DNS Clients and the Resolver

To be a DNS client, a machine must run the resolver. The resolver is neither a daemon nor a single program; rather, it is a set of dynamic library routines used by applications that need to know machine names. The resolver's function is to resolve users' queries. To do that, it queries a name server, which then returns either the requested information or a referral to another server. Once the resolver is configured, a machine can request DNS service from a name server.

When a machine's /etc/nsswitch.conf file specifies hosts: dns (or any other variant that includes dns in the hosts line), the resolver libraries are automatically used. If the nsswitch.conf file specifies some other name service before dns, that name service is consulted first for host information and only if that name service does not find the host in question are the resolver libraries used.

For example, if the hosts line in the nsswitch.conf file specifies hosts: nisplus dns, the NIS+ name service will first be searched for host information. If the information is not found in NIS+, then the DNS resolver is used. Since name services such as NIS+ and NIS only contain information about hosts in their own network, the effect of a hosts:nisplus dns line in a switch file is to specify the use of NIS+ for local host information and DNS for information on remote hosts out on the Internet.

There are two kinds of DNS clients:

• Client-only. A client-only DNS client does not run in.named; instead, it consults the resolver. The resolver knows about a list of name servers for the domain, to which queries are then directed.

• Client-server. A client-server uses the services provided by in.named to resolve queries forwarded to it by client-machine resolvers.

The Solaris operating environment includes the dynamic library routines that make up the resolver. Solaris Naming Setup and Configuration Guide, contains instructions for setting up a host as a DNS client.