source: freewrt/package/iptables/files/firewall.init@ 8b53bd7

#!/bin/sh

## Please make changes in /etc/firewall.user

. /etc/config/network
WAN="$wan_ifname"
LAN="$lan_ifname"

## CLEAR TABLES
for T in filter nat; 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

### 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
  iptables -A INPUT -p tcp --tcp-flags SYN SYN --tcp-option \! 2 -j  DROP

  #
  # insert accept rule or to jump to new accept-check table here
  #
  iptables -A INPUT -j input_rule

  # allow
  iptables -A INPUT ${WAN:+\! -i $WAN} -j ACCEPT        # allow from all interfaces except for wan
  iptables -A INPUT -p icmp     -j ACCEPT       # allow ICMP
  iptables -A INPUT -p gre      -j ACCEPT       # allow GRE

  # 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

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

  # allow
  iptables -A OUTPUT -j ACCEPT          #allow everything out

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

  #
  # insert accept rule or to jump to new accept-check table here
  #
  iptables -A FORWARD -j forwarding_rule

  # allow
  iptables -A FORWARD -i br0 -o br0 -j ACCEPT
  [ -z "$WAN" ] || iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT

  # 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
  [ -z "$WAN" ] || iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE

## USER RULES
[ -f /etc/firewall.user ] && . /etc/firewall.user
[ -n "$WAN" -a -e /etc/config/firewall ] && {
        awk -f /usr/lib/common.awk -f /usr/lib/firewall.awk /etc/config/firewall | ash
}