Ansible – Host Variable Reference Confusion

ansible

I'm new to Ansible, and found it a great tool, but I've stumbled upon a problem that I find very confusing.

In my inventory, there's just:

host1
host2
host3

host1 and host2 both have two network interfaces, one of them connected to host3 that is acting as a router. In my playbook, I wanted a task on host3 that would configure routing and test it via ping. In order to do so, I wanted to:

a) get IP addresses of network interfaces connected to host3 for both host1 and host2. Name of the interface connected to host3 is set via host variable 'routed_if' and it's IP via 'routed_ip4'.

b) Use these variables to set up routing and send few pings from host1 to host2 to verify it works.

I decided to debug these variables doing so:

(...)
- debug:
   var: "{{ hostvars['host1']['routed_ip4'] }}"

What I'm getting is:

ok: [host3] => {
"192.168.1.1/24": "VARIABLE IS NOT DEFINED!"
}

Which is very odd. The IP "192.168.1.1" is the correct value (for host1) set via host variable but it is being presented as though it's a key in dictionary. What I was expecting to get is just '192.168.1.1/24' (literally).

I read chapter of ansible manual for variables, but it didn't really help me that much.

Best Answer

In debug, either you use parameter msg

    msg: "{{ hostvars['host1']['routed_ip4'] }}"

, or parameter var

    var: hostvars['host1']['routed_ip4']

also, the dot notation should work

    var: hostvars.host1.routed_ip4

The syntax of the examples is self-explaining.

Related Solutions

How to see stdout of ansible commands

I think you can register the result to a variable, then print with debug.

- name: print to stdout
  command: echo "hello"
  register: hello

- debug: msg="{{ hello.stdout }}"

- debug: msg="{{ hello.stderr }}"

Ansible Host Variables – How to Access Host Variable of a Different Host with Ansible

You can access pretty much any inventory facts/variables by doing something like this:

{{ hostvars['foo.example.com']['ansible_eth0']['ipv4']['address'] }}

or, if you want to do it via an index into a group:

{{ hostvars[groups['collectors'][0]]['ansible_eth0']['ipv4']['address'] }}

The big trick is that you need to collect the facts for all the hosts/groups you're interested in. So you would want to modify your playbook that runs against the reporters group to include a no-op (dummy) task that is applied to the collectors group. That will cause Ansible to collect facts about the collectors hosts so that they can be accessed from the reporters group. So you might want to add something like this to the top of your reporters playbook:

- hosts: collectors
  name: Gather facts from collectors
  tasks: [ ]

The empty brackets basically mean that no tasks will be executed, but this will still force Ansible to gather facts about the collectors so that you can then reference them in the tasks that you run against your reporters.

Edit #1

It occurred to me that I should also mention that as of version 1.8 of Ansible, there is a fact-caching feature that is now available. Fact caching relies on a redis server to store facts between playbook runs. With it enabled, one playbook can reference facts that were obtained by another playbook that was run previously. The example the Ansible documentation gives:

Imagine, for instance, a very large infrastructure with thousands of hosts. Fact caching could be configured to run nightly, but configuration of a small set of servers could run ad-hoc or periodically throughout the day. With fact-caching enabled, it would not be necessary to “hit” all servers to reference variables and information about them.

Related Topic