Listing 1 -- Example IPTables firewall script

#!/bin/sh
#
# rc.firewall - Initial SIMPLE IP Firewall test script for 2.4.x
#
# Author: Oskar Andreasson <blueflux@koffein.net>
# (c) of BoingWorld.com, use at your own risk 
# Distribute Freely, but please acknowledge Oskar Andreasson &      
# BoingWorld.com.
# Cosmetic modifications by Duncan Napier, Aug 01, 2001
#

###########
# Configuration options, these will need to play with these in order 
# for it to work with your own setup.

# Enter in your own IPs. If you have a dynamically assigned IP, you may # want to modify the script to automatically load that in

LAN_IP_RANGE="192.168.0.0/24"
LAN_IP="192.168.0.2/32"
LAN_BCAST_ADRESS="192.168.0.255/32"
LOCALHOST_IP="127.0.0.1/32"
STATIC_IP="aaa.bbb.ccc.ddd/32"
INET_IFACE="eth0"
LAN_IFACE="eth1"
IPTABLES="/sbin/iptables"

#########
# Load all required IPTables modules if you compiled them as 
# loadable modules
#

#
# Load modules listed in /etc/modules.conf
# /sbin/depmod -a

# Adds some iptables targets like LOG, REJECT and MASQUARADE.
# Uncomment to load logging module
#/sbin/modprobe ipt_LOG

#/sbin/modprobe ipt_REJECT
#/sbin/modprobe ipt_MASQUERADE

#
# Support for owner matching
#
#/sbin/modprobe ipt_owner

#
# Support for connection tracking of FTP and IRC.
#
#/sbin/modprobe ip_conntrack_ftp
#/sbin/modprobe ip_conntrack_irc


#CRITICAL:  Enable IP forwarding since it is disabled by default.
#
echo "1" > /proc/sys/net/ipv4/ip_forward


# Dynamic IP users:
#
#   If you get your IP address dynamically from SLIP, PPP, or DHCP,    #   enable this option.  This enables dynamic-ip address hacking in IP #   MASQ, making the connection with Diald and similar programs much     #    easier.
#
#echo "1" > /proc/sys/net/ipv4/ip_dynaddr

# Enable simple IP FORWARDing and Masquerading
#
#  NOTE:  The following is an example for an internal LAN, where the 
#         LAN runs on eth1, and the Internet is on eth0.
#
#         Please change the network devices to match your own 
#         configuration.
#

$IPTABLES -t nat -A POSTROUTING -o $INET_IFACE -j MASQUERADE
$IPTABLES -A FORWARD -i $LAN_IFACE -j ACCEPT
$IPTABLES -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT 
$IPTABLES -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG --log-level DEBUG --log-prefix "IPT FORWARD packet died: "

#
# set default policies for the INPUT, FORWARD and OUTPUT chains
#

$IPTABLES -P INPUT DROP
$IPTABLES -P OUTPUT DROP
$IPTABLES -P FORWARD DROP

#
# Create separate chains for ICMP, TCP and UDP to traverse
#

$IPTABLES -N icmp_packets
$IPTABLES -N tcp_packets
$IPTABLES -N udpincoming_packets

#
# the allowed chain for TCP connections
#
# This chain will be utilised if someone tries to connect to an allowed
# port from the internet. If they are opening the connection, or if it's
# already established we ACCEPT the packages, if not we DROP them. This # is where the state matching is performed also, we allow ESTABLISHED 
# and RELATED packets. 

$IPTABLES -N allowed
$IPTABLES -A allowed -p TCP --syn -j ACCEPT
$IPTABLES -A allowed -p TCP -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A allowed -p TCP -j DROP

#
# ICMP rules
#

$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 0 -j ACCEPT
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 3 -j ACCEPT
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 5 -j ACCEPT
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

#
# TCP rules
#

$IPTABLES -A tcp_packets -p TCP -s 0/0 --dport 21 -j allowed
$IPTABLES -A tcp_packets -p TCP -s 0/0 --dport 22 -j allowed
$IPTABLES -A tcp_packets -p TCP -s 0/0 --dport 80 -j allowed
$IPTABLES -A tcp_packets -p TCP -s 0/0 --dport 113 -j allowed

#
# UDP ports
#

$IPTABLES -A udpincoming_packets -p UDP -s 0/0 --source-port 53 -j ACCEPT
$IPTABLES -A udpincoming_packets -p UDP -s 0/0 --source-port 123 -j ACCEPT
$IPTABLES -A udpincoming_packets -p UDP -s 0/0 --source-port 2074 -j ACCEPT
$IPTABLES -A udpincoming_packets -p UDP -s 0/0 --source-port 4000 -j ACCEPT

#
# PREROUTING chain.
#
# Do some checks for obviously spoofed IP's 
#

$IPTABLES -t nat -A PREROUTING -i $INET_IFACE -s 192.168.0.0/16 -j DROP
$IPTABLES -t nat -A PREROUTING -i $INET_IFACE -s 10.0.0.0/8 -j DROP
$IPTABLES -t nat -A PREROUTING -i $INET_IFACE -s 172.16.0.0/12 -j DROP


#
# INPUT chain
#
# establish the basic INPUT chain and filter the packets onto the 
# correct chains.
#


$IPTABLES -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets
$IPTABLES -A INPUT -p TCP -i $INET_IFACE -j tcp_packets
$IPTABLES -A INPUT -p UDP -i $INET_IFACE -j udpincoming_packets

$IPTABLES -A INPUT -p ALL -i $LAN_IFACE -d $LAN_BCAST_ADRESS -j ACCEPT
$IPTABLES -A INPUT -p ALL -d $LOCALHOST_IP -j ACCEPT
$IPTABLES -A INPUT -p ALL -d $LAN_IP -j ACCEPT
$IPTABLES -A INPUT -p ALL -d $STATIC_IP -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG --log-level DEBUG --log-prefix "IPT INPUT packet died: "

#
# OUTPUT chain
#
# establish the basic OUTPUT chain and filter them onto the correct 
#  chain
#

$IPTABLES -A OUTPUT -p ALL -s $LOCALHOST_IP -j ACCEPT
$IPTABLES -A OUTPUT -p ALL -s $LAN_IP -j ACCEPT
$IPTABLES -A OUTPUT -p ALL -s $STATIC_IP -j ACCEPT
$IPTABLES -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG --log-level DEBUG --log-prefix "IPT OUTPUT packet died: "



