Sieve Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track June 27, 2007
Expires: December 29, 2007
Sieve Extension: Externally Stored Lists
draft-melnikov-sieve-external-lists-00
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Copyright (C) The IETF Trust (2007).
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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . . 3
2. Extlists extension . . . . . . . . . . . . . . . . . . . . . 3
2.1. Capability Identifier . . . . . . . . . . . . . . . . . . . . 3
2.2. :list tagged argument to the 'address', 'header' and
'envelope' tests . . . . . . . . . . . . . . . . . . . . . . 3
2.3. :list tagged argument to the 'redirect' action . . . . . . . 4
2.4. Syntax of an externally stored list name . . . . . . . . . . 4
2.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . 7
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1. Introduction
This document specified an extension to the Sieve language defined by
[Sieve] 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].
1.1. Conventions used in this document
Conventions for notations are as in [Sieve] section 1.1, including
the use of [ABNF].
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].
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
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of values with names specified in key-list.
See Section 2.4 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 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], [ACAP] 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].
[[anchor6: Describe risks of external lists including thousands of
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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.
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6. References
6.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[Sieve] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", work in progress, draft-ietf-sieve-3028bis,
August 2006.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
6.2. Informative References
[ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP):
Technical Specification Road Map", RFC 4510, June 2006.
Author's Address
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
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