Have you ever wonder how to do like in secret agent movies : they knock the door with a secret sequence and the door opens! You can do it with your server too! (or RasPi ;)
##Prepare your firewall
edit /etc/iptables.test.rules
*filter
# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT