INTERNET-DRAFT Michael P. Armijo Levon Esibov August, 2000 Paul Leach Expires: February, 2001 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 other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Distribution of this memo is unlimited. It is filed as , and expires on February 25, 2001. Please send comments to the authors. 1. 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. 2. 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. 3. 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 4. 4. Locating LDAP servers through DNS LDAP 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: _._. where is always "ldap", and is a protocol that can be either "udp" or "tcp". 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 3. Note that "ldap" is the symbolic name for the LDAP service in Assigned Numbers[6], as required by [5]. 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 3, 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. 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. 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. 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. 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 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/ Expires February 25, 2001