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


   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


   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

Status of this Memo

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 20, 2010.

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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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",
   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


      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

   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

   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*",
   "*"), 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 ",<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"
                 header :list "from"
                        ",2009-05-28:mylist") {
           redirect :list ",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

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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

   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:

   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 <>

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

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.

              Murchison, K., "Sieve Email Filtering: Subaddress
              Extension", RFC 5233, January 2008.

              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


   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648

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