Running Ubuntu Server 12.04 (no GUI). What would be the best way to make kernel bring a network interface up only if it is physically plugged in? So, if it doesn't exist, just move on with initializing other interfaces (if any) and continue to the login screen, without "waiting for network configuration" delay.
E.g, I have a wireless USB key wlan9 (in /etc/network/interfaces):
auto wlan9 iface wlan9 inet dhcp wpa-ssid myssd wpa-psk mykey wpa-proto RSN wpa-pairwise CCMP wpa-group CCMP
I tried allow-hotplug instead of auto, in which case the interface doesn't get initialized automatically during the boot, and I have to do it manually with ifup wlan9. This is not exactly what I'm looking for.