I am trying to help a friend figure this out, so I don't know all the details, but I figured I would post on here to see any possibilities.
We have a custom application that communicates using a single TCP port between the client and server and want it accessible both locally and remotely. The strange thing is that it works when the client is remote(internet) but NOT locally. Port forwarding is set in the router, which is a Cisco E800, and running a port scan indicates that the relevant TCP port is open to both the internal network and to the internet, however, the application will not connect locally. It DOES connect remotely so we know the server software is set up correctly, etc. OS on both ends of the connection is Windows 7 with windows firewall turned off.
Is there anything else we can try or tests we can run to determine the problem? I have run port discovery scans and ran netstat on the server. It showed the remote connections as "established" and another one with the machine name as "listening" on the relevant port.
Sorry I can't be more detailed, nothing about this problem makes sense.
EDIT: Since it seems like everything is set correctly, correct ports are open, etc. and we also know the server is functioning properly because it works from the remote location, maybe it is time to try some kind of packet sniffing/monitoring so we can see what is going on on that port... Is there an easy way to do this? In this way we could see if an attempt is being made, if there is any differences between a local request and a remote request, etc.