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RFC 3088 - OpenLDAP Root Service An experimental LDAP referral s

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Network Working Group                                        K. Zeilenga
Request for Comments: 3088                           OpenLDAP Foundation
Category: Experimental                                        April 2001

                         OpenLDAP Root Service
                 An experimental LDAP referral service

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   The OpenLDAP Project is operating an experimental LDAP (Lightweight
   Directory Access Protocol) referral service known as the "OpenLDAP
   Root Service".  The automated system generates referrals based upon
   service location information published in DNS SRV RRs (Domain Name
   System location of services resource records).  This document
   describes this service.

1. Background

   LDAP [RFC2251] directories use a hierarchical naming scheme inherited
   from X.500 [X500].  Traditionally, X.500 deployments have used a
   geo-political naming scheme (e.g., CN=Jane
   Doe,OU=Engineering,O=Example,ST=CA,C=US).  However, registration
   infrastructure and location services in many portions of the naming
   hierarchical are inadequate or nonexistent.

   The construction of a global directory requires a robust registration
   infrastructure and location service.  Use of Internet domain-based
   naming [RFC2247] (e.g., UID=jdoe,DC=eng,DC=example,DC=net) allows
   LDAP directory services to leverage the existing DNS [RFC1034]
   registration infrastructure and DNS SRV [RFC2782] resource records
   can be used to locate services [LOCATE].

1.1.  The Glue

   Most existing LDAP implementations do not support location of
   directory services using DNS SRV resource records.  However, most
   servers support generation of referrals to "superior" server(s).
   This service provides a "root" LDAP service which servers may use as
   their superior referral service.

   Client may also use the service directly to locate services
   associated with an arbitrary Distinguished Name [RFC2253] within the
   domain based hierarchy.

     The mechanisms used by service are experimental.  The descriptions
     provided by this document are not definitive.  Definitive
     mechanisms shall be published in a Standard Track document(s).

2. Generating Referrals based upon DNS SRV RRs

   This service returns referrals generated from DNS SRV resource
   records [RFC2782].

2.1. DN to Domain Name Mapping

   The service maps a DN [RFC2253] to a fully qualified domain name
   using the following algorithm:

       domain = null;
       foreach RDN left-to-right        // [1]

           if not multi-valued RDN and
               RDN.type == domainComponent
               if ( domain == null || domain == "." )
               {   // start
                   domain = "";
               {   // append separator
                   domain .= ".";

               if ( RDN.value == "."  )
               {   // root
                   domain = ".";

               {   // append domainComponent
                   domain .= RDN.value;
           domain = null;


       Distinguished Name              Domain
       -----------------------------   ------------
       DC=example,DC=net               example.net
       UID=jdoe,DC=example,DC=net      example.net
       DC=.                            .            [2]
       DC=example,DC=net,DC=.          .            [3]
       DC=example,DC=.,DC=net          net          [4]
       DC=example.net                  example.net  [5]
       CN=Jane Doe,O=example,C=US      null
       UID=jdoe,DC=example,C=US        null
       DC=example,O=example,DC=net     net
       DC=example+O=example,DC=net     net
       DC=example,C=US+DC=net          null


   0) A later incarnation will use a Standard Track mechanism.

   1) A later incarnation of this service may use a right-to-left

   2) RFC 2247 does not state how one can map the domain representing
      the root of the domain tree to a DN.  We suggest the root of the
      domain tree be mapped to "DC=." and that this be reversable.

   3) RFC 2247 states that domain "example.net" should be mapped to the
      DN "DC=example,DC=net", not to "DC=example,DC=net,DC=.".  As it is
      not our intent to introduce or support an alternative domain to DN
      mapping, the algorithm ignores domainComponents to the left of

   4) RFC 2247 states that domain "example.net" should be mapped to the
      DN "DC=example,DC=net", not to "DC=example,DC=.,DC=net".  As it is
      not our intent to introduce or support an alternative domain to DN
      mapping, the algorithm ignores domainComponents to the left of
      "DC=." and "DC=." itself if further domainComponents are found to
      the right.

   5) RFC 2247 states that value of an DC attribute type is a domain
      component.  It should not contain multiple domain components.  A
      later incarnation of this service may map this domain to null or
      be coded to return invalid DN error.

