In the evolving landscape of Kubernetes, DNS plays a crucial role in ensuring smooth service discovery and communication within and outside the cluster. CoreDNS, a flexible and scalable DNS server, has been the default choice for Kubernetes since version 1.14. In this post, we’ll dive into CoreDNS’s role within Rancher-managed Kubernetes environments, particularly focusing on RKE1 and RKE2. We’ll explore common issues, troubleshooting techniques, and best practices to optimize your CoreDNS setup.

Overview of CoreDNS in Kubernetes

CoreDNS has replaced the legacy kube-dns in Kubernetes, bringing with it several advantages, including a plugin-based architecture, better integration with Kubernetes, and more flexibility in managing DNS configurations. Built in Go, CoreDNS is designed for high performance and is well-suited for containerized environments.

Why CoreDNS?

CoreDNS is built specifically for Kubernetes, leveraging a modular design that allows for extensive customization through plugins. Some key features include:

  • Plugin-Based Architecture: CoreDNS’s functionality is extended through plugins, which can be easily enabled or disabled as needed. This makes it highly flexible and adaptable to various environments.

  • Performance Optimizations: Written in Go, CoreDNS is lightweight and efficient, capable of handling high volumes of DNS queries with minimal latency.

  • Seamless Integration: As of Kubernetes v1.14, CoreDNS is the default DNS server, integrated natively within the Kubernetes ecosystem. This makes it a reliable and well-supported choice for DNS management.

For those new to CoreDNS or needing a refresher, the official CoreDNS documentation is a great place to start.

CoreDNS Configuration in RKE1/2

When deploying Kubernetes with Rancher’s RKE1 or RKE2, CoreDNS is the default DNS provider. However, Rancher offers several customization options to suit different operational needs:

CoreDNS in RKE1

In RKE1, CoreDNS configurations can be customized via the cluster.yaml file. Here are a few configuration scenarios:

  • Switching DNS Providers: You can switch back to kube-dns if needed, though CoreDNS is recommended for its modern architecture and flexibility.
  • NodeLocal DNSCache: Enabling NodeLocal DNSCache can reduce latency by allowing pods to query a DNS cache on the same node, rather than querying the CoreDNS service directly. This can be configured in the dns section of your cluster.yaml.
dns:
  provider: coredns
  nodelocal:
    ip_address: "169.254.20.10"

For more details, check out the RKE1 DNS documentation.

CoreDNS in RKE2

RKE2 handles DNS through HelmChartConfig, providing more granular control over CoreDNS deployment:

  • Disabling CoreDNS: If you need to disable CoreDNS, you can do so within the HelmChartConfig:
apiVersion: helm.cattle.io/v1
kind: HelmChartConfig
metadata:
  name: rke2-coredns
  namespace: kube-system
spec:
  valuesContent: |-
    disable: true
  • NodeLocal DNSCache: Similar to RKE1, you can enable NodeLocal DNSCache in RKE2 for improved DNS performance.

Additional details can be found in the RKE2 networking documentation.

Common Issues with CoreDNS

Despite its robustness, CoreDNS can encounter issues, particularly in complex or large-scale Kubernetes deployments. Understanding these issues and how to address them is key to maintaining a healthy cluster.

DNS Resolution Failures

DNS resolution failures are one of the most common issues in Kubernetes clusters. Symptoms include pods being unable to resolve DNS names, resulting in failed service communication.

Common Causes

  • Misconfigured Corefile: The Corefile, which controls CoreDNS behavior, may have errors, particularly in the kubernetes or forward plugins.
  • Network Issues: Network policies or firewalls might be blocking DNS traffic, or there may be issues with the underlying network infrastructure, such as CNI plugins.
  • Resource Constraints: CoreDNS pods may not have sufficient CPU or memory, leading to slow or failed DNS query processing.

Troubleshooting Tips

  • Check the Corefile Configuration: Ensure that the Corefile is correctly configured, especially the kubernetes and forward sections. Refer to the CoreDNS Configuration Guide for more details.
  • Verify Network Connectivity: Use tools like ping or traceroute to check connectivity between CoreDNS and other pods.
  • Monitor Resource Usage: Use kubectl top pods to check the CPU and memory usage of CoreDNS pods. If resources are insufficient, consider increasing the resource limits.

High Latency in DNS Queries

High latency can cause delays in service discovery, impacting the performance of applications running in the cluster.

Common Causes

  • Overloaded Nodes: If CoreDNS pods are running on nodes with high CPU or memory load, they may respond slowly to DNS queries.
  • Inefficient Forwarding Rules: Inefficient configurations in the forward plugin, such as forwarding all queries to a distant upstream DNS server, can cause delays.
  • Cache Misconfiguration: Lack of or misconfigured cache plugins may lead to repeated external DNS lookups for frequently queried domains.

Troubleshooting Tips

  • Optimize Forwarding Rules: Ensure that your forward plugin is configured to use fast and reliable upstream DNS servers. Consider using multiple upstream servers for redundancy.
  • Check Cache Settings: Review the cache settings in your Corefile to ensure they are appropriate for your environment. The CoreDNS Cache Plugin documentation provides guidance on configuring caching.

