Open VPN GUI (Windows) - VPN connected but data transfer fails

Open VPN

Issue

Open VPN GUI client connects to the Open VPN server, the user can successfully open server shares and browse folders but trying to open or transfer any file fails (connection times out). No errors or warnings were present in OpenVPN logs. The issue was only affecting one particular user and only when he was working from his home office. The same user with the same laptop could use the same VPN connection from other locations with no issues. My first thought was that it was the ISP interfering with some sort of Deep Packet Inspection firewall but that didn't appear to be the case.

Resolution

After some experimenting, I found that the issue was only present when OpenVPN was connecting via UDP (any port). When OpenVPN was connecting via any TCP port everything was working absolutely fine. After some more experimenting, I found that the issue was MTU (Maximum Transmission Unit) packet size. By default, OpenVPN sets MTU to 1450. The issue was resolved by reducing MTU to 1420. To do this I simply had to add the following line to OpenVPN config: mssfix 1420

February 2019
OpenVPN GUI
Windows 10 Home

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