Running telnet command on remote SSH session using JSch

Question

I am working on running telnet command on SSH shell session, for obtaining it I used JSch following the official example.

I wrote my own code following also this example on StackOverflow

this is my code:

package Utility;

import com.jcraft.jsch.Channel;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStreamReader;


public class JavaTelnet {

    public static void main(String[] arg) {
        try {
            System.out.println(telnetConnection(USER_ID,PASSWORD,REMOTE_HOST));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static String telnetConnection(String user, String password, String host) throws JSchException, Exception {
      JSch jsch=new JSch();
      Session session=jsch.getSession(user, host, 22);
      session.setPassword(password);
      // It must not be recommended, but if you want to skip host-key check,
       session.setConfig("StrictHostKeyChecking", "no");

      session.connect();
      //session.connect(30000);   // making a connection with timeout.

      Channel channel=session.openChannel("shell");

      channel.connect();

      DataInputStream dataIn = new DataInputStream(channel.getInputStream());
      BufferedReader reader = new BufferedReader(new InputStreamReader(dataIn));
      DataOutputStream dataOut = new DataOutputStream(channel.getOutputStream());

      dataOut.writeBytes("telnet localhost 4242\r\n");
/*
* telnet COMMANDS here
*/
      dataOut.writeBytes("exit\r\n");
       dataOut.writeBytes("logout\r\n");
       dataOut.flush();

      String line = reader.readLine();
      String result = line +"\n";

      while ((line= reader.readLine())!=null){
          result += line +"\n";

      }

      dataIn.close();
      dataOut.close();
      System.out.println("disconnecting....");
      channel.disconnect();
      session.disconnect();

      return "done";   

  }

}

It look like good but the strange thing is that it is working only in debug mode. If I run it the program don't reach his end. I think it is locking on the while loop but I don't understand why. Without the while loop the code reach his end but don't execute any shell command.

I am using Netbeans as IDE

Can you help me to find the problem?!

Answer 1

I got it.

line was never null in the while loop.

It remains the mystery about why it worked in debug.

I am posting my new code. I hope it can be helpful for everyone wants to make something similar.

package Utility;

import com.jcraft.jsch.Channel;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStreamReader;


public class JavaTelnet {

    public static void main(String[] arg) {
        try {
            System.out.println(telnetConnection(YOUR_COMMAND,YOUR_USER,YOUR_PASS,YOUR_HOST));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static String telnetConnection(String command, String user, String password, String host) throws JSchException, Exception {
      JSch jsch=new JSch();
      Session session=jsch.getSession(user, host, 22);
      session.setPassword(password);
      // It must not be recommended, but if you want to skip host-key check,
       session.setConfig("StrictHostKeyChecking", "no");

      session.connect(3000);
      //session.connect(30000);   // making a connection with timeout.

      Channel channel=session.openChannel("shell");

      channel.connect(3000);

      DataInputStream dataIn = new DataInputStream(channel.getInputStream());
      BufferedReader reader = new BufferedReader(new InputStreamReader(dataIn));
      DataOutputStream dataOut = new DataOutputStream(channel.getOutputStream());

      System.out.println("Starting telnet connection...");
      dataOut.writeBytes("telnet localhost 4242\r\n");
//      dataOut.writeBytes("enable\r\n");
      dataOut.writeBytes(command+"\r\n");
      dataOut.writeBytes("exit\r\n"); //exit from telnet
       dataOut.writeBytes("exit\r\n"); //exit from shell
       dataOut.flush();

      String line = reader.readLine();
      String result = line +"\n";

        while (!(line= reader.readLine()).equals("Connection closed by foreign host")){
          result += line +"\n";
      }

      dataIn.close();
      dataOut.close();
      channel.disconnect();
      session.disconnect();

      return result;   

  }

}

N.B. the shell is a Linux shell. In windows the "exit" command should be different