Solution for Ubuntu 12.04 and realtek N8101 NIC wired network not working

[chili555]

Let's verify a few things:

lspci -nn | grep 0280
dmesg | grep r8
lsmod | grep r8

Thanks.

[Owen_Mehegan]

Well, by sheer chance, I figured out what was wrong. In order to make debugging easier, I went to hook up a USB wifi adapter to this system, and I ran 'ip addr' to check what the interface name was. In the output of that I also noticed an eth1 interface. Given that the MAC addresses were completely dissimilar, I guessed that this might be my lost ethernet interface. I changed my /etc/network/interfaces file to reference eth1, saved, ran ifup eth1, and it came up perfectly! I removed the wifi and rebooted, and the eth1 is still fine. Very strange - why didn't it get identified as eth0?

For what it's worth, the lspci output with your grep filter would not have found anything. Here's the actual line for the NIC in my system:

01:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8101E/RTL8102E PCI Express Fast Ethernet controller [10ec:8136] (rev 05)

[chili555]

For what it's worth, the lspci output with your grep filter would not have found anything.

You are quite correct; I made a mistake and I apologize. It should have been:

lspci -nn | grep 0200

And then we'd see:

01:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8101E/RTL8102E PCI Express Fast Ethernet controller [10ec:8136] (rev 05)

Very strange - why didn't it get identified as eth0?

It happens once in a while; udev is an imperfect tool. You can certainly ignore it and, as you've seen, everything will work perfectly well. If you are super OCD and it bothers you, edit /etc/udev/rules.d/70-persistent-net.rules and also /etc/network/interfaces and immediately reboot. You seem pretty adept, so I'll leave the details to you. If you get stuck, post back and we'll be glad to help.