I am developing a custom server application hosting HTTP (though not HTML) requests on port TCP 5123. If I add a Windows firewall rule to allow incoming connections to this port for any program then it works fine but the moment I specify my program as the only allowed one the suddenly my clients can not get through any more. If it makes any difference, the program is a command line program at the moment and not yet a service.

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    Have you verified that the program Windows "sees" as being the recipient is actually your executable? Your application might be using a third-party interface, which is receiving the packets and passing them to the your code. You might try the free Nirsoft or Glasswire network monitors to determine that: nirsoft.net/utils/network_traffic_view.html and glasswire.com/download. – DrMoishe Pippik Jan 11 '15 at 16:04
  • @DrMoishePippik Ok, I'll have a look, as this is a .Net app using HttpListener, maybe the CLR is taking the hit. Is there anyway around that? – Gerharddc Jan 11 '15 at 16:48
  • Though I don't know of a workaround, would it be a security issue to allow any .Net application access to that port? – DrMoishe Pippik Jan 11 '15 at 21:36

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