This is so cool:
http://www.linuxguruz.com/iptables/howto/2.4routing-13.html
- /proc/sys/net/ipv4/ip_nonlocal_bind
- Set this if you want your applications to be able to bind to an address which doesn’t belong to a device on your system. This can be useful when your machine is on a non-permanent (or even dynamic) link, so your services are able to start up and bind to a specific address when your link is down.