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Versions: (draft-melnikov-sieve-external-lists) 00 01 02 03 04 05 06 07 08 09 10 RFC 6134

Sieve Working Group                                          A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                                B. Leiba
Expires: September 30, 2011                          Huawei Technologies
                                                          March 29, 2011


                Sieve Extension: Externally Stored Lists
                   draft-ietf-sieve-external-lists-06

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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 30, 2011.

Copyright Notice

   Copyright (c) 2011 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.    Other Uses for External Lists  . . . . . . . . . . . . . .  5
   2.5.    Syntax of an Externally Stored List Name . . . . . . . . .  5
   2.6.    Test valid_ext_list  . . . . . . . . . . . . . . . . . . .  6
   2.7.    Interaction with ManageSieve . . . . . . . . . . . . . . .  6
   2.8.    Examples . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.8.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.8.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . .  8
   2.8.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . . .  8

   3.      Security Considerations  . . . . . . . . . . . . . . . . .  8

   4.      IANA Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.1.    Registration of Sieve Extension  . . . . . . . . . . . . . 10
   4.2.    Registration of ManageSieve Capability . . . . . . . . . . 10
   4.3.    Registration of "ab" URI Scheme  . . . . . . . . . . . . . 11

   5.      Acknowledgements . . . . . . . . . . . . . . . . . . . . . 12

   6.      References . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.1.    Normative References . . . . . . . . . . . . . . . . . . . 12
   6.2.    Informative References . . . . . . . . . . . . . . . . . . 13

           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 14










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1.  Introduction

   This document specifies an extension to the Sieve language [RFC5228]
   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 [RFC4510], ACAP [RFC2244], 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 [RFC5228] section 1.1, including
   the use of [RFC5234].

   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 [RFC2119].


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) in 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.

   Comparators are not allowed together with the ":list" match type, so
   if both are specified in a test, that MUST result in an error.
   Queries done through list-specific mechanisms might have the effect
   of built-in comparators; for example, queries to certain lists might
   be case-sensitive, while queries to other lists might be done without



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   regard to case.

   Implementations MUST support the use of :list in "address",
   "envelope" and "header" tests.  Implementations that include the
   Variables extension [RFC5229] MUST also support its use in "string"
   tests.

   Implementations MAY support other tests but MUST raise an error
   (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
   [RFC5463]).

   For example, testing 'header ["to", "cc"]' against a list would cause
   each "to" and "cc" value, ignoring leading and trailing whitespace,
   to be queried.  If any value is found to belong to the list, the test
   returns "true".  If no value belongs to the list, the test returns
   "false".  Once a value is found in the list, there is no need for the
   query mechanism to look further.

   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 2.5 for the detailed description of syntax used for
   naming externally stored lists.

   The :list match type uses the concept of "match variables" as defined
   in Section 3.2 of the Variables extension [RFC5229].  Implementations
   that also support that extension MUST set the ${0} match variable to
   the value in the list that matched the query.  Other numbered match
   variables (${1}, ${2}, and so on) MAY be set with list-specific
   information that might be of use to the script.

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 the set of email addresses in the externally stored list
   named by the ext-list-name string.  This variant of the redirect



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   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.

   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 2.5 for the detailed description of syntax used for
   naming externally stored lists.

2.4.  Other Uses for External Lists

   The uses for external lists specified here represent the useful cases
   and situations at the time of this writing.  Other uses for external
   lists, using other Sieve features, might be devised in the future,
   and such uses can be described in extensions to this document.

2.5.  Syntax of an Externally Stored List Name

   A name of an externally stored list is always an absolute URI
   [RFC3986].  Implementations might find URIs such as LDAP [RFC4510],
   CardDAV [I-D.ietf-vcarddav-carddav], or Tag [RFC4151] to be useful
   for naming external lists.

   The "tag" URI scheme [RFC4151] 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 URI
   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



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   to end users.

   The "ab" URI scheme (in particular, the URI "ab:default"), defined in
   Section 4.3 MUST be supported.  The mandatory-to-implement URI "ab:
   default" gives access to the user's default address book (usually the
   user's personal address book).

   It's possible that a server will have no access to anything
   resembling an address book (perhaps in an implementation where
   address books are only client-side things), but the server can still
   provide access to other sorts of lists -- consider the list of dates
   in Example 2 (Section 2.8.2), or lists of important keywords and the
   like.  It might sometimes make sense to map "ab:default" into some
   available list, but that might not always be reasonable.  If there
   really is no concept of an address book in a particular server
   implementation, the server MAY support "ab:default" by having all
   matches to it fail.  Such an implementation SHOULD NOT be done except
   as a last resort.

