Block internet access before firewall starts at boot

I have an old Android phone with Ice Cream Sandwich and DroidWall as a firewall, which makes sure that apps can't use the 3G unless I enable it for that app. DroidWall starts on boot but there is a very brief time before it starts when any app can use the internet. What I would like to do is stop all incoming and outgoing internet access until DroidWall automatically runs on boot to put the firewall rules in place. DroidWall allows running a script when it starts so conceivably I could undo the block-everything rules when it starts. The phone is rooted but doesn't have a /etc/init.d/. Basically I want to make sure there is not even a second that traffic can get out before the firewall. 3G is expensive but I need it so I can't turn it off.