是否可以在不连接IP端口的情况下检查IP端口是否打开？

Question

So what I am trying to do is scan a local network for a specific port (4444) and then return whether or not that port is open or not. What I'm doing right now is this:

    private static boolean portIsOpen(String ip, int port, int timeout) {
        try {
            Socket socket = new Socket();
            socket.connect(new InetSocketAddress(ip, port), timeout);
            socket.close();
            return true;
        } catch (Exception ex) {
            return false;
        }
    }

I'm going to be using this method to search for servers on my local network (Think Minecraft's multiplayer and how it scans the local network for open games). My problem right now, is that the server side breaks when I use this method because the server just sends data right away when anything connects to it. Is it possible to check whether or not a port is open without connecting to it?

Answer 1

Is it possible to check whether or not a port is open without connecting to it?

In a word, No.

There is no standard mechanism that supports this. The only way you could do this is to design, implement and deploy a custom service to do this ... on all machines on the local network.

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As EJP points out, a well-written service should be able to cope with a client that immediately closes the TCP connection without sending or receiving data.

But if your aim is to do this without anyone noticing the probes ... good luck with that!