Server not Found in known_hosts: An Ultimate Guide for Dev

Hey there Dev! Are you struggling with the error message “server not found in known_hosts” while connecting to your server? Don’t worry, you’re not alone. This error message can be confusing and frustrating, especially for beginners. But don’t worry, in this comprehensive guide, we’ll cover everything you need to know about this error message and how to fix it.

What is known_hosts?

Before we dive into the “server not found in known_hosts” error message, let’s first understand what known_hosts is. Known_hosts is a file in your SSH directory that contains a list of all the servers that your machine has connected to and verified the host key for. This is done to prevent a man-in-the-middle (MITM) attack. When you try to connect to a server, SSH checks the server’s host key against the one in your known_hosts file to ensure that you’re connecting to the right server.

Why do we need known_hosts?

Known_hosts is an essential part of SSH. It prevents attackers from intercepting your connection and posing as the server. This is done through a process called host key verification, which involves comparing the server’s host key with the one stored in your machine’s known_hosts file. The host key is a unique identifier that’s used to verify that the server you’re connecting to is the one you intended to connect to. Without this verification, there’s a risk that an attacker could intercept your connection and steal your data or carry out other malicious activities.

Where is known_hosts located?

The known_hosts file is located in your SSH directory. The location of this directory depends on the operating system you’re using. If you’re using Linux or macOS, the directory is usually located in the ~/.ssh directory. If you’re using Windows, the directory is usually located in the C:\Users\username\.ssh directory.

What happens if a host key changes?

If the host key of a server changes, SSH will display a warning message and ask if you want to connect to the server. If you’re not sure why the host key has changed, it’s best to contact the system administrator of the server to verify the change. If you’re confident that the server has changed its host key for a legitimate reason, you can update your known_hosts file to include the new key.

What is the “server not found in known_hosts” error message?

The “server not found in known_hosts” error message occurs when SSH is unable to find the host key for a server in your known_hosts file. This can happen for a variety of reasons, including:

Reasons for the error message
The server is new and you haven’t connected to it before
You’re connecting to the server from a different machine
You’ve deleted the server’s entry from your known_hosts file
The server’s host key has changed

What does the error message mean?

When you see the “server not found in known_hosts” error message, it means that SSH is unable to verify the server’s host key. This could be because the key is missing from your known_hosts file or because the key has changed since you last connected to the server. In either case, SSH is unable to establish a secure connection to the server and will refuse to connect.

What should you do when you see the error message?

When you see the “server not found in known_hosts” error message, you have a few options:

1. You can add the server’s host key to your known_hosts file manually
2. You can delete your existing known_hosts file and allow SSH to create a new one
3. You can connect to the server with the “-o StrictHostKeyChecking=no” option to bypass host key verification

How do you add a host key to your known_hosts file?

To add a host key to your known_hosts file, you’ll need to connect to the server and verify the host key manually. You can do this by running the following command:

ssh-keyscan <server> >> ~/.ssh/known_hosts

This will append the server’s host key to your known_hosts file. Note that you should only do this if you’re confident that the server you’re connecting to is legitimate.

How do you delete your existing known_hosts file?

If you want to delete your existing known_hosts file and allow SSH to create a new one, you can do so by running the following command:

rm ~/.ssh/known_hosts

This will delete your existing known_hosts file. The next time you connect to a server, SSH will create a new file and add the server’s host key to it.

How do you connect to a server without host key verification?

If you want to connect to a server without host key verification, you can use the “-o StrictHostKeyChecking=no” option. This will tell SSH to connect to the server without verifying its host key. Note that this is not recommended, as it can leave you vulnerable to a man-in-the-middle attack.

Conclusion

So there you have it, Dev! Everything you need to know about the “server not found in known_hosts” error message. Remember, host key verification is an essential part of SSH and helps to keep your connections secure. If you do encounter this error message, it’s best to take the time to verify the server’s host key and add it to your known_hosts file manually. Stay safe and happy coding!

FAQ

Q. Why does the host key change?

The host key can change if the server’s configuration changes or if the server is reinstalled or reconfigured. It can also change if an attacker tries to intercept your connection and poses as the server.

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

A man-in-the-middle (MITM) attack is a type of cyber attack where an attacker intercepts your connection and poses as the server. This allows the attacker to steal your data or carry out other malicious activities.

Q. Is it safe to connect to a server without host key verification?

No, it’s not safe to connect to a server without host key verification. This leaves you vulnerable to a man-in-the-middle attack.

Q. How do I know if a server’s host key has changed?

If a server’s host key has changed, SSH will display a warning message when you try to connect to the server. If you’re not sure why the host key has changed, it’s best to contact the system administrator of the server to verify the change.

Q. Can I use SSH without known_hosts?

Yes, you can use SSH without a known_hosts file, but this is not recommended. Without host key verification, you’re leaving yourself vulnerable to a man-in-the-middle attack.