LDAP Server

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The Lightweight Directory Access Protocol, or LDAP, is an application protocol for querying and modifying directory services running over TCP/IP.

A directory is a set of objects with similar attributes organized in a logical and hierarchical manner. The most common example is the telephone directory, which consists of a series of names (either of persons or organizations) organized alphabetically, with each name having an address and phone number attached. Due to this basic design (among other factors) LDAP is often used by other services for authentication, despite the security problems this causes.

Other Tutorials
http://linuxwiki.riverworth.com/index.php?title=LDAP_Authentication