Sure -A FORWARDING -i internal -o external -j ACCEPT -A FORWARDING -m state --state RELATED,ESTABLISHED -j ACCEPT -A FORWADING -j REJECT Last one

Sure -A FORWARDING -i internal -o external -j ACCEPT -A FORWARDING -m state --state RELATED,ESTABLISHED -j ACCEPT -A FORWADING -j REJECT Last one can be done as policy on FORWARDING chain (ie. -P instead of -A), but in this way it is more explicit.