How to do Arithmetic Operations in Ansible

How to set host_key_checking=false in ansible inventory file?

 

^{Due to the fact that I answered this in 2014, I have updated my answer to account for more recent versions of ansible.}
Yes, you can do it at the host/inventory level (Which became possible on newer ansible versions) or global level:
inventory:
Add the following.

ansible_ssh_common_args='-o StrictHostKeyChecking=no'

host:
Add the following.

ansible_ssh_extra_args='-o StrictHostKeyChecking=no'

hosts/inventory options will work with connection type ssh and not paramiko. Some people may strongly argue that inventory and hosts is more secure because the scope is more limited.
global:
Ansible User Guide - Host Key Checking

• You can do it either in the /etc/ansible/ansible.cfg or ~/.ansible.cfg file:
  

  [defaults]
  host_key_checking = False
  

• Or you can setup and env variable (this might not work on newer ansible versions):
  

  export ANSIBLE_HOST_KEY_CHECKING=False 

  Yes, you can set this on the inventory/host level.

  ^{With an already accepted answer present, I think this is a better answer to the question on how to handle this on the inventory level. I consider this more secure by isolating this insecure setting to the hosts required for this (e.g. test systems, local development machines).}
  What you can do at the inventory level is add
  

  ansible_ssh_common_args='-o StrictHostKeyChecking=no'
  

  or
  

  ansible_ssh_extra_args='-o StrictHostKeyChecking=no'
  

  to your host definition (see Ansible Behavioral Inventory Parameters).
  This will work provided you use the ssh connection type, not paramiko or something else).
  For example, a Vagrant host definition would look like…
  

  vagrant ansible_port=2222 ansible_host=127.0.0.1 ansible_ssh_common_args='-o StrictHostKeyChecking=no'
  

  or
  

  vagrant ansible_port=2222 ansible_host=127.0.0.1 ansible_ssh_extra_args='-o StrictHostKeyChecking=no'
  

  Running Ansible will then be successful without changing any environment variable.
  

  $ ansible vagrant -i <path/to/hosts/file> -m ping
  vagrant | SUCCESS => {
      "changed": false, 
      "ping": "pong"
  }
  

  In case you want to do this for a group of hosts, here's a suggestion to make it a supplemental group var for an existing group like this:
  

  [mytestsystems]
  test[01:99].example.tld
  
  [insecuressh:children]
  mytestsystems
  
  [insecuressh:vars]
  ansible_ssh_common_args='-o StrictHostKeyChecking=no'