Host Key for Server Changed: What Dev Needs to Know

Dear Dev, if you have been managing servers, you may have come across a message that says: “WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!” This warning is a result of the host key for the server changing.

What is the Host Key for a Server?

The host key is a unique code that verifies the identity of a server when you connect to it using SSH or SFTP. This key is generated when the server is first set up, and it is used to encrypt the communication between the server and client applications.

When you connect to a server for the first time, your SSH or SFTP client will prompt you to verify the host key. If you accept the key, your client will save it and use it to authenticate the server in future connections.

What Happens When the Host Key Changes?

When the host key changes, it means that something has happened to the server that has caused it to regenerate a new key. This could be due to a number of reasons, including a software upgrade, a server migration, or a security breach.

When you try to connect to a server that has a changed host key, your SSH or SFTP client will display a warning message. This message is designed to alert you to a potential security issue, as it could mean that someone is attempting to intercept your connection.

How to Verify the New Host Key

If you see a warning message that the host key has changed, the first thing you should do is to contact the server administrator and confirm that the change is legitimate. Once you have verified the new host key, you can update your SSH or SFTP client to recognize the new key and continue your connection.

To update the host key in your SSH client, you can remove the old key from your known_hosts file and add the new key:

Action	Command
Remove old key	ssh-keygen -R [hostname]
Add new key	ssh-keyscan [hostname] >> ~/.ssh/known_hosts

FAQ about Host Key Changes

Q: Why did the host key change?

A: The host key may change for a number of reasons, including server upgrades and migrations, or security breaches.

Q: Is it safe to accept a new host key?

A: It is safe to accept a new host key as long as you can verify that the key is legitimate. Contact the server administrator to confirm the change.

Q: How do I update the host key in my SSH client?

A: To update the host key in your SSH client, remove the old key from your known_hosts file and add the new key using the ssh-keygen and ssh-keyscan commands.

Q: Can I ignore the warning message about the host key change?

A: Ignoring the warning message is not recommended, as it could indicate a potential security issue. Always verify the new host key before continuing your connection.

Q: What if I cannot verify the new host key?

A: If you cannot verify the new host key, it is best to contact the server administrator and investigate the reason for the key change.

Conclusion

Dev, understanding the host key for a server and what happens when it changes is crucial for maintaining the security and integrity of your connections. Always verify the new host key before continuing your connection, and contact the server administrator if you have any doubts or concerns.