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INTERNET-DRAFT Michael P. Armijo
<draft-ietf-ldapext-locate-01.txt> Levon Esibov
January, 2000 Paul Leach
Expires: July, 2000 Microsoft Corporation
<draft-ietf-ldapext-locate-02.txt> Levon Esibov
April, 2000 Paul Leach
Expires: October, 2000 Microsoft Corporation
R.L. Morgan
University of Washington
Discovering LDAP Services with DNS
......@@ -27,92 +29,121 @@ Status of this Memo
http://www.ietf.org/shadow.html.
Distribution of this memo is unlimited. It is filed as <draft-
ietf-ldapext-locate-01.txt>, and expires on July 4, 2000.
ietf-ldapext-locate-02.txt>, and expires on October 15, 2000.
Please send comments to the authors.
1. Abstract
This draft defines a way that native Internet LDAP servers can make
use of the DNS's knowledge base to provide clients a method to
resolve LDAP services for a given domain.
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. This may
be the X.500 directory itself, but the LDAP specification
explicitly allows it to be a different directory. Let us define
a "native LDAP server" to be one that is not a front end to the
X.500 directory service. Let us further define an "Internet based
organization" as one that has a domain name, and an "Internet LDAP
server" to be one containing a directory entries for such an
organization.
This draft defines a way that native Internet LDAP servers can make
use of the DNS's knowledge base to perform the same function, while
still supporting integration with the X.500 directory.
This draft builds on RFC 2247[2] to define a mechanism by which
collections of native Internet LDAP servers can be integrated to
create a directory service. That work supports this cause by
defining a mapping from an LDAP DN to a DNS name that can be
resolved to the address of a server holding the entry corresponding
to the DN. For example, the DN "CN=Fred,OU=Eng,DC=example,DC=net"
maps to the DNS name "example.net".
In an Internet context, many of the names about which users seek
information are DNS names, or include DNS names. A native LDAP based
directory service for the Internet should make it convenient to
process such names -- there is a huge social investment spanning two
decades to get to the point where names like
"john.doe@somewhere.example" and "http://www.example.net" can
appear in newspaper articles, TV commercials, and on billboards
and millions of people understand what to do with them. As a result,
we assume that Internet based organizations wish to preserve this
investment, yet also want to deploy directory services.
Widespread use of, and dependence on, LDAP services will require that
they are robust and scalable. Both of these features are typically
supported by replicated servers. Use of SRV records to locate LDAP
servers supports clients' use of replicated servers.
3. Locating LDAP servers through DNS
LDAP server location information is to be stored using DNS Service
Location Record (SRV)[6]. 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 the SRV protocol[6]. The name of this record always has
the following format:
_<Service>._<Proto>.<Domain>
where <Service> is always "ldap", <Proto> is a protocol that can
be either "udp" or "tcp", and <Domain> is the domain hosted by the
LDAP Server. Note that "ldap" is the symbolic name for the LDAP
service in Assigned Numbers [7], as required by the SRV Protocol[6].
Presence of such records enables clients to find the LDAP servers
using standard DNS query [3]. As an example, a client that searches
for an LDAP server in the example.net domain that supports TCP protocol
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.
The output domain name is initially empty. For each RDN component
of the DN, beginning with the first, if the attribute type is "DC",
then the attribute value is used as a domain name component (label).
The first such value becomes the most significant (i.e., rightmost)
domain name component, and successive values occupy less significant
positions (i.e., extending leftward), in order. If the attribute
type is not "DC", then processing stops. If the first RDN component
of the DN is not of type "DC" then the DN cannot be converted to a
domain name.
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:
_<Service>._<Proto>.<Domain>
where <Service> is always "ldap", and <Proto> is a protocol that can
be either "udp" or "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].
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 record:
_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.
_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.
The set of returned records may contain multiple records in the
case where multiple LDAP servers serve the same domain.
4. Security Considerations
5. Security Considerations
This document describes a method that uses DNS SRV records to
discover LDAP servers. All security considerations related to DNS
......@@ -120,28 +151,27 @@ Status of this Memo
considerations section in [6] for more details.
5. References
6. References
[1] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol(v3)". RFC 2251, December 1997.
[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol(v3)", RFC 2251, December 1997.
[2] S. Kille, M. Wahl, "Using Domains in LDAP/X.500 Distinguished
Names". RFC 2247, January 1998.
[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] P. Mockapetris, RFC 1034, DOMAIN NAMES - CONCEPTS AND FACILITIES,
November, 1987.
[4] P. Mockapetris, RFC 1035, DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION, November, 1987.
[3] Kille, S. and M. Wahl, "Using Domains in LDAP/X.500
Distinguished Names", RFC 2247, January 1998.
[5] T. Howes, M. Smith, "The LDAP URL Format". RFC 2255 December 1997.
[4] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC
1034, STD 13, November 1987.
[6] A. Gulbrandsen, P. Vixie, L. Esibov, "A DNS RR for specifying the
location of services (DNS SRV)".
http://www.ietf.org/internet-drafts/draft-ietf-
dnsind-rfc2052bis-05.txt, November 1999.
[5] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[7] J. Reynolds, J. Postel, "Assigned Numbers", STD 2, October 1994
[6] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
6. Authors' Addresses
......@@ -161,5 +191,15 @@ Status of this Memo
Redmond, WA 98052
levone@microsoft.com
Expires July, 2000
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 October, 2000
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