Have to run sudo dhclient eth0 automatically every boot

Ok so I have a way better answer then I had before.

How about we try this.

Open a terminal.

gksu gedit /etc/rc.local

Add: sudo dhclient eth0 like so.

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
sudo dhclient eth0
exit 0

Save & Close. Reboot and should work.

Your NetworkManager has for some reason commented out the necessary configuration to make DHCP pick up an IPv4 address. Maybe it was a bug or you configured it to turn off IPv4? This is why it isn't working.

# NetworkManager#iface eth0 inet dhcp

To resolve the problem, simply restore the configuration.

iface eth0 inet dhcp

Also check your NetworkManager configuration and make sure you didn't disable IPv4 on the interface.