Understanding and Resolving Git's "Host Key Verification Failed" Error with SSH

Introduction

When working with remote Git repositories over SSH, you might encounter an error message stating "Host Key Verification Failed." This issue occurs during the process of connecting to a repository and can be perplexing, especially if it is not happening on all machines in your team. Understanding the underlying mechanisms of host key verification in SSH and how Git interacts with this system will help you resolve this issue effectively.

What is Host Key Verification?

SSH (Secure Shell) uses cryptographic keys for secure communication between a client and a server. Each server has an associated public key, known as a "host key," which the client stores after their first connection. This mechanism ensures that subsequent connections are indeed with the intended server, preventing man-in-the-middle attacks.

When you attempt to connect via SSH, your system checks if it already knows the host’s public key by looking into a file named known_hosts. If the key doesn’t match or is absent from this file, SSH will warn you about "Host Key Verification Failed."

Causes of Host Key Verification Errors

First-time Connection: The server’s host key might not be in your client’s known_hosts file.
Changed Host Key: The server’s host key may have changed due to a legitimate reconfiguration or an unauthorized alteration.
Corrupted known_hosts File: This file can become corrupted, preventing the verification process from working correctly.

Resolving "Host Key Verification Failed" Error

Step 1: Verify SSH and Known Hosts Setup

Before addressing specific issues, ensure your SSH setup is correct:

Check for an existing ~/.ssh/known_hosts file. If it doesn’t exist, create the .ssh directory:
```
mkdir -p ~/.ssh
```

Step 2: Add or Update Host Keys

Option A: Manually Adding a Known Host Key

If you trust the new host key (e.g., after verifying changes with your server administrator), manually add it to your known_hosts file:

Use ssh-keyscan to fetch and append the host’s public key:
```
ssh-keyscan -t rsa domain.example >> ~/.ssh/known_hosts
```
Alternatively, connect using SSH directly which will prompt you to verify the host (answer "yes" when prompted):
```
ssh domain.example
```
When asked if you want to continue connecting, type yes and press Enter.

Option B: Removing Old Host Keys

If you suspect that a key has changed, remove the outdated entry from your known_hosts file:

Use ssh-keygen to delete the existing host key:
```
ssh-keygen -R domain.example
```
Reconnect or use SSH directly as described above to add the updated key.

Step 3: Using SSH Keys for Authentication

Consider configuring SSH keys for user authentication to avoid password prompts and enhance security:

Generate an RSA key pair:

ssh-keygen -t rsa -C "[email protected]"

Add your public key (~/.ssh/id_rsa.pub) to the remote server’s authorized keys or a service like GitHub.

Best Practices

Maintain Security: Only accept new host keys from trusted sources.
Regularly Review known_hosts: Periodically verify entries for authenticity and remove any that are no longer in use.
Use SSH Agents: To manage your SSH keys without entering passwords repeatedly, consider using ssh-agent.

Conclusion

The "Host Key Verification Failed" error is a security feature to ensure the integrity of SSH connections. By understanding how host key verification works and applying these solutions, you can resolve issues quickly and securely connect to remote Git repositories.