How to ignore ansible SSH authenticity checking?

Changing host_key_checking to false for all hosts is a very bad idea.

The only time you want to ignore it, is on "first contact", which this playbook will accomplish:

---
- name: Bootstrap playbook
  # Don't gather facts automatically because that will trigger
  # a connection, which needs to check the remote host key
  gather_facts: false

  tasks:
    - name: Check known_hosts for {{ inventory_hostname }}
      local_action: shell ssh-keygen -F {{ inventory_hostname }}
      register: has_entry_in_known_hosts_file
      changed_when: false
      ignore_errors: true
    - name: Ignore host key for {{ inventory_hostname }} on first run
      when: has_entry_in_known_hosts_file.rc == 1
      set_fact:
        ansible_ssh_common_args: "-o StrictHostKeyChecking=no"
    # Now that we have resolved the issue with the host key
    # we can "gather facts" without issue
    - name: Delayed gathering of facts
      setup:

So we only turn off host key checking if we don't have the host key in our known_hosts file.

Two options - the first, as you said in your own answer, is setting the environment variable ANSIBLE_HOST_KEY_CHECKING to False.

The second way to set it is to put it in an ansible.cfg file, and that's a really useful option because you can either set that globally (at system or user level, in /etc/ansible/ansible.cfg or ~/.ansible.cfg), or in an config file in the same directory as the playbook you are running.

To do that, make an ansible.cfg file in one of those locations, and include this:

[defaults]
host_key_checking = False

You can also set a lot of other handy defaults there, like whether or not to gather facts at the start of a play, whether to merge hashes declared in multiple places or replace one with another, and so on. There's a whole big list of options here in the Ansible docs.

---

Edit: a note on security.

SSH host key validation is a meaningful security layer for persistent hosts - if you are connecting to the same machine many times, it's valuable to accept the host key locally.

For longer-lived EC2 instances, it would make sense to accept the host key with a task run only once on initial creation of the instance:

- name: Write the new ec2 instance host key to known hosts
  connection: local
  shell: "ssh-keyscan -H {{ inventory_hostname }} >> ~/.ssh/known_hosts"

There's no security value for checking host keys on instances that you stand up dynamically and remove right after playbook execution, but there is security value in checking host keys for persistent machines. So you should manage host key checking differently per logical environment.

• Leave checking enabled by default (in ~/.ansible.cfg)
• Disable host key checking in the working directory for playbooks you run against ephemeral instances (./ansible.cfg alongside the playbook for unit tests against vagrant VMs, automation for short-lived ec2 instances)

You can pass it as command line argument while running the playbook:

ansible-playbook play.yml --ssh-common-args='-o StrictHostKeyChecking=no'

I found the answer, you need to set the environment variable ANSIBLE_HOST_KEY_CHECKING to False. For example:

ANSIBLE_HOST_KEY_CHECKING=False ansible-playbook ...