Chapter 25. LDAP—A Directory Service


25.1. LDAP versus NIS
25.2. Structure of an LDAP Directory Tree
25.3. Server Configuration with slapd.conf
25.4. Data Handling in the LDAP Directory
25.5. The YaST LDAP Client
25.6. Configuring LDAP Users and Groups in YaST
25.7. For More Information


The Lightweight Directory Access Protocol (LDAP) is a set of protocols designed to access and maintain information directories. LDAP can be used for numerous purposes, like user and group management, system configuration management, or address management. This chapter provides a basic understanding of how OpenLDAP works and how to manage LDAP data with YaST. While there are several implementations of the LDAP protocol, this chapter focuses entirely on the OpenLDAP implementation.

It is crucial within a networked environment to keep important information structured and quickly available. This can be done with a directory service that, like the common yellow pages, keeps information available in a well-structured, quickly searchable form.

In the ideal case, a central server keeps the data in a directory and distributes it to all clients using a certain protocol. The data is structured in a way that allows a wide range of applications to access it. That way, it is not necessary for every single calendar tool and e-mail client to keep its own database—a central repository can be accessed instead. This notably reduces the administration effort for the information. The use of an open and standardized protocol like LDAP ensures that as many different client applications as possible can access such information.

A directory in this context is a type of database optimized for quick and effective reading and searching:

The design of a directory service like LDAP is not laid out to support complex update or query mechanisms. All applications accessing this service should gain access quickly and easily.

Many directory services have previously existed and still exist both in Unix and outside it. Novell NDS, Microsoft ADS, Banyan's Street Talk, and the OSI standard X.500 are just a few examples. LDAP was originally planned as a lean flavor of DAP, the directory access protocol, which was developed for accessing X.500. The X.500 standard regulates the hierarchical organization of directory entries.

LDAP is a trimmed down version of DAP. Without losing the X.500 entry hierarchy, profit from LDAP's cross-platform capabilities and save resources. The use of TCP/IP makes it substantially easier to establish interfaces between a docking application and the LDAP service.

LDAP, meanwhile, has evolved and is increasingly employed as a stand-alone solution without X.500 support. LDAP supports referrals with LDAPv3 (the protocol version in package openldap2), making it possible to have distributed databases. The usage of SASL (simple authentication and security layer) is also new.

LDAP is not limited to querying data from X.500 servers, as it was originally planned. There is an open source server slapd, which can store object information in a local database. There is also an extension called slurpd, which is responsible for replicating multiple LDAP servers.

The openldap2 package consists of:


A stand-alone LDAPv3 server that administers object information in a BerkeleyDB-based database.


This program enables the replication of modifications to data on the local LDAP server to other LDAP servers installed on the network.

additional tools for system maintenance

slapcat, slapadd, slapindex

25.1. LDAP versus NIS

The Unix system administrator traditionally uses the NIS service for name resolution and data distribution in a network. The configuration data contained in the files in /etc and the directories group, hosts, mail, netgroup, networks, passwd, printcap, protocols, rpc, and services are distributed by clients all over the network. These files can be maintained without major effort because they are simple text files. The handling of larger amounts of data, however, becomes increasingly difficult due to nonexistent structuring. NIS is only designed for Unix platforms. This means it is not suitable as a centralized data administration tool in heterogeneous networks.

Unlike NIS, the LDAP service is not restricted to pure Unix networks. Windows servers (from 2000) support LDAP as a directory service. Novell also offers an LDAP service. Application tasks mentioned above are additionally supported in non-Unix systems.

The LDAP principle can be applied to any data structure that should be centrally administered. A few application examples are:

  • Employment as a replacement for the NIS service

  • Mail routing (postfix, sendmail)

  • Address books for mail clients, like Mozilla, Evolution, and Outlook

  • Administration of zone descriptions for a BIND9 name server

  • User authentication with Samba in heterogeneous networks

This list can be extended because LDAP is extensible, unlike NIS. The clearly-defined hierarchical structure of the data eases the administration of large amounts of data, because it can be searched better.