Creating an Ansible Playbook for Keeping DNS Client Settings

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This article shows how to create an Ansible playbook to maintain a consistent DNS configuration across multiple servers.

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By default, NetworkManager dynamically updates the /etc/resolv.conf file with the DNS settings from the active NetworkManager connection profiles

What is NetworkManager?

In some Linux distributions, the default networking service is provided by NetworkManager, which is a dynamic network control and configuration daemon to keep network devices and connections up and active when they are available. The traditional ifcfg type configuration files are still supported.

NetworkManager is installed by default on Red Hat Enterprise Linux. So, in our case, since we’re using a Linux distribution that has it by default, we’ll not install anything to use NetworkManager.

So, let’s get started and create an Ansible playbook to keep the DNS configuration consistent on all hosts.

Creating the Inventory

Before going forward, our environment is composed of:

• One Control VM (A CentOS 9 VM that runs Ansible).
• Six Managed VMs (Red Hat 8 VMs that Ansible manages).

So, Ansible needs to know which hosts to handle. Let’s create a directory to store the inventory file:

mkdir -p /root/ansible/inventory

Create the hosts.ini file:

touch /root/ansible/inventory/hosts.ini

And then, add the hosts to hosts.ini. In this case, for instance, our inventory file has some groups

[hpc2_login_nodes]    # This is a group
hpc2-login

[hpc2_head_nodes]     # This is a group
hpc2-head

[hpc2_compute_nodes]   # This is a group
hpc2-node[01:06]

[all:vars]             # This is a variable section
ansible_user=root      # This is a variable applies to all hosts

Creating the Playbook

Let’s create a directory to store the playbook files:

mkdir -p /root/ansible/playbooks

Inside the playbooks directory, generate the playbook file dns.yml:

touch /root/ansible/playbooks/dns.yml

And then, add the following content:

---
- name: Ensure DNS settings using NetworkManager
  hosts: all
  become: yes
  tasks:
    - name: Set DNS on interface
      community.general.nmcli:
        conn_name: "ens192"
        type: ethernet
        dns4:
          - "192.168.255.3"
        dns4_search:
          - "lab.local"
        state: present
      notify: Reactivate connection to apply DNS changes

  handlers:
    - name: Reactivate connection to apply DNS changes
      command: nmcli connection up ens192

Notes:

• In our case, for instance, all managed nodes have an interface “ens192”.
• Our DNS server IP is “192.168.255.3”.
• Our DNS domain name is “lab.local”.

Running the Playbook

To run the playbook:

ansilble-playbook -i inventory/hosts.ini playbooks/dns.yml

Output’s example – Considering that all managed hosts are in the desired state (no change):

PLAY [Ensure DNS settings using NetworkManager] *****************************************************************************************************************************

TASK [Gathering Facts] ******************************************************************************************************************************************************
ok: [hpc2-login]
ok: [hpc2-node03]
ok: [hpc2-node01]
ok: [hpc2-node02]
ok: [hpc2-head]
ok: [hpc2-node04]
ok: [hpc2-node05]
ok: [hpc2-node06]

TASK [Set DNS on interface] *************************************************************************************************************************************************
ok: [hpc2-head]
ok: [hpc2-login]
ok: [hpc2-node01]
ok: [hpc2-node02]
ok: [hpc2-node03]
ok: [hpc2-node04]
ok: [hpc2-node05]
ok: [hpc2-node06]

PLAY RECAP ******************************************************************************************************************************************************************
hpc2-head                  :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-login                 :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node01                :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node02                :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node03                :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node04                :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node05                :ok=2 changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node06                : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

If we access one managed node and inspect the DNS configuration, we can confirm that it is correct:

[root@hpc2-node01 ~]# cat /etc/resolv.conf
# Generated by NetworkManager
search lab.local
nameserver 192.168.255.3

Supposing that you have a demand to change the DNS IP from 192.168.255.3 to 10.10.10.10 on all managed hosts, you just need to change the IP in the Playbook and rerun it:

PLAY [Ensure DNS settings using NetworkManager] *****************************************************************************************************************************

TASK [Gathering Facts] ******************************************************************************************************************************************************
ok: [hpc2-node02]
ok: [hpc2-login]
ok: [hpc2-node01]
ok: [hpc2-node03]
ok: [hpc2-head]
ok: [hpc2-node04]
ok: [hpc2-node06]
ok: [hpc2-node05]

TASK [Set DNS on interface] *************************************************************************************************************************************************
changed: [hpc2-node02]
changed: [hpc2-node01]
changed: [hpc2-node03]
changed: [hpc2-login]
changed: [hpc2-head]
changed: [hpc2-node04]
changed: [hpc2-node05]
changed: [hpc2-node06]

RUNNING HANDLER [Reactivate connection to apply DNS changes] ****************************************************************************************************************
changed: [hpc2-login]
changed: [hpc2-node01]
changed: [hpc2-head]
changed: [hpc2-node02]
changed: [hpc2-node03]
changed: [hpc2-node04]
changed: [hpc2-node05]
changed: [hpc2-node06]

PLAY RECAP ******************************************************************************************************************************************************************
hpc2-head                  :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-login                 :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node01                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node02                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node03                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node04                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node05                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
hpc2-node06                :ok=3changed=2 unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

And inspecting the managed host again, we can confirm the change:

[root@hpc2-node01 ~]# cat /etc/resolv.conf
# Generated by NetworkManager
search lab.local
nameserver 10.10.10.10

That’s it for now 🙂