TCP port access and C++

Question

If a program is running on a Linux machine, is there a way for that program to scan for ports that are allowed through the firewall? for example, if a programmer wants to make a chat system, but the program needs to know what ports aren't being refused access to incoming connections by a user's firewall, is there a way to check for this in your code? A program may not fail to bind a socket to a port even if the firewall is blocking that same port from incoming connections. Is there a way to check for open firewall ports?

Sidenote: This is purely for educational purposes and free of bad intentions, to be clear I am writing a chat system, and during testing, I was unable to connect desktop->laptop until I manually opened a port via allowing it through my firewall. This seemed a bit off to me, and unlike something that a programmer's code should require a user to do. Not to mention I don't want to leave the few people using this code at risk (if leaving a port permanently open does so). So It seems like I would be better suited finding a way to utilize ports that are already open to incoming TCP connections.

sidesidenote: all clients are running fedora

Answer 1

You have stumbled onto the second biggest issue governing the creation of new Internet applications nowadays. The first biggest of course is NAT, which is a strongly related issue (and hopefully going away eventually because of IPv6).

And there is no easy answer. One good answer is UPnP, but that's not an easy answer, and by no means universal. My network doesn't run it.

Another answer is to somehow tunnel everything you do over https (or http if you must). But that's a huge pain for something like a chat application.