10.2.2.3. /etc/nsswitch.conf

The /etc/nsswitch.conf file defines the order in which to contact different name services. For Internet use, it is important that dns shows up in the "hosts" line:

[bob@tux ~] grep hosts /etc/nsswitch.conf
hosts:  files dns

This instructs your computer to look up hostnames and IP addresses first in the /etc/hosts file, and to contact the DNS server if a given host does not occur in the local hosts file. Other possible name services to contact are LDAP, NIS and NIS+.

More in man nsswitch.conf.