How do I troubleshoot Kubernetes networking issues?

Troubleshooting Kubernetes networking issues can be challenging because it involves multiple layers of abstraction, such as pods, services, network policies, and underlying infrastructure. Below is a systematic approach to resolving Kubernetes networking issues:

1. Understand the Scope

Identify the specific issue:
Are pods unable to communicate with each other?
Are services unreachable?
Is external access to the cluster broken?
Are DNS lookups failing?
Determine if the problem is affecting one pod, one namespace, or the entire cluster.

2. Verify Pod Networking

a. Check Pod Status

Use kubectl get pods and verify the pod status.
If the pod is not running, the issue might be related to scheduling, image pulling, or resource limits.

b. Inspect Pod IP Addresses

Use kubectl get pods -o wide to see the pod IPs.
Ensure pods have IP addresses assigned. If they don’t, there might be a problem with the CNI (Container Network Interface) plugin.

c. Test Pod-to-Pod Connectivity

Use tools like ping, curl, or telnet from inside a pod to test connectivity to other pods.
Example:
bash kubectl exec -it <pod-name> -- ping <target-pod-ip>
If this fails, check network policies, firewalls, or routing rules.

3. Verify Service Networking

a. Check Service Configuration

Use kubectl get svc and verify the configuration.
Ensure the service has the correct ClusterIP, NodePort, or LoadBalancer settings.
Example:
bash kubectl describe svc <service-name>

b. Test Service Connectivity

Use curl or wget to access the service from inside and outside the cluster.
Example:
bash kubectl exec -it <pod-name> -- curl <service-cluster-ip>:<port>
If this fails, check:
Service selectors: Ensure the service is selecting the correct pods.
Endpoints: Run kubectl get endpoints <service-name> and ensure there are endpoints listed.

c. Check External Access

If you’re using a LoadBalancer or Ingress, verify the external IP or DNS name.
Example:
bash kubectl get ingress
Test connectivity to the external IP/DNS from outside the cluster.

4. Verify DNS Resolution

a. Test DNS in Pods

Use tools like nslookup or dig from within a pod to check DNS resolution.
Example:
bash kubectl exec -it <pod-name> -- nslookup <service-name>

b. Check CoreDNS Logs

Inspect logs of the CoreDNS pods for errors.
Example:
bash kubectl logs -n kube-system <coredns-pod-name>

c. Verify CoreDNS Configuration

Check the ConfigMap for CoreDNS.
Example:
bash kubectl get cm -n kube-system coredns -o yaml

5. Check Network Policies

Use kubectl get networkpolicy to list the policies applied in the namespace.
Verify if there are restrictive network policies blocking traffic.
Example:
bash kubectl describe networkpolicy <policy-name>

6. Verify CNI Plugin

Check if the CNI plugin (e.g., Calico, Flannel, Cilium, etc.) is functioning correctly.

a. Check CNI Pod Logs

Inspect logs of the CNI pods.
Example:
bash kubectl logs -n kube-system <cni-pod-name>

b. Ensure CNI Configuration

Verify the CNI configuration files on the nodes (e.g., /etc/cni/net.d/).

c. Restart CNI Pods

Sometimes restarting the CNI pods resolves issues.
Example:
bash kubectl rollout restart deployment -n kube-system <cni-deployment-name>

7. Check Node Networking

a. Verify Node IPs

Ensure nodes have valid IP addresses and can communicate with each other.
Use kubectl get nodes -o wide to check node IPs.

b. Inspect Node-Level Network Configurations

Check firewall rules, routing tables, and network interfaces on the nodes.
Ensure kube-proxy is running correctly:
Example:
bash kubectl get pods -n kube-system | grep kube-proxy kubectl logs -n kube-system <kube-proxy-pod-name>

c. Test Node-to-Node Connectivity

Use ping or telnet from one node to another to ensure connectivity.

8. Debugging Tools

a. Tcpdump/Wireshark

Use tcpdump or Wireshark on nodes or pods to analyze network traffic.

b. Tracing and Logging

Use tools like traceroute or mtr to trace network paths.
Increase verbosity in kubectl for detailed output:
Example:
bash kubectl get pods --v=9

c. Kubernetes Debugging Tools

Install and use tools like k9s, kubectl-debug, or kubectl-trace for in-depth debugging.

9. Check Underlying Infrastructure

Ensure the underlying infrastructure (VMs, physical servers, cloud networking, etc.) is functioning correctly.
Verify security groups, firewalls, and routing rules in the cloud provider or on-premises environment.

10. Common Issues

Misconfigured Network Policies: Restrictive policies blocking traffic.
CNI Plugin Errors: Issues with the installation or configuration of the CNI plugin.
DNS Failures: CoreDNS misconfiguration or network issues preventing DNS resolution.
Service Misconfiguration: Incorrect selectors or missing endpoints.
Ingress Misconfiguration: Problems with ingress rules or LoadBalancer setup.

By following these steps systematically, you can identify and resolve Kubernetes networking issues effectively.