If you’ve recently migrated a Ubuntu virtual machine file from one computer to another, or from one virtualization product to another, you might have discovered that networking inside the VM was no longer working. As described in this Parallels KB article, what’s probably happened is that the random MAC address generated by the virtualization host has changed. The file: /etc/udev/rules.d/70-persistent-net.rules contains a record of the matching MAC address from the previous host.

I’ve just spent the afternoon trying to get a simple TCP connection established. The story is that I was configuring Nagios to check some HTTP services that are behind a Cisco PIX firewall. Nagios kept on getting timeout errors – when I tried by hand with telnet, I was getting the same problem, but only from the machine running Nagios. After setting up some logging on the firewall, I could see lines like: