The Resolver

DNS clients use the dynamic library routines, collectively called the resolver, to locate a remote host. The resolver queries the DNS database on a name server, which eventually returns the host name or IP address of the machine requested by the resolver. Because DNS name servers are clients of servers outside their local domains, they must also run the resolver.

The DNS name server uses several files to load its database. At the resolver level, it needs the file /etc/resolv.conf listing the addresses of the servers where it can obtain its information. The resolver reads this resolv.conf file to find the name of the local domain and the location of name servers. It sets the local domain name and instructs the resolver routines to query the listed name servers for information. Normally, each DNS client system on your network has a resolv.conf file in its /etc directory. (If a client does not have a resolv.conf file, it defaults to using a server at IP address 127.0.0.1.)

Whenever the resolver has to find the IP address of a host (or the host name corresponding to an address), the resolver builds a query package and sends it to the name servers listed in /etc/resolv.conf. The servers either answer the query locally or contact other servers known to them, ultimately returning the answer to the resolver.