[Ubuntu] After upgrade cannot access internet despite connection is up

I upgraded our server last night via apt-get update & apt-get upgrade, when the server reboot it cannot connect to internet and other network segment. It can access only all the resources in the same subnet.

There are a few solutions worth to try:

• ifdown the network interface and ifup it again
• when perform upgrade of Ubuntu, the system might modified /etc/resolv.conf to include 127.0.0.1 as nameserver. Sometimes this will mess thing up and you can't get online. You can either delete everything inside, or use google DNS: nameserver 8.8.8.8
• if you have bonding, you might want to add dns-nameserver 8.8.8.8 8.8.4.4 under your bond0
• restart your networking: restart networking (this could be different from reboot Ubuntu. Because my connection got fix by restarting the network, while reboot doesn't help at all

If you do not wish to tackle with all this stupid problem, my advice is only upgrade when you need it. As long as your server is running fine and the upgrade is not going to fix any issues, just keep it as it is.