If call bind with port 0 in Linux or Windows or OSX the OS will bind to a random unused port within the unrestricted port range. Whether there is any forwarding

If call bind with port 0 in Linux or Windows or OSX the OS will bind to a random unused port within the unrestricted port range. Whether there is any forwarding or firewall associated with that port is outside the scope of the code that opens the port. If the application is assigned a random port such as 55555 there could be forwarding or a firewall on that port at the OS level that the application is unaware of. If an application is opening random listening ports it just needs to record that information somewhere so clients can actually connect to it. In the case of using this for testing localhost servers the application can just print out the port number or save it to a file so that you can look it up easily without going through netstat or something. Opening a random listening port to serve content to clients which initiate communications to it is less common than the use case of initiating a communication to a listening port elsewhere and sending content through that socket, though both of these use cases open random local ports.