Sieve Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track B. Leiba
Expires: March 1, 2010 Huawei Technologies
August 28, 2009
Sieve Extension: Externally Stored Lists
draft-ietf-sieve-external-lists-01
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Abstract
The Sieve scripting language can be used to implement whitelisting,
blacklisting, and personal distribution lists. Currently, this
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requires that all members of such lists be hardcoded in the script
itself. Whenever a member of a 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 lists -- lists whose members are stored externally to the
script, such as using LDAP (RFC 4510), ACAP (RFC 2244), or relational
databases.
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 match type for "address", "envelope", and "header"
tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. :list tagged argument to the "redirect" action . . . . . . . 4
2.4. Syntax of an externally stored list name . . . . . . . . . . 5
2.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
This document specifies an extension to the Sieve language [Sieve]
for checking membership in an external list or for redirecting
messages to an external list of recipients. An "external list" is a
list whose members are stored externally to the Sieve script, such as
using LDAP [LDAP], ACAP [ACAP], or relational databases.
This extension adds a new match type to the "address", "envelope",
and "header" tests, and a new tagged argument to the "redirect"
action.
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 match type for "address", "envelope", and "header" tests
ABNF:
MATCH-TYPE =/ ":list"
The new ":list" match type changes the interpretation of the "key-
list" parameter (the second parameter) to the
"address"/"envelope"/"header" test. [[anchor4: This is not quite
right. Ned convinced me that this new match type can apply to other
tests as well.]] When the match type is ":list", the key-list
becomes a list of names of externally stored lists. The external
lists are queried, perhaps through a list-specific mechanism, and the
test evaluates to "true" if any of the specified values matches any
member of one or more of the lists.
For example, testing ':header ["to", "cc"]' against a list would
cause each "to" and "cc" value, ignoring leading and trailing
whitespace, to be queried. When any value is found to belong to the
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list, the queries may stop and the test returns "true". If no value
belongs to the list, the test returns "false".
For some lists, the Sieve engine might directly retrieve the list and
make its own comparison. Other lists might not work that way -- they
might provide a way to ask if a value is in the list, but not permit
retrieval of the list itself. It is up to the Sieve implementation
to understand how to interact with any supported list. If the Sieve
engine is permanently unable to query the list (perhaps because the
list doesn't support the required operation), the test MUST result in
a runtime error in the Sieve script.
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 addresses stored in the externally
stored list 'ext-list-name'. This variant of the redirect command
can be used to implement a personal distribution list.
Use of this feature requires that the list resolve to a list of email
addresses, and that the Sieve engine be able to enumerate those
addresses. [[anchor6: Alexey would like the option of allowing the
list handler to enumerate the addresses and do the redirect there.
(Ned seems to agree that this should be allowed.) Barry thinks
that's contrary to Sieve, which expects to queue the redirect action
for processing at a later stage, and that it would be a bad idea to
have the redirect happen in the list handler. The WG needs to
resolve this issue.]] In cases where, for example, a list contains
hashed email address values or an email address pattern ("sz*@
example.com", "*+ietf@example.net"), it will not be possible to
redirect to that list.
If the Sieve engine [[anchor7: or list handler?]] is permanently
unable to enumerate the list or the list does not resolve to email
addresses, the situation MUST result in a runtime error in the Sieve
script.
See Section 2.4 for the detailed description of syntax used for
naming externally stored lists.
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2.4. Syntax of an externally stored list name
A name of an externally stored list is always an absolute URI [URI].
Implementations might find URLs such as [LDAP], [CardDAV], or
[TAG-URI] to be useful for naming external lists.
The "tag" URI scheme [TAG-URI] 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 might not be very practical, due to its complexity. An
implementation can instead 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) and the prefix doesn't even need to be shown
to end users.
2.5. Examples
[[anchor8: Barry: This example looks wrong: the "envelope" test is
probably not right. Should it really be using the :list test? It's
testing :detail, so I think it should just be a simple test, maybe '
:is "mylist" ' or some such. No? Alexey: I think this test is
correct. This is checking for known plus addresses (parts).]]
Example 1 uses the "envelope" option [Sieve] and the "subaddress"
extension [Subaddress]:
require ["extlists", "envelope", "subaddress"];
# Submission from list members is sent to all members
if allof (envelope :detail :list "to"
"tag:example.com,2009-05-28:mylist",
header :list "from"
"tag:example.com,2009-05-28:mylist") {
redirect :list "tag:example.com,2009-05-28:mylist";
}
3. Security Considerations
Security considerations related to the "address"/"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
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list membership being down or otherwise inaccessible may alter the
result of Sieve processing. 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
either 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 or query 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 an appropriate level of confidentiality and authentication.
Usually, that will be at least the same level of confidentiality as
protocols/APIs used to retrieve Sieve scripts, but only the
implementation (or deployment) will know what is appropriate.
There's a difference, for example, between making an LDAP request on
a closed LAN that's only used for trusted servers (it may be that
neither encryption nor authentication is needed), on a firewalled LAN
internal to a company (it might be OK to skip encryption, depending
upon policy), and on the open Internet (encryption and authentication
are probably both required). It also matters whether the list being
accessed is private or public (no encryption or authentication may be
needed for public data, even on the Internet).
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 features that can be used to
minimize network traffic and load on the directory service. For
example, LDAP allows for search filters. Implementations SHOULD use
such features whenever they can.
Many organizations support external lists with thousands of
recipients. In order to avoid mailbombs when redirecting a message
to an externally stored 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-ietf-vcarddav-carddav, July 2009.
[LDAP] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
June 2006.
[Subaddress]
Murchison, K., "Sieve Email Filtering: Subaddress
Extension", RFC 5233, January 2008.
[TAG-URI] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, October 2005.
Authors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Barry Leiba
Huawei Technologies
USA
Phone: +1 646 827 0648
Email: barryleiba@computer.org
URI: http://internetmessagingtechnology.org/
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