Pod-to-Pod Communication Failures

In microservices architectures, DNS is crucial for service discovery. Failures in DNS can prevent pods from discovering and communicating with each other.

Common Causes

  • Network Policies: Kubernetes Network Policies might inadvertently block DNS traffic or pod-to-pod communication, especially in environments with restrictive security setups.
  • CoreDNS Configuration Errors: Misconfigurations in the Corefile, such as incorrect domain or zone settings, can cause DNS resolution failures.
  • Service Misconfigurations: Issues with Kubernetes service definitions, like incorrect ClusterIP addresses or missing selectors, can result in DNS records that do not point to the correct endpoints.

Troubleshooting Tips

  • Review Network Policies: Ensure that your Network Policies allow DNS traffic between pods. The Kubernetes Network Policy documentation can help you set up policies correctly.
  • Validate Service Configurations: Use kubectl describe service <service-name> to check that services are configured correctly and that DNS records are accurate.

CoreDNS Pod CrashLoopBackOff

CoreDNS pods may enter a CrashLoopBackOff state, disrupting DNS services cluster-wide.

Common Causes

  • Configuration Errors: Invalid syntax in the Corefile, such as misconfigured plugins or unsupported directives, can cause CoreDNS pods to crash on startup.
  • Resource Limits: Overly restrictive resource limits (CPU/memory) might cause CoreDNS pods to be terminated by the Kubernetes scheduler due to OOM (Out of Memory) conditions.
  • Conflicts with Other Services: Conflicts with other services or DNS solutions running in the cluster, such as another DNS server or service with overlapping port or domain configurations, can cause CoreDNS to fail.

Troubleshooting Tips

  • Check Pod Logs: Use kubectl logs <coredns-pod-name> to view the logs of the failing CoreDNS pod. Look for errors related to Corefile syntax or resource allocation.
  • Review Resource Limits: Consider increasing the resource limits for CoreDNS pods in the deployment configuration if they are being terminated due to resource constraints.

External DNS Resolution Failures

CoreDNS may fail to resolve external DNS names, impacting the ability of pods to communicate with services outside the Kubernetes cluster.

Common Causes

  • Forward Plugin Misconfiguration: Incorrectly configured forward plugin in the Corefile, such as incorrect IP addresses for upstream DNS servers or missing fallback servers.
  • Upstream DNS Server Issues: The upstream DNS servers might be down or unreachable due to network issues.
  • Network Segmentation: External DNS queries may be blocked by firewalls, network segmentation, or other security mechanisms within the organization’s infrastructure.

Troubleshooting Tips

  • Test External DNS Resolution: Use tools like nslookup or dig from within a pod to test external DNS resolution. Check that the forward plugin in the Corefile is pointing to the correct upstream DNS servers.
  • Verify Network Configuration: Ensure that firewalls or network security groups allow outbound DNS queries to external servers.

Troubleshooting Techniques

Effective troubleshooting is key to resolving CoreDNS issues quickly. Here are some advanced techniques to help:

Accessing and Interpreting Logs

Logs are crucial for diagnosing DNS issues. Ensure that logging plugins like log or errors are enabled in the Corefile to gather useful diagnostic information. You can use kubectl logs <coredns-pod-name> to access the logs of a CoreDNS pod.

Testing DNS Resolution

Use tools like nslookup and dig within pods to test DNS resolution and verify configurations. For example:

nslookup kubernetes.default.svc.cluster.local

This command checks if the kubernetes service can be resolved within the cluster. For more complex scenarios, refer to the Kubernetes DNS Debugging Guide.

Monitoring CoreDNS

Integrating CoreDNS with Prometheus/Grafana allows you to monitor key metrics such as cache hits, request counts, and errors. This helps in identifying performance bottlenecks and diagnosing issues quickly. Check out this guide on monitoring CoreDNS with Prometheus for more details.

Workarounds and Best Practices

Optimizing CoreDNS involves balancing performance and reliability. Here are some best practices to ensure your DNS setup is resilient and efficient:

Tuning CoreDNS Performance

Adjust cache sizes and TTL (Time-to-Live) settings in the Corefile to balance between caching and real-time resolution. For high-traffic environments, consider scaling CoreDNS pods to distribute the load effectively.

Handling High Traffic Scenarios

For clusters with high DNS traffic, it’s crucial to scale CoreDNS appropriately. This might involve running multiple replicas of CoreDNS pods or increasing the resource allocation for each pod. Additionally, consider implementing fallback mechanisms, such as configuring secondary DNS servers or enabling retry logic in applications.

Fallback Mechanisms

To enhance the resilience of DNS services, configure secondary DNS servers in the Corefile’s forward plugin. This ensures that DNS queries can still be resolved if the primary DNS server fails. Here’s an example configuration:

forward . 8.8.8.8 1.1.1.1 {
  max_concurrent 1000
}

Implementing retry mechanisms in your applications can also help mitigate the impact of temporary DNS failures.

Conclusion

By following these practices and being proactive in monitoring and troubleshooting, you can ensure that CoreDNS serves your Kubernetes clusters reliably and efficiently. Whether you’re dealing with DNS resolution failures or optimizing for high-traffic scenarios, understanding the intricacies of CoreDNS is key to maintaining a stable and performant Kubernetes environment.