14. The terms used to describe the various components of the Sieve language are taken from section 1.1.

The capability string associated with the extension defined in this document is "environment".

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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. 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 key-list 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. The "relational" extension 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. Environment items can be standardized or vendor-defined. An IANA registry is defined for both types of items.

The initial set of standardized environment items is as follows:

The primary DNS domain associated with the Sieve execution context, usually but not always a proper suffix of the host name. "evaluation-time" => Sieve processing is normally performing around or after the time of final delivery. This item provides additional information about the relationship to final delivery. Possible return values are "MTA", meaning the Sieve is being evaluated before final delivery, "MDA", meaning evaluation is occurring during final delivery", and "MUA", meaning evaluation is occurring after final delivery. "host" => The fully-qualified domain name of the host where the Sieve script is executing. "name" => The product name associated with the Sieve interpreter. "remote-host" => Host name of remote SMTP client, if applicable and available. "remote-ip" => IP address of remote SMTP client, if applicable and available. "version" => The product version associated with the Sieve interpreter. ]]> 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.

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. Environment item names starting with "vnd." SHOULD be followed by the name of the vendor and product, such as "vnd.acme.rocket-sled-status".

A registry of environment items is provided by IANA. Item names may be registered on a first-come, first-served basis. Extensions designed for interoperable use SHOULD be defined as standards track or IESG approved experimental RFCs.

The following template is to be used for registering new Sieve environment item names with IANA.

Multiple items and descripions MAY be specified in a single registration request. Both standardized and vender-defined items use this form.

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. All of the security considerations given in the base Sieve specification also apply to this extension.

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 &rfc.number; 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 . The initial contents of the registry are given in the following section.

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: evaluation-time Description: Time at which this Sieve processing is being performed. Capability name: host Description: The fully-qualified domain name of the host where the Sieve script is executing. Capability name: name Description: The product name associated with the Sieve interpreter. 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 &rfc.number; Contact address: Sieve discussion list <ietf-mta-filters@imc.org>