openwrt-xburst/openwrt/target/default/target_skeleton/etc/init.d/S45firewall

#!/bin/sh
. /etc/functions.sh
export WAN=$(nvram get wan_ifname)
export LAN=$(nvram get lan_ifname)

## CLEAR TABLES
for T in filter nat mangle; do
  iptables -t $T -F
  iptables -t $T -X
done

iptables -N input_rule
iptables -N output_rule
iptables -N forwarding_rule

iptables -t nat -N prerouting_rule
iptables -t nat -N postrouting_rule

### Port forwarding
# iptables -t nat -A prerouting_rule -p tcp --dport 22 -j DNAT --to 192.168.1.2
# iptables        -A forwarding_rule -p tcp --dport 22 -d 192.168.1.2 -j ACCEPT

### INPUT
###  (connections with the router as destination)

  # base case
  iptables -P INPUT DROP
  iptables -A INPUT -m state --state INVALID -j DROP
  iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

  # allow
  iptables -A INPUT -i \! $WAN -j ACCEPT	# allow from lan/wifi interfaces 
  iptables -A INPUT -p icmp -j ACCEPT		# allow ICMP
  iptables -A INPUT -p 47 -j ACCEPT		# allow GRE
  iptables -A INPUT -p tcp --syn --tcp-option \! 2 -j  DROP
  #
  # insert accept rule or to jump to new accept-check table here
  #
  iptables -A INPUT -j input_rule

  # reject (what to do with anything not allowed earlier)
  iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset
  iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable

### OUTPUT
### (connections with the router as source)

  # base case
  iptables -P OUTPUT DROP
  iptables -A OUTPUT -m state --state INVALID -j DROP
  iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

  # allow
  iptables -A OUTPUT -j ACCEPT		#allow everything out
  #
  # insert accept rule or to jump to new accept-check table here
  #
  iptables -A OUTPUT -j output_rule

  # reject (what to do with anything not allowed earlier)
  iptables -A OUTPUT -p tcp -j REJECT --reject-with tcp-reset
  iptables -A OUTPUT -j REJECT --reject-with icmp-port-unreachable

### FORWARDING
### (connections routed through the router)

  # base case
  iptables -P FORWARD DROP 
  iptables -A FORWARD -m state --state INVALID -j DROP
  iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
  iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

  # allow
  iptables -A FORWARD -i br0 -o br0 -j ACCEPT
  iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT
  #
  # insert accept rule or to jump to new accept-check table here
  #
  iptables -A FORWARD -j forwarding_rule

  # reject (what to do with anything not allowed earlier)
  # uses the default -P DROP

### MASQ
  iptables -t nat -A PREROUTING -j prerouting_rule
  iptables -t nat -A POSTROUTING -j postrouting_rule
  iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE