The caching technique described above is very important. After the time in the TTL has passed, the record is dropped from the cache, in order to ensure the latest information is maintained. Note that records read will include information about where to find the name servers for a particular zone. When this data is cached it becomes unnecessary to continually contact root servers. The clients in the process are called resolvers.
Applications use the resolvers to obtain an address mapping implemented as a system call.
In this section, you will learn how to configure a domain name server. First some of the concepts are presented. Each zone file contains many resource records. When name servers boot, they need to be told what functions they will perform. Here are now some configuration files.