   If the domain is null or ".", the service aborts further processing
   and returns noSuchObject.  Later incarnation of this service may
   abort processing if the resulting domain is a top-level domain.

2.2. Locating LDAP services

   The root service locates services associated with a given fully
   qualified domain name by querying the Domain Name System for LDAP SRV
   resource records.  For the domain example.net, the service would do a
   issue a SRV query for the domain "_ldap._tcp.example.net".  A
   successful query will return one or more resource records of the

     _ldap._tcp.example.net. IN SRV 0 0 389 ldap.example.net.

   If no LDAP SRV resource records are returned or any DNS error occurs,
   the service aborts further processing and returns noSuchObject.
   Later incarnations of this service will better handle transient

2.3. Constructing an LDAP Referrals

   For each DNS SRV resource record returned for the domain, a LDAP URL
   [RFC2255] is constructed.  For the above resource record, the URL
   would be:


   These URLs are then returned in the referral.  The URLs are currently
   returned in resolver order.  That is, the server itself does not make
   use of priority or weight information in the SRV resource records.  A
   later incarnation of this service may.

3. Protocol Operations

   This section describes how the service performs basic LDAP
   operations.  The service supports operations extended through certain
   controls as described in a later section.

3.1. Basic Operations

   Basic (add, compare, delete, modify, rename, search) operations
   return a referral result if the target (or base) DN can be mapped to
   a set of LDAP URLs as described above.  Otherwise a noSuchObject
   response or other appropriate response is returned.

3.2. Bind Operation

   The service accepts "anonymous" bind specifying version 2 or version
   3 of the protocol.  All other bind requests will return a non-
   successful resultCode.  In particular, clients which submit clear
   text credentials will be sent an unwillingToPerform resultCode with a
   cautionary text regarding providing passwords to strangers.

   As this service is read-only, LDAPv3 authentication [RFC2829] is not

3.3. Unbind Operations

   Upon receipt of an unbind request, the server abandons all
   outstanding requests made by client and disconnects.

3.4. Extended Operations

   The service currently does recognize any extended operation.  Later
   incarnations of the service may support Start TLS [RFC2830] and other

3.5. Update Operations

   A later incarnation of this service may return unwillingToPerform for
   all update operations as this is an unauthenticated service.

4. Controls

   The service supports the ManageDSAit control.  Unsupported controls
   are serviced per RFC 2251.

4.1. ManageDSAit Control

   The server recognizes and honors the ManageDSAit control [NAMEDREF]
   provided with operations.

   If DNS location information is available for the base DN itself, the
   service will return unwillingToPerform for non-search operations.
   For search operations, an entry will be returned if within scope and
   matches the provided filter.  For example:

       c: searchRequest {

       s: searchEntry {
           dn: DC=example,DC=net
           objectClass: referral
           objectClass: extensibleObject
           dc: example
           ref: ldap://ldap.example.net:389/
           associatedDomain: example.net
       s: searchResult {

   If DNS location information is available for the DC portion of a
   subordinate entry, the service will return noSuchObject with the
   matchedDN set to the DC portion of the base for search and update

       c: searchRequest {

       s: searchResult {

5. Using the Service

   Servers may be configured to refer superior requests to

   Though clients may use the service directly, this is not encouraged.
   Clients should use a local service and only use this service when
   referred to it.

   The service supports LDAPv3 and LDAPv2+ [LDAPv2+] clients over
   TCP/IPv4.  Future incarnations of this service may support TCP/IPv6
   or other transport/internet protocols.

6. Lessons Learned

6.1. Scaling / Reliability

   This service currently runs on a single host.  This host and
   associated network resources are not yet exhausted.  If they do
   become exhausted, we believe we can easily scale to meet the demand
   through common distributed load balancing technics.  The service can
   also easily be duplicated locally.

6.2. Protocol interoperability

   This service has able avoided known interoperability issues in
   supporting variants of LDAP.

6.2.1. LDAPv3

   The server implements all features of LDAPv3 [RFC2251] necessary to
   provide the service.

6.2.2. LDAPv2

   LDAPv2 [RFC1777] does not support the return of referrals and hence
   may not be referred to this service.  Though a LDAPv2 client could
   connect and issue requests to this service, the client would treat
   any referral returned to it as an unknown error.

