Comment 18 for bug 123559

The exact same issue exists on Ubuntu Server Edition, however it is quite different on that platform.

There is NO /etc/udev/rules.d/70-persistent-net.rules, the closest thing is "85-ifupdown.rules", but it only defines some default actions to perform whenever an ethernet interface is found, you cannot add your own SUBSYSTEM=="net" rules (as per the bottom of post 11 by Serge for instance).

Instead we have a file called /etc/iftab.

This file is incredibly unintelligent. Why? Because it hardcodes device names to mac addresses, so instead of the first ethernet address getting eth0, it reserves that name for a specific mac address. In effect this means that if you install Ubuntu Server Edition on 1 PC with one Mac address, then move it to PC #2 with a different mac address, then /etc/iftab will be assigning the ethernet interface of that computer to eth1, because eth0 is reserved to the mac address of the OLD computer. That is absolutely ridiculous!

Why is it ridiculous? Well, because /etc/network/interfaces is setup to configure internet access and dhcp on eth0, so if you change computers you are left without network connectivity unless you manually do "dhclient eth1".

I had to spend 3 hours tracking down this issue with lots of help from the people in #linux, and tried everything. I looked at loaded driver modules, I looked at config files everywhere, I edited /etc/network/interfaces. What happened was that while the system was booting it referred to my ethernet device as "eth0", but as soon as the system was finished booting it was suddenly called "eth1".

In the end I had an idea that perhaps eth0 was reserved for my old mac address (on the computer I used when I installed Ubuntu Server Edition on this flash drive). This hadn't occured to me earlier because that is a retarded design, linux is meant to be able to run on different hardware simply by putting the harddrive in another computer, and locking down the eth0 name by mac address is pure stupidity in that regard. Even more so because you're unable to connect to the network by default after moving your usb drive to another computer.

So what I did was navigate to /etc and type grep -R -i "00:16:35:7E:74:69", which is the mac address from PC #1 where I installed Ubuntu from, and got these results:

root@ubuntu:/etc# grep -R -i "00:16:35:7E:74:69"
/etc/iftab:eth0 mac 00:11:2f:92:4a:8d arp 1

I went into that file and put my NEW (PC #2) mac address there, so it looked like this:

root@ubuntu:/etc# cat /etc/iftab
# This file assigns persistent names to network interfaces.
# See iftab(5) for syntax.

eth0 mac 00:16:35:7E:74:69 arp 1

Now my interface is being assigned to eth0 and I get network connectivity right away. I could have solved this by changing /etc/network/interfaces to eth1 instead, but that would have been accepting the problem.

I propose that you remove this /etc/iftab hard-coding, and leave the hard-coding up to the user!

All it serves to do right now is locking down the installation so that you cannot change computers/network interface.

I hope that this is of help to everyone else, it seems like quite a common problem but I had never seen this particular (Ubuntu Server Edition) solution!

All the best,
Christopher Hilding