I have an client application that is unable to connect with it's server counterpart over a VPN connection. Server is Windows SBS 2011, client is Windows 7 64bit Enterprise. Using Windows VPN client. I can ping the server and browse it's drives, but when the client application attempts to connect it fails immediately with a series of generic reasons (e.g. "Maybe the server isn't on" and "Maybe you are specifying an invalid IP address"). After speaking with the application support people they had me run a testing client/server pair of apps on the server and workstation to verify UDP traffic was being transmitted. The test failed.

So now I need to figure out whether or not UDP traffic is passing through, and if not what I need to do to resolve that.

  • 1
    Believe it or not the several reasons could be quite important. – user9517 Jan 23 '14 at 17:12
  • "So now I need to figure out whether or not UDP traffic is passing through" - It seems to me that you've already determined that it isn't. – joeqwerty Jan 23 '14 at 17:28
  • Wireshark/tcpdump, plus whatever your favorite tools is for generating traffic. – Zoredache Jan 23 '14 at 17:40
  • @Lain - good point, I have edited my question to clarify. There is nothing telling in the errors reported, they are borderline insulting in the simplicity. ;) – Steve K Jan 23 '14 at 19:57

I use netcat, you can download the windows version here

On one end of the VPN start a listener (-l for listen -u for UDP):

nc -lu <port>

On the other end test the UDP connection, text entered after issuing the nc command should appear on the listener terminal:

nc -u <litenerIP> <port>

here's a local test run in 2 separate terminals:


$ sudo nc -lu 99



$ sudo nc -u 99

  • Great help! I setup the pair on the LAN and everything worked as expected. I then ran netcat from behind the VPN and the string is not echoed on the listener. So UDP is blocked. I will post another question asking how to resolve this now. Thanks again! – Steve K Jan 23 '14 at 20:06

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