How do I use JSch with strict host key checking and known_hosts validation?

When using JSch (Java Secure Channel) to connect to an SSH server, you can enable strict host key checking and validate the server against a known_hosts file. By default, strict host key checking ensures that your application will only connect to SSH servers that are already listed in the known_hosts file. If the server’s key is not present or doesn’t match, the connection will fail.

Here’s how you can implement strict host key checking and configure the use of a known_hosts file with JSch:

Step 1: Enable Strict Host Key Checking and Set Known Hosts

Below is an example of how to configure JSch with strict host key checking:

package org.kodejava.jsch;

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

import java.util.Properties;

public class JSchStrictHostKeyCheckingExample {
   public static void main(String[] args) {
      String username = "username";
      String host = "example.com";
      int port = 22; // default SSH port
      String privateKeyPath = "/path/to/your/private/key";
      String knownHostsPath = "/path/to/your/known_hosts";

      try {
         // Initialize JSch
         JSch jsch = new JSch();

         // Set private key if authentication requires it
         jsch.addIdentity(privateKeyPath);

         // Set the known_hosts file for host key verification
         jsch.setKnownHosts(knownHostsPath);

         // Create SSH session
         Session session = jsch.getSession(username, host, port);

         // Set session properties for strict host key checking
         Properties config = new Properties();
         config.put("StrictHostKeyChecking", "yes"); // Enables strict host key checking
         session.setConfig(config);

         // Connect to the SSH server
         session.connect();

         System.out.println("Connected securely with strict host key checking.");

         // Perform your operations (e.g., execute commands, transfer files, etc.)

         // Disconnect from the SSH server
         session.disconnect();
         System.out.println("Disconnected from server.");

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

Explanation:

  1. StrictHostKeyChecking:
    • Setting the StrictHostKeyChecking property to "yes" will enforce strict validation of the host’s key against the known_hosts file.
    • If the host is not in the known_hosts file or if the key does not match, the connection will fail.
  2. Known Hosts File:
    • Use jsch.setKnownHosts(knownHostsPath) to specify the path to the known_hosts file. This file stores the public host keys of remote servers that you trust.
  3. Private Key:
    • If the SSH server requires private key authentication, use jsch.addIdentity(privateKeyPath) to add your private key.
  4. Session Configuration:
    • Other common configuration options (set in Properties) may include:
      • PreferredAuthentications: Specify the preferred authentication methods (e.g., publickey,password,keyboard-interactive).
      • UserKnownHostsFile: Alternative way to point to the known_hosts file.
  5. Error Handling:
    • If the server’s host key is not present in the known_hosts file or does not match, you will encounter an error similar to:
    com.jcraft.jsch.JSchException: reject HostKey: your-host
    

    This means the server’s public key either needs to be added to the known_hosts file or matches an incorrect entry.

Step 2: Generating/Updating the known_hosts File

To manually add a host key to the known_hosts file:

Run the following command on any system with SSH installed:

ssh-keyscan -H your-host >> /path/to/known_hosts
  • -H: Hashes the hostname before storing it in the known_hosts file.
  • Replace your-host with the actual hostname or IP address of the server.

Common Issues and Debugging

  1. Host Key Verification Failed:
    • Ensure the server’s public key exists in the known_hosts file.
    • Ensure the correct knownHostsPath is specified in your code.
  2. Permission Denied:
    • Check your username, private key path, and associated permissions.
    • Make sure your private key is readable and properly associated with the user on the server.
  3. Logging Debug Information:
    JSch provides detailed logs for debugging. You can enable verbose logging as below:

    JSch.setLogger(new com.jcraft.jsch.Logger() {
          public boolean isEnabled(int level) { return true; }
          public void log(int level, String message) { System.out.println(message); }
      });
    

By following this approach, you can securely connect to an SSH server while leveraging strict host key checking and known_hosts validation.


Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>

Maven Central

How do I transfer files with resume support over SFTP using JSch?

When transferring files over SFTP using JSch, resuming partially transferred files (either uploads or downloads) can be implemented by handling offsets for files that are already partially transferred.

This guide explains how to:

  • Resume downloads by continuing from the last transferred byte of a local file.
  • Resume uploads by appending to a remote file.

