University Observatory Munich

Faculty of Physics at the Ludwig-Maximilians-University

Reducing the MTU of the VPN interface

If you have successfully activated the VPN and were able to connect with VNC, but the VNC viewer only shows a black screen instead of the remote desktop, you may need to reduce the MTU (maximum transmission unit) of the VPN interface in order for the network packets to be transmitted through the VPN.

Windows

1. Open a cmd window as administrator. (Open the start menu, type “cmd”, and at “Command prompt” click on “Run as administrator”.)
2. In the cmd window, type

   netsh interface ipv4 show interfaces
   

3. Note the first number (Idx) on the line containing “OpenVPN Wintun”.
4. Type

   netsh interface ipv4 set subinterface Idx mtu=500
   

   where Idx is the number noted above for the VPN interface.
5. If you now repeat the

   netsh interface ipv4 show interfaces
   

   command, it should list “500” as the MTU of the VPN interface.