Pod network down? How to debug CNI in Kubernetes
Are your pods unable to communicate? Learn how to isolate network failures in Kubernetes using native tools and CNI diagnostics to restore connectivity.
Summary
- Kubernetes network failures often stem from routing issues in the CNI or restrictive network policies.
- Inspecting veth interfaces allows validating if traffic is correctly leaving the container.
- Tools like tcpdump are essential for capturing packets and identifying silent drops between nodes.
- Incorrect IPAM configurations often cause conflicts preventing pods from starting or connecting.
- Logs from CNI plugins frequently reveal critical configuration errors not visible via standard commands.
What is CNI and why does it fail?
The Container Network Interface (CNI) is the brain managing connectivity between your containers. In Kubernetes, it defines how each pod gets an IP address and how packets travel across the network. When a pod cannot reach another, the issue is rarely in your application code; it lies in how the CNI translates those intentions into hardware reality. If you have ever dealt with stuck pods, you know that debugging needs to be surgical.
Validating IP addressing
Before blaming the network, check the basics: are pods getting the correct IP? Kubernetes uses a component called IPAM (IP Address Management) to assign addresses within the range defined in your cluster. If the IP range is exhausted, the pod will remain in an initialization loop. Use kubectl get pod -o wide to verify the assigned IP and whether it belongs to the expected subnet. If no IP is assigned, the error lies in the Kubelet's communication with the network plugin.
Inspecting veth interfaces
Each container has a virtual network interface (veth) connecting its isolated namespace to the node's bridge. To investigate the packet path, you need to access the cluster node and list the interfaces. Once you identify the pod's interface, you can use diagnostic tools to observe real traffic. Learning how to use tcpdump to inspect network traffic is vital for verifying if packets are actually leaving the source node interface.
Packet capture and deep diagnostic
If the interface is up but communication fails, the issue might lie in routing levels or firewalls (iptables/nftables). Often, network rules injected by the CNI conflict with each other. The best way to validate is to perform a ping or curl between pods while capturing traffic on the node. Mastering how to use tcpdump to capture network packets on Linux is a mandatory skill for any engineer dealing with production infrastructure.
Final thoughts
Debugging CNI requires patience and a clear vision of how packets move through the kernel. Most of the time, the issue is a misconfigured route or a firewall rule blocking cross-node traffic. By mastering interface inspection and packet analysis, you drastically reduce downtime.
Always remember to check the specific documentation for your CNI plugin, such as Calico, Flannel, or Cilium. The way they handle traffic varies, but the diagnostic tools on the node's operating system remain the same.