I think the most reliable way to do that is by setting your
resolv.conf to be immutable. Once you set it the way you want, you simply run
sudo chattr +i /etc/resolv.conf
To edit it again, you need to remove the immutable flag:
sudo chattr -i /etc/resolv.conf
The application that puts that message in
/etc/resolv.conf is called
resolvconf. You can remove your
resolvconf package with (I'm assuming debian or ubuntu here, but I think
resolvconf is a debian thing):
sudo apt-get remove resolvconf
I remove it on my computers because I have found that I am more likely to be able to connect with proper
/etc/resolv.conf settings in strange situations (hotel or airport wifi, specifically) when I do not have the
resolvconf package installed.
resolvconf also does other things like DNS caching, and probably some other things I feel I can live without. You might look into what all it does before you remove it.
resolvconf will not guarantee that your
/etc/resolv.conf remains unchanged. Other programs, like
dhclient will make edits to
/etc/resolv.conf. If you run a network manager, that might also edit
/etc/resolv.conf. I don't have much experience with network managers.
chattr solution should prevent those other programs from making edits, and is probably right for your server situation. If you were using it on a laptop, situations may arise where important changes to your
/etc/resolv.conf would be blocked. An example would be when accessing hotel wifi, where a modification to your
/etc/resolv.conf is needed for access to a certain locally hosted url before gain access to the proper internet). If you can remember in those situations to
chattr -i /etc/resolv.conf and reconnect to the wifi, then it shouldn't be a problem.