Comment 25 for bug 50099

If ifup says "interface $IF already configured", but it's really not configured on boot, then perhaps the file /var/run/network/ifstate is stale. ifdown $IF; ifup $IF fixes the problem, but if the problem persists, check that file to see if that interface is listed before /etc/init.d/networking runs. Try a live or install CD, or boot Ubuntu with 'init=/bin/bash' (and no splash) on the kernel commandline.

If 'ifconfig' lists the interface but it does not work, then must have been configured once already.