How can I detect a free port on the server by code from client side?

Question

Is there any way by which i can find out free port on the server by executing a code from client computer.

Example for opening a port on my local machine:

ServerSocket localmachine = new ServerSocket(0);
int port = localmachine.getLocalPort();

This way I can retrieve a free port on my local machine.

Now suppose I have my own server: http://xyz.com

How can I get a free port for the server by running a program from client?

Basically I want to initiate a socket connection between client and a server. Which will be further used for communication purpose.

Answer 1

The port information is only available to the server itself, so you'll need to run a program on the server to find a free port and send it to the client.

If you help us understand why you want to do this, we might be able to give a better answer.

Answer 2

Although I'm not sure what/why you will accomplish with this, you can just try to connect to the server with regular client sockets. If you do that in a loop you can probe all the ports. This may/may not tell you if the port is occupied, since there may be firewalls blocking certain ports. At least you can see if you see the port.

Tools like nmap will of course give you much more sophisticated analysis.

Answer 3

A free port implies it is a port you can used. The only ports a client can use on a server are ports which are open on the server.

However, I assume you mean you want to find an unused port. This sounds like you are trying to do something which doesn't need to be done if you were to do whatever you are trying to a do a standard way.

However, to answer your question literally.

public static int unusedPort(String hostname) throws IOException {
    while (true) {
        int port = (int) (65536 * Math.random());
        try {
            Socket s = new Socket(hostname, port);
            s.close();
        } catch (ConnectException e) {
            return port;
        } catch (IOException e) {
            if (e.getMessage().contains("refused"))
                return port;
            throw e;
        }
    }
}