1 INTERNET-DRAFT Michael P. Armijo
2 <draft-ietf-ldapext-locate-04.txt> Levon Esibov
3 August, 2000 Paul Leach
4 Expires: February, 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.
15 Internet-Drafts are working documents of the Internet Engineering
16 Task Force (IETF), its areas, and its working groups. Note that
17 other groups may also distribute working documents as Internet-
20 Internet-Drafts are draft documents valid for a maximum of six months
21 and may be updated, replaced, or obsoleted by other documents at any
22 time. It is inappropriate to use Internet- Drafts as reference
23 material or to cite them other than as "work in progress."
25 The list of current Internet-Drafts can be accessed at
26 http://www.ietf.org/ietf/1id-abstracts.txt
28 The list of Internet-Draft Shadow Directories can be accessed at
29 http://www.ietf.org/shadow.html.
31 Distribution of this memo is unlimited. It is filed as <draft-
32 ietf-ldapext-locate-04.txt>, and expires on February 25, 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. The DN is processed in
96 right-to-left order (i.e., beginning with the first RDN in the
97 sequence of RDNs). An RDN is able to be converted if it (1)
98 consists of a single AttributeTypeAndValue; (2) the attribute type
99 is "DC"; and (3) the attribute value is non-null. If it can be
100 converted, the attribute value is used as a domain name component
101 (label). The first such value becomes the rightmost (i.e., most
102 significant) domain name component, and successive converted RDN
103 values extend to the left. If an RDN cannot be converted,
104 processing stops. If the output domain name is empty when
105 processing stops, the DN cannot be converted into a domain name.
109 cn=John Doe,ou=accounting,dc=example,dc=net
111 The client would convert the DC components as defined above into
116 The determined DNS name will be submitted as a DNS query using the
117 algorithm defined in section 4.
120 4. Locating LDAP servers through DNS
122 LDAP server location information is to be stored using DNS Service
123 Location Record (SRV)[5]. The data in a SRV record contains the DNS
124 name of the server that provides the LDAP service, corresponding
125 Port number, and parameters that enable the client to choose an
126 appropriate server from multiple servers according to the algorithm
127 described in [5]. The name of this record has the following format:
129 _<Service>._<Proto>.<Domain>
131 where <Service> is always "ldap", and <Proto> is a protocol that can
132 be either "udp" or "tcp". <Domain> is the domain name formed by
133 converting the DN of a naming context mastered by the LDAP Server
134 into a domain name using the algorithm in Section 3. Note that
135 "ldap" is the symbolic name for the LDAP service in Assigned
136 Numbers[6], as required by [5].
138 Presence of such records enables clients to find the LDAP servers
139 using standard DNS query [4]. A client (or server) seeking an LDAP
140 server for a particular DN converts that DN to a domain name using
141 the algorithm of Section 3, does a SRV record query using the DNS
142 name formed as described in the preceding paragraph, and interprets
143 the response as described in [5] to determine a host (or hosts) to
144 contact. As an example, a client that searches for an LDAP server
145 for the DN "ou=foo,dc=example,dc=net" that supports the TCP protocol
146 will submit a DNS query for a set of SRV records with owner name:
148 _ldap._tcp.example.net.
150 The client will receive the list of SRV records published in DNS
151 that satisfy the requested criteria. The following is an example of
154 _ldap._tcp.example.net. IN SRV 0 0 389 phoenix.example.net.
156 The set of returned records may contain multiple records in the case
157 where multiple LDAP servers serve the same domain. If there are no
158 matching SRV records available for the converted DN the client SHOULD
159 NOT attempt to 'walk the tree' by removing the least significant
160 portion of the constructed fully qualified domain name.
163 5. Security Considerations
165 DNS responses can typically be easily spoofed. Clients using this
166 location method SHOULD ensure, via use of strong security
167 mechanisms, that the LDAP server they contact is the one they
168 intended to contact. See [7] for more information on security
169 threats and security mechanisms.
171 This document describes a method that uses DNS SRV records to
172 discover LDAP servers. All security considerations related to DNS
173 SRV records are inherited by this document. See the security
174 considerations section in [5] for more details.
179 [1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
180 Protocol(v3)", RFC 2251, December 1997.
182 [2] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory Access
183 Protocol (v3): UTF-8 String Representation of Distinguished
184 Names", RFC 2253, December 1997.
186 [3] Kille, S. and M. Wahl, "Using Domains in LDAP/X.500
187 Distinguished Names", RFC 2247, January 1998.
189 [4] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
190 1034, STD 13, November 1987.
192 [5] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
193 specifying the location of services (DNS SRV)", RFC 2782,
196 [6] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
199 [7] Wahl, M., Alvestrand, H., Hodges, J. and Morgan, R.,
200 "Authentication Methods for LDAP", RFC 2829, May 2000.
203 7. Authors' Addresses
208 micharm@microsoft.com
221 University of Washington
226 Phone: +1 206 221 3307
227 EMail: rlmorgan@washington.edu
228 URI: http://staff.washington.edu/rlmorgan/
230 Expires February 25, 2001