Sieve Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track July 5, 2009
Expires: January 6, 2010
Sieve Extension: Externally Stored Lists
draft-melnikov-sieve-external-lists-02
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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
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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.
ToDo
o Need a way to advertise supported URI schemas in ManageSieve and
ihave.
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 . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . 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
A name of an externally stored list is always an absolute URI.
Implementations might find URL such as [LDAP], [CardDAV] URL), or
[TAG-URI] to be useful for naming external lists.
The "tag" URI scheme can be used to represent opaque, but user
friendlier identifiers. Resolution of such identifiers is going to
be implementation specific and it can help in hiding the 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 generic LDAP URL syntax is not going
to be very practical, due to its complexity. However such
implementation can use a fixed tag URI prefix such as "tag:
example.com,<date>:" (where <date> can be, for example, a date
generated once on installation of the web interface and left
untouched upon upgrades) instead and the prefix doesn't even need to
be shown to end users.
2.5. Examples
Example 1:
require ["extlists"];
# Submission from list members is sent to all members
if allof (envelope :detail :list "to"
"tag:example.com,2009-05-28:mylist",
header :contains :list "from"
"tag:example.com,2009-05-28:mylist") {
redirect :list "tag:example.com,2009-05-28:mylist";
}
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3. Security Considerations
Security considerations related to the "envelope"/"header" tests and
"redirect" action discussed in [Sieve] also apply to this document.
A failure to retrieve data due to the server storing the external
list membership being down or otherwise inaccessible may alter the
result of Sieve processing. So implementations SHOULD treat a
temporary failure to retrieve or verify external list membership in
the same manner as a temporary failure to retrieve a Sieve script.
For example, if the Sieve script is stored in the Lightweight
Directory Access Protocol (LDAP) and the script can't be retrieved
when a message is processed, then the agent performing Sieve
processing can, for example, assume that the script doesn't exist or
delay message delivery until the script can be retrieved
successfully. External list memberships should be treated as if they
are a part of the script itself, so a temporary failure to retrieve
them should be handled in the same way as a temporary failure to
retrieve the Sieve script itself.
Protocols/APIs used to retrieve/verify external list membership MUST
provide at least the same level of confidentiality as protocols/APIs
used to retrieve Sieve scripts. For example, if Sieve scripts are
retrieved using LDAP secured with Transport Layer Security (TLS)
encryption, then the protocol used to retrieve external list
membership must use a comparable mechanism for providing connection
confidentiality. In particular, the protocol used to retrieve
external list membership must not be lacking encryption.
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 and load on the directory service. For
example LDAP allows for search filters.
Many organizations support external lists with thousands of
recipients. 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.
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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, Dave Cridland, Cyrus Daboo, Pete Resnick for ideas,
comments and suggestions.
6. References
6.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[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", RFC 5228, January 2008.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
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6.2. Informative References
[ACAP] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[CardDAV] Daboo, C., "vCard Extensions to WebDAV (CardDAV)", work in
progress, draft-daboo-carddav, May 2007.
[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
June 2006.
[TAG-URI] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, October 2005.
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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