In this post I am going to show you how to test GRE Protocol is open and working when setting up PPTP VPN Servers. GRE Protocol being blocked on network infrastructure is one of the most common issues faced when setting up a PPTP VPN Server.
The first step is to stop the VPN Server. This is done by right clicking the VPN server in Routing and Remote Access and clicking "Stop" under All Tasks. This is done because our testing tool also listens on TCP1723 so we need to ensure this port is no longer in use on the server in question.
The first step is to stop the VPN Server. This is done by right clicking the VPN server in Routing and Remote Access and clicking "Stop" under All Tasks. This is done because our testing tool also listens on TCP1723 so we need to ensure this port is no longer in use on the server in question.
Next download our testing tools called pptpsrv.exe and pptpclnt.exe. These tools are available in the Windows XP SP2 support tools available from the following URL:
Copy the pptpsrv.exe file to C:\Windows\System32 on your PPTP VPN Server then run it from an Administrative Command Prompt. Ensure Windows Firewall does not block it. Run the tool on the server and it will begin listening on TCP1723.
Next run the pptpclnt.exe from another remote computer and target it against the PPTP VPN Server like this:
pptpclnt.exe 192.168.2.11
My local IP address of my PPTP VPN Server is 192.168.2.11. Then when prompted enter in some sample text to send through the TCP1723 port socket. I wrote "Hello this is clint".
Next go back to the PPTP VPN Server and verify the text was received. We can see that the VPN Server received the text "Hello this is clint" and then went onto the second test of closing down the port socket TCP1723 and beginning a socket for GRE Protocol test.
The client automatically goes on to test GRE protocol. GRE Protocol on my local network is currently blocked by our Cisco Infrastructure (L3 Switch/Router) and as a result we get a Socket Failure error.
This means I need to go back to the Cisco Infrastructure, resolve this problem then test again.
Hope this post has been helpful for troubleshooting PPTP VPN servers.
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