Deep Dive into CoreDNS: Best Practices, Optimization, and Troubleshooting

In the dynamic world of Kubernetes, DNS is a critical component that facilitates seamless service discovery and communication, both within and beyond the cluster. Since Kubernetes 1.14, CoreDNS has emerged as the go-to DNS solution due to its flexibility and scalability. This article will address typical challenges, effective troubleshooting methods, and optimization strategies to enhance your CoreDNS configuration.

Overview of CoreDNS in Kubernetes

CoreDNS has taken over as the default DNS solution in Kubernetes, replacing the older kube-dns system. It has several advantages, including a plugin-based architecture, better integration with Kubernetes, and more flexibility in managing DNS configurations. Built in Go, CoreDNS is built to deliver high performance, making it an excellent fit for containerized environments.

Why CoreDNS?

CoreDNS is specifically designed for Kubernetes, utilizing a modular architecture that supports extensive customization via plugins. Some key features include:

Plugin-Based Architecture: CoreDNS allows users to enable or disable plugins as needed, offering exceptional flexibility to adapt to diverse use cases and environments.
Optimized Performance: Built with Go, CoreDNS is lightweight and efficient, ensuring it can manage high DNS query loads with low latency.
Native Kubernetes Integration: Since Kubernetes v1.14, CoreDNS has been the default DNS server, providing seamless integration and robust support within the Kubernetes ecosystem.

For those new to CoreDNS or looking to revisit the basics, the official CoreDNS documentation is an excellent resource to explore.

Common Issues with CoreDNS

CoreDNS is known for its reliability, but CoreDNS also can encounter issues, particularly in complex or large-scale Kubernetes setups. Being aware of these challenges and having strategies to troubleshoot them is crucial for maintaining a well-functioning cluster.

DNS Resolution Failure

One of the most prevalent issues in Kubernetes environments is DNS resolution failures. This typically occurs when pods cannot resolve DNS names, causing service communication to break down and impacting cluster functionality.

Common Cause

Misconfigured Corefile: Errors in the Corefile, which dictates CoreDNS behavior, can occur, especially in configurations related to the kubernetes or forward plugins.
Network Issues: DNS traffic might be blocked by network policies, firewalls, or problems with the underlying network infrastructure, such as issues with CNI plugins.
Resource Constraints: Insufficient CPU or memory allocation for CoreDNS pods can result in delayed or failed DNS query processing.

Troubleshooting Tips

Review Corefile Configuration: Double-check the Corefile for accuracy, paying close attention to the kubernetes and forward plugin sections. For detailed guidance, consult the official CoreDNS Configuration Guide.
Test Network Connectivity: Use diagnostic tools like ping or traceroute to verify communication between CoreDNS pods and other components in the cluster.
Monitor Resource Allocation: Use the kubectl top pods command to track CPU and memory usage of CoreDNS pods. If resource limits are too low, adjust them to ensure optimal performance.

High Latency in DNS Queries

High latency in DNS resolution can lead to delays in service discovery, which may degrade the performance of cluster-based applications.

Common Cause

Overloaded Nodes: When CoreDNS pods are deployed on nodes experiencing high CPU or memory usage, their ability to respond to DNS queries promptly may be compromised.
Inefficient Forwarding Rules: Poorly optimized configurations in the forward plugin, such as routing all queries to a remote upstream DNS server, can introduce significant delays.
Cache Misconfiguration: Missing or improperly configured cache plugins can result in unnecessary external DNS lookups for domains that are frequently accessed.

Troubleshooting Tips

Improve Forwarding Configuration: Adjust the forward plugin to point to high-performance and stable upstream DNS servers. Using multiple servers can enhance redundancy and reliability.
Verify Cache Settings: Ensure the cache settings in your Corefile are properly configured for your workload. The CoreDNS Cache Plugin documentation offers valuable insights for optimizing caching.

Pod-to-Pod Communication Failures

In microservices-based systems, DNS plays a vital role in enabling service discovery. When DNS fails, pods may lose the ability to locate and communicate with one another, disrupting the entire architecture.

