#!/bin/sh
#
# usbnet.sh - script to setup the N900 as an ip router
#
# Author:   Mirakels 
#
# Date:     may 2010
#
# This script and documentation sets up the N900 as a router
# between your PC and the wlan/cellphone on your n900.
#
# We need to setup the usb device, allow forwarding and
# masquerade all traffic going outside...
# to get simple dns working on your PC we als need to update
# dnsmasq on the n900.
# 
#
# Preparations
# ============
# So you need the iptables package (apt-get install iptables)
# and a kernel that supports iptables (don't know if the
# default kernel supports iptables. I use the titan overclock
# kernel)
#
# Note the preparation woork needs to be done as root...
# 
#
# SETUP USB0
# ----------
# Changed usb0 definition in /etc/network/interfaces
# from:
#      auto usb0
#      iface usb0 inet static
#              address 192.168.2.15
#              netmask 255.255.255.0
#              gateway 192.168.2.14
#
# to:
#      auto usb0
#      iface usb0 inet static
#              address 192.168.50.1
#              netmask 255.255.255.0
#
# I just picked the 192.168.50.x subnet to act as the intermediate
# network. It should be a subnet that is not used within your campus.
#
# SETUP DNSMASQ
# -------------
# Changed dnsmasq exec line in /etc/event.d/dnsmasq
# from:
#      exec /usr/sbin/dnsmasq -k -i lo -a 127.0.0.1 -z 
#
# to:
#      exec /usr/sbin/dnsmasq -k -i lo -a 127.0.0.1 -z -i usb0
#
# So it also listens on the usb0 network interface for dns queries
#
#
# With above steps the n900 is prepared to act as a router.
#
# STARTUP
# =======
# Connect the n900 with the usb cable to your laptop.
# When the n900 asks for 'mass storage' or 'PCsuite',
# select 'PCsuite'.
#
# Run this script without options, e.g.:
#
#       sudo gainroot
#       /home/user/usbnet.sh
#
# This will start he usb0 network device, allow
# ip forwarding and sets up masquerading.
#
# Now it is time to do some steps on your laptop/PC.
# The steps depend on your laptop OS. For my Fedora 12
# system I will see a new network interface called usb0.
# All I need to do now as root is:
#
#   ifconfig usb0 192.168.50.2
#   route add default gw 192.168.50.1
#   echo "nameserver 192.178.50.1" > /etc/resolv.conf
#
# and I have internet access from the laptop through the n900
# while the n900 still can be used as fully functional network
# device and phone.
#
# The nice thing in this setup is that I can still monitor the
# total traffic  through the cell phone network via the n900
# dataplan tool.
#
# PS: I tried to defined the usb0 network in network manager
#     but then I need a MAC address of the usb0 device to define
#     the connection. Unfortunately the MAC address seems to
#     be a random address so changes every time the usb connection
#     is recreated.
#
#
# To stop acting as a router run this script again with some
# dummy command line argument
#
#       sudo gainroot
#       /home/user/usbnet.sh done

if [ -z "$1" ] ; then
    echo "Starting network and forwarding through usb0"
    ifup usb0
    echo 1 > /proc/sys/net/ipv4/ip_forward
    iptables -t nat -A POSTROUTING -o gprs0  -j MASQUERADE
    iptables -t nat -A POSTROUTING -o wlan0  -j MASQUERADE
else
    echo "Stopping network and forwarding through usb0"
    ifdown usb0
    echo 0 > /proc/sys/net/ipv4/ip_forward
    iptables -t nat -D POSTROUTING -o gprs0  -j MASQUERADE
    iptables -t nat -D POSTROUTING -o wlan0  -j MASQUERADE
fi

#
# The End
#