   Queries against address books SHOULD be done without regard to case.

2.6.  Test valid_ext_list

   Usage: valid_ext_list <ext-list-names: string-list>

   The "valid_ext_list" test is true if all of the external list names
   in the ext-list-names argument are supported, and they are valid both
   syntactically (including URI parameters) and semantically (including
   implementation-specific semantic restrictions).  Otherwise the test
   returns false.

   This test MUST perform exactly the same validation of an external
   list name as would be performed by the "header :list" test.

2.7.  Interaction with ManageSieve

   This extension defines the following new capability for ManageSieve
   (see [RFC5804] section 1.7):

   EXTLISTS - A space-separated list of URI schema parts [RFC3986] for
   supported externally stored list types.  This capability MUST be
   returned if the corresponding Sieve implementation supports the
   "extlists" extension defined in this document.

   This also extends the ManageSieve ABNF as follows:






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   single-capability  =/ DQUOTE "EXTLISTS" DQUOTE SP ext-list-types CRLF
           ; single-capability is defined in [RFC5804]

   ext-list-types  = string
           ; space separated list of URI schema parts
           ; for supported externally stored list types.
           ; MUST NOT be empty.

2.8.  Examples

2.8.1.  Example 1

   This example uses a personal address book, along with the Spamtest
   [RFC5235] and Relational [RFC5231] extensions to give a different
   level of spam tolerance to known senders.

       require ["envelope", "extlists", "fileinto", "spamtest",
                "relational", "comparator-i;ascii-numeric"];
       if envelope :list "from" "ab:default"
         { /* Known: allow high spam score */
           if spamtest :value "ge" :comparator "i;ascii-numeric" "8"
             {
               fileinto "spam";
             }
         }
       elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3"
         { /* Unknown: less tolerance in spam score */
           fileinto "spam";
         }

   The same example can also be written another way, if the Variables
   extension [RFC5229] is also supported:

       require ["envelope", "extlists", "fileinto", "spamtest",
           "variables", "relational", "comparator-i;ascii-numeric"];
       if envelope :list "from" "ab:default" {
         set "limit" "8";  /* Known: allow high spam score */
       } else {
         set "limit" "3";  /* Unknown: less tolerance in spam score */
       }
       if spamtest :value "ge" :comparator "i;ascii-numeric" ${limit} {
         fileinto "spam";
       }








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2.8.2.  Example 2

   This example uses the "currentdate" test [RFC5260] and a list
   containing the dates of local holidays.  If today is a holiday, the
   script will notify [RFC5435] the user via XMPP [RFC5437] about the
   message.

       require ["extlists", "date", "enotify"];
       if currentdate :list "date"
          "tag:example.com,2011-01-01:localHolidays" {
          notify "xmpp:romeo@im.example.com";
       }

2.8.3.  Example 3

   This example also uses the "envelope" option [RFC5228] and the
   Subaddress extension [RFC5233].  If mail is sent with the list name
   as a subaddress of the recipient (to, say, "alexey+mylist"), and the
   message comes from a member of the list, it will be redirected to all
   members of the list.  Variants of this technique might be useful for
   creating private mailing lists.

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

       # Submission from list members is sent to all members
       if allof (envelope :detail "to" "mylist",
                 header :list "from"
                        "tag:example.com,2010-05-28:mylist") {
           redirect :list "tag:example.com,2010-05-28:mylist";
       }


3.  Security Considerations

   Security considerations related to the "address"/"envelope"/"header"
   tests and "redirect" action discussed in Sieve [RFC5228] also apply
   to this document.

   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 [RFC4510] 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



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   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.

   Note in particular that it can be too easy for a script to use
        redirect :list "ab:default";
   to send messages to "everyone in your address book", and one can
   easily imagine both intentional and accidental abuse.  The situation
   can be even worse for, say, "ab:corporate".  Warnings, as well as
   enforced limits, are appropriate here.


4.  IANA Considerations





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4.1.  Registration of Sieve Extension

   The following template specifies the IANA registration of the Sieve
   extension specified in this document.  This information should be
   added to the list of sieve extensions given on
   http://www.iana.org/assignments/sieve-extensions.