Handling Download with Resume Support

To resume a download:

  1. Check the current size of the local file.
  2. Skip already downloaded bytes from the remote file using InputStream.skip().
  3. Append remaining content to the local file.

Code for Resuming Download

package org.kodejava.jsch;

import com.jcraft.jsch.*;
import java.io.*;

public class SFTPResumeDownload {

   public static void main(String[] args) {
      String host = "sftp.example.com";
      String username = "user";
      String password = "password";
      String localFile = "local/path/to/file.txt";
      String remoteFile = "/remote/path/to/file.txt";

      JSch jsch = new JSch();
      Session session = null;
      ChannelSftp sftpChannel = null;

      try {
         // Setup SFTP connection
         session = jsch.getSession(username, host, 22);
         session.setPassword(password);
         session.setConfig("StrictHostKeyChecking", "no"); // Disable key checking
         session.connect();

         Channel channel = session.openChannel("sftp");
         channel.connect();
         sftpChannel = (ChannelSftp) channel;

         // Resume download logic
         File file = new File(localFile);
         long localFileSize = file.exists() ? file.length() : 0;
         long remoteFileSize = sftpChannel.lstat(remoteFile).getSize();

         if (localFileSize >= remoteFileSize) {
            System.out.println("File already fully downloaded.");
            return;
         }

         try (InputStream inputStream = sftpChannel.get(remoteFile);
              OutputStream outputStream = new FileOutputStream(file, true)) {
            inputStream.skip(localFileSize); // Skip downloaded portion

            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = inputStream.read(buffer)) != -1) {
               outputStream.write(buffer, 0, bytesRead);
            }

            System.out.println("Download resumed and completed.");
         }
      } catch (Exception e) {
         e.printStackTrace();
      } finally {
         if (sftpChannel != null) sftpChannel.disconnect();
         if (session != null) session.disconnect();
      }
   }
}

Handling Upload with Resume Support

To resume an upload:

  1. Check the size of the remote file.
  2. Skip already uploaded bytes from the local file.
  3. Use the ChannelSftp.APPEND flag to append remaining bytes to the remote file.

Code for Resuming Upload

public static void resumeUpload(ChannelSftp sftpChannel, String localFile, String remoteFile) throws SftpException, IOException {
    File file = new File(localFile);
    long remoteFileSize = 0;

    try {
        remoteFileSize = sftpChannel.lstat(remoteFile).getSize(); // Check remote file size
    } catch (SftpException e) {
        System.out.println("Remote file does not exist. Starting upload from the beginning.");
    }

    System.out.println("Resuming upload from byte: " + remoteFileSize);

    try (InputStream inputStream = new FileInputStream(file)) {
        inputStream.skip(remoteFileSize); // Skip already uploaded bytes

        // Append mode upload
        sftpChannel.put(inputStream, remoteFile, ChannelSftp.APPEND);
        System.out.println("Resume upload completed.");
    }
}

Explanation of Key Steps

  1. Session Setup:
    • A secure session is established with the SFTP server using user credentials.
    • StrictHostKeyChecking is disabled for simplicity, but proper key validation is recommended for production.
  2. Resume Logic:
    • Download: The remote file is read as an InputStream, skipping already downloaded bytes. The local file is opened in append mode.
    • Upload: The local file is read as an InputStream, skipping already uploaded bytes, and the put method with ChannelSftp.APPEND is used to continue the upload.
  3. Error Handling:
    • If the remote file or local file does not exist, appropriate error handling ensures either the upload/download starts from the beginning or exits gracefully.
  4. File Integrity: To ensure file integrity, consider validating the file with hash checks or checksums after transfer.

Notes

  • Increase the buffer size (byte[] buffer = new byte[1024]) for better performance for larger files.
  • Consider implementing retries or reconnect logic if the SFTP session disconnects during a transfer.
  • Always confirm proper permissions for writing to the destination and reading from the source.

Conclusion

The above solution demonstrates how to implement resumable file transfer via SFTP using JSch. It ensures efficient and reliable file transfers by avoiding redundant retransmission of already transferred data.


Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>

Maven Central

How do I handle interactive prompts and keyboard-interactive authentication using JSch?

When working with JSch (Java Secure Channel) for SSH connections, handling interactive prompts and keyboard-interactive authentication requires implementing the UserInfo and UIKeyboardInteractive interfaces provided by JSch. These interfaces allow you to interact with the user to gather necessary input for authentication (like passwords, passphrases, or other interactive challenges like 2FA).

