Domain Names

A domain name is the name assigned to a group of systems on a local network that share DNS administrative files. A domain name is required for the network information service database to work properly.

Default Domain Name

DNS obtains your default domain name from your resolv.conf file.

• If the resolv.conf file is not available, or does not identify a default domain, and if your enterprise-level naming service is either NIS+ or NIS, the Sun implementation of DNS obtains the default domain name from those services.

• If resolv.conf is not available or does not provide a domain name and you are not running either NIS+ or NIS, you must either provide a resolv.conf file on each machine that does specify the domain or set the LOCALDOMAIN environment variable.

Trailing Dots in Domain Names

When working with DNS-related files, follow these rules regarding the trailing dot in domain names:

• Use a trailing dot in domain names in hosts, hosts.rev, named.ca, and named.local data files. For example, sales.doc.com. is correct for these files.

• Do not use a trailing dot in domain names in named.boot or resolv.conf files. For example, sales.doc.com is correct for these files.

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. 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.

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.

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 naming service before dns, that naming service is consulted first for host information and only if that naming 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+ naming service will first be searched for host information. If the information is not found in NIS+, then the DNS resolver is used. Since naming 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.