debian:~# more /usr/local/bin/iptablerouter
#!/bin/sh
PATH=/usr/sbin:/sbin:/bin:/usr/bin
#
# delete all existing rules.
#
iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -X
# Always accept loopback traffic
iptables -A INPUT -i lo -j ACCEPT
# Allow established connections, and those not coming from the outside
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW -i ! eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# Allow outgoing connections from the LAN side.
iptables -A FORWARD -i eth0 -o eth0 -j ACCEPT
# Masquerade.
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# Don't forward from the outside to the inside.
iptables -A FORWARD -i eth0 -o eth1 -j REJECT
# Enable routing.
echo 1 > /proc/sys/net/ipv4/ip_forward