Use JSch sudo example and Channel.setPty for running sudo command on remote host

Question

I have used JSch Sudo example under following link:

http://www.jcraft.com/jsch/examples/Sudo.java.html

And changed it a bit and get rid of all the dialogs as I have to use it for EC2 instances using PuTTY.

Now my code looks like this:

import com.jcraft.jsch.*;
import java.awt.*;
import javax.swing.*;
import java.io.*;

public class sudo{
  public static void main(String[] arg){
    try{
      JSch jsch=new JSch();

      String host=null;
      if(arg.length>0){
        host=arg[0];
      }

      String privateKey = "my private key.pem";

      jsch.addIdentity(privateKey, "");
      Session session=jsch.getSession("ec2-user", "xx.xx.xx.xx", 22);

      session.setPassword("");
      java.util.Properties config = new java.util.Properties();
      config.put("StrictHostKeyChecking", "no");
      session.setConfig(config);

      session.connect();

      String command="sudo mkdir /data";
      String sudo_pass="";

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

      // man sudo
      // -S The -S (stdin) option causes sudo to read the password from the
      // standard input instead of the terminal device.
      // -p The -p (prompt) option allows you to override the default
      // password prompt and use a custom one.
      ((ChannelExec)channel).setCommand("sudo -S -p '' "+command);

      InputStream in=channel.getInputStream();
      OutputStream out=channel.getOutputStream();
      ((ChannelExec)channel).setErrStream(System.err);

      channel.connect();

      out.write((sudo_pass+"\n").getBytes());
      out.flush();

      byte[] tmp=new byte[1024];
      while(true){
        while(in.available()>0){
          int i=in.read(tmp, 0, 1024);
          if(i<0)break;
          System.out.print(new String(tmp, 0, i));
        }
        if(channel.isClosed()){
          System.out.println("exit-status: "+channel.getExitStatus());
          break;
        }
        try{Thread.sleep(1000);}catch(Exception ee){}
      }
      channel.disconnect();
      session.disconnect();
    }
    catch(Exception e){
      System.out.println(e);
    }
  }
}

But I am getting the error

sorry you must have a tty to run sudo

I also tried to use ((ChannelExec) channel).setPty(true) but I can run the program once but next time, for same EC2 instance, I get the following error but for new instance it works fine for first time again.

com.jcraft.jsch.JSchException: Session.connect: java.io.IOException: End of IO Stream Read
sudo mkdir /data
Exception in thread "main" com.jcraft.jsch.JSchException: session is down
        at com.jcraft.jsch.Session.openChannel(Session.java:791)

And then also I could not ssh the remote host from command line as well.

Can someone please guide me that what I need to do to run sudo command on remote host.

Answer 1

I have similar code in a project I am working on, and was getting the same error. I resolved this using the setPty(true) as you did.

I think you're getting this error because you don't close out the streams in your code. If you are using Java 1.7, you can use a resource block as follows:

try( InputStream in=channel.getInputStream() ) {
    try( OutputStream out = channel.getOutputStream() ) {
    ...
    }
}

or the try...finally block pattern from past versions.

Answer 2

After setting

((ChannelExec) channel).setPty(true)

the reported problem have in my code got solved.

Answer 3

By default, SUDO is configured to require a TTY. That is, SUDO is expected to be run from a login shell.

That's probably because your /etc/sudoers file (or any file it includes) has:

Defaults requiretty

But the option -S in sudo will make it read from Standard input.

-S    The -S (stdin) option causes sudo to read the password from
      the standard input instead of the terminal device.  The pass-
      word must be followed by a newline character.

Jsch exploits the same and tries to send the password. If you want to Jsch to work without prompting the password , you need to disable the requiretty from the sudoers file...using visudo command as follows

Defaults !requiretty

or

#Defaults requiretty