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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.  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].


.  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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.  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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.  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.


.  Acknowledgements

   Thanks to Alexandros Vellis, Barry Leiba, Nigel Swinson, Kjetil
   Torgrim Homme, Dave Cridland, Cyrus Daboo, Pete Resnick for ideas,
   comments and suggestions.


.  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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