Ansible: Handling Multiple Hosts via SSH

PROBLEM

To run Ansible playbook in multiple hosts via SSH.

SOLUTION

Configuring SSH environment

Ensure SSH keypair exists on the current machine (ex: ~/.ssh/id_rsa for private key and ~/.ssh/id_rsa.pub for public key). If you do not have one, create one:

ssh-keygen

Copy the public key (ex: ~/.ssh/id_rsa.pub) to each remote host’s ~/.ssh/authorized_keys. If this file doesn’t exist, create it.

Ensure the current machine’s .ssh/ directory and file have correct permission.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

In the current machine’s /etc/hosts, add all remote hosts.

127.0.0.1      localhost  # current machine
192.168.1.100  donkeykong # remote host 1
192.168.1.200  supermario # remote host 2

In each remote host, enable the remote login and grant yourself the access to this service.

Enabling Remote Login on Mac

Test SSH connection to remote host to ensure they work first before working on Ansible playbook.

ssh user@donkeykong
ssh user@supermario

Creating Ansible Playbook

Create ansible.cfg and define the location of inventory file.

[defaults]
inventory = inventory.yml

Create inventory.yml and define both localhost and remote hosts.

all:
  hosts:
    localhost:
      ansible_connection: local
    donkeykong:
      ansible_user: user
      ansible_ssh_private_key_file: ~/.ssh/id_rsa
    supermario:
      ansible_user: user
      ansible_ssh_private_key_file: ~/.ssh/id_rsa

Run a test to ensure the connection to remote hosts are successful.

ansible all -i inventory.yml -m ping

If successful, the output looks something like this:

localhost | SUCCESS => {
    "changed": false,
    "ping": "pong"
}
donkeykong | SUCCESS => {
    "changed": false,
    "ping": "pong"
}
supermario | SUCCESS => {
    "changed": false,
    "ping": "pong"
}

Create main.yml with a very simple task.

- name: all-hosts
  hosts: all
  tasks:
    - name: Capture current dir
      shell: pwd
      register: output

    - name: Display output
      debug: msg='{{ output.stdout }}'

Run the playbook.

ansible-playbook main.yml

If successful, the output looks something like this:

PLAY [all-hosts] *******************************************************************************************************

TASK [Gathering Facts] *************************************************************************************************
ok: [localhost]
ok: [donkeykong]
ok: [supermario]

TASK [Capture current dir] *********************************************************************************************
changed: [localhost]
changed: [donkeykong]
changed: [supermario]

TASK [Display output] **************************************************************************************************
ok: [localhost] => {
    "msg": "/Users/user/myshittycode"
}
ok: [donkeykong] => {
    "msg": "/Users/user"
}
ok: [supermario] => {
    "msg": "/Users/user"
}

PLAY RECAP *************************************************************************************************************
donkeykong                 : ok=3    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
localhost                  : ok=3    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0  
supermario                 : ok=3    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

Controlling the Hosts

Sometimes, you want finer controls on what tasks to be ran in certain hosts.

To run in just one host (ex: donkeykong):

- name: one-host
  hosts: donkeykong
  tasks:
    - ...

To run in all remote hosts except localhost:

- name: all-hosts-except-localhost
  hosts: all:!localhost
  tasks:
    - ...