Chapter 28. Lightweight Directory Access Protocol (LDAP)

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The Lightweight Directory Access Protocol (LDAP) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to as "X.500 Lite." The X.500 standard is a directory that contains hierarchical and categorized information, which could include information such as names, addresses, and phone numbers.
Like X.500, LDAP organizes information in a hierarchal manner using directories. These directories can store a variety of information and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.
In many cases, LDAP is used as a virtual phone directory, allowing users to easily access contact information for other users. But LDAP is more flexible than a traditional phone directory, as it is capable of referring a querent to other LDAP servers throughout the world, providing an ad-hoc global repository of information. Currently, however, LDAP is more commonly used within individual organizations, like universities, government departments, and private companies.
LDAP is a client/server system. The server can use a variety of databases to store a directory, each optimized for quick and copious read operations. When an LDAP client application connects to an LDAP server, it can either query a directory or attempt to modify it. In the event of a query, the server either answers the query locally, or it can refer the querent to an LDAP server which does have the answer. If the client application is attempting to modify information within an LDAP directory, the server verifies that the user has permission to make the change and then adds or updates the information.
This chapter refers to the configuration and use of OpenLDAP 2.0, an open source implementation of the LDAPv2 and LDAPv3 protocols.

28.1. Why Use LDAP?

The main benefit of using LDAP is that information for an entire organization can be consolidated into a central repository. For example, rather than managing user lists for each group within an organization, LDAP can be used as a central directory accessible from anywhere on the network. And because LDAP supports Transport Layer Security (TLS), sensitive data can be protected from prying eyes.


Due to the vulnerability described in Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings, Red Hat recommends that you do not rely on the SSLv3 protocol for security. OpenLDAP is one of the system components that do not provide configuration parameters that allow SSLv3 to be effectively disabled. To mitigate the risk, it is recommended that you use the stunnel command to provide a secure tunnel, and disable stunnel from using SSLv3.
LDAP also supports a number of back-end databases in which to store directories. This allows administrators the flexibility to deploy the database best suited for the type of information the server is to disseminate. Because LDAP also has a well-defined client Application Programming Interface (API), the number of LDAP-enabled applications are numerous and increasing in quantity and quality.

28.1.1. OpenLDAP Features

OpenLDAP includes a number of important features.
  • LDAPv3 Support — OpenLDAP supports Simple Authentication and Security Layer (SASL), and Transport Layer Security (TLS) among other improvements. Many of the changes in the protocol since LDAPv2 are designed to make LDAP more secure.
  • IPv6 Support — OpenLDAP supports the next generation Internet Protocol version 6.
  • LDAP Over IPC — OpenLDAP can communicate within a system using interprocess communication (IPC). This enhances security by eliminating the need to communicate over a network.
  • Updated C API — Improves the way programmers can connect to and use LDAP directory servers.
  • LDIFv1 Support — Provides full compliance with the LDAP Data Interchange Format (LDIF) version 1.
  • Enhanced Stand-Alone LDAP Server — Includes an updated access control system, thread pooling, better tools, and much more.
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