Directory Servers
Directory Server:
A directory service organizes computerized content and runs on a directory server computer. It is not to be confused with the directory itself, which is the database that holds the information about objects that are to be managed by the directory service. The directory service is the interface to the directory and provides access to the data that is contained in that directory.
From: en.wikipedia.org/wiki/Directory_server
Likewise Open
Likewise Open is an open source, community project sponsored by Likewise Software to provide fast & easy integration into networks utilizing Microsoft's Active Directory domains. Likewise Open is licensed under the Version 3 of the GNU General Public License so you can download it for free.
Key features
Joins Linux systems to Active Directory domains in a single step from the command line or from a GUI
Authenticates users with a single user name and password on both Windows and Linux systems
Enforces the same password policies for Linux users as for Windows users
Support for multiple forests with one-way and two-way cross forest trusts
Single sign-on for SSH and Putty
Single UID automatically assigned
Credential caching in the event of temporary loss of connectivity to the Active Directory Domain Controller
No Active Directory schema changes involved
Most recent Linux distributions supported
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