🕸 Kubernetes Troubleshooting: Your Go-To Guide for Common Issues

As a DevOps Engineer, your ability to troubleshoot Kubernetes issues efficiently can significantly impact the stability and availability of your applications. Kubernetes is a powerful and complex orchestration platform that abstracts much of the complexity of managing containers. However, when things go wrong, pinpointing the issue and finding a solution can be a challenging task.

In this article, we’ll dive deep into some of the most common Kubernetes issues you might encounter and walk through the step-by-step process of how to identify and resolve them. This guide will not only help you troubleshoot issues but also give you the knowledge needed to better understand the inner workings of Kubernetes.

1. Pods Stuck in Pending State

One of the first issues you may encounter is seeing pods stuck in a Pending state. A pod remains in this state when the Kubernetes scheduler cannot place it on a node, and it hasn’t started executing the container.

Root Causes:

• Insufficient Resources: The node may not have enough CPU or memory to accommodate the pod’s resource requests.
• Node Affinity or Taints: The pod may have node affinity rules or be restricted from scheduling due to taints on nodes.
• Unbound PVCs: If your pod uses persistent storage, the associated PersistentVolumeClaim (PVC) may not be bound to a suitable PersistentVolume (PV).

Troubleshooting Steps:

Check Pod Events: Describe the pod to view any related events or error messages.

kubectl describe pod <pod-name>

The Events section will often show why the pod is pending. For example, you might see messages like "Insufficient CPU" or "No nodes available with the required affinity."

Check Node Resources: If the issue is related to resources, check the available resources on the nodes.

kubectl get nodes -o wide
kubectl top nodes

If nodes are running out of resources, scale up your cluster or modify the pod’s resource requests to fit within the available capacity.

Node Affinity and Taints: Ensure that the pod’s node affinity or tolerations are not too restrictive. If the pod can only be scheduled on specific nodes and those nodes are unavailable or tainted, it will remain pending.

kubectl describe node <node-name>

PersistentVolumeClaim Binding: If the pod uses persistent storage, verify that the PVC is bound to a PV. A pod will not start if its PVC is in the Pending state.

kubectl get pvc kubectl describe pvc <pvc-name>

Solutions:

• Scale the Cluster: Add more nodes to provide the necessary CPU and memory.
• Adjust Resource Requests: Modify the pod’s resource requests and limits to fit within available capacity.
• Relax Node Affinity: Remove or adjust node affinity and tolerations to allow the pod to be scheduled on more nodes.
• Ensure PVC Binding: Make sure the PVC is correctly bound to a PersistentVolume.

2. CrashLoopBackOff: The Pod Keeps Restarting

The CrashLoopBackOff status is another frequent issue, indicating that a pod is crashing and Kubernetes is attempting to restart it repeatedly. This can be one of the most frustrating problems to solve because the error message itself doesn’t provide much insight into why the container is failing.

Root Causes:

• Application Crashes: The application inside the container is failing, causing the container to stop unexpectedly.
• Configuration Issues: Missing environment variables, incorrect file paths, or misconfigured services can cause the application to fail.
• Liveness/Readiness Probes: If Kubernetes health checks (probes) are misconfigured, they may cause the container to be killed and restarted unnecessarily.

Troubleshooting Steps:

Check Pod Logs: Start by looking at the logs of the crashing container to understand why it is failing.

kubectl logs <pod-name>

The logs will often show errors such as missing environment variables, dependency failures, or other application-level issues.

Describe the Pod: Describing the pod will show you if there are any additional events indicating what is wrong.

kubectl describe pod <pod-name>

Check Liveness/Readiness Probes: If the pod restarts too frequently, it might be due to misconfigured liveness or readiness probes. These probes may declare the container as unhealthy too early. Example:

livenessProbe:
  httpGet:
    path: /healthz
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10

If your application takes longer to start, you may need to increase the initialDelaySeconds or tweak the probe configuration.

Check Resource Limits: If resource limits are too low, Kubernetes may kill the container due to resource exhaustion (Out Of Memory or CPU throttling). Check if the pod is hitting resource limits.

kubectl describe pod <pod-name>

Solutions:

• Fix Application Errors: Use the log output to fix any application-level errors causing the crash.
• Adjust Probes: Modify the liveness/readiness probe configuration to give the application enough time to start.
• Increase Resource Limits: If the pod is crashing due to resource exhaustion, increase the memory and CPU limits.
• Test Locally: If possible, run the container outside Kubernetes (locally or in a test environment) to ensure the application runs without issues.

