VMWare : Loosing eth0 after you've copied your VM

Background

Here's a bit of weird-behavior I've noticed when working with some of our production virtual machines (running Gentoo Linux) here at work.

In order to update the OS's on our virtual machines, I will copy them to my local machine, power them on, update them, and then push the updated OS's back out into production at the earliest convenience.

When you copy the VM from one location to another, VMWare notices this and asks you "Hey, it looks like this machine has been physically moved or copied, do you want me to create a new VM-UUID?" If you answer in the affirmative, VMWare internally regenerates any unique-identifiers tied to this virtual machine. The one thing that's really noticeable is that any virtual ethernet adapters get their MAC addresses changed.

The problem I've experienced is that when you power on the new-UUID'd VM, you no longer have an ethernet adapter. Gentoo tries to bring-up eth0 and it says "network interface eth0 does not exist" and "Please verify hardware or kernel module (driver)"

Explanation

"So, what's going on?"

Try a couple things:

• If you run lspci you should still see the ethernet adapter.


• If you run 'dmesg' and should see the kernel find the network card and it even calls it eth0

"So, where does eth0 go?"

Try running ifconfig -a. I bet you now have an eth1 and it's MAC address matches the newly-generated virtual MAC address specified in the virtual machine's .vmx file.

"Oh great, so every time I copy the VM I need to update the system configs to use the new eth1, or eth2, etc!?!?!"

No, hush, I'm getting to the answer.

The problem stems from the linux distro 'remembering' the MAC address of the network adapter and expecting it to be the same between boots. In the case of our Gentoo VM's, it's udev that mucks this up.

"Ok, fine, it's udev's fault. We know it's broken because it's expecting the ethernet adapter to have a MAC address that it no longer has. What to do?

The Answer

To fix this problem you need to tell your linux distro the VM's new MAC address. How you do this can vary by distro. In my spelunking, I found a few ways:

• In Gentoo do one of the following: (Do #1, it's the easiest.)
  
  1. Delete /etc/udev/rules.d/70-persistent-net.rules and reboot. Your eth0 should be back.
     • 2007/09/13 Update: This almost-always works for me. But, for some reason, sometimes it seems to confuse udev even more; after rebooting, I'll have an eth2 or eth3. When this happens, I end up following #2, making sure the udev config file has 'eth0' listed, and not eth1, eth2, or eth3.
  2. Edit /etc/udev/rules.d/70-persistent-net.rules (or whatever it's named) to match your new MAC address and reboot. Your eth0 should be back.
• Other distros:
  • Look for, (and edit if you find,) /etc/iftab
  • Look for, and delete, then reboot /etc/udev/rules.d/25-iftab.rules
  • Look for, (and edit if you find,) /etc/sysconfig/network-scripts/ifcfg-eth0

Give Credit Where Credit Is Due

I got hints from a number of pages, but in the end, it was the folks over at the VMWare discussion forums for the win:
VMWare Discussion Forums