Domain Controller

On Microsoft Servers, a domain controller (DC) is a server computer that responds to security authentication requests (logging in, checking permissions, etc.) within a Windows domain. A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.

With Windows NT 4 Server, one domain controller per domain was configured as the primary domain controller (PDC); all other domain controllers were backup domain controllers (BDC).

Because of the critical nature of the PDC, best practices dictated that the PDC should be dedicated solely to domain services, and not used for file, print or application services that could slow down or crash the system. Some network administrators took the additional step of having a dedicated BDC online for the express purpose of being available for promotion if the PDC failed.

A BDC could authenticate the users in a domain, but all updates to the domain (new users, changed passwords, group membership, etc.) could only be made via the PDC, which would then propagate these changes to all BDCs in the domain. If the PDC was unavailable (or unable to communicate with the user requesting the change), the update would fail. If the PDC was permanently unavailable (e.g. if the machine failed), an existing BDC could be promoted to be a PDC.

Windows 2000 and later versions introduced Active Directory ("AD"), which largely eliminated the concept of PDC and BDC in favor of multi-master replication. However, there are still several roles that only one domain controller can perform, called the Flexible single master operation roles. Some of these roles must be filled by one DC per domain, while others only require one DC per AD forest. If the server performing one of these roles is lost, the domain can still function, and if the server will not be available again, an administrator can designate an alternate DC to assume the role in a process known as "seizing" the role.

In Windows NT 4, one DC serves as the primary domain controller (PDC). Others, if they exist, are usually a backup domain controller (BDC). The PDC is typically designated as the "first". The "User Manager for Domains" is a utility for maintaining user/group information. It uses the domain security database on the primary controller. The PDC has the master copy of the user accounts database which it can access and modify. The BDC computers have a copy of this database, but these copies are read-only. The PDC will replicate its account database to the BDCs on a regular basis. The BDCs exist in order to provide a backup to the PDC, and can also be used to authenticate users logging on to the network. If a PDC should fail, one of the BDCs can then be promoted to take its place. The PDC will usually be the first domain controller that was created unless it was replaced by a promoted BDC.

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