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INTERNET-DRAFT                                         Michael P. Armijo
<draft-ietf-ldapext-locate-08.txt>                          Levon Esibov
June 5, 2002                                                  Paul Leach
Expires: December 5, 2002                          Microsoft Corporation
							     R.L. Morgan
						University of Washington

Discovering LDAP Services with DNS

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, and its working groups.  Note that
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Distribution of this memo is unlimited.  It is filed as <draft-
   ietf-ldapext-locate-08.txt>, and expires on December 5, 2002.
   Please send comments to the authors.

Abstract

A Lightweight Directory Access Protocol (LDAP) request must be
   directed to an appropriate server for processing.  This document
   specifies a method for discovering such servers using information in
   the Domain Name System.

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1. Introduction

The LDAPv3 protocol [1] is designed to be a lightweight access
   protocol for directory services supporting X.500 models.  As a
   distributed directory service, the complete set of directory
   information (known as the Directory Information Base) is spread
   across many different servers.  Hence there is the need to
   determine, when initiating or processing a request, which servers
   hold the relevant information.  In LDAP, the Search, Modify, Add,
   Delete, ModifyDN, and Compare operations all specify a Distinguished
   Name (DN) [2] on which the operation is performed.  A client, or a
   server acting on behalf of a client, must be able to determine the
   server(s) that hold the naming context containing that DN, since
   that server (or one of that set of servers) must receive and process
   the request.  This determination process is called "server
   location".  To support dynamic distributed operation, the
   information needed to support server location must be available via
   lookups done at request processing time, rather than, for example,
   as static data configured into each client or server.

It is possible to maintain the information needed to support server
   location in the directory itself, and X.500 directory deployments
   typically do so.  In practice, however, this only permits location
   of servers within a limited X.500-connected set.  LDAP-specific
   methods of maintaining server location information in the directory
   have not yet been standardized.  This document defines an
   alternative method of managing server location information using the
   Domain Name System. This method takes advantage of the global
   deployment of the DNS, by allowing LDAP server location information
   for any existing DNS domain to be published by creating the records
   described below.  A full discussion of the benefits and drawbacks of
   the various directory location and naming methods is beyond the
   scope of this document.

RFC 2247[3] defines an algorithm for mapping DNS domain names into
   DNs.  This document defines the inverse mapping, from DNs to DNS
   domain names, based on the conventions in [3], for use in this
   server location method.  The server location method described in
   this document is only defined for DNs that can be so mapped, i.e.,
   those DNs that are based on domain names.  In practice this is
   reasonable because many objects of interest are named with domain
   names, and use of domain-name-based DNs is becoming common.

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 RFC 2119 [9].

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2. Mapping Distinguished Names into Domain Names

This section defines a method of converting a DN into a DNS domain
   name for use in the server location method described below.  Some
   DNs cannot be converted into a domain name.  Converted DNs result 
   in a fully qualified domain name.

The output domain name is initially empty.  The DN is processed in
   right-to-left order (i.e., beginning with the first RDN in the
   sequence of RDNs).  An RDN is able to be converted if it (1)
   consists of a single AttributeTypeAndValue; (2) the attribute type
   is "DC"; and (3) the attribute value is non-null.  If it can be
   converted, the attribute value is used as a domain name component
   (label).  The first such value becomes the rightmost (i.e., most
   significant) domain name component, and successive converted RDN
   values extend to the left.  If an RDN cannot be converted,
   processing stops.  If the output domain name is empty when
   processing stops, the DN cannot be converted into a domain name.

For DN:

cn=John Doe,ou=accounting,dc=example,dc=net

The client would convert the DC components as defined above into 
   DNS name:

example.net

The determined DNS name will be submitted as a DNS query using the 
   algorithm defined in section 3.

3. Locating LDAPv3 servers through DNS

LDAPv3 server location information is to be stored using DNS Service
   Location Record (SRV)[5].  The data in a SRV record contains the DNS
   name of the server that provides the LDAP service, corresponding
   Port number, and parameters that enable the client to choose an
   appropriate server from multiple servers according to the algorithm
   described in [5].  The name of this record has the following format:

_<Service>._<Proto>.<Domain>.

where <Service> is "ldap", and <Proto> is "tcp". <Domain> is the
   domain name formed by converting the DN of a naming context mastered
   by the LDAP Server into a domain name using the algorithm in
   Section 2.  Note that "ldap" is the symbolic name for the LDAP
   service in Assigned Numbers[6], as required by [5].

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Presence of such records enables clients to find the LDAP servers
   using standard DNS query [4].  A client (or server) seeking an LDAP
   server for a particular DN converts that DN to a domain name using
   the algorithm of Section 2, does a SRV record query using the DNS
   name formed as described in the preceding paragraph, and interprets
   the response as described in [5] to determine a host (or hosts) to
   contact. As an example, a client that searches for an LDAP server
   for the DN "ou=foo,dc=example,dc=net" that supports the TCP protocol
   will submit a DNS query for a set of SRV records with owner name:

_ldap._tcp.example.net.

The client will receive the list of SRV records published in DNS
   that satisfy the requested criteria.  The following is an example of
   such a record:

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

The set of returned records may contain multiple records in the case
   where multiple LDAP servers serve the same domain.  If there are no 
   matching SRV records available for the converted DN the client SHOULD 
   NOT attempt to 'walk the tree' by removing the least significant 
   portion of the constructed fully qualified domain name.

4.  IANA Considerations

This document does not require any IANA actions.

5. Security Considerations

DNS responses can typically be easily spoofed.  Clients using this
   location method SHOULD ensure, via use of strong security
   mechanisms, that the LDAP server they contact is the one they
   intended to contact.  See [7] for more information on security
   threats and security mechanisms.

When using LDAP with TLS the client MUST check the server's name,
   as described in section 3.6 of [RFC 2830].  As specified there, the
   name the client checks for is the server's name before any
   potentially insecure transformations, including the SRV record
   lookup specified in this memo.  Thus the name the client MUST check
   for is the name obtained by doing the mapping step defined in
   section 2 above.  For example, if the DN "cn=John
   Doe,ou=accounting,dc=example,dc=net" is converted to the DNS name
   "example.net", the server's name MUST match "example.net".

This document describes a method that uses DNS SRV records to 
   discover LDAP servers.  All security considerations related to DNS
   SRV records are inherited by this document.  See the security 
   considerations section in [5] for more details.

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6. References

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

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

[3]  Kille, S. and M. Wahl, "Using Domains in LDAP/X.500
        Distinguished Names", RFC 2247, January 1998.

[4]  Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
        1034, STD 13, November 1987.

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

[6]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
        1700, October 1994.

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

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

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

7. Authors' Addresses

Michael P. Armijo
   One Microsoft Way
   Redmond, WA 98052
   micharm@microsoft.com

Paul Leach
   One Microsoft Way
   Redmond, WA 98052
   paulle@microsoft.com

Levon Esibov
   One Microsoft Way
   Redmond, WA 98052
   levone@microsoft.com

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RL "Bob" Morgan
   University of Washington
   4545 15th Ave NE
   Seattle, WA  98105
   US

Phone: +1 206 221 3307
   EMail: rlmorgan@washington.edu
   URI:   http://staff.washington.edu/rlmorgan/

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INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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10.  Expiration Date

This document is filed as <draft-ietf-ldapext-locate-08.txt>, and 
   expires December 5, 2002.

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