ssh disable strict checking

Guri Bee

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18-10-2024

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SSH: Disabling Strict Host Key Checking – A Guide to Security and Convenience

When you connect to a remote server using SSH, it's essential to verify the server's identity. This verification process, known as "strict host key checking", ensures you're not connecting to a malicious imposter. However, in some situations, you might need to temporarily disable this strict check for convenience or when dealing with dynamic environments.

Understanding Strict Host Key Checking

By default, SSH clients maintain a list of known host keys. When you connect to a server for the first time, SSH prompts you to add the server's key to this list. In subsequent connections, SSH checks the server's key against this list. If there's a mismatch, SSH warns you, potentially preventing connection.

Why Disable Strict Host Key Checking?

While crucial for security, strict host key checking can be inconvenient. Here are some scenarios where you might consider disabling it:

• Dynamic Environments: When working with environments where servers are frequently provisioned or reconfigured, host keys change frequently. Strict checking would require constant re-verification, hindering workflow.
• Automation Scripts: Automating SSH tasks in scripts becomes challenging with strict checking enabled, as scripts might fail when encountering unknown host keys.
• Development and Testing: When working on development or testing servers that frequently change, strict checking can be disruptive.

Disabling Strict Host Key Checking - The Options

There are several ways to disable strict host key checking, each with its own trade-offs:

1. Using the -o StrictHostKeyChecking=no Flag

• Recommended for: One-time connections or testing environments.

• Example:

  ssh -o StrictHostKeyChecking=no user@server
  

• Caution: This option only disables strict checking for the current session.

2. Modifying the SSH Config File

• Recommended for: Persistent disabling for specific servers or environments.

• Location: ~/.ssh/config

• Example:

  Host my_server
    HostName my_server.example.com
    User my_user
    StrictHostKeyChecking no
  

• Caution: This method disables strict checking for all connections to my_server.example.com with user my_user.

3. Manually Adding the Host Key

• Recommended for: Adding a known host key to the known_hosts file.

• Location: ~/.ssh/known_hosts

• Example:

  ssh-keyscan my_server.example.com >> ~/.ssh/known_hosts
  

• Caution: Ensure you're connecting to the correct server as adding a malicious server's key can compromise your security.

Important Considerations

• Security Risks: Disabling strict host key checking weakens your security posture. It's crucial to understand the risks and use this approach cautiously.
• Alternatives: If you frequently deal with changing server environments, consider using tools like Ansible, Puppet, or Chef for managing configurations and host keys.
• Document Your Actions: Always document when and why you disabled strict host key checking.

Conclusion

Disabling strict host key checking is a powerful tool for convenience but comes with significant security implications. It's crucial to understand the risks, consider alternative approaches, and only use it when necessary. When in doubt, err on the side of caution and keep strict host key checking enabled.