The easiest to add or edit the dns servers your server uses is to edit the file at /etc/resolv.conf and then add dns entries in the following manner
nameserver 184.108.40.206 nameserver 220.127.116.11 nameserver 18.104.22.168 nameserver 22.214.171.124
So I’ve gone and set one each of Google’s DNS servers and OpenDNS servers and then their secondary IP’s.
Restart network and you’re golden.
service network restart //this command also works on CentOS 7