openwrt-xburst/root/etc/init.d/S45firewall

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

WAN=$(nvram_get wan_ifname)

IPT=/usr/sbin/iptables

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

$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 INPUT -p icmp -j ACCEPT 
$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 -m state --state INVALID -j DROP 
$IPT -t filter -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT 
$IPT -t filter -A FORWARD -i $WAN -m state --state NEW,INVALID -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