The Inside Playbook

Dynamic inventory plugin collection for network device management

September 28, 2022 by Eric McLeroy

network device management blog

Tackling the complexities of enterprise inventories

One common challenge our customers face is the need to track hosts from multiple sources: LDAP, cloud providers, and enterprise CMDB systems. Using a dynamic inventory allows users to integrate with these systems and update the Ansible inventory as it varies over time, with hosts spinning up and shutting down in response to business demands.

Ansible supports two ways to connect with external inventory: Inventory plugins and inventory scripts. 

Today we are going to cover dynamic inventory plugins as a Collection for network device management through an /etc/hosts file. This same type of setup can be used for creating any dynamic inventory using different items from /etc/hosts files to ini files or even csv’s. 

 

The first mission: Where is the source of truth?

We are going to start by figuring out the source of truth of the inventory we want to import. 

If you want to test and use this inventory plugin you can find the code in this Github repository: 

https://github.com/jmcleroy/inventoryplugin.git

In this case, it will be an /etc/hosts file externally stored in the Github/Gitlab inventory plugin repo as a test, in a similar fashion this file can also be pulled from a server with the correct host file with all of your network devices. In this Git repo we are going to make the necessary changes to permit a multi-vendor setup and we will use keywords from the naming to trigger grouping capabilities.

The hosts file will look like this:

% cat hosts 
10.25.25.25 ansible.cisco.external
10.25.26.26 ansible2.juniper.external
10.26.35.52 ansible3.arista.external

From there we will have the hierarchy for the Collections plugin. This is new to Ansible starting in 2.11+ and Red Hat Ansible Automation Platform 2.1.

The hierarchical structure looks like this:

inventoryplugin % tree 
.
├── README.md
├── collections
│   └── ansible_collections
│       └── ansible
│           └── network
│               └── plugins
│                   └── inventory
│                       └── etc_hosts.py
├── etc_hosts.yml
└── hosts

6 directories, 4 files

 

The second mission: Using the plugin

After we define our source of truth, we have to add the playbook to call this new plugin. There are some items you need to set, however within the ansible.cfg., you will need to set the Collections path see following code:

#
#inject_facts_as_vars = True

# Paths to search for collections, colon separated
collections_paths = ~/.ansible/collections:/usr/share/ansible/collections:/Users/jmcleroy/PycharmProjects/inventoryplugin/collections

# Paths to search for roles, colon separated
#roles_path = ~/.ansible/roles:/usr/share/ansible/roles:/etc/ansible/roles

# Host key checking is enabled by default
host_key_checking = False

Also you will need to set the inventory path see below:

# set plugin path directories here, separate with colons
#action_plugins     = /usr/share/ansible/plugins/action
#become_plugins     = /usr/share/ansible/plugins/become
#cache_plugins      = /usr/share/ansible/plugins/cache
#callback_plugins   = /usr/share/ansible/plugins/callback
#connection_plugins = /usr/share/ansible/plugins/connection
#lookup_plugins     = /usr/share/ansible/plugins/lookup
inventory_plugins  = /usr/share/ansible/plugins/inventory:/Users/jmcleroy/PycharmProjects/inventoryplugin/collections/ansible_collections/ansible/network/plugins/inventory
#vars_plugins       = /usr/share/ansible/plugins/vars
#filter_plugins     = /usr/share/ansible/plugins/filter
#test_plugins       = /usr/share/ansible/plugins/test
#terminal_plugins   = /usr/share/ansible/plugins/terminal
#strategy_plugins   = /usr/share/ansible/plugins/strategy

Here is a snippet of the Python plugin:

DOCUMENTATION = '''
    name: Etc_Hosts Inventory
    plugin_type: inventory
    author:
      - Eric McLeroy (@jmcleroy)
    short_description: Dynamic inventory plugin for a etc/hosts network file.
    version_added: "n/a"
    extends_documentation_fragment:
      - constructed
    options:
        plugin:
            description: Token that ensures this is a source file for the plugin.
            required: True
            choices: ['ansible.network.etc_hosts']
        file_path:
            description:
                - The path to the etc/hosts file.
                - This can be either an absolute path, or relative to inventory file.
            required: True
    requirements:
        - python >= 2.7
'''
EXAMPLES = r'''
# example etc_hosts.yml file
---
plugin: ansible.network.etc_hosts
file_path: /etc/hosts
'''
 
from ansible.errors import AnsibleError, AnsibleParserError
from ansible.plugins.inventory import BaseFileInventoryPlugin, Constructable
from string import digits
 
import os
import re
 
 
class InventoryModule(BaseFileInventoryPlugin, Constructable):
 
    NAME = 'ansible.network.etc_hosts'
 
    def verify_file(self, path):
        super(InventoryModule, self).verify_file(path)
        return path.endswith(('etc_hosts.yml', 'etc_hosts.yaml'))
 
    def parse(self, inventory, loader, path, cache=True):
        super(InventoryModule, self).parse(inventory, loader, path)
        self._read_config_data(path)
 
        hosts_file_in = self.get_option('file_path')
        if os.path.isabs(hosts_file_in):
            hosts_file = hosts_file_in
        else:
            hosts_file = os.path.join(os.path.dirname(path), hosts_file_in)
        file=open(hosts_file, 'r')
        lines=file.readlines()
        for line in lines:
            group_name = line.split(' ')[1]
            group_name = re.split("\.|-", group_name)[1].rstrip('0123456789')
            self.inventory.add_group(group_name)
            host_name = self.inventory.add_host(line.split(' ')[1].strip(), group_name)
            self.inventory.set_variable(host_name, 'ansible_host' , line.split()[0])

Next we will use the plugin to show the output as created by the plugin that took the ini file from above and dynamically created groups from a variable.

[emcleroy@rhel3 etcinventory]$ ansible-inventory --inventory etc_hosts.yml --list
{
    "_meta": {
        "hostvars": {
            "ansible.cisco.external": {
                "ansible_host": "10.25.25.25"
            },
            "ansible2.juniper.external": {
                "ansible_host": "10.25.26.26"
            },
            "ansible3.arista.external": {
                "ansible_host": "10.26.35.52"
            }
        }
    },
    "all": {
        "children": [
            "arista",
            "cisco",
            "juniper",
            "ungrouped"
        ]
    },
    "arista": {
        "hosts": [
            "ansible3.arista.external"
        ]
    },
    "cisco": {
        "hosts": [
            "ansible.cisco.external"
        ]
    },
    "juniper": {
        "hosts": [
            "ansible2.juniper.external"
        ]
    }
}

 

What can I do next?

You can view a step by step example on how to use the plugin in the following video demonstration:

 

Whether you are beginning your automation journey or are a seasoned veteran, there are a variety of resources to enhance your automation knowledge:
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Topics:
Network Automation


 

Eric McLeroy

Eric is a Principal Specialist Solutions Architect at Red Hat, focused on Ansible Network for the TME organization. He is ever expanding his knowledge of networking and Ansible and how it can help Red Hat customers solve more complex issues using Red Hat Ansible Automation Solution.

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