How can I detect Open Ports on a router/firewall?

I'm writing a client/server application it is necessary for my users to have the necessary listening ports open on their router/firewall.


Is there any method that I can use to detect this? I'm willing to write a seperate server application that they can ping, and then that application can try a TCP connection back to the original app. If it times out and fails, I can assume that their ports are being blocked, and can send out a UDP message?

I'm new at all this, but my applications are complicated and I need to automate things as much as possible for the end user.

brandoom

Yes, you can use a server app to connect to. If you can connect , of course the port is not blocked. You can assume that if you cannot connect you are being blocked. However this is not also true with UDP on the port. For UPD you have to send a message then get one back. Since there is no guaranteed delivery you have to attempt several times before determining that the port is blocked.

I am not sure how you got the PING in there. That help you not at all. That will only tell you if a device with that IP address will answer. Now if that device with that IP address doesn't answer, it means nothing because it may be in 'Stealth' Mode and willl not answer to ping, but is still there.