SSH_MSG_UNIMPLEMENTED and Handling Unknown SSH Messages

What is SSH_MSG_UNIMPLEMENTED ?

The "SSH_MSG_UNIMPLEMENTED" message is part of the SSH (Secure Shell) protocol and is used to handle situations where an SSH server receives a packet with a message number that it does not recognize or support. When an SSH server receives a message it cannot process, it should respond with an SSH_MSG_UNIMPLEMENTED message to inform the client.

Here's how it works:

1. The SSH client sends a request or command to the SSH server by sending an SSH message with a specific message number.

2. If the SSH server does not support or recognize the message number it receives, it should reply with an SSH_MSG_UNIMPLEMENTED message. This message includes the sequence number of the original request, indicating which message is unimplemented.

3. The SSH client receives the SSH_MSG_UNIMPLEMENTED message and may choose to retransmit the original message or take appropriate action based on the server's response.

This mechanism allows for graceful handling of unsupported or unrecognized SSH messages and helps in maintaining the integrity and reliability of the SSH connection. It is part of the SSH protocol's error and notification handling mechanisms.

Keep in mind that this message is typically handled by the SSH library or software itself and is not something that users generally need to interact with directly. It is used internally to ensure proper communication between the SSH client and server.

How to handle this ?

In Java, you can handle the "SSH_MSG_UNIMPLEMENTED" message when using an SSH library or framework that provides support for SSH communication. One popular library for SSH in Java is JSch. Below, I'll provide a simplified example of how to handle an "SSH_MSG_UNIMPLEMENTED" message using JSch:

1. First, make sure you have JSch added to your project's dependencies.

2. Create a Java program that establishes an SSH connection to the server and sends SSH messages.

Here's an example:

import com.jcraft.jsch.*;

public class SSHExample {
    public static void main(String[] args) {
        String host = "example.com";
        String user = "your-username";
        String password = "your-password";
        int port = 22;

        try {
            JSch jsch = new JSch();
            Session session = jsch.getSession(user, host, port);
            session.setPassword(password);
            session.setConfig("StrictHostKeyChecking", "no");

            session.connect();

            // Create an SSH channel (for example, an SSH shell channel)
            ChannelShell channel = (ChannelShell) session.openChannel("shell");
            channel.connect();

            // Send an SSH message (for demonstration purposes)
            channel.getOutputStream().write("Some SSH command".getBytes());
            channel.getOutputStream().flush();

            // Handle SSH_MSG_UNIMPLEMENTED
            byte[] buffer = new byte[1024];
            int bytesRead;
            while (true) {
                bytesRead = channel.getInputStream().read(buffer);
                if (bytesRead < 0) {
                    break;
                }
                String response = new String(buffer, 0, bytesRead);
                System.out.println("Received response: " + response);

                // Check if this is an SSH_MSG_UNIMPLEMENTED message
                if (response.startsWith("SSH_MSG_UNIMPLEMENTED")) {
                    System.out.println("Received SSH_MSG_UNIMPLEMENTED");
                    // Handle the unimplemented message here if needed
                }
            }

            // Close the channel and session when done
            channel.disconnect();
            session.disconnect();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In this example, we use JSch to establish an SSH connection, open a shell channel, and send an SSH message. The code then continuously reads from the channel's input stream, checks if the received response starts with "SSH_MSG_UNIMPLEMENTED," and handles it accordingly.

Please note that this is a simplified example for demonstration purposes, and in a real-world application, you would handle SSH_MSG_UNIMPLEMENTED more robustly based on your specific requirements. Additionally, you should consider error handling and security best practices when working with SSH in your Java application.