Which port to sniff for packets

Question

Hi ppl, i been tasked to write a basic packet sniffer program using Java to sniff packets off my school's student LAn for one of my subject's assignment, topic which is on network protocol analysis.
My plan is to write several type of packet classes like TCP,IP,UDP
then i got my own sniffer program which will sniff packets off the LAN , detect type of packet and then instantiate of the above objects and the user can then manipulate.
The GUI and implementation is rather fundamental to me, but i am rather puzzled or loss at how can i actually sniff the packets off the LAN. Is there a specfic class in the Java API, or port number which i can specify that i can capture the packets for analysis ?
Hope you ppl can pump in your suggestions on the capturing data info..
thanks
wilson

Answer 1

The easiest way you could do this is by creating a socket to the computer you want to scan.
Than use the getLocalPort() method and the getPort() method to get the values back.
You could use this script:
import java.net.*;
import java.io.*;
import java.sql.*;
import java.util.*;
public class portListen extends Thread {
   /* Public interface ******************************************************/
   static public void main( String argv[] ) {
      String cPortFrom;
      String cPortDesc;
      System.out.println( "\nOpening portListen.ini ...\n" );
      try {
         RandomAccessFile RAF;
         RAF = new RandomAccessFile("portListen.ini","r");
         String cIPFrom = "127.0.0.1";
         if( argv.length>0 )
            cIPFrom = argv[0];
         String cLine = "";
         cLine = RAF.readLine();
         while( cLine!=null ){
            StringTokenizer token = new StringTokenizer( cLine, "," );
            cPortFrom="";
            cPortDesc="";
            if( token.hasMoreTokens() )
               cPortFrom = token.nextToken();
            if( token.hasMoreTokens() )
               cPortDesc = token.nextToken();
            if( cPortFrom.length()>0 ){
               portListen server = new portListen( cIPFrom, cPortFrom, cPortDesc );
               server.start();
            cLine = RAF.readLine();
         RAF.close();
         System.out.println( "\nAll system ready" );
      } catch(Exception e) {
         e.printStackTrace();
         System.out.println( "\nError during opening of portListen.ini  " +e.toString() );
   private int nPortFrom;
   private String cInServer;
   private String cPortDesc;
   public portListen( String cIPFrom, String inPort, String cPDesc ){
      cInServer  = cIPFrom;
      nPortFrom  = Integer.parseInt(  inPort );
      cPortDesc  = cPDesc;
      System.out.println( "   listen [" +cInServer +":" +nPortFrom +":" +cPortDesc +"] " );
   public void run(){
      try {                                  // port, maxrequest, address
         ServerSocket sock = new ServerSocket( nPortFrom, 5, InetAddress.getByName(cInServer) );
         while( true ){
            Socket socket = sock.accept();
            //System.out.println( " User at [" +socket.getLocalPort() +":" +cPortDesc +"] retreive information" );
            System.out.println( " New user from [" +socket.getInetAddress() +":" +socket.getPort() +"] to [" +socket.getLocalAddress()  +":" +socket.getLocalPort() +":" +cPortDesc +"]" );
            socket.close();
      } catch (Throwable e) {
         e.printStackTrace();
         System.out.println( e.getMessage() );
         System.out.println( "Error in listen [" +nPortFrom +"] " );
}I got this code from this url: http://www.baccan.it/index.php?sezione=javaportlistener&email=si
Hope this helps you out a bit. Good luck.