12.04 wired network doesn't work RTL8111/8168B

Reaktek: http://www.realtek.com/downloads/downloadsView.aspx?Langid=1&PNid=13&PFid=5&Level=5&Conn=4&DownTypeID=3&GetDown=false#2

I had the same problem. I solved it by adding (the graphical way) a new network connection, that automatically connects with "Automatic DHCP" as the setting. It seems that this default had been removed?!

Update - My issue was unrelated to the orginal author's. The instructions I provided below disabled Network Manager. This circumvents dnsmasq, which had been inaccessable due to iptables blocking localhost-to-localhost UDP traffic.

Is this a DNS issue? Can you ping 72.14.204.101 but get nothing from host google.com?

Edit /etc/network/interfaces to look like:

auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp

Then:

$ sudo ifdown eth0
$ sudo ifup eth0

This happened to me after upgrading from 11.10 to 12.04. For some reason that last line of the interfaces file had been commented out.