This recipe will explain the shortest setup possible when using OpenVPN. For this setup two computers are used that are connected over a network (LAN or Internet). We will use both a TUN-style network and a TAP-style network and will focus on the differences between them. A TUN device is used mostly for VPN tunnels where only IP-traffic is used. A TAP device allows full Ethernet frames to be passed over the OpenVPN tunnel, hence providing support for non-IP based protocols such as IPX and AppleTalk.
While this may seem useless at first glance, it can be very useful to quickly test whether OpenVPN can connect to a remote system.
Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Windows XP SP3 and OpenVPN 2.1.1.
We launch the server (listening)-side OpenVPN process for the TUN-style network:
[root@server]# openvpn --ifconfig 10.200.0.1 10.200.0.2 \ --dev tun
Then we launch the client-side OpenVPN process:
[WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \ --ifconfig 10.200.0.2 10.200.0.1 --dev tun \ --remote openvpnserver.example.com
The following screenshot shows how a connection is established:
As soon as the connection is established, we can ping the other end of the tunnel.
Next, we stop the tunnel by pressing the F4 function key in the Command window and we restart both ends of the tunnel using the TAP device:
We launch the server (listening)-side OpenVPN process for the TAP-style network:
[root@server]# openvpn --ifconfig 10.200.0.1 255.255.255.0 \ --dev tap
Then we launch the client-side OpenVPN process:
[WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \ --ifconfig 10.200.0.2 255.255.255.0 --dev tap \ --remote openvpnserver.example.com
The connection is established and we can again ping the other end of the tunnel.
The server listens on UDP port 1194, which is the OpenVPN default port for incoming connections. The client connects to the server on this port. After the initial handshake, the server configures the first available TUN device with IP address 10.200.0.1 and it expects the remote end (Peer address) to be 10.200.0.2.
The client does the opposite: after the initial handshake, the first TUN or TAP-Win32 device is configured with IP address 10.200.0.2. It expects the remote end (Peer address) to be 10.200.0.1. After this, the VPN is established.
In case of a TAP-style network, the server configures the first available TAP device with the IP address 10.200.0.01 and netmask 255.255.255.0. Similarly, the client is configured with IP address 10.200.0.2 and netmask 255.255.255.0.
In the previous example, we chose the UDP protocol. For this example, it would not have made any difference if we had chosen the TCP protocol, provided that we do that on the server side (the side without --remote):
[root@server]# openvpn --ifconfig 10.200.0.1 10.200.0.2 \ –-dev tun --proto tcp-server
And also on the client side:
[root@server]# openvpn --ifconfig 10.200.0.2 10.200.0.1 \ --dev tun --proto tcp-client
It is now possible to run non-IP traffic over the tunnel. For example, if AppleTalk is configured correctly on both sides, we can query a remote host using the aecho command:
aecho openvpnserver 22 bytes from 65280.1: aep_seq=0. time=26. ms 22 bytes from 65280.1: aep_seq=1. time=26. ms 22 bytes from 65280.1: aep_seq=2. time=27. ms
A tcpdump -nnel -i tap0 shows that the type of traffic is indeed non-IP based AppleTalk.