Ansible - Defining Variables As Dictionaries

6 minute read

This post is to go through an example of defining Ansible variables as dictionaries rather than lists. I wanted to throw this together as I have not found a decent example of doing this.

NOTE: Dictionaries are composed of key: value pairs.

When using Ansible you will generally find examples which use lists for variable definitions rather than dictionaries. So below I will show an example of a list and then compare it to a dictionary.

In this example I will be defining what would end up becoming the /etc/network/interfaces file on a Debian based system. The example of the list definitions will be a short excerpt compared to the example of using dictionaries.

Lists

When using lists all definitions begin with a "- ":

NOTE: Notice the space after the -.

network_interfaces:
  - name: enp0s3
    configure: true
    method: dhcp
    parameters:
      - param: pre-up sleep
        val: 2
  - name: enp0s8
    configure: true
    method: static
    address: 192.168.250.10
    netmask: 255.255.255.0
  - name: enp0s9
    configure: true
    comment: bond0 member
    method: manual
    parameters:
      - param: bond_master
        val: bond0
  - name: enp0s10
    configure: true
    comment: bond0 member
    method: manual
    parameters:
      - param: bond_master
        val: bond0
  - name: enp0s16
    configure: true
    comment: br0 member
    method: manual

As you can see from the above definitions this tends to be the traditional way of defining variables in Ansible. (Even for me!!)

Dictionaries

When using dictionaries the defintions are in a simple key: value pair.

NOTE: Notice the space after the :.

Below is an example of the same definitions in our list example but changed to be used as dictionaries.

network_interfaces:
  enp0s3:
    configure: true
    method: dhcp
    parameters:
      pre-up: sleep 2
  enp0s8:
    address: 10.0.0.100
    configure: true
    gateway: 10.0.0.1
    method: static
    netmask: 255.255.255.0
  enp0s09:
    configure: true
    method: manual
    parameters:
      bond_master: bond0
  enp0s10:
    configure: true
    method: manual
    parameters:
      bond_master: bond0
  enp0s16:
    configure: true
    method: manual
  enp0s17:
    configure: true
    method: manual
    parameters:
      pre-up: ifconfig $IFACE up
      post-down: ifconfig $IFACE down
  lo:
    configure: true
    method: loopback

As you can see, the format is extremely different but tends to be a bit cleaner looking in my opinion.

Usage

So how could we use dictionaries along with a Jinja2 template to generate our /etc/network/interfaces file? Let’s look at the example below.

Definitions

Let’s first define a more elaborate set of variables using dictionaries.

---
network_bonds:
  bond0:
    address: 192.168.1.10
    comment: Bond Group 0
    configure: true
    method: static
    netmask: 255.255.255.0
    parameters:
      bond_mode: active-backup
      bond_miimon: 100
      primary: enp0s9
      slaves:
        - enp0s9
        - enp0s10

network_bridges:
  br0:
    address: 192.168.1.11
    comment: Bridge 0
    configure: true
    method: static
    netmask: 255.255.255.0
    gateway: 192.168.1.1
    parameters:
     bridge_stp: off
     bridge_fd: 0
     ports:
       - enp0s16

network_interfaces:
  enp0s3:
    configure: true
    method: dhcp
    parameters:
      pre-up: sleep 2
  enp0s8:
    address: 10.0.0.100
    configure: true
    gateway: 10.0.0.1
    method: static
    netmask: 255.255.255.0
  enp0s09:
    configure: true
    method: manual
    parameters:
      bond_master: bond0
  enp0s10:
    configure: true
    method: manual
    parameters:
      bond_master: bond0
  enp0s16:
    configure: true
    method: manual
  enp0s17:
    configure: true
    method: manual
    parameters:
      pre-up: ifconfig $IFACE up
      post-down: ifconfig $IFACE down
  lo:
    configure: true
    method: loopback

We have now defined some interfaces, bonds, and bridges.

Template

Let’s now look at an example Jinja2 template on how to use our defintions to generate our /etc/network/interfaces file.

NOTE: Pay close attention to some of the Jinja2 voodoo going on here. There are a few interesting tricks going on.

