It could be a variety of issues. Thus, it can be worthwhile to use lower level networking tools, such as netcat
to serve a port on the server, and try connecting with netcat
from the client side. This can help simplify the problem by removing any potential problems introduced by an application's network programming code. If the problem is identified as being a network issue, then there are a few things to check:
- Verify firewall exceptions on the server.
- Verify firewall exceptions in the server's network gateway.
- If the server and client are on different networks, with the client trying to connect to the server's external IP, then verify that the server's gateway knows what traffic to route to the server. This may require setting up rules, such as port forwarding, in the routing device.
- If the server and client are on the same network, but the client is trying to connect to the server through the network's external IP, then verify that the gateway supports looping back internal traffic destined to the external IP.
- Use a network analyzer tool, such as Wireshark, to verify that the time to live field in the packets is high enough that it will not be discarded.