Common Cause

Network Policies: Kubernetes Network Policies can unintentionally restrict DNS traffic or communication between pods, particularly in clusters with strict security configurations.
CoreDNS Configuration Errors: Mistakes in the Corefile, such as incorrect domain or zone configurations, can lead to DNS resolution issues.
Service Misconfigurations: Errors in Kubernetes service definitions, such as incorrect ClusterIP assignments or missing selectors, can cause DNS records to point to the wrong endpoints.

Troubleshooting Tips

Inspect Network Policies: Verify that your Network Policies permit DNS traffic between pods. Refer to the Kubernetes Network Policy documentation for guidance on configuring policies accurately.
Check Service Configurations: Use the command kubectl describe service <service-name> to ensure services are properly set up and that DNS records are correctly mapped.

CoreDNS Pod CrashLoopBackOff

CoreDNS pods might end up in a CrashLoopBackOff state, causing DNS services to fail across the entire cluster.

Common Cause

Configuration Errors: Incorrect syntax in the Corefile, such as improperly configured plugins or unsupported directives, can prevent CoreDNS pods from starting successfully.
Resource Limits: If resource limits (CPU/memory) are too restrictive, CoreDNS pods may be terminated by the Kubernetes scheduler, often due to Out of Memory (OOM) errors.
Service Conflicts: CoreDNS may fail if there are conflicts with other services or DNS solutions in the cluster, such as overlapping port assignments or domain configurations.

Troubleshooting Tips

Analyze Logs: Run kubectl logs <coredns-pod-name> to check the logs of the problematic CoreDNS pod. Look for clues such as Corefile errors or insufficient resource messages.
Increase Resource Allocation: If CoreDNS pods are failing due to resource limits, update the deployment configuration to provide more CPU and memory resources.

External DNS Resolution Failures

CoreDNS might struggle to resolve external DNS names, which can prevent pods from communicating with services outside the Kubernetes cluster.

Common Cause

Forward Plugin Misconfiguration: Errors in the forward plugin configuration, such as incorrect IP addresses for upstream DNS servers or the absence of fallback servers, can cause resolution failures.
Upstream DNS Server Problems: The upstream DNS servers might be unavailable or unreachable due to network outages or connectivity issues.
Network Restrictions: External DNS queries could be blocked by firewalls, network segmentation, or other security measures implemented in the organization's infrastructure.

Troubleshooting Tips

Validate External DNS Queries: Run nslookup or dig from a pod to test external DNS resolution. Make sure the forward plugin in the Corefile is set up with accurate upstream DNS server addresses.
Inspect Network Policies: Verify that firewalls or network security settings are not blocking outbound DNS traffic to external servers.

Troubleshooting Techniques

Efficient troubleshooting is essential for resolving CoreDNS issues. Here are some advanced methods to diagnose and fix problems:

Analyzing Logs

Logs are crucial for identifying DNS-related problems. Make sure logging plugins like log or errors are enabled in the Corefile. Use the command kubectl logs <coredns-pod-name> to view the logs of a specific CoreDNS pod.

Testing DNS Resolution

Tools like nslookup and dig can be used within pods to test DNS resolution and validate configurations. For example:

nslookup kubernetes.default.svc.cluster.local

This command verifies whether the kubernetes service can be resolved within the cluster. For more advanced troubleshooting, consult the Kubernetes DNS Debugging Guide.

Monitoring CoreDNS

Integrating CoreDNS with monitoring tools like Prometheus and Grafana enables you to track critical metrics such as cache hits, request counts, and error rates. This approach helps identify performance issues and resolve them efficiently. For a step-by-step guide, see this resource on monitoring CoreDNS with Prometheus.

When to Use Optimized CoreDNS

Enhanced DNS configurations are particularly beneficial in the following scenarios:

High Traffic Loads: Clusters experiencing heavy network traffic may face DNS bottlenecks, resulting in slower response times and potential request drops.
Microservices Architectures: In environments with a large number of microservices, efficient DNS resolution is critical to support the constant and intricate communication between services.
Global Deployments: For organizations operating Kubernetes clusters across multiple regions, optimized DNS ensures fast and reliable resolution regardless of geographical location.
Continuous Deployment Pipelines: In setups where applications are frequently updated or redeployed, a robust DNS configuration helps prevent service discovery issues during deployment cycles.

