Linux - Shorewall firewall with PPTP VPN dialup client

PPTP VPN uses protocol GRE on two way, both inbound and outbound. Setting up the firewall to allow PPTP VPN dialup client (e.g. Ubuntu's NetworkManager) to use protocol GRE is just a few steps in adding rules and interfaces. This post is based on the previous post "How to set up Shorewall firewall (alternate to FireStarter)", which is a quick tutorial on setting up Shorewall firewall on a single network interface. Here it is :

First step

Add the necessary interface into /etc/shorewall/interfaces :

net ppp0 -

Second step

Add/change the firewall rules in /etc/shorewall/rules :

ACCEPT fw net gre
ACCEPT net fw gre
ACCEPT fw net tcp 1723
ACCEPT net fw tcp 1723

Voilla !!!