Something

#!/bin/bash

IPT='/sbin/iptables'

EXTIF=eth0
INTIF1=eth1

$IPT        -P INPUT       DROP
$IPT        -P OUTPUT      DROP
$IPT        -P FORWARD     DROP

CHAINS=`cat /proc/net/ip_tables_names 2>/dev/null`
for i in $CHAINS;
do
 $IPT -t $i -F
done
for i in $CHAINS;
do
 $IPT -t $i -X
done
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

$IPT -A INPUT    -i $EXTIF -p icmp -j ACCEPT
$IPT -A OUTPUT   -o $EXTIF -p icmp -j ACCEPT

$IPT -A INPUT    -i $EXTIF -p tcp --dport 22 -m state --state NEW -j ACCEPT
$IPT -A OUTPUT   -o $EXTIF -p tcp --dport 22 -m state --state NEW -j ACCEPT
$IPT -A FORWARD  -i $EXTIF -p tcp --dport 22 -m state --state NEW -j ACCEPT