Best Practice Optimizing CoreDNS

Optimizing CoreDNS involves balancing performance and reliability. Here are some best practices for a resilient and efficient DNS setup:

Scale CoreDNS Appropriately

Properly scaling CoreDNS ensures it can handle high query volumes, preventing performance bottlenecks in larger or more dynamic Kubernetes environments. One effective way to scale CoreDNS is by using a Horizontal Pod Autoscaler (HPA). Here's how you can implement it:

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: coredns
  namespace: kube-system
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: coredns
  minReplicas: 2
  maxReplicas: 10
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 70

Deploy Node-local DNS Cache

Implementing a node-local DNS cache can significantly reduce DNS lookup latency by handling queries directly at the node level. Below are the steps to deploy a node-local DNS cache using a DaemonSet:

1. Create Manifest

Click to expand Manifests nodelocaldns.yaml

apiVersion: v1
kind: ServiceAccount
metadata:
  name: node-local-dns
  namespace: kube-system
  labels:
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
---
apiVersion: v1
kind: Service
metadata:
  name: kube-dns-upstream
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/name: "KubeDNSUpstream"
spec:
  ports:
  - name: dns
    port: 53
    protocol: UDP
    targetPort: 53
  - name: dns-tcp
    port: 53
    protocol: TCP
    targetPort: 53
  selector:
    k8s-app: kube-dns
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: node-local-dns
  namespace: kube-system
  labels:
    addonmanager.kubernetes.io/mode: Reconcile
data:
  Corefile: |
    __PILLAR__DNS__DOMAIN__:53 {
        errors
        cache {
                success 9984 30
                denial 9984 5
        }
        reload
        loop
        bind __PILLAR__LOCAL__DNS__ __PILLAR__DNS__SERVER__
        forward . __PILLAR__CLUSTER__DNS__ {
                force_tcp
        }
        prometheus :9253
        health __PILLAR__LOCAL__DNS__:8080
        }
    in-addr.arpa:53 {
        errors
        cache 30
        reload
        loop
        bind __PILLAR__LOCAL__DNS__ __PILLAR__DNS__SERVER__
        forward . __PILLAR__CLUSTER__DNS__ {
                force_tcp
        }
        prometheus :9253
        }
    ip6.arpa:53 {
        errors
        cache 30
        reload
        loop
        bind __PILLAR__LOCAL__DNS__ __PILLAR__DNS__SERVER__
        forward . __PILLAR__CLUSTER__DNS__ {
                force_tcp
        }
        prometheus :9253
        }
    .:53 {
        errors
        cache 30
        reload
        loop
        bind __PILLAR__LOCAL__DNS__ __PILLAR__DNS__SERVER__
        forward . __PILLAR__UPSTREAM__SERVERS__
        prometheus :9253
        }
---
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: node-local-dns
  namespace: kube-system
  labels:
    k8s-app: node-local-dns
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
spec:
  updateStrategy:
    rollingUpdate:
      maxUnavailable: 10%
  selector:
    matchLabels:
      k8s-app: node-local-dns
  template:
    metadata:
      labels:
        k8s-app: node-local-dns
      annotations:
        prometheus.io/port: "9253"
        prometheus.io/scrape: "true"
    spec:
      priorityClassName: system-node-critical
      serviceAccountName: node-local-dns
      hostNetwork: true
      dnsPolicy: Default  # Don't use cluster DNS.
      tolerations:
      - key: "CriticalAddonsOnly"
        operator: "Exists"
      - effect: "NoExecute"
        operator: "Exists"
      - effect: "NoSchedule"
        operator: "Exists"
      containers:
      - name: node-cache
        image: registry.k8s.io/dns/k8s-dns-node-cache:1.24.0
        resources:
          requests:
            cpu: 25m
            memory: 5Mi
        args: [ "-localip", "__PILLAR__LOCAL__DNS__,__PILLAR__DNS__SERVER__", "-conf", "/etc/Corefile", "-upstreamsvc", "kube-dns-upstream" ]
        securityContext:
          capabilities:
            add:
            - NET_ADMIN
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
        - containerPort: 9253
          name: metrics
          protocol: TCP
        livenessProbe:
          httpGet:
            host: __PILLAR__LOCAL__DNS__
            path: /health
            port: 8080
          initialDelaySeconds: 60
          timeoutSeconds: 5
        volumeMounts:
        - mountPath: /run/xtables.lock
          name: xtables-lock
          readOnly: false
        - name: config-volume
          mountPath: /etc/coredns
        - name: kube-dns-config
          mountPath: /etc/kube-dns
      volumes:
      - name: xtables-lock
        hostPath:
          path: /run/xtables.lock
          type: FileOrCreate
      - name: kube-dns-config
        configMap:
          name: kube-dns
          optional: true
      - name: config-volume
        configMap:
          name: node-local-dns
          items:
            - key: Corefile
              path: Corefile.base
---
# A headless service is a service with a service IP but instead of load-balancing it will return the IPs of our associated Pods.
# We use this to expose metrics to Prometheus.
apiVersion: v1
kind: Service
metadata:
  annotations:
    prometheus.io/port: "9253"
    prometheus.io/scrape: "true"
  labels:
    k8s-app: node-local-dns
  name: node-local-dns
  namespace: kube-system
spec:
  clusterIP: None
  ports:
    - name: metrics
      port: 9253
      targetPort: 9253
  selector:
    k8s-app: node-local-dns

