masq

#!/bin/sh

# Set up IP masquerading to the Raspberry Pi.
# Partially adapted from
# http://www.ibiblio.org/pub/linux/docs/howto/other-formats/html_single/Masquerading-Simple-HOWTO.html

pi_net=192.168.2
ext_iface=wlan0   # Would be nice to figure this out
pi_iface=eth0

# Don't actually need to know the Pi's intended address, just the network.
#pi_addr=97

myaddr=$(ifconfig ${ext_iface} | grep 'inet addr:' | sed -e 's/.*addr://' -e 's/ *Bcast:.*//')
echo My ${ext_iface} address is $myaddr

# Set up IP masquerading:
modprobe ipt_MASQUERADE
iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -t nat -A POSTROUTING -o ${ext_iface} -j SNAT --to $myaddr
echo 1 > /proc/sys/net/ipv4/ip_forward

# A little security: set it up so that the pi can send us anything,
# but outside hosts can't:
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW ! -i ${ext_iface} -j ACCEPT
iptables -P INPUT DROP

# Bring up eth0 so we can actually talk:
ifconfig ${pi_iface} ${pi_net}.1 up