Simple IPv4 firewall

#!/bin/bash

WAN="eth0"
LAN="eth1"

echo 1 > /proc/sys/net/ipv4/ip_forward

echo 1 > /proc/sys/net/ipv4/tcp_syncookies
 # TCP SYN Cookie Protection

iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -X

iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT
iptables -P FORWARD DROP

iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE
    #NAT

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT

iptables -A INPUT -i $LAN -j ACCEPT
iptables -A FORWARD -i $LAN -o $LAN -j ACCEPT

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
    #loopback

iptables -A INPUT -p icmp -j ACCEPT
iptables -A OUTPUT -p icmp -j ACCEPT
iptables -A FORWARD -p icmp -j ACCEPT
    #icmp

iptables -A INPUT -i $WAN -j REJECT

iptables -A INPUT -m state --state INVALID -j DROP

iptables -A INPUT -j REJECT --reject-with icmp-admin-prohibited