In CentOS or Red Hat Enterprise Linux (RHEL) 7, you can find your /etc/resolv.conf file, which holds all nameserver configurations for your server, to be overwritten by the NetworkManager.
If you check the content of /etc/resolv.conf, it may look like this.
$ cat /etc/resolv.conf # Generated by NetworkManager search mydomain.tld nameserver 8.8.8.8
The NetworkManager will assume it has the rights to control /etc/resolv.conf, if it finds a DNS related configuration in your interface configuration file.
$ grep DNS /etc/sysconfig/network-scripts/ifcfg-* DNS1="8.8.8.8" IPV6_PEERDNS="yes"
To prevent Network Manager to overwrite your resolv.conf changes, remove the DNS1, DNS2, … lines from /etc/sysconfig/network-scripts/ifcfg-*.
Now, you can manually change the /etc/resolv.conf file again, and you should be good to go. NetworkManager will no longer overwrite your DNS nameserver configurations.