6.2 DC Locator
One of the
fundamental issues for clients in any NOS environment is finding the
most optimal domain controller (DC) to authenticate against. The
process under Windows NT was not very efficient and could cause
clients to authenticate to domain controllers in the least optimal
location. With Active Directory, clients use DNS to locate domain
controllers via the DC locator process. To illustrate at a high level
how the DC locator process works, we will describe an example where a
client has moved from one location to another and needs to find a DC:
A client previously located in Site A logs in from Site B. When the client boots up, it thinks it is still in Site A, so it
proceeds to contact a DC in Site A using DNS unless the server name
was previously cached. The DC in Site A receives the request and realizes that the client
should now be talking to a DC in Site B due to its IP address
changing. If the server does not cover Site B, it will return the
clients new site in the reply. The client will then perform a DNS lookup to find a DC in Site B. The client then contacts the DC in Site B. Three things can happen:
the DC responds and authenticates the client; the DC fails to respond
(it could be down), and the client attempts to use a different DC in
Site B; or the DC fails to respond, and the client searches and fails
to find another DC in Site B, instead turning back to the DC in Site
A and authenticating with the original server.
The two main things that are needed to support the DC locator process
are proper definition of the site topology in Active Directory and
containment of all the necessary Active Directory related resource
records in DNS. In the next section, we will describe the purpose of
the resource records used in Active Directory. For a more detailed
description of how the DC locator process works, including the
specific resource records that are queried during the process, check
out Microsoft Knowlede Base (KB) article 247811 "How
Domain Controllers Are Located in Windows" and
Microsoft KB article 314861 "How Domain Controllers
Are Located in Windows XP" at http://support.microsoft.com.
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