3. Pods in ContainerCreating State

Another common issue is seeing pods stuck in the ContainerCreating state for an extended period. This indicates that Kubernetes is trying to create the container but something is preventing it from completing the process.

Root Causes:

• Image Pull Errors: Kubernetes is unable to pull the container image from the registry. This can be due to incorrect image names, tags, or lack of access (e.g., authentication failure for private registries).
• PersistentVolume Issues: The pod may be waiting for a PersistentVolume to be mounted, but the volume is either unavailable or misconfigured.
• Node Disk Space: The node might have insufficient disk space, preventing the container from being created.

Troubleshooting Steps:

Describe the Pod: Describing the pod will give you details on why the container is stuck in the ContainerCreating state.

kubectl describe pod <pod-name>

Look for events such as ErrImagePull or ImagePullBackOff.

Check Image Pull Errors: If the problem is related to pulling the container image, you’ll see errors like ImagePullBackOff or ErrImagePull.

• Verify the image exists in the registry.
• Check that the image pull secret is correctly configured (if using a private registry).

kubectl get secrets

Check PersistentVolumeClaim: If the pod is waiting for a PVC, ensure that the PVC is bound to a suitable PV.

kubectl get pvc 
kubectl describe pvc <pvc-name>

Inspect Node Resources: Check if the node has enough disk space or if any issues are reported with the container runtime (e.g., Docker).

sudo systemctl status docker

Solutions:

• Fix Image Issues: Ensure the image name and tag are correct, and the image exists in the registry. If using a private registry, configure the image pull secret correctly.
• PVC Binding: Ensure the PVC is bound to a PersistentVolume.
• Node Disk Space: If the node is out of disk space, free up space or add more storage to the node.

4. DNS Issues: Pods Unable to Resolve Hostnames

Kubernetes uses CoreDNS to provide service discovery and DNS resolution within the cluster. DNS issues can cause problems when pods cannot resolve hostnames for services or external addresses, leading to application failures.

Root Causes:

• CoreDNS Failure: CoreDNS pods may be down or misconfigured, leading to DNS resolution failures.
• Network Policies: Network policies or firewalls might block DNS traffic (UDP on port 53).
• ConfigMap Misconfiguration: The CoreDNS ConfigMap could be misconfigured, pointing to incorrect upstream DNS servers.

Troubleshooting Steps:

1. Check CoreDNS Status: Verify that CoreDNS is running correctly by listing the pods in the kube-system namespace.

kubectl get pods -n kube-system

Inspect CoreDNS Logs: Check the logs for any errors or misconfigurations.

kubectl logs -n kube-system <coredns-pod-name>

Test DNS Resolution: Use a busybox pod to test DNS resolution.

kubectl run busybox --image=busybox --rm -it -- nslookup kubernetes.default

Review CoreDNS ConfigMap: Ensure the CoreDNS ConfigMap is properly configured with correct DNS settings.

kubectl get configmap -n kube-system coredns -o yaml

Solutions:

• Restart CoreDNS Pods: If CoreDNS is down or misbehaving, try restarting the pods.
• Update ConfigMap: Fix any misconfigurations in the CoreDNS ConfigMap, such as incorrect upstream DNS servers.
• Network Policies: Ensure network policies and firewalls are not blocking DNS traffic within the cluster.

Conclusion

As a DevOps Engineer, your ability to effectively troubleshoot Kubernetes issues is critical to maintaining the reliability and performance of your applications. Kubernetes can present many challenges, but with a structured approach to diagnosing and fixing problems, you’ll be able to resolve issues quickly and efficiently.

The key is to gather as much information as possible through logs, events, and pod descriptions, and use that data to pinpoint the root cause. Whether it’s a CrashLoopBackOff, pending pods, DNS failures, or image pull issues, the steps outlined in this guide should help you on your way to becoming a Kubernetes troubleshooting expert.

Remember: Kubernetes troubleshooting is as much an art as it is a science. The more issues you encounter and resolve, the better you’ll become at maintaining a healthy, resilient Kubernetes environment.