Sieve Working Group                                          A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                            July 5, 2009                                B. Leiba
Expires: January 6, March 1, 2010                               Huawei Technologies
                                                         August 28, 2009

                Sieve Extension: Externally Stored Lists

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   The Sieve scripting language can be used for implementing of to implement whitelisting, blacklisting
   blacklisting, and personal distribution lists.
   Currently  Currently, this
   requires that all members of such lists be hardcoded in the script
   itself.  Whenever a member of such 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 mailing lists, i.e. list lists -- lists whose members are stored externally to the
   script, for example in such as using LDAP (RFC 4510), ACAP (RFC 2244) 2244), or a relational database.


   o  Need a way to advertise supported URI schemas in ManageSieve and

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' match type for "address", "envelope", and
         'envelope' "header"
         tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.3.  :list tagged argument to the 'redirect' "redirect" action  . . . . . . . 4
   2.4.  Syntax of an externally stored list name  . . . . . . . . . . 4 5
   2.5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5

   3.    Security Considerations . . . . . . . . . . . . . . . . . . . 5

   4.    IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7

   5.    Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 6 7

   6.    References  . . . . . . . . . . . . . . . . . . . . . . . . . 6 7
   6.1.  Normative References  . . . . . . . . . . . . . . . . . . . . 6 7
   6.2.  Informative References  . . . . . . . . . . . . . . . . . . . 7

         Author's Address  . 8

         Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . 7 8

1.  Introduction

   This document specified specifies an extension to the Sieve language defined by [Sieve]
   for checking membership in an externally stored external list or for
   sending redirecting
   messages to a an external list of recipients recipients.  An "external list" is a
   list whose members are stored externally to the Sieve script. script, such as
   using LDAP [LDAP], ACAP [ACAP], or relational databases.

   This extension adds a new tagged argument match type to the "header" "address", "envelope",
   "envelope" "header" tests, and a new tagged argument 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",
   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' match type for "address", "envelope", and 'envelope' "header" 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>


      MATCH-TYPE  =/ ":list"

   The new ":list" tagged argument, if present, match type changes the interpretation of the "key-list" "key-
   list" parameter (the second parameter) to the 'address'/'header'/'envelope' test
   "address"/"envelope"/"header" test. [[anchor4: This is not quite
   right.  Ned convinced me that this new match type can apply to become other
   tests as well.]]  When the match type is ":list", the key-list
   becomes a list of names of externally stored lists.  E.g.  The external
   lists are queried, perhaps through a list-specific mechanism, and the
   test with the ":list" parameter evaluates to true "true" if the
   value of any of the named headers, ignoring leading and trailing
   whitespace, specified values matches any
   member of one or more externally stored 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
   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 values the list itself.  It is up to the Sieve implementation
   to understand how to interact with names specified 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 key-list. 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' "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 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*@", "*"), 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

   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 [URI].
   Implementations might find URL URLs such as [LDAP], [CardDAV] URL), [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 is might not going
   to be very practical, due to its complexity.  However such  An
   implementation can instead use a fixed tag URI prefix such as "tag:,<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

   [[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: 1 uses the "envelope" option [Sieve] and the "subaddress"
   extension [Subaddress]:

       require ["extlists"]; ["extlists", "envelope", "subaddress"];

       # Submission from list members is sent to all members
       if allof (envelope :detail :list "to"
                 header :contains :list "from"
                        ",2009-05-28:mylist") {
           redirect :list ",2009-05-28:mylist";

3.  Security Considerations

   Security considerations related to the "envelope"/"header" "address"/"envelope"/"header"
   tests and "redirect" action discussed in [Sieve] also apply to this

   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  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) [LDAP] and the script can't be retrieved when a
   message is processed, then the agent performing Sieve processing can, for example, 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.  For scripts, but only the
   implementation (or deployment) will know what is appropriate.
   There's a difference, for example, if Sieve scripts are
   retrieved using between making an LDAP secured with Transport Layer Security (TLS)
   encryption, then the protocol 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 retrieve external list
   membership must use a comparable mechanism for providing connection
   confidentiality.  In particular, the protocol used company (it might be OK to retrieve
   external skip encryption, depending
   upon policy), and on the open Internet (encryption and authentication
   are probably both required).  It also matters whether the list membership must not being
   accessed is private or public (no encryption or authentication may be lacking encryption.
   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 facilities 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, 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:

   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 <>

   This information should be added to the list of sieve extensions
   given on

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.

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. draft-ietf-vcarddav-carddav, July 2009.

   [LDAP]     Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510,
              June 2006.

              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.

Author's Address

Authors' Addresses

   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX


   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648