Create a file called /etc/resolv.conf write fill in the contents:

nameserver 

For example if your want to use googles dns-service:

nameserver 8.8.8.8
nameserver 8.8.4.4

This is likely caused by DHCP configuration when you first installed Ubuntu. Try this 3-step process to handle this auto configuration issue.

First

Edit your interface configuration, which is located in: /etc/network/interfaces

Add this line below iface lo inet loopback:

dns-nameservers yourdns youraltdns

As an example for Google DNS, you may want to use this:

dns-nameservers 8.8.8.8 8.8.4.4

Second

Edit your DHCP configuration file, located at:

/etc/dhcp/dhclient.conf

Mark the syntax as a comment using # on every line or simply remove every request name-server. In 16.04, you may not be required to make any changes here.

Third

Restart your networking by using this command :

/etc/init.d/networking restart

In 16.04:

sudo ifdown -a
sudo ifup -a