Script to setup a basic ruleset for iptables, used on my raspberrypi firewall.

pirewall.sh

#!/bin/sh
# * eth0 = Internet interface (DHCP)
# * eth1 = LAN interface (192.168.1.0/24)
# * eth2 = DMZ interface (172.16.0.0/16)
# * Traffic open from firewall to internet
# * Traffic open and translated from LAN and DMZ to internet
# * Traffic open from LAN to Firewall
# * Traffic open from LAN to DMZ
# * Traffic open from internet to a DMZ web server

PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
EXT_IP=`ifconfig eth0 | grep 'inet addr:' | cut -d":" -f2 | awk '{ print $1}'`

### SYSCTL

# Set required kernel params
echo -n '1' > /proc/sys/net/ipv4/ip_forward
echo -n '0' > /proc/sys/net/ipv4/conf/all/accept_source_route
echo -n '0' > /proc/sys/net/ipv4/conf/all/accept_redirects
echo -n '1' > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
echo -n '1' > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses

### IPTABLES

## INIT

# Flush previous rules, delete chains and reset counters
iptables -F
iptables -X
iptables -Z
iptables -t nat -F

# Default policies
iptables -P INPUT   DROP
iptables -P OUTPUT  DROP
iptables -P FORWARD DROP

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

# Enable stateful rules
iptables -A INPUT   -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT  -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Drop invalid state packets
iptables -A INPUT   -m conntrack --ctstate INVALID -j DROP
iptables -A OUTPUT  -m conntrack --ctstate INVALID -j DROP
iptables -A FORWARD -m conntrack --ctstate INVALID -j DROP

## INPUT

# Enable all incoming from LAN to firewall
iptables -A INPUT -i eth1 -m conntrack --ctstate NEW -j ACCEPT

## OUTPUT

# Enable all outgoing traffic to internet
iptables -A OUTPUT -o eth0 -m conntrack --ctstate NEW -j ACCEPT

## FORWARD

# No routable networks behind eth0, need to masq
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# LAN to DMZ
iptables        -A FORWARD     -o eth2 -i eth1 -m conntrack --ctstate NEW -j ACCEPT

# LAN to Internet
iptables        -A FORWARD     -o eth0 -i eth1 -m conntrack --ctstate NEW -j ACCEPT

# DMZ to Internet
iptables        -A FORWARD     -o eth0 -i eth2 -m conntrack --ctstate NEW -j ACCEPT

# Internet HTTP to DMZ server 172.16.0.1
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 \
                              -j DNAT --to-destination 172.16.0.1

iptables        -A FORWARD    -i eth0 -p tcp \
                              --dport 80 \
                              -o eth2 -d 172.16.0.1    \
                              -m conntrack --ctstate NEW -j ACCEPT

# NAT loopback - LAN HTTP to/from DMZ server 172.16.0.1
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -d $EXT_IP \
                              -p tcp --dport 80 \
                              -j DNAT --to-destination 172.16.0.1

iptables -t nat -A POSTROUTING -d 172.16.0.1 -s 192.168.1.0/24 \
                              -p tcp --dport 80 \
                              -j SNAT --to-source $EXT_IP