Change DNS server with bash

Hello @Geekologist,

By default on DigitalOcean Ubuntu 16.04 /etc/network/interfaces is populated with Google Public DNS - 8.8.8.8 - 8.8.4.4. If you open etc/network/interfaces with any text editor you will see following:

# This file describes the network interfaces available on your
# system and how to activate them. For more information, see
# interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
        address 111.111.111.111
        netmask 255.255.255.0
        gateway 111.111.111.0
        up ip addr add 10.17.0.6/16 dev eth0
        dns-nameservers 8.8.8.8 8.8.4.4

Red values could be different in you case. Note the dns-nameservers 8.8.8.8 8.8.4.4 - it says to use Google Public DNS. You can change it with simple command - sed.

sed -i '/dns-nameservers 8.8.8.8 8.8.4.4/c\        dns-nameservers 1.2.3.4 1.2.3.5' /etc/network/interfaces

You can learn more about sed at GNU docs Change red values in command to reflect your desired DNS settings. After you execute it, it will change values. When you open /etc/network/interfaces you will notice changes.

You can also use this as User data on Droplet Creation:

#!/bin/bash

sed -i '/dns-nameservers 8.8.8.8 8.8.4.4/c\        dns-nameservers 1.2.3.4 1.2.3.4' /etc/network/interfaces

Note: this applies for DigitalOcean Ubuntu 16.04 droplets. If you use another host, or using it on your own machine, /etc/network/interfaces could be different and can cause command to fail!