systemd-resolved provide DNS resolver facility in newer version of Linux servers running systemd.
I prefer manually editing /etc/resolv.conf and add resolvers. If you are running your own DNS server, then you need to disable systemd-resolved as it listen on port 53, with out disabling the service, you can’t start your DNS server.
To disable systemd-resolved, run
wget https://raw.githubusercontent.com/serverok/server-setup/master/tools/disable-systemd-resolved.sh bash disable-systemd-resolved.sh
Or do the following.
systemctl disable systemd-resolved.service systemctl stop systemd-resolved
Now create /etc/resolv.conf file
rm -f /etc/resolv.conf tee /etc/resolv.conf << END nameserver 8.8.8.8 nameserver 1.1.1.1 END
See resolv.conf, dns