OpenVPN client and daemon binary. More information: https://openvpn.net/.
sudo openvpn path/to/client.conf
sudo openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
sudo openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1
openvpn --genkey secret path/to/key
sudo openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --secret path/to/key
sudo openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --secret path/to/key