Sieve Extension: Externally Stored Lists
draft-melnikov-sieve-external-lists-00

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Abstract

Sieve scripting language can be used for implementing of whitelisting, blacklisting and personal distribution lists. Currently this requires that all members of such lists be hardcoded in the script itself. Whenever a member of such list is added or deleted, the script needs to be updated and possibly uploaded to a mail server.

This document defines a Sieve extension for accessing externally stored mailing lists, i.e. list whose members are stored externally to the script, for example in LDAP (RFC 4510), ACAP (RFC 2244) or a relational database.

Table of Contents

1.  Introduction
1.1.  Conventions used in this document

2.  Extlists extension
2.1.  Capability Identifier
2.2.  :list tagged argument to the 'address', 'header' and 'envelope' tests
2.3.  :list tagged argument to the 'redirect' action
2.4.  Syntax of an externally stored list name
2.5.  Examples

3.  Security Considerations

4.  IANA Considerations

5.  Acknowledgements

6.  References
6.1.  Normative References
6.2.  Informative References

§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

This document specified an extension to the Sieve language defined by [Sieve] (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” August 2006.) for checking membership in an externally stored list or for sending messages to a list of recipients stored externally to the Sieve script.

This extension adds a new tagged argument to the "header" and "envelope" tests, and to the "redirect" action [Sieve] (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” August 2006.).

1.1.  Conventions used in this document

Conventions for notations are as in [Sieve] (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” August 2006.) section 1.1, including the use of [ABNF] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.).

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 [Kwds] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Extlists extension

2.1.  Capability Identifier

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

2.2.  :list tagged argument to the 'address', 'header' and 'envelope' tests

Usage:

address [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE] [:list] <header-list: string-list>
<key-list: string-list>

Usage:

header [COMPARATOR] [MATCH-TYPE] [:list] <header-names: string-list>
<key-list: string-list>

Usage:

envelope [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE] [:list] <envelope-part: string-list>
<key-list: string-list>

The new ":list" tagged argument, if present, changes interpretation of the "key-list" parameter to the 'address'/'header'/'envelope' test to become a list of names of externally stored lists. E.g. the "header" test with the ":list" parameter evaluates to true if the value of any of the named headers, ignoring leading and trailing whitespace, matches any member of one or more externally stored lists of values with names specified in key-list.

See Section 2.4 (Syntax of an externally stored list name) for the detailed description of syntax used for naming externally stored lists.

2.3.  :list tagged argument to the 'redirect' action

Usage:

redirect :list <ext-list-name: string>

The "redirect" action with the ":list" argument is used to send the message to one or more email address stored in the externally stored list 'ext-list-name'. This variant of the redirect command can be used to implement a personal distribution list.

See Section 2.4 (Syntax of an externally stored list name) for the detailed description of syntax used for naming externally stored lists.

2.4.  Syntax of an externally stored list name

[anchor5] (Specify ABNF. Dave Cridland has suggested to use a leading ':' for "opaque" identifiers.)

A name of an externally stored list is either an absolute URL (e.g. an [LDAP] (Zeilenga, K., “Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map,” June 2006.), [ACAP] (Newman, C. and J. Myers, “ACAP -- Application Configuration Access Protocol,” November 1997.) or CalDAV URL) or an opaque identifier. Interpretation of opaque identifiers is left up to implementations. They can hide complexity of an implementation from end users. For example, an implementation can provide a web interface for managing lists of users stored in LDAP. Requiring users to know syntax for LDAP URLs is just not very practical.

2.5.  Examples

Example 1:
    require ["extlists"];

    # Submission from list members is sent to all members
    if allof (envelope :detail :list "to" "mylist",
              header :contains :list "from" "mylist") {
        redirect :list "mylist";
    }

3.  Security Considerations

Security considerations are discussed in [Sieve] (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” August 2006.).

[anchor6] (Describe risks of external lists including thousands of recipients.) [anchor7] (Describe how to handle outages of servers providing access to an externally stored list.)

Implementations of this extensions should keep in mind that matching values against an externally stored list can be IO and/or CPU intensive. This can be used to deny service to the mailserver and/or to servers providing access to externally stored mailing lists. A naive implementation, such as the one that tries to retrieve content of the whole list to perform matching can make this worse. But note that many protocols that can be used for accessing externally stored lists support flexible searching facilities that can be used to minimize network traffic. For example LDAP allows for search filters.

In order to avoid mailbombs, when redirecting a message to an externally stored mailing list, implementations SHOULD enforce limits on the number of recipients and/or on domains to which such recipients belong.

4.  IANA Considerations

The following template specifies the IANA registration of the notify Sieve extension specified in this document:

To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: extlists
Description: adds the ':list' tagged argument to 'address', 'header' and 'envelope' tests, and to the 'redirect' action. The ':list' argument changes address/header/envelope test to match values against values stored in one or more externally stored list. The ':list' argument to the redirect action changes the redirect action to forward the message to email addresses stored in the externally stored list.
RFC number: this RFC
Contact address:
    The Sieve discussion list <ietf-mta-filters@imc.org>

This information should be added to the list of sieve extensions given on http://www.iana.org/assignments/sieve-extensions.

5.  Acknowledgements

Thanks to Alexandros Vellis, Barry Leiba, Nigel Swinson, Kjetil Torgrim Homme, Pete Resnick for ideas, comments and suggestions.

6.  References

6.1. Normative References

6.2. Informative References

Author's Address

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