Question

I’ve created a simple method in attempts to determine if a socket is open on a remote machine. Here is my code:

public static Boolean isPortAvailable( int port, String bindAddr  ) {
    try { 
        System.out.println("IP: " + InetAddress.getByName(bindAddr ));
        ServerSocket srv = new ServerSocket(port, 0, InetAddress.getByName(bindAddr ) );  

        srv.close();  
        srv = null;  
        return true;  

    } catch (IOException e) {  
        return false;  
    }
}  

Passing it these two args:

String bindAddr  = "remotemachinename";
int port = 1719;

It continues to tell me the port is not available but if I try netstat -a on the machine I see it’s clearly NOT in use. Am I missing something?

Answer 1

If you want to contact a listening (server) socket you should use a Socket not a ServerSocket. See the ServerSocket javadoc, the third parameter of the ServerSocket constructor is a local address to bind to, not a remote address to connect to.

Try this:

try {
    InetAddress addr = InetAddress.getByName(bindAddr);
    Socket sock = new Socket();
    SocketAddress sockaddr = new InetSocketAddress(addr, port);
    int timeout = 1000;   // wait for 1 second = 1000ms (adapt to your use case)
    sock.connect(sockaddr, timeout);
} catch (UnknownHostException e) {
} catch (SocketTimeoutException e) {
    // nobody's listening or willing to accept our connection...
} catch (IOException e) {
}