The Server’s Host Key is Not Cached: A Comprehensive Guide for Dev

Hello Dev! In this article, we will discuss one of the common issues encountered in SSH connections – “The server’s host key is not cached.” We will explore what this error means, its root cause, and possible solutions. Additionally, we will provide a step-by-step guide on how to fix this error on different operating systems.

Understanding the Error: The Server’s Host Key is Not Cached

When trying to connect to a server via SSH, you may encounter the error message “The server’s host key is not cached. You have no guarantee that the server is the computer you think it is.” This message means that the client can’t verify the authenticity of the server because its host key is not cached.

A host key is a cryptographic key used to authenticate a server during the SSH connection setup. When a client connects to a server, it verifies the host key received from the server with the one it has cached. If the keys match, the client can trust that it’s connecting to the right server. If the keys don’t match or the client doesn’t have the server’s host key cached, the client can’t verify the server’s authenticity, and the connection is aborted.

The Root Cause of the Error

The error message “The server’s host key is not cached” occurs when the client doesn’t have the server’s host key cached. This can happen when you’re connecting to a server for the first time, or the server has regenerated its host key.

The client caches the host key of a server by storing it in the known_hosts file. This file is located in the user’s home directory and contains a list of known hosts and their host keys. When a client connects to a server, it checks the known_hosts file for the server’s host key before establishing a connection. If the server’s host key matches the cached one, the connection is established.

Possible Solutions to the Error

There are various solutions to fix the error “The server’s host key is not cached.” Here are some of them:

1. Add the Host Key Manually

If you trust the server, you can add its host key manually to the known_hosts file. To do this, you need to know the server’s host key. You can obtain it by connecting to the server via SSH and checking its fingerprint. Once you have the host key, you can add it to the known_hosts file manually.

Steps to Add Host Key Manually
Step 1: Open the terminal or command prompt on your computer.
Step 2: Type the following command: ssh-keygen -F <server-ip>
Step 3: Verify that the output displays the server’s fingerprint.
Step 4: Add the server’s host key to the known_hosts file by typing the following command: ssh-keyscan <server-ip> >> ~/.ssh/known_hosts
Step 5: Try connecting to the server via SSH again.

2. Disable Strict Host Key Checking

If you don’t want to add the host key manually, you can disable strict host key checking. This option tells the SSH client to automatically add new host keys to the known_hosts file without prompting the user, effectively bypassing the “The server’s host key is not cached” error. However, this option poses a security risk as it allows man-in-the-middle attacks to occur.

To disable strict host key checking, you need to edit the SSH configuration file. The location of this file varies depending on your operating system.

On Linux and macOS:

Open the terminal and type the following command: sudo nano /etc/ssh/ssh_config

Find the line that says: #StrictHostKeyChecking ask

Change it to: StrictHostKeyChecking no

Save and exit by pressing Ctrl+X and then Y.

On Windows:

Open the Command Prompt and type the following command: notepad C:\Program Files\Git\etc\ssh\ssh_config

Find the line that says: #StrictHostKeyChecking ask

Change it to: StrictHostKeyChecking no

Save and exit by pressing Ctrl+S and then Alt+F4.

3. Clear the Known Hosts File

If neither adding the host key manually nor disabling strict host key checking works, you can try clearing the known_hosts file. This file may contain outdated or incorrect host keys that cause the “The server’s host key is not cached” error.

To clear the known_hosts file, you need to delete its contents. The location of this file varies depending on your operating system.

On Linux and macOS:

Open the terminal and type the following command: sudo rm ~/.ssh/known_hosts

On Windows:

Open the Command Prompt and type the following command: del %USERPROFILE%\.ssh\known_hosts

FAQ

1. What is a Host Key?

A host key is a cryptographic key used to authenticate a server during the SSH connection setup. It’s a public key that is unique to each server and changes when the server is reinstalled or its configuration changes.

2. What is the Known Hosts File?

The known_hosts file is a file located in the user’s home directory that contains a list of known hosts and their host keys. When a client connects to a server, it checks the known_hosts file for the server’s host key before establishing a connection. If the server’s host key matches the cached one, the connection is established.

3. What is Strict Host Key Checking?

Strict host key checking is a security feature of SSH that makes the client verify the authenticity of a server by comparing its host key with the one it has cached. If the host keys don’t match or the client doesn’t have the server’s host key cached, the connection is aborted.

4. Is Disabling Strict Host Key Checking Safe?

No, disabling strict host key checking poses a security risk as it allows man-in-the-middle attacks to occur. You should only disable it if you trust the server and can’t add its host key manually.

5. How Do I Obtain a Server’s Host Key?

You can obtain a server’s host key by connecting to the server via SSH and checking its fingerprint. The fingerprint is a string of characters that uniquely identifies the server’s host key.

Conclusion

In conclusion, the “The server’s host key is not cached” error is a common issue encountered in SSH connections. It occurs when the client doesn’t have the server’s host key cached. We’ve provided different solutions to fix this error, including adding the host key manually, disabling strict host key checking, and clearing the known_hosts file. Remember that SSH is a powerful tool, and ensuring the authenticity of the server is critical for secure communication.