Internet-Draft                                                Ryan Moats
draft-ietf-ldapext-ldap-taxonomy-05                       Lemur Networks
Expires in six months                                     Roland Hedberg
Track: Informational                                           Catalogix
                                                               July 2001

         A Taxonomy of Methods for LDAP Clients Finding Servers
           Filename: draft-ietf-ldapext-ldap-taxonomy-05.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   There are several different methods for a LDAP client to find a LDAP
   server. This draft discusses these methods and provides pointers for
   interested parties to learn more about implementing a particular

1. Introduction

   The Lightweight Directory Access Protocol (LDAP) [1] can be used to
   build "islands" of servers that are not a priori tied into a single
   Directory Information Tree (DIT.) Here, it is necessary to determine
   how a client can discover LDAP servers. This documents discusses the
   currently available methods and provides pointers for interested
   parties to learn more about implementing a particular method.

   This draft documents only those methods that are currently being
   pursued in the IETF.  Other methods have been considered for this

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   problem and the history of these other methods are presented in the

2. Methods

2.1 Client Configuration

   The simplest method of enabling a LDAP client to discover LDAP
   servers is for the client administrator to configure the client with
   a list of known LDAP servers (and associated base objects) to send
   queries to.  While this method has the advantage of being correct
   (initially), it adds the requirement that the list of initial servers
   be kept small and constant.  Otherwise, the required client update
   process won't scale.

2.2 Well known DNS aliases

   If the DIT uses a naming scheme similar to that in RFC 2377 [2], then
   it is possible to build the DNS names of potential servers using well
   known DNS aliases, like those documented in RFC 2219 [3].  When a
   different naming scheme is used, it is also possible to build
   potential server names based on the client's fully qualified domain
   name or local (within the organization or country) environment.

   One shortcoming of this method are that it is not exact.  Multiple
   DNS lookups and LDAP protocol operations may be necessary to find the
   proper LDAP server to serve the client requests.  To support client
   roaming, it is necessary that either the RFC 2377 (or similar) naming
   scheme be used or that roaming be implemented through tunnels.

   Because this method uses DNS, it inherits all the security
   considerations of using DNS to discover LDAP servers: see the
   security consideration in [3] for more details.

2.3 Service Location Protocol

   If a client supports the service location protocol [4], it could use
   a SLP query for LDAP servers.  The SLP template that is used to
   describe LDAP servers is presented in [5], and requires that the
   servers announce themselves using SLP and this template.

   Using this method inherits the scaling and security considerations
   for the service location protocol, which are documented further in

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2.4 Referrals

   In LDAPv3, servers can return referrals to the client if the server
   has knowledge of where a query might be satisfiable.  Two ways of
   deploying referral information are deploying a LDAP knowledge server
   or exchanging CIP index objects [6] between servers.

   A LDAP knowledge server would hold cross references to possibly
   hundreds of other LDAP  servers, so that a client would only need to
   know about its local LDAP server and the knowledge server.  As an
   optimization, the local LDAP server could also act as a knowledge

   If CIP index objects are exchanged between LDAP servers, then those
   objects can also carry URL information for providing referrals to
   clients. Here, the client would only need to know about the local
   server. Using CIP index objects inherits the security considerations
   of CIP: see [6, 7, 8] for more details.

   In either of these cases, the local LDAP server could be determined
   using another of the methods discussed.

2.5 Using SRV records

   RFC 2052 [12] defined SRV records for DNS, which bound a host name
   and port to a label in the DNS. This makes it possible for a client
   to look up information about a supported protocol for a domain and
   get back a  weighted list of fully qualified domain names and ports
   for where that protocol is supported.  For more information, see

3. Implementation

   The Norwegian Directory Forum plans to start a service based on a
   central LDAP service containing contact information for every
   organization within Norway [10]. If an organization has more
   information about its sub-units, employees or functions that it wants
   to publish it can do so by placing this information in a publicly
   available LDAP server and providing the management of the central
   service with a pointer (URL) to this server.

   The TISDAG project is running a test service based on the TISDAG
   specification [11]. This service gathers indices from connected White
   Pages Service Providers using CIP Tagged Index Objects [9].  The
   rationale for this service is that by supplying the name of a person
   or a function/role to the service it will return pointers to where
   more information can be found about persons/functions with that name.

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   The European cofunded project DESIRE ( is designing a
   system to use a LDAP server that communicates with a referral index
   that in turn, uses CIP Tagged Index Objects [9] and is fed by LDAP
   crawlers.  DANTE plans to set up a European infrastructure of such
   referral index servers.

4. References

   Request For Comments (RFC) and Internet Draft documents are available
   from numerous mirror sites.

[1]         M. Wahl, T. Howes, S. Kille, Lightweight Directory Access
            Protocol (v3), RFC 2251, December 1997.

[2]         A. Grimstad, R. Huber, S. Sataluri, M. Wahl, Naming Plan for
            Internet Directory-Enabled Applications, RFC 2377, September

[3]         M. Hamilton, R. Wright, "Use of DNS Aliases for Network Ser-
            vices," RFC 2219 (Also BCP 17), October 1997.

[4]         E. Guttman, C. Perkins, J. Veizades, M. Day, "Service Loca-
            tion Protocol, Version 2," RFC 2608, June 1999.

[5]         J. Wood, R. Tam, "The LDAP Service Type," SVRLOC Template

[6]         J. Allen, M. Mealling, "The Architecture of the Common
            Indexing Protocol (CIP)," RFC 2651, August 1999.

[7]         J. Allen, M. Mealling, "MIME Object Definitions for the Com-
            mon Indexing Protocol (CIP)," RFC 2652, August 1999.

[8]         J. Allen, P. Leach, R. Hedberg, "CIP Transport Protocols,"
            RFC 2653, August 1999.

[9]         R. Hedberg, B. Greenblatt, R. Moats, M. Wahl, "A Tagged
            Index Object for use in the Common Indexing Protocol," RFC
            2654, August 1999.

[10]        R. Hedberg, H. Alverstrand, "Technical Specification, The
            Norwegian Directory of Directories (NDD)," http://

[11]        R. Hedberg, L. Daigle, "Technical Infrastructure for Swedish
            Directory Access Gateways (TISDAG)," Internet Draft (work in
            progress), February 2000.

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[12]        A. Gulbrandsen, P. Vixie, "A DNS RR for specifying the loca-
            tion of services (DNS SRV)," RFC 2052, October 1996.

[13]        M. Armijo, L. Esibov, P. Leach, "Discovering LDAP Services
            with DNS," Internet Draft (work in progress), July 1999.

5. Author's Addresses

      Ryan Moats                         Roland Hedberg
      Lemur Networks                     Catalogix
      15621 Drexel Circle                Dalsveien 53
      Omaha, NE 68135                    0775 Oslo
      USA                                Norway
      Email:    Email:

Appendix A. Historical Methods

A.1 Discovery

   The discovery approach was to use a combination of other methods pre-
   sented in this taxonomy along with storing either the search DN or a
   related URL in the DNS in some way.  Using both TXT or NAPTR records
   in the DNS were considered.  This approach requires an administrator
   to configure the DNS with necessary information.  Further, the idea
   of storing standards based information (either a DN or an URL) in a
   DNS RR has been an extremely controversial one in the IETF.

A.2 DHCP extensions

   Another proposed method was to use DHCP to deliver information about
   LDAP server to a DHCP client. This would require that such informa-
   tion be configured into the DHCP server and that the client use DHCP
   to load host configuration information. While there has been some
   nascent interest in this method, there has been no interest in imple-
   mentation of this approach.

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