10.4.1 The resolver

The job of finding out what IP addresses are associated with a particular domain name is the job of a resolver. The most commonly used resolver is the set of functions that go by that name (resolver(3)) in the GNU C Library. Another is the FireDNS resolver which comes in the libfiredns package. There are others.

How the GNU LIBC resolver resolves names is governed by the hosts line in the /etc/nsswitch.conf configuration file. This line lists the services that should be used to resolve a name: e.g., dns, files, nis, nisplus. See nsswitch.conf(5). Insofar as the files service is used, the behavior of the resolver is also governed by the /etc/hosts configuration file. See hosts(5).

All of the above files are static and can be edited with your favorite editor.

Insofar as the dns service is used, the behavior of the resolver is also governed by the /etc/resolv.conf configuration file. See resolv.conf(5). One of the important functions of resolv.conf is to list the IP addresses of nameservers that will be contacted to resolve the name. This list often has to depend upon the network environment and the network environment may change from time to time while your computer is running. Programs such as pppd and dhclient are able to manipulate resolv.conf to add and remove lines, but these features do not always work properly and they conflict with one another. The resolvconf package solves the problem better by providing a standard framework for updating this file. See Managing nameserver information – resolvconf, Section 10.4.2.