Java Tips

Java tips

Java Port Scanner

Here is a nice program to scan your localhost to see if you have any unexpected open ports (the first 100, but you can easily change this).





import java.net.*; 

import java.io.*; 

public class TCPscan { 

  public static void main(String [] args) { 

    int    from = args.length > 0 ? Integer.parseInt(args[0]) : 0; 

    int    to   = args.length > 1 ? Integer.parseInt(args[1]) : 100; 

    String host = args.length > 2 ? args[2] : "localhost"; 

    try {
        InetAddress ipAd = InetAddress.getByName(host); 

         for (int i = from; i <= to; i++) { 

          try { 

            Socket s = new Socket(ipAd, i); 

            System.out.println("TCP server on port " + 

                                        i + " of " + host); 

          s.close(); 

        } catch (IOException e) { 

          // must not be a server on port i 

        } 

      } 

    } catch (UnknownHostException e) { System.err.println(e); } 

  } 

}

Don't forget the classpath

When you find the following error:

     Exception in thread "main" java.lang.NoClassDefFoundError:

Remember, the current directory is probably excluded from the places where the JVM is looking for files.

Just add the current directory to the "classpath" (abbreviated cp) in the following way :

     java -cp . [Name of class containing the main method]