Example simple iptable ruleset

#! /bin/sh
# /etc/iptables.bak

# Let's save typing & confusion with variables
IPTABLES=/sbin/iptables

# Flush active rules and custom tables
$IPTABLES --flush
$IPTABLES --delete-chain

# set the defaults so that by-default incoming packets are explicitly allowed;
$IPTABLES -P INPUT ACCEPT
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT ACCEPT