The Name Service Switch configuration file is at /etc/nssswitch.conf. In this file will be lines that list the database name followed by one or more words. In this example, this line tells the system to first use the /etc/hosts file for IP address to hostname resolution, and then use DNS second. If you were to issue command ping -c4 example.com, first the /etc/hosts file would be checked for a line that contains the IP address of example.com.

hosts:  files dns

In this example, the passwd command will first check the /etc/passwd file, and then sss. As an example, if the passwd user1 command is used to update the password for user1, first the /etc/passwd file will be checked for a record for user1, and then use SSSD second.

passwd:  files sss

Notice in this prior examples, that instead of /etc/hosts and /etc/passwd being listed, just the word files was used. When the word files is used, following are the paths to the files that are used.

aliases     /etc/aliases
ethers      /etc/ethers
group       /etc/group
hosts       /etc/hosts
initgroups  /etc/group
netgroup    /etc/netgroup
networks    /etc/networks
passwd      /etc/passwd
protocols   /etc/protocols
publickey   /etc/publickey
rpc         /etc/rpc
services    /etc/services
shadow      /etc/shadow