The Server’s Host Key Does Not Match The One Cached

Hello Dev, we are glad to have you here. Today, we will be discussing one of the most common errors faced by server administrators – “the server’s host key does not match the one cached”. We understand how frustrating and confusing this error can be, and that’s why we have created this journal article to guide you through troubleshooting and resolving this error. So, let’s get started!

Understanding The Error Message

Before we dive into the troubleshooting process, let’s first understand the error message. This error occurs when the server’s host key has changed since you last connected to it, and your SSH client is warning you that the identity of the server has not been verified. Hence, your client cannot establish a secure connection and throws up the error message.

This error message indicates that either the server has been reinstalled or the host key has been regenerated for security purposes. In any case, it is important to verify the new host key to ensure that you are connecting to the correct server and not a man-in-the-middle attacker.

Why Is The Host Key Important?

The host key is a critical component of the SSH protocol that ensures the authenticity of the server. It is a unique cryptographic key that is generated when the server is first installed, and it is used to verify the identity of the server during subsequent connections. Whenever you connect to a server, your client verifies the host key to ensure that you are connecting to the same server that you connected to previously. If the host key does not match, it means that the server’s identity has not been verified, and the connection is not secure.

Common Causes Of The Error

Now that we have a basic understanding of the error message and the importance of the host key, let’s explore the common causes of the error:

Troubleshooting The Error

Now that we know the common causes of the error, let’s explore the troubleshooting steps:

Step 1: Verify The Host Key

The first step in troubleshooting this error is to verify the new host key. You can do this by opening a terminal and running the following command:

ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub

This command will display the fingerprint of the new host key. Compare this fingerprint with the one stored in your client’s known_hosts file. If they match, you can safely connect to the server. If they do not match, it means that the host key has changed, and you will need to update the known_hosts file with the new host key.

Step 2: Update The Known_Hosts File

If the host key has changed, you will need to update the known_hosts file with the new key. You can do this by using the ssh-keygen command. Here’s how:

ssh-keygen -R <hostname or IP address>ssh-keyscan <hostname or IP address> >> ~/.ssh/known_hosts

The first command removes the old key from the known_hosts file, and the second command adds the new key to the file. Once you have updated the known_hosts file, you should be able to connect to the server without any issues.

Step 3: Disable Host Key Checking

If you are absolutely sure that the server’s identity has not been compromised and you do not want to verify the host key each time, you can disable host key checking in your SSH client. Here’s how:

ssh -o "UserKnownHostsFile=/dev/null" -o "StrictHostKeyChecking=no" <username>@<hostname or IP address>

This command will disable host key checking and allow you to connect to the server without verifying the host key. However, we strongly advise against this as it can compromise the security of your connection.

FAQs

Q1. What is the known_hosts file?

The known_hosts file is a file that SSH clients use to store the fingerprints of hosts that they have connected to previously. When you connect to a host for the first time, your client will store the host’s fingerprint in the known_hosts file. The next time you connect to the same host, your client will verify the fingerprint to ensure that you are connecting to the same host.

Q2. Can I use SSH without host key checking?

Yes, you can use SSH without host key checking, but we strongly advise against it as it can compromise the security of your connection. SSH uses host key checking to ensure that you are connecting to the correct server and not a man-in-the-middle attacker.

Q3. Can I regenerate the host key?

Yes, you can regenerate the host key for security purposes. However, you will need to update the known_hosts file with the new key to avoid the “server’s host key does not match the one cached” error.

Q4. What is a man-in-the-middle attack?

A man-in-the-middle attack is an attack where an attacker intercepts communication between two parties and alters it in some way. In the context of SSH, a man-in-the-middle attacker could intercept your SSH connection and pretend to be the server to steal your credentials or other sensitive information.

Q5. Can I prevent the “server’s host key does not match the one cached” error?

No, you cannot prevent the error from occurring if the host key has changed. However, you can reduce the likelihood of the error by updating the known_hosts file regularly and verifying the host key each time you connect to a server.

Conclusion

In conclusion, the “server’s host key does not match the one cached” error is a common error faced by server administrators. It occurs when the server’s host key has changed since you last connected to it, and your SSH client is warning you that the identity of the server has not been verified. To troubleshoot this error, you need to verify the host key, update the known_hosts file, or disable host key checking. We hope that this article has been helpful in guiding you through the troubleshooting process. Thank you for reading!