1. /etc/nsswitch.conf

The /etc/nsswitch.conf file configures how the system uses various databases and name resolution mechanisms. The first field of entries in this file identifies the name of the database. The second field defines a list of resolution mechanisms in the order in which the system attempts to resolve queries on the database.

The following example hosts definition from /etc/nsswitch.conf indicates that the system first attempts to resolve host names and IP addresses by querying files (that is, /etc/hosts) and, if that fails, next by querying a DNS server, and last of all, by querying NIS+ (NIS version 3) :

hosts: files dns nisplus