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Can I open any TCP/UDP ports when using an Applet?

Hi,

I want to use JAVA applet for communication between Client/Server. The client has to listen to port 162 and connect (send messages) to the server port no. 161. The connection is over UDP.
I am aware to JAVA applets' restrictions, the client applet is planned to perform the connection only to/from the host, which it was download from. My question is: can I connect to any port on this host (the server) and listen to any port on my local host (the client)?

Code example or any other example will be usefull too.
Thanks,
Amir.
gooliver@yahoo.com
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AmirG
Asked:
AmirG
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1 Solution
 
jpk041897Commented:
Yes, you can open any port for UDP provided there are no firewalls involved.

If firewalls are involved you would be prety much forced to either open the UDP port on the firewall or tunnel your packets over TCP port 80.

As far as a UDP server (with no firewall) is involved, your code would look like this:

import java.net.*;
import java.io.*;

// START main1
class EchoServer extends Thread {
    private DatagramSocket sock = null;
    EchoServer(int port) {
      try {
          sock = new DatagramSocket(port);
      } catch (SocketException e) {
          e.printStackTrace();
      }
    }
    public void run() {
      if (sock == null)
          return;

      byte[] inbuf = new byte[1024];
      DatagramPacket request = new DatagramPacket(inbuf, inbuf.length);
      try {
          while (true) {
            sock.receive(request);
            sock.send(request); // just return what was sent
          }
      } catch (IOException e) {
          e.printStackTrace();
      }
    }
}
// END main1

class Main {
    static int echo_port = 1257;

// START main2
public static void echo(String msg, InetAddress dst, int port) {
    byte[] inbuf = new byte[1024];  // default size
    byte[] outbuf = msg.getBytes();

    try {
      DatagramSocket client = new DatagramSocket(); // any port
      DatagramPacket request = new DatagramPacket(outbuf, outbuf.length,
          dst, port);
      DatagramPacket reply = new DatagramPacket(inbuf, inbuf.length);
      client.send(request);
      client.receive(reply);
      client.close();
      System.out.println(new String(reply.getData()));
    } catch (SocketException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
}
// END main2

    public static void main(String[] args) {
      if (args.length != 1) {
          System.err.println("Usage: java Main <message>");
          System.exit(1);
      }

      // start server
      EchoServer srv = new EchoServer(echo_port);
      srv.setDaemon(true);
      srv.start();

      try {
          String msg = "\n";
          InetAddress dst = InetAddress.getLocalHost();

          echo(args[0], dst, echo_port);

      } catch (UnknownHostException e) {
          System.err.println("Host not found: " + e);
      }

// START local
try {
    DatagramSocket client = new DatagramSocket();
    System.out.println("Using port number " + client.getLocalPort());
    System.out.println("Using address " + client.getLocalAddress());
// UNCOM      ...
    client.close();
} catch (SocketException e) {
    e.printStackTrace();
}
// END local
    }
}


The client would look like:

import java.net.*;
import java.io.*;

class Main {
    public static String dgExchange(String msg, InetAddress dst, int port) {
        byte[] outbuf = msg.getBytes();
        byte[] inbuf = new byte[256];  // default size

        try {
            // Send datagram
            DatagramPacket request =
                new DatagramPacket(outbuf, outbuf.length, dst, port);
            DatagramSocket sock = new DatagramSocket();
            sock.send(request);

            // Wait for reply
            DatagramPacket reply = new DatagramPacket(inbuf, inbuf.length);
            sock.receive(reply);

            System.out.println(
                "Received packet from:" + reply.getAddress() +
                " port: " + reply.getPort() +
                " length: " + reply.getLength());

            sock.close();
            return (new String(reply.getData()));
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return (null);
    }

    public static void main(String[] args) {
        try {
            String msg = "\n";
            int port = 13;
            InetAddress dst = InetAddress.getLocalHost();

            if (args.length > 0) {
                port = Integer.parseInt(args[0]);
                if (args.length >= 2)
                    msg = args[1];
                if (args.length == 3)
                    dst = InetAddress.getByName(args[2]);
            }

            System.out.println(dgExchange(msg, dst, port));
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }
}


Hope this helps.
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AmirGAuthor Commented:
Please ignore the firewall problem, is it possible to open port no. 162 on my local machine (the applet side) for listening and sending messages to port 161 on the server side?
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AmirGAuthor Commented:
Please ignore the firewall problem, is it possible to open port no. 162 on my local machine (the applet side) for listening and sending messages to port 161 on the server side?
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jpk041897Commented:
Yes, though for an applet, certain security restrictions apply.

The browsers security sandbox by default, as of JDK 1.2, will only allow opening port 80.

On JDK 1.1 you only need to pack your applet in a JAR file and sign it. On JDK 1.2 and above you also need to add a security policy file to the JAR file in order to be allowed to open port 161.

The server might or might not allow the opening of the port depending on architecture. For example, on a moderatly secure UNIX system you would have to modify some network configuration files to open the port at the OS level and on JDK 1.3 you would also need to create a Java security manifest.
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stimpyjcatCommented:
On a Unix machine, ports below 1024 require superuser (root) access.

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AmirGAuthor Commented:
And if I control also the server OS (I use Apache web-sever on RTOS), is it possible to open standard ports below 1024?  
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stimpyjcatCommented:
It likely depends on the configuration of RTOS, but usually embedded OSes don't default to a lot of restrictions.  You should just make sure there isn't already a process using the port to which you wish to bind.

netstat ("netstat -a") is a common tool that will show you which ports are in use.

Port 161/162 is not a good choice, since it is used by SNMP, and is likely to be either in use already or interpreted as SNMP instead of *your* messages on the other end.

Unless you are actually trying to implement some form of snmp.
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stimpyjcatCommented:
BTW, jpk explained the other restrictions you'll run into when attempting to use a listening socket where the applet is downloaded (client).

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sudhakar_koundinyaCommented:


No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:


--  points to jpk


Please leave any comments here within the next seven days.
 
PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!
 
sudhakar_koundinya
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