Configuring IPSec VPN Connection Between FreeBSD and OpenBSD

Although there are some good tutorials on setting VPN connections using IPSec in FreeBSD-to-FreeBSD or OpenBSD-to-OpenBSD configurations, only tutorials on setting FreeBSD-to-OpenBSD IPSec tunnels are at least partly outdated, mostly due to changes introduced in OpenBSD 3.8.

In this tutorial I'll demonstrate how to configure such a VPN connection (tunneling mode) using racoon and isakmpd IKE daemons along with x509 certificates. The IP addresses used throughout this tutorial are:
- gateway A running FreeBSD with internal IP address: 192.168.0.1, and external IP: 10.0.0.1,
- gateway B running OpenBSD with internal IP address: 192.168.1.1, and external IP: 10.0.0.2.

For the purpose of this tutorial I have used FreeBSD 6.2-RELEASE and OpenBSD 4.1-RELEASE operating systems, but the configuration covered here should be applicable in any reasonably recent version as well.