Here’s a step-by-step process:


Steps to Handle Interactive Prompts

  1. Implement the UserInfo Interface:
    This interface is used to provide and verify user credentials. For example, request a password or passphrase during authentication.
  2. Implement the UIKeyboardInteractive Interface:
    This interface is used for keyboard-interactive authentication. This mechanism often includes dynamic prompts (e.g., security questions, OTP codes, etc.).
  3. Attach the Implementation to the Session Object:
    Set your UserInfo implementation to the session using session.setUserInfo().
  4. Connect to the Session:
    Once everything is set up, open the session and proceed with connecting to the host.

Code Example

Here’s an example of how to handle both interactive prompts and keyboard-interactive authentication using JSch:

package org.kodejava.jsch;

import com.jcraft.jsch.*;

public class JschKeyboardInteractiveExample {
   public static void main(String[] args) {
      String username = "username";
      String host = "example.com";
      int port = 22;

      JSch jsch = new JSch();
      try {
         Session session = jsch.getSession(username, host, port);

         // Set a UserInfo implementation
         session.setUserInfo(new MyUserInfo());

         // Connect to the session
         session.connect();

         System.out.println("Connected to the host successfully!");

         // Do your operations (e.g., execute commands) here...

         session.disconnect();
         System.out.println("Disconnected from the host.");
      } catch (Exception e) {
         e.printStackTrace();
      }
   }

   // Custom UserInfo implementation for interactive prompts
   public static class MyUserInfo implements UserInfo, UIKeyboardInteractive {
      private String password;

      // Constructor to provide password (or use a Scanner to collect input)
      public MyUserInfo() {
         // Replace this with actual input collection if required
         this.password = "password"; // Set your password here
      }

      @Override
      public String getPassword() {
         return password;
      }

      @Override
      public boolean promptYesNo(String message) {
         System.out.println("Prompt Yes/No: " + message);
         // Assuming 'Yes' for simplicity; implement actual logic if needed
         return true;
      }

      @Override
      public String getPassphrase() {
         return null; // Not using a passphrase for this example
      }

      @Override
      public boolean promptPassphrase(String message) {
         System.out.println("Prompt Passphrase: " + message);
         return false; // No passphrase in this example
      }

      @Override
      public boolean promptPassword(String message) {
         System.out.println("Prompt Password: " + message);
         return true; // Assuming the password is already set
      }

      @Override
      public void showMessage(String message) {
         System.out.println("Message: " + message);
      }

      @Override
      public String[] promptKeyboardInteractive(String destination, String name, String instruction, String[] prompt, boolean[] echo) {
         System.out.println("Keyboard Interactive Authentication:");
         System.out.println("Destination: " + destination);
         System.out.println("Name: " + name);
         System.out.println("Instruction: " + instruction);

         String[] responses = new String[prompt.length];
         for (int i = 0; i < prompt.length; i++) {
            System.out.println("Prompt: " + prompt[i]);
            // Collect input from the user (hardcoded for this example)
            responses[i] = this.password; // Assuming password for simplicity
         }
         return responses;
      }
   }
}

Explanation of Key Parts in the Code

  1. UserInfo Methods:
    • getPassword(): Returns the password string (hard-coded or dynamically retrieved).
    • promptYesNo(String): Handles Yes/No prompts (like accepting host key verification).
    • getPassphrase() and promptPassphrase(String): Used if dealing with private key authentication and a passphrase is necessary.
    • showMessage(String): Displays generic messages from the server or library to the user.
  2. UIKeyboardInteractive Methods:
    • promptKeyboardInteractive(...): Handles keyboard-interactive authentication challenges.
      Prompts can include questions for passwords, 2FA, CAPTCHA, etc.
  3. Session Configuration:
    The setUserInfo() method attaches your custom implementation to the session, enabling interactive behavior during the connection process.