###
# Beginning of network interfaces
###

{% for int in network_interfaces %}
{%   set _int = network_interfaces[int] %}
{%   if _int['configure'] %}
{%     if _int['comment'] is defined %}
# {{ _int['comment'] }}
{%     endif %}
auto {{ int }}
iface {{ int }} inet {{ _int['method'] }}
{%     if _int['method']|lower == "static" or _int['method']|lower == "manual" %}
{%       if _int['address'] is defined %}
  address {{ _int['address'] }}
{%       endif %}
{%       if _int['netmask'] is defined %}
  netmask {{ _int['netmask'] }}
{%       endif %}
{%       if _int['gateway'] is defined %}
  gateway {{ _int['gateway'] }}
{%       endif %}
{%     endif %}
{%   endif %}
{%   if _int['parameters'] is defined %}
{%     for k, v in _int['parameters'].iteritems() %}
  {{ k }} {% if v is not iterable or v is string %}{{ v }}{% elif v is iterable and v is not string %}{{ v|join(" ") }}{% endif %}

{%     endfor %}
{%   endif %}

{% endfor %}
###
# End of network interfaces
###

###
# Beginning of network bonds
###

{% for bond in network_bonds %}
{%   set _bond = network_bonds[bond] %}
{%   if _bond['configure'] %}
{%     if _bond['comment'] is defined %}
# {{ _bond['comment'] }}
{%     endif %}
auto {{ bond }}
iface {{ bond }} inet {{ _bond['method'] }}
{%     if _bond['method']|lower == "static" or _bond['method']|lower == "manual" %}
{%       if _bond['address'] is defined %}
  address {{ _bond['address'] }}
{%       endif %}
{%       if _bond['netmask'] is defined %}
  netmask {{ _bond['netmask'] }}
{%       endif %}
{%       if _bond['gateway'] is defined %}
  gateway {{ _bond['gateway'] }}
{%       endif %}
{%     endif %}
{%     if _bond['parameters'] is defined %}
{%       for k, v in _bond['parameters'].iteritems() %}
  {{ k }} {% if v is not iterable or v is string %}{{ v }}{% elif v is iterable and v is not string %}{{ v|join(" ") }}{% endif %}

{%       endfor %}
{%     endif %}
{%   endif %}

{% endfor %}

###
# Beginning of network bonds
###

{% for bridge in network_bridges %}
{%   set _bridge = network_bridges[bridge] %}
{%   if _bridge['configure'] %}
{%     if _bridge['comment'] is defined %}
# {{ _bridge['comment'] }}
{%     endif %}
auto {{ bridge }}
iface {{ bridge }} inet {{ _bridge['method'] }}
{%     if _bridge['method']|lower == "static" or _bridge['method']|lower == "manual" %}
{%       if _bridge['address'] is defined %}
  address {{ _bridge['address'] }}
{%       endif %}
{%       if _bridge['netmask'] is defined %}
  netmask {{ _bridge['netmask'] }}
{%       endif %}
{%       if _bridge['gateway'] is defined %}
  gateway {{ _bridge['gateway'] }}
{%       endif %}
{%     endif %}
{%     if _bridge['parameters'] is defined %}
{%       for k, v in _bridge['parameters'].iteritems() %}
  {{ k }} {% if v is not iterable or v is string %}{{ v }}{% elif v is iterable and v is not string %}{{ v|join(" ") }}{% endif %}

{%       endfor %}
{%     endif %}
{%   endif %}

{% endfor %}
###
# End of network bonds
###

Final Configuration

Now that we have our dictionaries defined and our Jinja2 template defined, we can see what our final configuration would look like below:

###
# Beginning of network interfaces
###

auto enp0s3
iface enp0s3 inet dhcp
  pre-up sleep 2

auto enp0s8
iface enp0s8 inet static
  address 10.0.0.100
  netmask 255.255.255.0
  gateway 10.0.0.1

auto enp0s09
iface enp0s09 inet manual
  bond_master bond0

auto enp0s10
iface enp0s10 inet manual
  bond_master bond0

auto enp0s16
iface enp0s16 inet manual

auto enp0s17
iface enp0s17 inet manual
  pre-up ifconfig $IFACE up
  post-down ifconfig $IFACE down

auto lo
iface lo inet loopback

###
# End of network interfaces
###

###
# Beginning of network bonds
###

# Bond Group 0
auto bond0
iface bond0 inet static
  address 192.168.1.10
  netmask 255.255.255.0
  bond_mode active-backup
  bond_miimon 100
  primary enp0s9
  slaves enp0s9 enp0s10


###
# Beginning of network bonds
###

# Bridge 0
auto br0
iface br0 inet static
  address 192.168.1.11
  netmask 255.255.255.0
  gateway 192.168.1.1
  bridge_stp off
  bridge_fd 0
  ports enp0s16

###
# End of network bonds
###

So there you have it, we now have a very cleanly defined /etc/network/interfaces file by using Ansible variables defined as dictionaries.

Enjoy!

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