openwrtv4/openwrt/target/default/target_skeleton/etc/init.d/S45firewall

#!/bin/sh
. /etc/functions.sh

export WAN=$(nvram get wan_ifname)
export IPT=/usr/sbin/iptables

for T in filter nat mangle ; do
  $IPT -t $T -F
  $IPT -t $T -X
done

# initial rules
$IPT -t filter -A INPUT -m state --state INVALID -j DROP
$IPT -t filter -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
$IPT -t filter -A FORWARD -m state --state INVALID -j DROP 
$IPT -t filter -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT 

if [ -d /etc/firewall.d ]; then 
	for fw in /etc/firewall.d/??*; do
		[ -x $fw ] && $fw
	done
fi

# defaults

$IPT -t filter -A INPUT -p icmp -j ACCEPT
$IPT -t filter -A INPUT -p 47 -j ACCEPT # allow GRE
$IPT -t filter -A INPUT -i $WAN -p tcp --syn --tcp-option \! 2 -j  DROP
$IPT -t filter -A INPUT -i $WAN -p tcp -j REJECT --reject-with tcp-reset 
$IPT -t filter -A INPUT -i $WAN -j REJECT --reject-with icmp-port-unreachable 

$IPT -t filter -A FORWARD -i br0 -o br0 -j ACCEPT
$IPT -t filter -A FORWARD -i $WAN -m state --state NEW -j DROP
$IPT -t filter -A FORWARD -o $WAN -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

$IPT -t nat -A POSTROUTING -o $WAN -j MASQUERADE