This script is to turn on/off NATting through to a VPN server on Mac.

natvpn.sh

#!/bin/bash
# This script to turn on/off NATting through to a VPN server.
# Example would be turning the mac into a wireless router, and routing all the incoming
# traffic to the VPN server.
# Thanks to http://rodrigo.sharpcube.com/2010/06/20/using-and-sharing-a-vpn-connection-on-your-mac/

case "$1" in
    on)
        echo "Turning NAT VPN on."
        natd -interface tun0
        ipfw -f flush
        ipfw add divert natd ip from any to any via tun0
        ipfw add pass all from any to any
        sysctl -w net.inet.ip.forwarding=1
    ;;
    
    off)
        echo "Turning NAT VPN off."
        killall natd
        ipfw -f flush
        sysctl -w net.inet.ip.forwarding=0
    ;;
    
    *)
        echo "To use the script, you can either turn the NAT VPN on or off:"
        echo "./natvpn.sh on"
        echo "./natvpn.sh off"
    ;;

esac
exit 0