How to Ignore Ansible SSH Authenticity Checking in Ansible

How to Ignore Ansible SSH Authenticity Checking

When managing servers using Ansible, one of the common issues encountered is SSH authenticity checking. While it’s designed to ensure secure communication, it can interrupt automation workflows—especially when dealing with dynamic, frequently changing servers. This blog will guide you through the methods to bypass SSH authenticity checking, complete with practical examples and precautions to take.

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What is SSH Authenticity Checking in Ansible?

SSH authenticity checking is a security feature that verifies the identity of the remote server before establishing a connection. This check ensures the server’s identity matches what’s stored in the ~/.ssh/known_hosts file. If there’s a mismatch or if the server’s key is not recognized, SSH prompts for confirmation or fails altogether.

While this mechanism is crucial for secure connections, it can cause issues in scenarios like:

• Dynamic environments: Where server IPs or hostnames frequently change.

• Automation pipelines: Where manual intervention is not feasible.

• Ephemeral infrastructure: Where servers exist only for short durations.

Let’s explore how to bypass this check safely.

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Methods to Ignore SSH Authenticity Checking

Below are three commonly used methods to disable SSH host key verification in Ansible. Each comes with examples for different use cases.

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1. Pass SSH Options via ansible_ssh_extra_args

You can configure Ansible to pass additional SSH arguments for specific hosts. This is particularly useful when you want to override settings for certain servers without affecting the global configuration.

Example:

In your inventory file, add the ansible_ssh_extra_args variable:

all:
  hosts:
    dev_server:
      ansible_host: 192.168.1.101
      ansible_user: devuser
      ansible_ssh_extra_args: "-o StrictHostKeyChecking=no"

    test_server:
      ansible_host: 192.168.1.102
      ansible_user: testuser
      ansible_ssh_extra_args: "-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null"

Here:

• StrictHostKeyChecking=no bypasses the host key verification.

• UserKnownHostsFile=/dev/null prevents the host keys from being written to the known_hosts file.

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2. Update the ansible.cfg File

For a more global approach, you can modify the ansible.cfg file to include SSH options. This method is ideal for applying the same configuration across multiple playbooks.

Example:

Edit your ansible.cfg file to include the following:

[ssh_connection]
ssh_args = -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null

This configuration ensures that:

• SSH host key verification is disabled.

• No new host keys are added to the known_hosts file.

When to Use:

• Shared environments: When multiple users or teams use the same Ansible configuration.

• CI/CD pipelines: To ensure consistency across automated deployments.

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3. Use Environment Variables

For one-time or temporary setups, environment variables provide a quick and effective solution.

Example:

Export the SSH arguments as an environment variable before running your playbook:

export ANSIBLE_SSH_ARGS="-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null"
ansible-playbook deploy.yml

This method is ideal for:

• Ad hoc tasks: When you need to temporarily disable SSH checks for a single session.

• Scripting: When wrapping Ansible commands in scripts.

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4. Use a Custom SSH Wrapper Script

For advanced use cases, you can create a custom SSH wrapper script that includes the desired options. Ansible can be configured to use this script instead of the default SSH binary.

Example:

Create a script named ssh_no_check:

#!/bin/bash
/usr/bin/ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null "$@"

Make it executable:

chmod +x ssh_no_check

Then, configure Ansible to use this script by setting the ANSIBLE_SSH_EXECUTABLE environment variable:

export ANSIBLE_SSH_EXECUTABLE=/path/to/ssh_no_check
ansible-playbook deploy.yml

This approach is useful when you need granular control over SSH behavior.

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Risks of Ignoring SSH Authenticity Checking

Disabling SSH authenticity checking removes an important layer of security. While it simplifies workflows, it also introduces risks such as:

1. Man-in-the-middle (MITM) attacks: An attacker could impersonate a trusted host and intercept sensitive data.

2. Reduced accountability: Without verified keys, it’s harder to track and audit server identities.

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Best Practices for Secure Automation

If you must disable SSH authenticity checking, follow these precautions:

1. Limit exposure: Only disable SSH checks in trusted environments, such as private networks or development/test setups.

2. Use role-based access: Restrict the permissions of SSH users to minimize potential damage from compromised connections.

3. Re-enable checks in production: Always validate host keys in production environments to maintain security.

4. Monitor connections: Use logging and monitoring tools to detect suspicious activities.

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Conclusion

Ignoring SSH authenticity checking in Ansible can save time and reduce friction in automation workflows, especially for dynamic or ephemeral environments. However, it’s crucial to balance convenience with security. By following the methods and best practices outlined in this guide, you can bypass SSH checks responsibly and maintain the integrity of your infrastructure.