run ssh-keyscan to add keys to known_hosts. This is a playbook for ansible

ssh_keyscan.yml

---
- hosts: all
  gather_facts: no
  sudo: no
  tasks:
  - name: run ssh-keyscan to add keys to known_hosts
    local_action: shell ssh-keyscan {{ ansible_ssh_host }} >> ~/.ssh/known_hosts

Ditch the pipes.

- name: Scan for SSH host keys.
  local_action:
    module: shell
    cmd: ssh-keyscan 192.168.1.1 2>/dev/null
  changed_when: False
  register: ssh_scan

- name: Update known_hosts.
  local_action:
    module: known_hosts
    key: "{{ item }}"
    name: "{{ ansible_host }}"
  with_items: "{{ ssh_scan.stdout_lines }}"

@DanielDavis5 this is a good solution as you don't need to use extra hacks. Though to me it was not working if I was redirecting to /dev/null, all the output would be registered to stderr. If I keep ssh-keyscan some-host only, then it works fine.