2. Apply Manifest

kubedns=`kubectl get svc kube-dns -n kube-system -o jsonpath={.spec.clusterIP}`
domain=cluster.local
localdns=169.254.20.10
sed -i "s/__PILLAR__LOCAL__DNS__/$localdns/g; s/__PILLAR__DNS__DOMAIN__/$domain/g; s/__PILLAR__DNS__SERVER__/$kubedns/g" ~/k3s/dev/test/nodelocaldns/nodelocaldns.yaml
kubectl apply -f nodelocaldns.yaml

💡

Value domain is "cluster.local" by default. Value localdns is the local listen IP address chosen for NodeLocal DNSCache.

3. Edit Configmap CoreDNS

$ kubectl edit cm -n kube-system coredns
...
        prometheus :9153
        forward . 169.254.20.10 {  # Add this line
            prefer_udp
        }
    }

4. Restart CoreDNS

kubectl rollout restart deployment -n kube-system coredns
kubectl get deployment -n kube-system coredns
kubectl get pod -n kube-system

Utilize Health Checks

Adding readiness and liveness probes to CoreDNS deployments helps maintain service reliability and ensures quick recovery from failures. Below is an example of how to configure health checks for the CoreDNS deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
spec:
  template:
    spec:
      containers:
      - name: coredns
        image: rancher/mirrored-coredns-coredns:1.10.1
        livenessProbe:
          httpGet:
            path: /health
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          periodSeconds: 10
          successThreshold: 1
          timeoutSeconds: 1
          failureThreshold: 3
        readinessProbe:
          httpGet:
            path: /ready
            port: 8181
            scheme: HTTP
          periodSeconds: 2
          successThreshold: 1
          timeoutSeconds: 1
          failureThreshold: 3

Manage DNS Records Effectively

Properly managing DNS records, such as using wildcards and TTL settings strategically, can improve DNS resolution speed and accuracy. However, avoid overusing wildcard DNS records, as they can introduce security risks and make DNS management more complex. Optimize TTL values to strike a balance between cache efficiency and the need for timely updates. Below is an example configuration for the CoreDNS Corefile:

example.com {
    file /etc/coredns/example.com.db {
        reload 30s
    }
    ttl 60
}

Conclusion

By adopting these best practices and staying proactive in monitoring and troubleshooting, you can ensure that CoreDNS operates reliably and efficiently in your Kubernetes clusters. Whether you're addressing DNS resolution issues or optimizing for high-traffic environments, a deep understanding of CoreDNS is essential for maintaining a stable and high-performing Kubernetes setup.