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I’m writing a Java application like AIM where I want a lookup server to help two clients connect to each other with the ServerSocket class. Unfortunately a serversocket needs an open port or it will be blocked by your firewall. Without having the clients manually change their firewall settings is there a way I can either find an open port, open a port (it’s okay if it asks for permission) I just don’t want every client to manually have to change firewall settings. Thanks!
Creating a ServerSocket is what opens the port on the server end (assuming the port is NOT also blocked on the firewall). On many systems, port numbers <1024 are not blocked by the firewall, and therefore often don’t require custom configuration. However, since many ISPs have been more and more stringent with what ports they block within their network, many pieces of software have moved to operating over port 80. Why port 80? It’s the port used for HTTP, and therefore pretty much open (at firewalls) 100% of the time for any service that operates on the web.
To answer the question of how to find an open port, port scanners perform this function. Basically all they do is try to establish a connection on a range of ports (say port 1-1024). When a connection is successful (in Java you would see this as a good connection over a Socket), you can consider that port “open”. You don’t need to exchange any data in order to make the connection, you just need to establish the connection, and then close it.
Also, if there is no server software of any kind running on the server, it won’t appear “open”, even if it isn’t blocked. Without a successful Socket (TCP) connection, you won’t know what ports are not blocked by user’s firewall.
To reiterate, I’m switching back and forth between two related, but separate concepts. Firewalls can block/open ports, or have port forwarding. That’s not the same as a port being “open” for connections on a given machine. In order for a machine to accept connections on a given port, there must be a piece of software listening for a connection on that port, otherwise no connection can be made.