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System Administration📅 March 15, 2021⏱️ 7 minutes min read

How to Fix SSH Error: Host Key Verification Failed

Learn how to resolve the SSH 'Host key verification failed' error when connecting to VPS or servers. Comprehensive guide with step-by-step solutions, security considerations, and best practices for SSH connection management.

👤 Author: Linux-ID Team
How to Fix SSH Error: Host Key Verification Failed

Introduction

SSH (Secure Shell) is a network protocol commonly used for remote access to VPS or servers. However, sometimes when attempting to connect to a VPS or server, you may encounter an error message like "Host key verification failed", preventing you from establishing a remote connection.

The "Host key verification failed" error occurs because SSH maintains a security mechanism to prevent man-in-the-middle attacks. When you first connect to a server, SSH stores the server's host key in a local file called known_hosts. If the server's host key changes (due to OS reinstallation, server replacement, or other reasons), SSH detects this mismatch and blocks the connection for security purposes.

Understanding SSH Host Key Verification

What is SSH Host Key Verification?

SSH host key verification is a security feature that:

  • Authenticates server identity - Ensures you're connecting to the intended server
  • Prevents man-in-the-middle attacks - Protects against connection interception
  • Maintains connection integrity - Verifies the server hasn't been compromised

When Does This Error Occur?

The "Host key verification failed" error typically happens when:

  • Server OS reinstallation - New installation generates new host keys
  • Server hardware replacement - Different physical server with same IP
  • Host key rotation - Server administrator updates keys for security
  • Network configuration changes - IP address or hostname modifications
  • Compromised known_hosts file - Local file corruption or modification

How to Fix SSH Host Key Verification Failed Error

There are several secure methods to resolve this issue. Choose the appropriate solution based on your specific situation and security requirements.

Method 1: Using Command Line (Recommended)

The most straightforward and secure method is to remove the problematic host key using the SSH command-line tools.

Step 1: Identify the Command from Error Message

When you encounter the SSH error, look for a message similar to this:

bash 
user@computer:~> ssh [email protected]
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:iFK9mZ5BXuYzsliAoTyENojPWQj5sKxrTylJlNliKoI.
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/user/.ssh/known_hosts:13
You can use following command to remove the offending key:
ssh-keygen -R 10.10.100.1 -f /home/user/.ssh/known_hosts
ECDSA host key for 10.10.100.1 has changed and you have requested strict checking.
Host key verification failed.

Step 2: Execute the Removal Command

Use the command provided in the error message:

bash 
ssh-keygen -R 10.10.100.1 -f /home/user/.ssh/known_hosts

Command Breakdown:

  • ssh-keygen -R - Remove host key from known_hosts file
  • 10.10.100.1 - IP address of the problematic server
  • -f /home/user/.ssh/known_hosts - Path to your known_hosts file

Step 3: Reconnect to the Server

After removing the old host key, attempt to reconnect:

bash 
ssh [email protected]

You'll see a prompt asking to verify the new host key:

bash 
The authenticity of host '10.10.100.1 (10.10.100.1)' can't be established.
ECDSA key fingerprint is SHA256:iFK9mZ5BXuYzsliAoTyENojPWQj5sKxrTylJlNliKoI.
Are you sure you want to continue connecting (yes/no/[fingerprint])?

Type yes to accept and store the new host key.

Method 2: Manual known_hosts File Editing

If you prefer to manually edit the file or need more control over the process:

Step 1: Open the known_hosts File

bash 
vim ~/.ssh/known_hosts

Step 2: Locate and Remove the Problematic Entry

From the error message, identify the line number (e.g., line 13 in the example above). Navigate to that line and delete it entirely.

Example of a known_hosts entry:

text 
[10.10.100.1]:2222 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBKkToWAUheuqt876tDOrWwDSH5HbL0TLMw5MAsOsLqCnb3jRn5oxIJn2yECB1HPamglO/m79o2W8mAGAjypyFkw=

Step 3: Save and Exit

Save the file and exit the editor (in Vim: :wq).

Step 4: Reconnect

Try connecting again and accept the new host key when prompted.

