Introduction

When configuring a new Rocky VM in my home lab, I decided to experiment with setting the hostname dynamically based on the DNS resolution.

Here’s an overview of my process:

Initial Attempts with the dig Command

My first step involved preparing the environment for DNS queries. I installed the dnspython package to facilitate DNS lookups:

[admin@workstation ansible_activity]$ pip install dnspython

Following that, I updated the community.general collection to ensure all Ansible modules were up-to-date:

[admin@workstation ansible_activity]$ ansible-galaxy collection install community.general
Process install dependency map
Starting collection install process
Installing 'community.general:8.0.1' to '/home/admin/.ansible/collections/ansible_collections/community/general'

These installations were necessary prerequisites for utilizing the dig command within Ansible. However, the dig command did not return the expected DNS name, prompting me to switch to nslookup.

Setting the Hostname

Extracting Output from nslookup

To accurately retrieve the DNS name of the host, I utilized a regular expression within Ansible. Here’s the regex I applied:

regex_search('name\\s+=\\s+(.*)', '\\1')

Breaking down the regex components:

  • regex_search is an Ansible filter that conducts a regex pattern search within a string.
  • name signals the literal string we’re searching for in the input.
  • \\s+ identifies one or more whitespace characters.
  • = is the literal equal sign.
  • \\s+ (repeated) indicates additional whitespace characters following the equal sign.
  • (.*) captures all subsequent characters, denoting the target DNS name.

The entire pattern name\\s+=\\s+(.*) is designed to locate the line that begins with “name”, followed by whitespace, an equal sign, more whitespace, and finally the DNS name.

  • \\1 is a backreference to the content captured by the regex.
  • | first ensures that if multiple matches are found, only the first (and most relevant) result is used.

Ansible Playbook Incorporating Regex

The following playbook snippet implements the aforementioned regex pattern to retrieve and set the DNS name:

- name: Get the DNS name for the host using nslookup
  command: "nslookup {{ inventory_hostname }}"
  register: nslookup_output

- name: Get the DNS name for the host
  set_fact:
    dns_hostname: "{{ nslookup_output.stdout | regex_search('name\\s+=\\s+(.*)', '\\1') }}" 

- name: Set the hostname using shell
  shell: "hostnamectl set-hostname {{ dns_hostname }}"

This playbook performs a DNS lookup, extracts the DNS name using the defined regex, and then uses that name to set the system’s hostname.

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