Make an SSH connection with a remote server using Java

Sometimes, we might need to connect to a different machine to execute a few commands or transfer files to it. This article will discuss how to connect to a remote machine/server using Java.

We have also made a video on this topic, which you can find at the end of this article. Please feel free to check it out.

Adding dependencies

We will use the JSch library to connect to the remote server. So, we will add its dependency here –

		<!-- https://mvnrepository.com/artifact/com.jcraft/jsch -->
		<dependency>
			<groupId>com.jcraft</groupId>
			<artifactId>jsch</artifactId>
			<version>0.1.55</version>
		</dependency>

Now, we will attempt to connect to a remote server. There are typically two methods for doing so –

Using Password
Using key

Let’s see both of them one by one.

Making an SSH connection with Jsch using a password

So, let us see what we need to connect to a remote server. Well, there are only a few things-

Remote server IP
Remote server port ( The port on which you want to connect, say, 33000 )
Username
Password

Below is the sample code to connect with your server using the password –

import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.JSchException;

public class Codekru {

	public static void main(String[] args) {
		
		try {
			JSch jsch = new JSch();
			String username = "codekru"; // your username
			String host = "127.0.0.1"; // your remote server address
			int port = 33000; // your remote server port
			String password = "root"; // your username's password

			Session session = jsch.getSession(username, host, port);
			session.setConfig("PreferredAuthentications", "publickey,keyboard-interactive,password");
			session.setPassword(password);
			session.setTimeout(15000);
			session.connect();
		} catch (JSchException e) {
			e.printStackTrace();
		}
	}

}

This is how you can establish an SSH connection using a password. Let’s now move on to creating a connection using a key.

Making an SSH connection using key

We will store the private key named “codekru.pem” in the “/Users/codekru/key” directory and use it to establish an SSH connection with the remote server.

import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.JSchException;

public class Codekru {
	public static void main(String[] args) {
		try {

			JSch jsch = new JSch();
	        String user = "codekru"; // your username
	        String host = "127.0.0.1"; // your remote server address
	        int port = 33000; // your remote server port
	        String yourKeyName = "/Users/codekru/key/codekru.pem";
	         
	        jsch.addIdentity(yourKeyName);
	        Session session = jsch.getSession(user, host, port);
	        session.setTimeout(15000);
	        session.connect();

		} catch (JSchException e) {
			e.printStackTrace();
		}

	}
}

You will probably get the below error –

Exception in thread "main" java.lang.RuntimeException: com.jcraft.jsch.JSchException: UnknownHostKey: 127.0.0.1.

This is because SSH isn’t able to validate the identity of the remote server. So, how do we fix it?

Well, there are 2 ways –

One is to bypass the checking of identity by setting the “StrictHostKeyChecking” value as “no” ( not recommended )
And second is by using the known_hosts file

Using StrictHostKeyChecking

import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.JSchException;

public class Codekru {
	public static void main(String[] args) {
		try {

			JSch jsch = new JSch();
	        String user = "codekru"; // your username
	        String host = "127.0.0.1"; // your remote server address
	        int port = 33000; // your remote server port
	        String yourKeyName = "/Users/codekru/key/codekru.pem";
	         
	        jsch.addIdentity(yourKeyName);
            
	        Session session = jsch.getSession(user, host, port);
            session.setConfig("StrictHostKeyChecking","no");
	        session.setTimeout(15000);
	        session.connect();

		} catch (JSchException e) {
			e.printStackTrace();
		}

	}
}

Now, we should be able to connect with the server, but this way is not recommended. Because it poses various security concerns and is more prone to man-in-the-middle attacks. So, let’s see the other way now.

Using known_hosts file

Run the below commands in your terminal –

ssh-keyscan -t rsa <Remote_Machine_IP> >> ~/.ssh/known_hosts

This command retrieves the RSA key from the remote machine and appends it to the known_hosts file.

Then, we can tell our program the path of the known_hosts file so that the server’s identity can be validated using the known_hosts file.

jSch.setKnownHosts("/.ssh/known_hosts");

Whole code

import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.JSchException;

public class Codekru {
	public static void main(String[] args) {
		try {

			JSch jsch = new JSch();
	        String user = "codekru"; // your username
	        String host = "127.0.0.1"; // your remote server address
	        int port = 33000; // your remote server port
	        String yourKeyName = "/Users/codekru/key/codekru.pem";
	         
	        jsch.addIdentity(yourKeyName);
            jSch.setKnownHosts("/.ssh/known_hosts");

	        Session session = jsch.getSession(user, host, port);
	        session.setTimeout(15000);
	        session.connect();

		} catch (JSchException e) {
			e.printStackTrace();
		}

	}
}

And again, we would be able to make an SSH connection with the remote machine.

Here is the video showing how to connect to a remote machine using Java.

What if we used an invalid key? What exception will we get?

We will get an invalid privatekey JSchException.

com.jcraft.jsch.JSchException: invalid privatekey: [B@1b2c6ec2
	at com.jcraft.jsch.KeyPair.load(KeyPair.java:948)
	at com.jcraft.jsch.KeyPair.load(KeyPair.java:561)
	at com.jcraft.jsch.IdentityFile.newInstance(IdentityFile.java:40)
	at com.jcraft.jsch.JSch.addIdentity(JSch.java:406)
	at com.jcraft.jsch.JSch.addIdentity(JSch.java:366)

What if we used the wrong password?

Here, we will get an Auth fail JSchException.

com.jcraft.jsch.JSchException: Auth fail
	at com.jcraft.jsch.Session.connect(Session.java:519)
	at com.jcraft.jsch.Session.connect(Session.java:183)