Troubleshooting MTU and tun-mtu issues
One of the more advanced features of OpenVPN is the ability to tune the network parameters of both the TUN (or TAP) adapter and the parameters of the encrypted link itself. This is a frequent cause of configuration mistakes, leading to low performance or even the inability to successfully transfer data across the VPN tunnel. This recipe will show what happens if there is an MTU (Maximum Transfer Unit) mismatch between the client and the server and how this mismatch can cause the VPN tunnel to fail only under certain circumstances.
Getting ready
Set up the client and server certificates using the first recipe from Chapter 2, Client-server IP-only Networks. For this recipe, the server computer was running CentOS 6 Linux and OpenVPN 2.3.11, and the client was running Fedora 22 Linux and OpenVPN 2.3.11. Keep the client configuration file, basic-udp-client.conf, handy along with the configuration file, basic-udp-server.conf, from the Server-side routing recipe...
