The com.jcraft.jsch.JSchException: UnknownHostKey error typically occurs when using JSch, a Java implementation of SSH2, and the SSH server's host key is not recognized or has changed since the last time it was seen.
When JSch connects to an SSH server for the first time, it checks the server's host key against a list of known host keys stored in its known_hosts file. If the server's host key is not found or does not match any of the known host keys, JSch raises an UnknownHostKey exception.
To resolve this issue, you have a few options:
Adding the Host Key Manually: If you trust the SSH server and know that its host key has changed legitimately (e.g., the server has been reinstalled or updated), you can manually add the new host key to the known_hosts file. The known_hosts file is typically located in the .ssh directory in the user's home directory.
Disabling Host Key Verification (Not Recommended for Production): You can disable host key verification in JSch by setting the StrictHostKeyChecking property to no. However, this approach is not recommended for production environments as it opens the possibility of man-in-the-middle attacks.
Implementing Host Key Verification Callback: JSch allows you to implement a HostKeyVerifier interface to provide custom host key verification logic. You can use this interface to verify the server's host key programmatically and decide whether to accept or reject it based on your application's requirements.
Here's a basic example of how to implement a HostKeyVerifier:
import com.jcraft.jsch.HostKey;import com.jcraft.jsch.JSch;import com.jcraft.jsch.JSchException;import com.jcraft.jsch.Session;import com.jcraft.jsch.UserInfo;
public class SSHExample { public static void main(String[] args) { try { JSch jsch = new JSch(); Session session = jsch.getSession("username", "hostname", 22); // Set UserInfo (optional) UserInfo ui = new MyUserInfo(); session.setUserInfo(ui); // Set HostKeyVerifier session.setHostKeyVerifier(new MyHostKeyVerifier()); session.connect(); // Perform operations with the session session.disconnect(); } catch (JSchException e) { e.printStackTrace(); } }}class MyUserInfo implements UserInfo { // Implement UserInfo methods (if needed)}class MyHostKeyVerifier implements com.jcraft.jsch.HostKeyVerifier { @Override public boolean verify(String s, int i, byte[] bytes1, byte[] bytes2) { // Implement host key verification logic here return true; // Return true to accept the host key }}
Replace "username", "hostname", and 22 with your SSH username, hostname, and port respectively. Implement the verify method in MyHostKeyVerifier to verify the host key based on your requirements.
By using one of these methods, you should be able to resolve the UnknownHostKey issue in JSch.