6.2.3. LDAPv2+

   LDAPv2+ [LDAPv2+] provides a number of extensions to LDAPv2,
   including referrals.  LDAPv2+, like LDAPv3, does not require a bind
   operation before issuing of other operations.  As the referral
   representation differ between LDAPv2+ and LDAPv3, the service returns
   LDAPv3 referrals in this case.  However, as commonly deployed LDAPv2+
   clients issue bind requests (for compatibility with LDAPv2 servers),
   this has not generated any interoperability issues (yet).

   A future incarnation of this service may drop support for LDAPv2+
   (and LDAPv2).

6.2.4. CLDAP

   CLDAP [RFC1798] does not support the return of referrals and hence is
   not supported.

7. Security Considerations

   This service provides information to "anonymous" clients.  This
   information is derived from the public directories, namely the Domain
   Name System.

   The use of authentication would require clients to disclose
   information to the service.  This would be an unnecessary invasion of

   The lack of encryption allows eavesdropping upon client requests and
   responses.  A later incarnation of this service may support
   encryption (such as via Start TLS [RFC2830]).

   Information integrity protection is not provided by the service.  The
   service is subject to varies forms of DNS spoofing and attacks.  LDAP
   session or operation integrity would provide false sense of security
   concerning the integrity of DNS information.  A later incarnation of
   this service may support DNSSEC and provide integrity protection (via
   SASL, TLS, or IPSEC).

   The service is subject to a variety of denial of service attacks.
   The service is capable of blocking access by a number of factors.
   This capability have yet to be used and likely would be ineffective
   in preventing sophisticated attacks.  Later incarnations of this
   service will likely need better protection from such attacks.

8. Conclusions

   DNS is good glue.  By leveraging of the Domain Name System, global
   LDAP directories may be built without requiring a protocol specific
   registration infrastructures.

   In addition, use of DNS service location allows global directories to
   be built "ad hoc".  That is, anyone with a domain name can
   participate.  There is no requirement that the superior domain

9. Additional Information

   Additional information about the OpenLDAP Project and the OpenLDAP
   Root Service can be found at <http://www.openldap.org/>.

10. Author's Address

   Kurt Zeilenga
   OpenLDAP Foundation

   EMail: kurt@openldap.org

11. Acknowledgments

   Internet hosting for this experiment is provided by the Internet
   Software Consortium <http://www.isc.org/>.  Computing resources were
   provided by Net Boolean Incorporated <http://www.boolean.net/>.  This
   experiment would not have been possible without the contributions of
   numerous volunteers of the open source community.  Mechanisms
   described in this document are based upon those introduced in
   [RFC2247] and [LOCATE].


   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1777]  Yeong, W., Howes, T. and S. Kille, "Lightweight Directory
              Access Protocol", RFC 1777, March 1995.

   [RFC1798]  Young, A., "Connection-less Lightweight Directory Access
              Protocol", RFC 1798, June 1995.

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

   [RFC2247]  Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
              Sataluri, "Using Domains in LDAP/X.500 Distinguished
              Names", RFC 2247, January 1998.

   [RFC2251]  Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
              Access Protocol (v3)", RFC 2251, December 1997.

   [RFC2253]  Wahl, M., Kille, S. and T. Howes, "Lightweight Directory
              Access Protocol (v3): UTF-8 String Representation of
              Distinguished Names", RFC 2253, December 1997.

   [RFC2255]  Howes, T. and M. Smith, "The LDAP URL Format", RFC 2255,
              December 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2829]  Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
              "Authentication Methods for LDAP", RFC 2829, May 2000.

   [RFC2830]  Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory
              Access Protocol (v3): Extension for Transport Layer
              Security", RFC 2830, May 2000.

   [LOCATE]   IETF LDAPext WG, "Discovering LDAP Services with DNS",
              Work in Progress.

   [LDAPv2+]  University of Michigan LDAP Team, "Referrals within the
              LDAPv2 Protocol", August 1996.

   [NAMEDREF] Zeilenga, K. (editor), "Named Subordinate References in
              LDAP Directories", Work in Progress.

   [X500]     ITU-T Rec. X.500, "The Directory: Overview of Concepts,
              Models and Service",  1993.

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