The “Unknown Server” error in Paramiko typically arises when the SSH client (your Python script using Paramiko) cannot verify the identity of the SSH server it’s trying to connect to. This is a crucial security measure to prevent man-in-the-middle attacks. Here’s a breakdown of common causes and solutions:
1. Missing or Incorrect Host Key
Paramiko needs to know the server’s public key to verify its identity. If the key isn’t known, the “Unknown Server” error occurs.
Solution:
The safest approach is to manually add the server’s host key to your known_hosts file. You can typically retrieve this key via ssh-keyscan:
ssh-keyscan your_server_addressThis will output the server’s key. Add this line to your ~/.ssh/known_hosts file. Alternatively, you can use Paramiko’s AutoAddPolicy (with caution!):
import paramiko
client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy()) # Use with caution!
client.connect('your_server_address', username='your_username', password='your_password')
# ... rest of your code ...
client.close()
Warning: Using AutoAddPolicy bypasses host key verification and opens you up to potential security risks if a malicious actor intercepts the connection. Only use it in trusted environments or for testing purposes. Manually verifying the host key is always the recommended approach.
2. Incorrect Server Address or Port
Double-check that you’re using the correct hostname or IP address and port number for the SSH server.
Solution:
Carefully review the connection details. A simple typo can lead to this error. Ensure the server is running and accessible on the specified port (default SSH port is 22).
3. Firewall Issues
A firewall on either the client or server side might be blocking the SSH connection.
Solution:
Verify that the firewall allows outbound connections on the port you’re using (usually 22). Check the firewall rules on both your local machine and the server.
4. SSH Server Configuration
In rare cases, the SSH server itself might be misconfigured, preventing connections.
Solution:
If you have access to the server, check the SSH server configuration file (usually /etc/ssh/sshd_config) for any errors. Ensure that the server is listening on the correct port and that the necessary authentication methods are enabled.
5. Network Connectivity Problems
General network issues, like DNS resolution problems or network outages, can also cause this error.
Solution:
Test your network connectivity. Can you ping the server? Can you resolve its hostname? Check for any network problems that might be preventing the connection.
Debugging Tips
- Print the full exception traceback for more details:
import traceback
try:
# ... your Paramiko code ...
except Exception as e:
traceback.print_exc()