DNS on Windows Server 2003, 3rd Edition

Running a Secondary Name Server

You need to set up another name server for robustness. You can (and probably will) set up more than two name servers. Two servers are the minimum. If you have only one name server and it goes down, no one can look up names in your zone. A second name server splits the load with the first server or handles the whole load if the first server is down. You could set up another primary master name server, but we don’t recommend it. Set up a secondary name server instead.

How does a server know if it is a primary master or a secondary for a zone? The DNS server configuration information in the Registry tells the server it is a primary master or a secondary on a per zone basis. The NS records don’t tell us which server is the primary master for a zone and which servers are secondaries for a zone—they only say who the servers are. (Globally, DNS doesn’t care; as far as the actual name resolution goes, secondary servers are as good as primary master servers.)

What is different between a primary master name server and a secondary name server? The crucial difference is where the server gets its data. A primary master name server reads its data from files. A secondary name server loads its data over the network from another name server. This process is called a zone transfer.

A secondary name server is not limited to loading zones from a primary master name server; a secondary can load from another secondary. The big advantage of secondary name servers is that you maintain only one set of zone datafiles: the ones on the primary master name server. You don’t have to worry about synchronizing the files among name servers; the secondaries do that for you.

A secondary name server doesn’t need to retrieve all of its datafiles over the network; the cache.dns file is the same as on a primary master, so you’ll need a local copy on the secondary. Fortunately, the DNS server installation process includes this file.