We introduced resolvers way back in Chapter 2, but we didn't say much more about them. The resolver, you'll remember, is the client half of the Domain Name System. It's responsible for translating a program's request for host information into a query to a name server and for translating the response into an answer for the program.
We haven't done any resolver configuration yet because the occasion for it hasn't arisen. When we set up our name servers in Chapter 4, the resolver's default behavior worked just fine for our purposes. But if we'd needed the resolver to do more than or behave differently from the default, we would have had to configure the resolver.
There's one thing we should mention up front: what we describe in the next few sections is the behavior of the Windows 2000, Windows XP, and Windows Server 2003 resolvers. There are lots of other resolvers, though. Every version of Windows has its own resolver, and the configuration and behavior of each one is slightly different. Unix hosts normally use some variant of the BIND resolver, discussed in O'Reilly's DNS and BIND, and many Unix vendors have extended their resolvers' functionality. Still, the basic concepts behind the operation of each resolver are quite common.
 Installing a Service Pack can also change resolver behavior.