The wired internet connection by default is configured properly in Ubuntu installation. We can directly dial-up via sudo pppoeconf, or connect to a cable which already on internet. But sometimes wired connection stop working because of kernel upgraded, installed a special program, or even something to do with wireless.

Here’s a solution that fixed my wired problem while I was trying access internet via a router(that has already access to internet) and before I directly pppoe dial-up. It also works for kernel upgrade reason, but not sure for every situation.

1.) First run 

ifconfig -a

and the problem’s always that no assigned ip from cable on eth0. Use 

sudo dhclient eth0

and it works again, but it stops working after reboot.

2.) Edit the file “/etc/network/interfaces” and change it looks like this:

# The loopback network interface

auto lo

iface lo inet loopback

# The primary network interface

auto eth0

iface eth0 inet dhcp

BTW: if you have the wireless connection up, that might interfere with wired connection, you can try wicd-network-manager(available in Ubuntu Software Center) to configure the wireless connection.