To Quote NORMYZO on another (2008) thread:
NetworkManager will get DNS information where you tell it to get DNS from; you can either enter the DNS servers manually into your /etc/sysconfig/network-scripts/ifcfg-* files with the keys DNS1, DNS2, DNS3 (which NM will pick up and use automatically if the device is managed by NM), or you can use nm-connection-editor to add your preferred DNS servers to the connection.
google for this and you will find many instances. On my servers I do not install network manager. If it has been installed, simply removing it does NOT resolve all dynamic networking issues, as there are other things that come with it, but that helps.
Network Manager has some very nice features for a mobile machine (laptop, etc.) that are a nightmare for a server. If you have things you want locked in, either clean NM from the system and manage networking directly, or manage your network settings the NM way.
In your case, making it work the NM way seems the proper option.