It is unlikely that the DNS server is a problem, as you say that you are providing the external IP address directly to the client program. I would guess that your server is not fully operational. You can check this by putting a hub on the external port and connecting your PC directly (not through the Internet) to the hub, and attempting to start the VPN. If it works locally, but not through the net, you may need to check with your ISP to ensure that VPN traffic is not being blocked outside of your location.
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