Set DNS Servers:
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 X'es with your own IPs and the DNS server of choise, and when this is done, run this command to update the settings:
sudo ifdown eth0 && sudo ifup eth0
Hope this helps!