Output Examples

  • If the server uses simple password authentication:
Connected to the host successfully!
Disconnected from the host.
  • If the server uses keyboard-interactive challenges:
Keyboard Interactive Authentication:
Destination: example.com
Name: SSH Server
Instruction: Please respond to the following prompts:
Prompt: Password

Notes and Best Practices

  1. Password Storage Security:
    Avoid hardcoding sensitive credentials in the code. Use environment variables, encrypted vaults, or secure input methods.
  2. Dynamic Input Collection:
    Replace hardcoded strings with dynamic input collection (e.g., Scanner or a GUI dialog).
  3. Error Handling:
    Handle exceptions for cases like invalid credentials, session interruptions, and server-side configuration issues.
  4. Host Key Checking:
    JSch may require host key checking. Either configure the known hosts file or handle it manually in the promptYesNo method.

This approach allows you to securely and efficiently handle interactive prompts and keyboard authentication while using JSch in Java.


Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>

Maven Central

How do I create a reusable SSH connection pool with JSch in a multithreaded application?

Creating a reusable SSH connection pool using JSch in a multithreaded application involves managing connections efficiently and ensuring thread safety. JSch (Java Secure Channel) does not natively provide a connection pooling feature, so you have to implement it manually using a pooling library or write your own pooling logic.

Below is the step-by-step guide to implementing a reusable SSH connection pool with JSch.

1. Define an SSH Connection Pool

You can use a thread-safe pool, such as Java’s BlockingQueue, to manage SSH connections. Here’s how:

Define a Connection Pool Manager

package org.kodejava.jsch;

import com.jcraft.jsch.*;
import java.util.concurrent.*;

public class SSHConnectionPool {
    private final BlockingQueue<Session> pool;
    private final JSch jsch;
    private final String username;
    private final String host;
    private final int port;
    private final String password; // or private key if using key-based authentication

    public SSHConnectionPool(int poolSize, String username, String password, 
                             String host, int port) throws JSchException {
        this.pool = new LinkedBlockingQueue<>(poolSize); // Thread-safe pool
        this.jsch = new JSch();
        this.username = username;
        this.host = host;
        this.port = port;
        this.password = password;

        for (int i = 0; i < poolSize; i++) {
            pool.offer(createSession()); // Initialize the pool with SSH sessions
        }
    }

    private Session createSession() throws JSchException {
        Session session = jsch.getSession(username, host, port);
        session.setPassword(password);

        // Configuration - Disable strict host checking for simplicity
        java.util.Properties config = new java.util.Properties();
        config.put("StrictHostKeyChecking", "no");
        session.setConfig(config);

        session.connect();
        return session;
    }

    public Session borrowSession() throws InterruptedException {
        return pool.take(); // Borrow a session from the pool
    }

    public void returnSession(Session session) {
        if (session != null) {
            pool.offer(session); // Return session to the pool
        }
    }

    public void close() {
        // Close all sessions and clear the pool
        for (Session session : pool) {
            session.disconnect();
        }
        pool.clear();
    }
}

2. Usage in a Multi-Threaded Application

You can now use SSHConnectionPool in a multithreaded environment. For every task, borrow a session, perform the necessary operations, and return the session to the pool.

Example

package org.kodejava.jsch;

import com.jcraft.jsch.*;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class SSHPoolDemo {
    public static void main(String[] args) {
        try {
            // Create a pool with 5 connections
            SSHConnectionPool pool = new SSHConnectionPool(5, "username", 
                    "password", "example.com", 22);

            // Thread pool for executing tasks
            ExecutorService executorService = Executors.newFixedThreadPool(10);

            for (int i = 0; i < 10; i++) {
                executorService.submit(() -> {
                    Session session = null;
                    try {
                        // Borrow a session
                        session = pool.borrowSession();

                        // Execute commands via ChannelExec
                        ChannelExec channel = (ChannelExec) session.openChannel("exec");
                        channel.setCommand("echo Hello, World!");
                        channel.setInputStream(null);
                        channel.setErrStream(System.err);

                        channel.connect();

                        // Read the output
                        try (var input = channel.getInputStream()) {
                            int data;
                            while ((data = input.read()) != -1) {
                                System.out.print((char) data);
                            }
                        }

                        channel.disconnect();
                    } catch (Exception e) {
                        e.printStackTrace();
                    } finally {
                        // Return the session to the pool
                        pool.returnSession(session);
                    }
                });
            }

            // Shutdown thread pool after tasks are complete
            executorService.shutdown();

            // Clean up the connection pool
            pool.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

3. Notes

  • Thread Safety: LinkedBlockingQueue ensures thread-safe access to the pool.
  • Session Validity: Before returning a session to the pool, consider checking if it is still alive. JSch does not reconnect automatically if a session is disconnected.
  • Connection Configuration: You can use private key authentication by adding:
jsch.addIdentity("/path/to/private_key");
  • Resource Cleanup: Always close the pool properly to avoid resource leaks.

