1 INTERNET-DRAFT Michael P. Armijo
2 <draft-ietf-ldapext-locate-03.txt> Levon Esibov
4 Expires: January, 2001 Microsoft Corporation
6 University of Washington
8 Discovering LDAP Services with DNS
12 This document is an Internet-Draft and is in full conformance with
13 all provisions of Section 10 of RFC2026.
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31 Distribution of this memo is unlimited. It is filed as <draft-
32 ietf-ldapext-locate-03.txt>, and expires on January 14, 2001.
33 Please send comments to the authors.
38 A Lightweight Directory Access Protocol (LDAP) request must be
39 directed to an appropriate server for processing. This document
40 specifies a method for discovering such servers using information in
41 the Domain Name System.
46 The LDAPv3 protocol [1] is designed to be a lightweight access
47 protocol for directory services supporting X.500 models. As a
48 distributed directory service, the complete set of directory
49 information (known as the Directory Information Base) is spread
50 across many different servers. Hence there is the need to
51 determine, when initiating or processing a request, which servers
52 hold the relevant information. In LDAP, the Search, Modify, Add,
53 Delete, ModifyDN, and Compare operations all specify a Distinguished
54 Name (DN) [2] on which the operation is performed. A client, or a
55 server acting on behalf of a client, must be able to determine the
56 server(s) that hold the naming context containing that DN, since
57 that server (or one of that set of servers) must receive and process
58 the request. This determination process is called "server
59 location". To support dynamic distributed operation, the
60 information needed to support server location must be available via
61 lookups done at request processing time, rather than, for example,
62 as static data configured into each client or server.
64 It is possible to maintain the information needed to support server
65 location in the directory itself, and X.500 directory deployments
66 typically do so. In practice, however, this only permits location
67 of servers within a limited X.500-connected set. LDAP-specific
68 methods of maintaining server location information in the directory
69 have not yet been standardized. This document defines an
70 alternative method of managing server location information using the
71 Domain Name System. This method takes advantage of the global
72 deployment of the DNS, by allowing LDAP server location information
73 for any existing DNS domain to be published by creating the records
74 described below. A full discussion of the benefits and drawbacks of
75 the various directory location and naming methods is beyond the
76 scope of this document.
78 RFC 2247[3] defines an algorithm for mapping DNS domain names into
79 DNs. This document defines the inverse mapping, from DNs to DNS
80 domain names, based on the conventions in [3], for use in this
81 server location method. The server location method described in
82 this document is only defined for DNs that can be so mapped, i.e.,
83 those DNs that are based on domain names. In practice this is
84 reasonable because many objects of interest are named with domain
85 names, and use of domain-name-based DNs is becoming common.
88 3. Mapping Distinguished Names into Domain Names
90 This section defines a method of converting a DN into a DNS domain
91 name for use in the server location method described below. Some
92 DNs cannot be converted into a domain name. Converted DNs result
93 in a fully qualified domain name.
95 The output domain name is initially empty. For each RDN component
96 of the DN, beginning with the rightmost and working left, if the
97 attribute type is "DC", then the attribute value is used as a domain
98 name component (label).
99 The first such value becomes the most significant (i.e., rightmost)
100 domain name component, and successive values occupy less significant
101 positions (i.e., extending leftward), in order. If the attribute
102 type is not "DC", then processing stops. If the final RDN component
103 of the DN is not of type "DC" then the DN cannot be converted to a
108 cn=John Doe,ou=accounting,dc=example,dc=net
110 The client would convert the DC components as defined above into
115 The determined DNS name will be submitted as a DNS query using the
116 algorithm defined in section 4.
119 4. Locating LDAP servers through DNS
121 LDAP server location information is to be stored using DNS Service
122 Location Record (SRV)[5]. The data in a SRV record contains the DNS
123 name of the server that provides the LDAP service, corresponding
124 Port number, and parameters that enable the client to choose an
125 appropriate server from multiple servers according to the algorithm
126 described in [5]. The name of this record has the following format:
128 _<Service>._<Proto>.<Domain>
130 where <Service> is always "ldap", and <Proto> is a protocol that can
131 be either "udp" or "tcp". "_ldap._tcp" applies to services
132 compatible with LDAPv2 [7] or LDAPv3 [1]. "_ldap._udp"
133 applies to services compatible with CLDAP [8]. <Domain> is
134 the domain name formed by converting the DN of a naming context
135 mastered by the LDAP Server into a domain name using the algorithm in
136 Section 3. Note that "ldap" is the symbolic name for the LDAP service
137 in Assigned Numbers[6], as required by [5].
139 Presence of such records enables clients to find the LDAP servers
140 using standard DNS query [4]. A client (or server) seeking an LDAP
141 server for a particular DN converts that DN to a domain name using
142 the algorithm of Section 2, does a SRV record query using the DNS
143 name formed as described in the preceding paragraph, and interprets
144 the response as described in [5] to determine a host (or hosts) to
145 contact. As an example, a client that searches for an LDAP server
146 for the DN "ou=foo,dc=example,dc=net" that supports the TCP protocol
147 will submit a DNS query for a set of SRV records with owner name:
149 _ldap._tcp.example.net.
151 The client will receive the list of SRV records published in DNS
152 that satisfy the requested criteria. The following is an example of
155 _ldap._tcp.example.net. IN SRV 0 0 389 phoenix.example.net.
157 The set of returned records may contain multiple records in the case
158 where multiple LDAP servers serve the same domain. If there are no
159 matching SRV records available for the converted DN the client SHOULD
160 NOT attempt to 'walk the tree' by removing the least significant
161 portion of the constructed fully qualified domain name.
164 5. Security Considerations
166 This document describes a method that uses DNS SRV records to
167 discover LDAP servers. All security considerations related to DNS
168 SRV records are inherited by this document. See the security
169 considerations section in [6] for more details.
174 [1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
175 Protocol(v3)", RFC 2251, December 1997.
177 [2] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory Access
178 Protocol (v3): UTF-8 String Representation of Distinguished
179 Names", RFC 2253, December 1997.
181 [3] Kille, S. and M. Wahl, "Using Domains in LDAP/X.500
182 Distinguished Names", RFC 2247, January 1998.
184 [4] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
185 1034, STD 13, November 1987.
187 [5] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
188 specifying the location of services (DNS SRV)", RFC 2782,
191 [6] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
194 [7] Yeong, W., Howes, T. and Kille, S., "Lightweight Directory Access
195 Protocol", RFC 1777, March 1995
197 [8] Young, A., "Connection-less Lightweight Directory Access Protocol",
201 7. Authors' Addresses
206 micharm@microsoft.com
219 University of Washington
224 Phone: +1 206 221 3307
225 EMail: rlmorgan@washington.edu
226 URI: http://staff.washington.edu/rlmorgan/
228 Expires January, 2001