About Network Manager
Do not worry for the comment after the # symbol that states “Generated by NetworkManager” that was valid before we disabled DHCP on our CentOS 7 machine, since then, our server is using an static IP Address and also static or manually configured DNS settings.
Resolv.conf file settings
The settings for the “resolv.conf” file are really simple, just two lines as you can see below.
It is quite common that the Network Manager overwrites the DNS settings, if that is your case then follow the steps listed on this article to permanently disable Network Manager.