OpenVPN is a free and open source software VPN solution. It allows secure point-to-point or site-to-site connections with routed or bridged configurations and remote access facilities.

OpenVPN is a free and open source software application that implements VPN techniques for creating secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses SSL/TLS security for encryption and is capable of traversing network address translators and firewalls.

It's architecture is build upon 4 principles:

  • Encryption
  • Authentication
  • Networking
  • Security

More info is to be found on the Open-Source site of VPN.
A very thorough tutorial can be found on Shorewall.