Sieve Working Group                                          A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                            July 6, 2007
Expires: January 7, 2008


                Sieve Extension: Externally Stored Lists
                 draft-melnikov-sieve-external-lists-01

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Copyright Notice

   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.

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 . . . . . . . . . . . . . . . . . . . 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 [CardDAV] 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, Dave Cridland, Cyrus Daboo, 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.

   [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.


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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