   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:  Sieve mailing list <sieve@ietf.org>

4.2.  Registration of ManageSieve Capability

   The following requests IANA to register a new ManageSieve Capability
   according to the IANA registration template specified in [RFC5804]:

   To: iana@iana.org

   Subject:  ManageSieve Capability Registration

   Capability name:  extlists

   Description:  This capability is returned if the server supports the
       "extlists" [RFCXXXX] Sieve extension.

   Relevant publications:  this RFC, Section 2.7

   Person & email address to contact for further information:  Sieve
       mailing list <sieve@ietf.org>

   Author/Change controller:  IESG







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4.3.  Registration of "ab" URI Scheme

   The following requests IANA to register a new URI scheme according to
   the IANA registration template specified in [RFC4395]:

   URI scheme name:  ab

   Status:  Permanent

   URI scheme syntax:
       paburi = "ab:" addrbook [ "?" extensions ]
       addrbook = segment
            ; <segment> defined in [RFC3986]
       extensions = query
            ; <query> defined in [RFC3986]

   URI scheme semantics:  "ab" URIs are used for designating references
       to address books.  An address book is an internal concept used by
       different applications (such as Sieve interpreters) for
       describing a list of named entries, and may be translated into
       other types of address books, such as LDAP Groups.  Address books
       may be private or shared; they may be personal, organizational,
       or perhaps even "crowdsourced".

   Encoding considerations:  Percent-encoding is allowed in "segment"
       and "query" components.  Internationalization is handled by IRI
       processing.

   Intended usage:  An "ab" URI is designed to be used internally by
       applications for referencing address books.  Each URI is intended
       to represent a grouping of addresses that can be logically
       thought of as one "book".  Any given address can belong to more
       than one book -- that is, can be referred to by more than one
       URI.

       The URI "ab:default" is a reserved name that MUST be implemented,
       representing a default grouping (book) of addresses.  Other
       names, representing the same or other groupings MAY be
       implemented.  For example, an implementation might have the
       following URIs:

       *   ab:personal -- a book representing the user's personal
           address book.

       *   ab:friends -- a subset of ab:personal, defined by the user.

       *   ab:family -- a subset of ab:personal, defined by the user.




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       *   ab:company -- a book representing user's company's address
           book.

       *   ab:department -- a subset of ab:company, defined by the
           company.

       *   ab:co-workers -- a subset of ab:company, defined by the user.

       *   ab:default -- the default address book, a reference to ab:
           personal.

   Applications and/or protocols that use this URI scheme name:
       Currently only the Sieve External List extension is using this
       URI scheme.  Email clients that use URIs internally might find
       this URI scheme to be useful as well.

   Interoperability considerations:  Applications are only REQUIRED to
       support "ab:default".

   Security considerations:  Applications SHOULD ensure appropriate
       restrictions are in place to protect sensitive information that
       might be revealed by "ab" URIs from access or modification by
       untrusted sources.

   Relevant publications:  this RFC

   Contact:  Sieve mailing list <sieve@ietf.org>

   Author/Change controller:  IETF/IESG


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.  Kristin Hubner also
   helped greatly with the examples.


6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,



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              RFC 3986, January 2005.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, October 2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5804]  Melnikov, A. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, July 2010.

6.2.  Informative References

   [I-D.ietf-vcarddav-carddav]
              Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
              draft-ietf-vcarddav-carddav-10 (work in progress),
              November 2009.

   [RFC2244]  Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510,
              June 2006.

   [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.

   [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:
              Relational Extension", RFC 5231, January 2008.

   [RFC5233]  Murchison, K., "Sieve Email Filtering: Subaddress
              Extension", RFC 5233, January 2008.

   [RFC5235]  Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
              Extensions", RFC 5235, January 2008.

   [RFC5260]  Freed, N., "Sieve Email Filtering: Date and Index
              Extensions", RFC 5260, July 2008.

   [RFC5435]  Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,



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              "Sieve Email Filtering: Extension for Notifications",
              RFC 5435, January 2009.

   [RFC5437]  Saint-Andre, P. and A. Melnikov, "Sieve Notification
              Mechanism: Extensible Messaging and Presence Protocol
              (XMPP)", RFC 5437, January 2009.

   [RFC5463]  Freed, N., "Sieve Email Filtering: Ihave Extension",
              RFC 5463, March 2009.


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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