Idempotently set upstream nameservers in dns addon

General Discussions microk8s
1

I’m running an old version of microk8s (1.18), and need update the DNS nameserver to Azure’s one so that my Private Link connection resolves correctly. Newer versions support enable dns:1.2.3.4 but alas I’m one version behind and am not in a place where I can upgrade.

Changing the DNS after the fact is easy enough to do via microk8s kubectl -n kube-system edit configmap/coredns and editting out the 8.8.8.8, 8.8.4.4:

apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health {
          lameduck 5s
        }
        ready
        log . {
          class error
        }
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . 168.63.129.16
        cache 30
        loop
        reload
        loadbalance
    }
kind: ConfigMap

However I don’t see how I could do this programmatically (i.e. via Ansible). Can I create this ConfigMap file somewhere and tell microk8s to use it? I can’t find any configuration/yaml in /var/snap that relates to this change.

I’m very new to Kubernetes so apologies in advance if I’m asking something obvious. I need to be able to do this programmatically so I can apply the same steps across multiple environments.

2

Hi you can try some of the techniques diacussed here, such as exporting the yaml using kubectl get cm | sed. ... | kubectl apply -f - ....

3

Awesome, thanks for the pointers!

I’ve now created a simple Ansible playbook to do this for me, sharing below in case it’s useful for others:

---
- name: Update coredns nameservers
  hosts: jupyter
  vars:
    nameservers: 168.63.129.16 # Azure

  tasks:
  - name: Read coredns configmap
    command: microk8s kubectl get configmap/coredns -n kube-system -o yaml
    register: coredns_config_stdout
    changed_when: no
    become: yes
    become_user: "{{ app_user }}"

  - name: Parse configmap yaml into usable variable
    set_fact:
      coredns_config: "{{ coredns_config_stdout.stdout | from_yaml }}"

  - name: Replace 'forward' nameservers with our config
    set_fact:
      new_coredns_config:
        data:
          Corefile: "{{ coredns_config.data.Corefile | regex_replace('forward \\. ([0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+ *)+\\n', 'forward . ' + nameservers + ' \\n' ) }}"

  - name: Create coredns configmap file
    copy:
      content: "{{ coredns_config | combine(new_coredns_config) | to_yaml }}"
      dest: "/home/{{ app_user }}/coredns.configmap.yaml"
    become: yes
    when: coredns_config.data.Corefile != new_coredns_config.data.Corefile

  - name: Load in new coredns configmap
    command: microk8s kubectl apply -f "/home/{{ app_user }}/coredns.configmap.yaml"
    when: coredns_config.data.Corefile != new_coredns_config.data.Corefile
    become: yes
    become_user: "{{ app_user }}"
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