Method 3: Advanced Solutions

Verify Host Key Authenticity

Before accepting a new host key, especially in production environments, verify its authenticity:

1. Contact Server Administrator

  • Confirm the server has been reinstalled or keys have been rotated
  • Request the new key fingerprint for verification

2. Use Alternative Connection Method

  • Access the server via console (if available)
  • Compare the actual host key on the server

3. Check Server Host Key

bash 
ssh-keyscan -t ecdsa 10.10.100.1

Bypass Host Key Checking (Temporary Only)

⚠️ Security Warning: Only use this method temporarily and in trusted environments.

bash 
ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null user@server

Parameters:

  • StrictHostKeyChecking=no - Disables host key verification
  • UserKnownHostsFile=/dev/null - Prevents storing the key

Security Considerations and Best Practices

✅ Best Practices

  1. Always Verify Host Keys

    • Confirm server changes with administrators
    • Compare fingerprints when possible
    • Never blindly accept new keys in production

  2. Maintain SSH Configuration

    • Regularly backup your .ssh directory
    • Use SSH config files for complex setups
    • Implement key rotation policies

  3. Monitor SSH Connections

    • Log SSH access attempts
    • Use fail2ban for brute force protection
    • Implement proper firewall rules

  4. Use Key-Based Authentication

    • Prefer SSH keys over passwords
    • Use strong key algorithms (ED25519, RSA 4096)
    • Protect private keys with passphrases

⚠️ Security Warnings

  1. Never Ignore Host Key Changes

    • Always investigate unexpected key changes
    • Consider potential security breaches
    • Document all legitimate key changes

  2. Avoid Permanent Bypass Solutions

    • Don't disable host key checking permanently
    • Use temporary bypasses only for troubleshooting
    • Re-enable security features after resolution

Troubleshooting Common Issues

Issue 1: Permission Denied After Key Removal

Cause: SSH key authentication might be disabled or misconfigured.

Solution:

bash 
# Check SSH server configuration
sudo grep -E "(PasswordAuthentication|PubkeyAuthentication)" /etc/ssh/sshd_config

# Enable password authentication temporarily if needed
sudo systemctl restart sshd

Issue 2: Multiple Known_hosts Files

Cause: System-wide and user-specific known_hosts files might conflict.

Solution:

bash 
# Remove from system-wide file
sudo ssh-keygen -R hostname -f /etc/ssh/ssh_known_hosts

# Remove from user file
ssh-keygen -R hostname -f ~/.ssh/known_hosts

Issue 3: Non-standard SSH Port

Cause: Server uses non-standard SSH port, requiring special syntax.

Solution:

bash 
# For non-standard ports, use bracket notation
ssh-keygen -R [hostname]:port

# Example for port 2222
ssh-keygen -R [10.10.100.1]:2222

Preventing Future Issues

1. SSH Configuration Management

Create an SSH config file (~/.ssh/config) for better connection management:

bash 
Host myserver
    HostName 10.10.100.1
    User admin
    Port 22
    StrictHostKeyChecking ask
    UserKnownHostsFile ~/.ssh/known_hosts

2. Automated Host Key Management

For dynamic environments, consider:

  • Using SSH certificates instead of host keys
  • Implementing centralized key management
  • Using configuration management tools (Ansible, Puppet)

3. Documentation and Monitoring

  • Document all server changes and key rotations
  • Implement monitoring for SSH connection failures
  • Maintain an inventory of server host keys

Conclusion

The SSH "Host key verification failed" error is a security feature, not a bug. While it can be frustrating, it protects you from potential security threats. By understanding the error and following the proper resolution steps, you can maintain both connectivity and security.

Key Takeaways:

  • Always investigate unexpected host key changes
  • Use the ssh-keygen -R command for safe key removal
  • Verify new host keys before accepting them
  • Never permanently disable host key checking in production
  • Implement proper SSH security practices

Remember, when managing VPS and servers, security should always be your top priority. If you encounter SSH errors and need professional assistance, consider consulting with experienced system administrators or managed service providers who can help maintain both security and accessibility.

For complex server management needs, professional managed services can provide expert guidance on SSH security, server configuration, and infrastructure management while ensuring your systems remain secure and accessible.

Tags:

sshlinuxserversecuritytroubleshootingtutorial

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