Application is not able to communicate over TCP port

Question

I have installed a client to communicate with a server.

The application uses two TCP ports for communication (lets call them "A" and "B"). I have opened the ports on the server and the client via the Windows firewall (both inbound and outbound), and created a firewall rule on the hardware firewall to allow them to communicate through these ports.

On Friday through yesterday morning, the client was able to communicate with the server. After some unknown event, the client no longer communicates with the server.

Reading the launch log from the client, the application gets hung up on port A and times out. I verified the Windows firewall rules: the inbound and outbound rules are still active and configured properly. After a reboot, no change to the rules.

Running netstat -aon before the application times out doesn't list either port as listening. My remote control software also does not list A or B in the Network/Open Ports list. The hardware firewall still has the rule active. The server device appears to be working correctly; the server application is listening on A and B.

Help! I don't know what to look for next.

Answer 1

Running netstat -aon before the application times out doesn't list either port as listening

Sounds like the app is somehow broken. May need the developer's help to debug it.

--please don't forget to upvote and Accept as answer if the reply is helpful--

Answer 2

Hello,

For a port to be Listening needs to have an application or running process that uses it. In this situation (since nothing has changed on the settings) the most likely that the remote application is not using the port. For instance, you can get the same behavior in a SQL Server when the service is stopped, the ports will not be listening on the server.

I can also recommend you Portqry, the Microsoft tool to check connections and active ports: https://www.microsoft.com/en-us/download/details.aspx?id=24009

Hope this helps with your query,

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--If the reply is helpful, please Upvote and Accept as answer--

Answer 3

Kinda sorta solved. We had the ability to switch which device was the server and which was the client so we went with that and it appears to be working. For now.

The machines are in a weird state of being managed by two separate IT divisions; one that's very hands-off but strict, and us, who just try to solve problems. The original server machine had the strict division's software firewall installed, and they wouldn't probe deep enough to see if that was the block; the assumption is that they were the issue, since we kindly uninstalled our software firewall to test (I know, it's very inadvisable to have two installed but I don't get a say in that).

To summarize - still don't know what the problem was, however the assumption is that one of the two software firewalls installed was blocking one of the ports, but we can't verify since it's managed by a third-party.

Thanks for the responses, hope this helps someone in the future.