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.
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Please send comments to the authors.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
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
[email protected]
Paul Leach
One Microsoft Way
Redmond, WA 98052
[email protected]
Levon Esibov
One Microsoft Way
Redmond, WA 98052
[email protected]
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RL "Bob" Morgan
University of Washington
4545 15th Ave NE
Seattle, WA 98105
US
Phone: +1 206 221 3307
EMail: [email protected]
URI: http://staff.washington.edu/rlmorgan/
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INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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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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