Paramiko 'Unknown Server' Error: How to Resolve SSH Connection Issues?

Paramiko "Unknown Server": Troubleshooting and Resolution

When attempting to establish SSH connections using Paramiko, users may encounter an "Unknown Server" exception like the one below:

paramiko.SSHException: Unknown server 127.0.0.1

This occurs when the server's host key is not recognized by Paramiko. To resolve this issue, the missing_host_key_policy attribute of the SSHClient object needs to be set to an appropriate value.

By default, Paramiko's policy is to reject all unknown servers, ensuring that connections are only made to known and trusted hosts. To override this behavior, the following code can be used:

client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())

The AutoAddPolicy instructs Paramiko to automatically add the server's host key to the list of known hosts, making it trusted for future connections. Alternatively, one can specify a different policy or write their own implementation tailored to specific requirements.

Once the appropriate policy is set, connections to unknown servers can be established without encountering the "Unknown Server" exception. Additionally, hosts can be added to the trusted hosts list using the get_host_keys() and save() methods of the SSHClient object. This allows hosts to be saved and loaded for subsequent use.

By understanding the role of the missing_host_key_policy and using the AutoAddPolicy or a custom implementation, developers can effectively handle unknown servers and establish secure SSH connections using Paramiko.

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