In SSH (Secure Shell), the host key is crucial as it uniquely identifies each SSH server.
This key forms the basis for secure, encrypted connections between the server and its clients. When a client connects to an SSH server for the first time, the server’s host key is stored in the client’s known_hosts file.
This key is then used in future connections to authenticate the server’s identity and ensure secure communication.
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How to Find the Host Key in Linux
Understanding how to locate the host key of an SSH server in Linux is important for verifying the server’s identity and for troubleshooting purposes.
Finding the Local SSH Server’s Host Key:
- Access the SSH Server’s Host Key Files: SSH host keys are typically stored in /etc/ssh/. File names vary based on the key type (e.g., ssh_host_rsa_key.pub, ssh_host_ecdsa_key.pub).
- View the Host Key: Use the cat command to display the public part of the host key, for example:
Finding a Remote SSH Server’s Host Key:
Use ssh-keyscan to fetch the public keys of a remote SSH server:
This command retrieves the public keys for comparison with the known_hosts file.
Remote Host Key Has Changed Error: Explanation and Fix
The “Remote Host Key Has Changed” error in SSH indicates a mismatch between the current server’s host key and the one previously recorded in the client’s known_hosts file.
This discrepancy can occur due to legitimate reasons like a reinstalled operating system or an updated SSH server package. However, it may also signal a potential security threat, such as a man-in-the-middle attack.
How to Fix:
- Verify the Change: Confirm the legitimacy of the change in the host key. Investigate further if there’s any doubt about the change’s authenticity.
- Remove the Old Host Key: Use the command ssh-keygen -R [hostname_or_IP] to remove the old key from the known_hosts file.
- Reconnect to the Server: After removing the old key, reconnect with the command ssh [username]@[hostname_or_IP]. You will be prompted to accept the new host key, which should be carefully verified before acceptance.
- Key Re-Added: Once the new key is accepted, it is added to the known_hosts file, securing future connections to the server.
Resolving the “Remote Host Key Has Changed” error involves verifying the cause of the key change, removing the outdated key, and updating the known_hosts file with the new key.
This process is essential for maintaining the integrity and security of SSH connections.