What's the default /etc/network/interfaces?

For eth0 with dhcp:

# The loopback network interface
auto lo eth0
iface lo inet loopback

# The primary network interface
iface eth0 inet dhcp

For eth0 static:

# The loopback network interface
auto lo eth0
iface lo inet loopback

# The primary network interface
iface eth0 inet static
    address 192.168.10.33
    netmask 255.255.255.0
    broadcast 192.168.10.255
    network 192.168.10.0
    gateway 192.168.10.254 
dns-nameservers 192.168.10.254

link: http://www.iasptk.com/ubuntu/20202-ubuntu-server-networking-configuration-dhcp-address-static-ip-address-second-ip-address-or-virtual-ip-address

Mine looks like:

auto lo
iface lo inet loopback

Following the Debian network configuration page, if you're just using DHCP then all you need is something like:

auto eth0
allow-hotplug eth0
iface eth0 inet dhcp