In a previous installment of this article, you learned about caching only DNS servers. In this article you will learn about the other two DNS server roles - primary DNS servers and secondary DNS servers.

What do you need to remember about a primary DNS server? Well, a primary DNS server hosts the working copy of a zone file. If you need to make changes to the zone file, it must be done from the server that is designated as the primary server for that zone. For those of you who are familiar with Windows NT 4.0, this is similar to how the Primary Domain Controller (PDC) maintains the working copy of the directory database. Once a server is configured as a primary DNS server for a zone, it is said to be authoritative for that domain. Also, a single DNS server can be the primary DNS server for multiple zones.

A secondary server gets all its zone information from a master DNS server. The secondary DNS server hosts a read-only copy of the zone file, which it gets from the primary DNS server or another secondary DNS server. Through a process known as a zone transfer, the master DNS server sends a copy of the zone file to the secondary server. Secondary DNS servers add some level of fault tolerance. In the event that the primary DNS server is unavailable, host names can still be resolved. However, no changes can be made to the zone file during this time (remember it stores a read only copy).

Now on the topic of zone transfers - Windows Server 2003 supports both full and incremental zone transfers. Incremental zone transfers only replicate the changes to the zone file, as opposed to the entire zone file. This of course, results in less network traffic.

When determining whether to add a secondary DNS server, you should recognize the following benefits:

• Fault Tolerance. Because the secondary server has a copy of the zone file, name resolution can continue should the primary DNS server be unavailable.

• Reduction in Name Resolution Traffic. Secondary servers can be placed in remote locations with a large number of users. Clients can then resolve hostnames locally instead of having to contact a primary DNS using a WAN link
• Load Balancing. Name resolution services for a zone can be provided by the secondary server as well, thereby reducing the load placed on the primary DNS server.