Sieve Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track B. Leiba
Expires: November 20, 2010 Huawei Technologies
May 19, 2010
Sieve Extension: Externally Stored Lists
draft-ietf-sieve-external-lists-02
Abstract
The Sieve scripting language can be used to implement whitelisting,
blacklisting, personal distribution lists, and other sorts of list
matching. Currently, this 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. From IETF 77 minutes: "Can use ihave to test if a URI is
valid, both that the scheme is supported and that the URI can be
retrieved/queried."
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 20, 2010.
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Copyright Notice
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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 supported 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 apply to supported 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 supported tests
ABNF:
MATCH-TYPE =/ ":list"
; only valid for supported tests
The new ":list" match type changes the interpretation of the "key-
list" parameter (the second parameter) to supported tests. 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.
Implementations MUST support "address", "envelope" and "header"
tests. Implementations that include the Variables extension
[Variables] MUST also support "string" tests.
Implementations MAY support other tests but MUST raise an error
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(which SHOULD be a compile-time error, but MAY be a runtime error)
when a script uses :list with a test for which it is not supported.
To maintain interoperability, other tests that can be used with :list
SHOULD be documented in a specification that defines a capability
string that can be tested (in a "require" statement, or using ihave
[Ihave]).
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
list, the queries 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.
For this feature to work, one of the following conditions has to be
true:
1. The list resolves to a list of email addresses, and the Sieve
engine is able to enumerate those addresses.
2. The list handler is able to take care of the redirection on
behalf of the Sieve engine.
In cases where, for example, a list contains hashed email address
values or an email address pattern ("sz*@example.com",
"*+ietf@example.net"), the Sieve engine will not be able to redirect
to that list, and responsibility must pass to the list handler.
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If neither the Sieve engine nor the list handler can enumerate (or
iterate) 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.
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] MUST be supported, and 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
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.
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External list memberships ought to be treated as if they are an
integral part of the script, so a temporary failure to access an
external list SHOULD be handled in the same way as a temporary
failure to retrieve the Sieve script itself.
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 (perhaps the LDAP server is unavailable),
then the Sieve engine might delay message delivery until the script
can be retrieved successfully. Similarly, if an external list is
stored in LDAP and that LDAP server is unavailable, the Sieve engine
would take the same action -- delay message delivery and try again
later.
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 extension 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' match type to certain Sieve tests, and
the ':list' argument to the 'redirect' action. The ':list' match
type changes tests to match values against values stored in one or
more externally stored lists. 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 <sieve@ietf.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, Nigel Swinson, Ned Freed, Kjetil Torgrim
Homme, Dave Cridland, Cyrus Daboo, Pete Resnick, and Robert Burrell
Donkin 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.
[TAG-URI] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, October 2005.
[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, November 2009.
[Ihave] Freed, N., "Sieve Email Filtering: Ihave Extension",
RFC 5463, March 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.
[Variables]
Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
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
Phone: +1 646 827 0648
Email: barryleiba@computer.org
URI: http://internetmessagingtechnology.org/
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