In this article, we will analyze the causes of DNS resolution failures in Kubernetes, explore debugging techniques, and provide best practices to ensure stable and reliable network communication within Kubernetes clusters.

Understanding Kubernetes DNS Resolution Failures

DNS resolution failures in Kubernetes occur when pods fail to resolve service names or external domains. Common causes include:

• Misconfigured CoreDNS settings.
• Network policies blocking DNS traffic.
• High load on the CoreDNS service leading to query timeouts.
• Incorrect service discovery due to missing or misconfigured headless services.
• Conflicts between Kubernetes DNS and external DNS resolvers.

Common Symptoms

• Pods unable to communicate with other services using service names.
• Frequent Temporary failure in name resolution errors.
• Intermittent connectivity issues with external services.
• Delayed pod startup times due to failing DNS lookups.

Diagnosing Kubernetes DNS Issues

1. Checking CoreDNS Pod Status

Ensure that CoreDNS is running without errors:

kubectl get pods -n kube-system -l k8s-app=kube-dns

2. Testing DNS Resolution from a Pod

Use nslookup or dig to verify DNS resolution inside a pod:

kubectl exec -it mypod -- nslookup myservice.default.svc.cluster.local

3. Inspecting CoreDNS Logs

Check for errors in CoreDNS logs:

kubectl logs -n kube-system -l k8s-app=kube-dns

4. Verifying DNS Configurations in Pods

Check the DNS settings assigned to a pod:

kubectl exec mypod -- cat /etc/resolv.conf

5. Testing External DNS Resolution

Ensure external DNS queries are working:

kubectl exec -it mypod -- nslookup google.com

Fixing Kubernetes DNS Resolution Issues

Solution 1: Restarting CoreDNS

Restart CoreDNS to resolve transient failures:

kubectl delete pod -n kube-system -l k8s-app=kube-dns

Solution 2: Scaling CoreDNS for High Traffic

Increase CoreDNS replicas to handle more queries:

kubectl scale deployment -n kube-system coredns --replicas=3

Solution 3: Allowing DNS Traffic in Network Policies

Ensure DNS traffic is not blocked by a network policy:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-dns
  namespace: default
spec:
  podSelector:
    matchLabels: {}
  policyTypes:
  - Egress
  egress:
  - to:
    - namespaceSelector: {}
    ports:
    - protocol: UDP
      port: 53

Solution 4: Using Headless Services for Stateful Applications

Ensure headless services are correctly defined:

apiVersion: v1
kind: Service
metadata:
  name: my-headless-service
spec:
  clusterIP: None
  selector:
    app: my-app

Solution 5: Manually Configuring DNS Servers

Specify external DNS servers if needed:

apiVersion: v1
kind: Pod
metadata:
  name: test-dns
spec:
  dnsPolicy: None
  dnsConfig:
    nameservers:
      - 8.8.8.8
      - 8.8.4.4

Best Practices for Reliable Kubernetes DNS

• Monitor CoreDNS logs to detect failures early.
• Ensure proper network policies allow DNS traffic.
• Scale CoreDNS in high-load environments.
• Use headless services for stateful applications.
• Manually configure external DNS resolvers if needed.

Conclusion

Intermittent DNS resolution failures in Kubernetes can disrupt microservices and break critical application workflows. By monitoring CoreDNS, managing network policies, and scaling DNS resources appropriately, DevOps teams can maintain stable and reliable Kubernetes networking.

FAQ

1. Why are my Kubernetes services not resolving?

CoreDNS misconfigurations, blocked network policies, or failing CoreDNS pods may be causing DNS resolution failures.

2. How do I check if CoreDNS is running properly?

Run kubectl get pods -n kube-system -l k8s-app=kube-dns to check CoreDNS pod status.

3. Can network policies block DNS resolution?

Yes, if UDP traffic on port 53 is not allowed, pods may fail to resolve DNS queries.

4. How can I manually test DNS resolution inside a pod?

Use kubectl exec -it mypod -- nslookup myservice.default.svc.cluster.local to test internal service resolution.

5. Should I use headless services in Kubernetes?

Yes, headless services are useful for stateful applications where direct pod communication is needed.