Initialization of DNS Service
There are thirteen root server clusters that are authoritative for queries to the global DNS root zone. The root servers hold the lists of names and addresses for the authoritative servers for all top-level domains. Every name lookup must either start with a query to a root server or use information that was once obtained from a root server.
The root servers have the official names a.root-servers.net to m.root-servers.net. However, to look up the IP address of a root server from these names, a DNS resolver must first be able to look up a root server to find the address of an authoritative server for the .net DNS zone. Clearly this creates a circular dependency, so the address of at least one root server must be known by a host in order to bootstrap access to the DNS. This is usually done by shipping the addresses of all known DNS root servers as a file with the computer operating system: the IP addresses of some root servers will change over the years, but only one correct address is needed for the resolver to obtain the current list of name servers. This file is called named.cache in the BIND nameserver reference implementation and a current version is officially distributed by ICANN's InterNIC.
Once the address of a single functioning root server is known, all other DNS information can be discovered recursively, and the address of any domain name may be found.
Read more about this topic: DNS Root Zone
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