I have been asked to configure a firewall between the public and private parts of a network for a small UK charity. The only imposed requirements is that the firewall must allow access using VPN into the private area for correctly configured users and all the machines inside the private area must be able to operate normally as if connected via a NAT modem to the internet. We want the private network facilities to be available via the VPN to these users from home.
We have a dual ported Linux box running Ubuntu 12.04 LTS Server to act as the firewall and vpn connection point. I have followed the directions for the Shorewall dual ported http://www.shorewall.net/two-interface.htm set-up and I have OpenVPN roadwarrior http://www.shorewall.net/OPENVPN.html working. However, no matter what configuration options I use I end up with all the machines unable to connect out to the internet.
I would publish the configuration files but with OpenVPN, Shorewall, dchp and eventually dns there are quite a number involved and a few others are also indirectly implicated like networking.
As I believe this is a common requirement I will be very happy to publish a How To here once I have finished the configuration with explanations of all the settings I understand in the hope of helping others.