DNS supports a few different server configurations. For the exam, you need to know the difference between the server roles and when you would want to use one over the other. So let’s take a look at the different server roles supported by DNS.

The three main server roles supported by DNS include:

• Caching-only server

• Primary server
• Secondary server

The first type of DNS server you can configure is a caching only DNS server. All DNS servers maintain a cache.dns file that contains a list of all Internet root servers. Any time a DNS server resolves a hostname to an IP address, the information is added to the cache file. The next time a DNS client needs to resolve the hostname, the information can be retrieved from the cache as opposed to the Internet.

Caching-only servers however do not contain any zone information. This is the difference between a primary/secondary DNS server and a caching only DNS server. The main purpose of a caching-only server (other than providing name resolution) is to build the cache file as names are resolved. They resolve hostnames, cache the information, and return the results to the client. Just remember that caching only DNS servers do not store any zone data.

This type of DNS server is ideal in situations where network traffic is a concern. Again, because there is no zone information, there is no zone transfer traffic generated from replication. Hostname traffic is also reduced as the cache file is built up.

For the exam, you need to know when caching-only servers should be implemented. Caching-only servers are useful when there are remote locations that have slow WAN links. Configuring a caching-only server in these locations can reduce network traffic that would normally be generated between primary and secondary DNS servers, and speed up hostname resolution once the cache file is established.