Ansible/Hosts/AWS

How to use Ansible Hosts File with AWS

The hosts inventory file is treated as static when managing our own infrastructure, but with AWS this information will become out of date quickly. Ansible can get information via the AWS API, but has to know to do that.

Ansible can be put in dynamic inventory mode by making the inventory file (playbooks/hosts executable):

chmod +x playbooks/hosts

The script must accept two command line flags:

--host=<hostname>   show host details
--list              list groups

For example, Ansible will call the inventory script like so:

$ ./dynamic.py --host=vagrant2

Example dynamic inventory script

Let's walk through an example.

List of hosts:

For vagrant machines, vagrant status gives output about which machines are running. This is information we want to provide to the dynamic inventory file to create a list of hosts.

vagrant status --machine-readable

which is a lot of info in CSV format that can be parsed.

Next, we can use the vagrant ssh-config command, which outputs information that is formatted for an SSH config file, and parse it using pamiko (a Python library for doing SSH-related things - in this case, parsing an SSH config file and turning it into a python dictionary).

#!/usr/bin/env python
# Adapted from Mark Mandel's implementation
# https://github.com/ansible/ansible/blob/stable-2.1/contrib/inventory/vagrant.py
# License: GNU General Public License, Version 3 <http://www.gnu.org/licenses/>
import argparse
import json
import paramiko
import subprocess
import sys


def parse_args():
    parser = argparse.ArgumentParser(description="Vagrant inventory script")
    group = parser.add_mutually_exclusive_group(required=True)
    group.add_argument('--list', action='store_true')
    group.add_argument('--host')
    return parser.parse_args()


def list_running_hosts():
    cmd = "vagrant status --machine-readable"
    status = subprocess.check_output(cmd.split()).rstrip()
    hosts = []
    for line in status.split('\n'):
        (_, host, key, value) = line.split(',')[:4]
        if key == 'state' and value == 'running':
            hosts.append(host)
    return hosts


def get_host_details(host):
    cmd = "vagrant ssh-config {}".format(host)
    p = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE)
    config = paramiko.SSHConfig()
    config.parse(p.stdout)
    c = config.lookup(host)
    return {'ansible_host': c['hostname'],
            'ansible_port': c['port'],
            'ansible_user': c['user'],
            'ansible_private_key_file': c['identityfile'][0]}


def main():
    args = parse_args()
    if args.list:
        hosts = list_running_hosts()
        json.dump({'vagrant': hosts}, sys.stdout)
    else:
        details = get_host_details(args.host)
        json.dump(details, sys.stdout)

if __name__ == '__main__':
    main()