Implementing DNS Governance in OpenShift with Red Hat Advanced Cluster Management

DNS Governance for OpenShift Beginners: A Friendly Guid

Red Hat Advanced Cluster Management (RHACM) acts as a centralized controller to govern DNS configurations across distributed OpenShift clusters. In large-scale environments, DNS serves as the critical ‘phonebook’ where a single configuration error can trigger a total service resolution failure.

Why This Matters

While OpenShift automates service discovery via the DNS Operator and CoreDNS, technical reality often involves configuration drift or operator degradation that remains undetected until a cluster-wide outage occurs. Implementing a governance framework shifts DNS from a black-box background service to a monitored asset, balancing the safety of ‘Inform’ mode for configuration integrity with the necessity of ‘Enforce’ mode for critical alerting rules.

Key Insights

• OpenShift DNS utilizes CoreDNS as a DaemonSet, ensuring every node has a local resolver to translate service names like my-app.production.svc.cluster.local into IP addresses.
• RHACM policies provide a mechanism to verify that the DNS Operator resource is not in a ‘degraded’ state, which is the primary indicator of subsystem failure.
• Corefile integrity checks ensure that critical CoreDNS plugins, including forward, errors, health, and cache, are consistently present in the configuration.
• Resource exhaustion monitoring compares the number of expected DNS pods against available pods to identify ‘Running’ but non-functional instances.
• Governance policies typically use ‘Inform’ mode for DNS to prevent accidental automated outages, while ‘Enforce’ mode is reserved for deploying Prometheus alerting rules.

Working Examples

Commands to clone the policy repository and apply DNS governance policies to the RHACM hub cluster.

git clone https://github.com/tosin2013/dns-policy-config.git
oc apply -f demo/namespace.yaml
oc apply -f demo/clusterset-binding.yaml
oc apply -f policies/dns/operator-health-check.yaml
oc apply -f policies/dns/corefile-integrity.yaml
oc apply -f policies/dns/resource-exhaustion.yaml
oc apply -f policies/observability/dns-alerting-rule.yaml

Placement resource used to target specific managed clusters for DNS policy application.

apiVersion: cluster.open-cluster-management.io/v1beta1
kind: Placement
metadata:
  name: dns-policy-placement
  namespace: dns-governance-policies
spec:
  predicates:
  - requiredClusterSelector:
      labelSelector:
        matchExpressions:
        - key: name
          operator: In
          values:
          - your-cluster-name

Practical Applications

• Use Case: Multi-cluster health monitoring where RHACM verifies CoreDNS pod availability across Dev, Staging, and Production environments.
• Pitfall: Using ‘Enforce’ mode on Corefile policies which can lead to automated cluster-wide resolution failure if the policy template is misconfigured.
• Use Case: Automated alerting deployment by enforcing Prometheus rules that page engineers when the DNS Operator status becomes degraded.
• Pitfall: Relying solely on pod ‘Running’ status without verifying the ClusterOperator status, which may hide logic-level DNS failures.

References:

• https://dev.to/tosin2013/dns-governance-for-openshift-beginners-a-friendly-guid-172n
• https://github.com/tosin2013/dns-policy-config.git