You need to configure the /etc/network/interfaces file if you want to change your DNS server via the command line.
It should look something like this:
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.X.X
netmask 255.255.255.0
gateway 192.168.X.X
dns-nameservers X.X.X.X
If you have more than one DNS server, just add a space between each:
dns-nameservers X.X.X.X Y.Y.Y.Y Z.Z.Z.Z
Just replace the Xs, Ys, and Zs with your own IPs of the DNS servers of choice, and when this is done, run this command to update the settings:
sudo ifdown eth0 && sudo ifup eth0
Hope this helps!