By following this setup, you can create a reusable and thread-safe SSH connection pool in a multithreaded application.


Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>

Maven Central

How do I implement a secure SSH proxy tunnel using JSch?

To implement a secure SSH proxy tunnel using JSch (Java Secure Channel library), you can follow these steps. JSch is a Java library designed to perform SSH operations like creating tunnels, port forwarding, and other remote operations.

Here’s a detailed implementation guide:

1. Code for Creating an SSH Proxy Tunnel

Here’s how you can create a local-to-remote port forwarding (a tunnel) using JSch:

package org.kodejava.jsch;

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

public class SSHProxyTunnel {
   public static void main(String[] args) {
      String sshHost = "example.com";
      int sshPort = 22;
      String sshUser = "username";
      String sshPassword = "password";
      String remoteHost = "remote.server.com";
      int localPort = 8080;   // Local port to bind
      int remotePort = 80;    // Remote port to forward to

      Session session = null;
      try {
         // Create JSch instance
         JSch jsch = new JSch();

         // Create a session with the SSH server
         session = jsch.getSession(sshUser, sshHost, sshPort);
         session.setPassword(sshPassword);

         // Avoid asking for key confirmation
         session.setConfig("StrictHostKeyChecking", "no");

         // Connect to the SSH server
         System.out.println("Connecting to SSH server...");
         session.connect();

         // Setup local port forwarding
         int assignedPort = session.setPortForwardingL(localPort, remoteHost, remotePort);
         System.out.println("SSH Tunnel established:");
         System.out.println("LocalPort: " + localPort + " -> RemoteHost: " + remoteHost + ":" + remotePort);
         System.out.println("AssignedPort: " + assignedPort);

         System.in.read();
      } catch (Exception e) {
         e.printStackTrace();
      } finally {
         // Cleanup and disconnect
         if (session != null && session.isConnected()) {
            session.disconnect();
         }
      }
   }
}

2. Explanation

  • SSH Server (Jump Host): The sshHost is the host of the jumphost (or bastion) server you will connect to using SSH.
  • Remote Server (Backend Host): The remoteHost is the internal server you want to connect to through the SSH server, using the tunnel.
  • Local Port: The port on your local machine that acts as an entry point to the proxy tunnel.
  • Remote Port: The port on the remote server that your request should be forwarded to.

3. How Port Forwarding Works

  1. Local Port Forwarding: session.setPortForwardingL(localPort, remoteHost, remotePort) forwards traffic to a local port (e.g., port 8080 on your machine) through the SSH server and to the remote server and port you specify. For example, accessing http://localhost:8080 would route traffic to remote.server.com:80 through the SSH tunnel.

4. Security Enhancements

Here are some best practices to improve the security of your implementation:

  • Key Authentication: Use an SSH key instead of a password for authentication. This can be done by calling jsch.addIdentity("path-to-private-key"):
jsch.addIdentity("/path/to/private-key");
  • StrictHostKeyChecking: Avoid turning off strict host key checking (StrictHostKeyChecking=no) in production. Configure trusted known hosts instead.
jsch.setKnownHosts("/path/to/known_hosts");
  • Close Resources: Ensure session.disconnect() is always called, preferably in a try-with-resources block or a finally block.

5. Advanced Configuration (Optional)

  • Using a Proxy: If the SSH server is behind a proxy, you can use ProxySOCKS5 or ProxyHTTP to configure the proxy.
  • Timeouts: Set connection and session timeouts for better handling of connection issues:
session.setTimeout(30000); // Timeout in milliseconds

6. Testing the Tunnel

  1. Run the program.
  2. Open your browser or terminal and access http://localhost:8080.
  3. You should see the data served by remote.server.com:80.

Example Use Case

You could use this setup to securely connect to a database on a remote server (e.g., Postgres or MySQL) without exposing the server directly to the internet.


Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>

Maven Central