Manually flush iptables in rescue mode

grml won't actually have loaded your hosts' iptables rules, so they won't be available.

If you followed some popular guides and are using NetworkManager, then you might find the rules located in /etc/iptables.*, so removing them (or providing an empty file) upon startup might be your best bet.

It all boils down to how you saved the rules to begin with, so any details on how you got there to begin with would be helpful.

You could also try appending this segment to your server's startup, probably /etc/rc.local:

#!/bin/sh -e
sudo iptables -X
sudo iptables -t nat -F
sudo iptables -t nat -X
sudo iptables -t mangle -F
sudo iptables -t mangle -X
sudo iptables -P INPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -P OUTPUT ACCEPT

Please remember that when in grml, you're running another version of Linux, so you'd have to mount your disk, and modify those files, not the running grml environment.