Should at least one DC host a DNS server per AD site?

Many of Microsoft's AD best practices white papers suggest that at least one domain controller (DC) host a DNS server per Active Directory site that is deployed. In making that suggestion, the papers don't necessarily back the suggestion up with any technical reasons. Do you agree with the white papers? If so, why?

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DNS is critical to the operation of AD. If there is no DNS, AD cannot properly register its services in the DNS and the ability to utilize those services is effectively disabled.

So, say you have a site in London and a site in New York. If the link between the two is down and there is only a DNS server in NY, despite the fact that the London DC is up and running, only those systems with cached DNS information will be able to find the server. Thus, your AD in London becomes disabled. Of course, if London is a single-broadcast domain, the workstation may still be able to locate the server, but the performance will be greatly degraded. So, if you put a DNS on the DC in London, even though the link to NY goes down, the London DNS is able to serve as locator for all of services the client systems might need.

Now the reason we use DCs is because you can utilize Active Directory zones, which are automatically replicated along with the AD schema information to all DCs.

This was first published in March 2003

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