Q

Checking port settings

My company has implemented a Point-to-Point VPN for all of our offices. Our London office keeps losing connection. I'd like to be able to check their router and see if the proper ports are opened. BT owns the router and won't let us configure it and I have a feeling they're being less then truthful about the config. How can I check to see the port settings? Hi Frank, I see this problem all the time when trying to set up a VPN where...

someone else controls the router or firewall. So you're not alone. Although it does sometimes occur that a large carrier may be misleading you, it's more than likely that the person you were talking to is simply mistaken.

A relatively straightforward way to see if the port is open and forwarding properly is to:

  1. Set up a server inside your firewall and/or router that can accept telnet requests.
  2. Assign the server the IP address that you would have normally assigned to your VPN gateway and have it listen on the appropriate ports for telnet requests.
  3. From inside the firewall, on the same network, establish a telnet session with the server. If this works, you're ready for the outside test. If not, make sure that you have everything set up correctly before continuing.
  4. From outside the firewall, initiate a telnet session to the server address (if it is a public address) or to the firewall address (if you're using port forwarding and/ a server has a private IP address). If the telnet session works, then ports are open. If not, then they are closed or not forwarding properly.
This was first published in October 2002

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