Kubernetes Networking Deep Dive: Surviving the Overlay Chaos

Let’s be honest: Kubernetes networking is magic until it breaks. Then it becomes a black hole that swallows weekends. I’ve spent the last decade watching bright-eyed sysadmins deploy a cluster, only to panic when CrashLoopBackOff hits because a Pod can’t resolve DNS across nodes.

It is February 2025. If you are still relying on default iptables rules for routing traffic in a cluster with more than 50 nodes, you are doing it wrong. The overhead is killing your throughput. In this deep dive, we aren't discussing basics. We are ripping apart the networking layer, looking at CNI selection, MTU fragmentation, and why your physical infrastructure—specifically latency within Norway—matters more than your YAML configurations.

The CNI War is Over: eBPF Won

Years ago, we debated Flannel vs. Weave. Today, for any serious production workload in Europe, the choice usually narrows down to Cilium or highly tuned Calico. Why? Because iptables is a bottleneck. It was never designed to handle the churn of ephemeral containers. Every service update triggers a table reload that locks the kernel. At scale, this is a disaster.

In 2025, we use eBPF (Extended Berkeley Packet Filter). It allows the kernel to process packets without the context switch overhead of traversing the full TCP/IP stack for every hop.

Deploying Cilium for High Performance

Here is a standard production configuration I use for high-throughput clusters. Note the kube-proxy-replacement strict mode. We are bypassing kube-proxy entirely to shave off latency.

helm install cilium cilium/cilium --version 1.16.0 \
  --namespace kube-system \
  --set kubeProxyReplacement=strict \
  --set k8sServiceHost=${API_SERVER_IP} \
  --set k8sServicePort=${API_SERVER_PORT} \
  --set hubble.relay.enabled=true \
  --set hubble.ui.enabled=true \
  --set ipam.mode=kubernetes

Pro Tip: If you are running on CoolVDS, ensure your underlying VM interface has checksum offloading enabled. Some virtualization platforms drop packets if the checksums look weird inside VXLAN tunnels. We optimize our KVM stack to handle this transparently, but it's a common trap elsewhere.

The "Speed of Light" Problem: Latency in Norway

You can optimize your CNI all day, but you cannot beat physics. If your Kubernetes nodes are spread across cheap VPS providers with poor peering, your microservices will stutter.

Let’s look at the math. A round trip from a user in Oslo to a server in Frankfurt is roughly 15-20ms. That’s fine for a monolithic web app. But in a microservices architecture, where a single user request might trigger 50 internal service-to-service calls (Request A -> Auth B -> Database C -> Cache D), that latency compounds.

20ms x 50 calls = 1 second of pure network lag.

This is unacceptable. This is why hosting locally in Norway isn't just about GDPR and Datatilsynet compliance; it's a performance requirement. On CoolVDS NVMe instances located in Oslo, the latency to local users and between nodes is often sub-2ms. That same request chain drops to 100ms total.

Network Policies: The "Zero Trust" Reality

By default, Kubernetes allows all traffic. Any pod can talk to any pod. If a hacker breaches your frontend, they have a straight line to your database. In 2025, leaving this open is negligence.

We use NetworkPolicies to lock this down. However, traditional policies are hard to debug. Here is a restrictive policy that allows DNS lookups (crucial!) but blocks everything else by default, then explicitly permits ingress to a backend service.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: backend-policy
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend-api
  policyTypes:
  - Ingress
  - Egress
  ingress:
  - from:
    - podSelector:
        matchLabels:
          app: frontend-ui
    ports:
    - protocol: TCP
      port: 8080
  egress:
  - to:
    - namespaceSelector:
        matchLabels:
          name: kube-system
    ports:
    - protocol: UDP
      port: 53

Optimizing the Node Kernel

Kubernetes nodes are just Linux servers. If you don't tune the sysctl parameters, you will hit connection tracking limits during DDoS attacks or high traffic spikes. We see this constantly with high-traffic Magento or gambling sites.

Apply this sysctl configuration via a DaemonSet or directly on your CoolVDS nodes:

# /etc/sysctl.d/99-k8s-networking.conf

# Increase the connection tracking table size
net.netfilter.nf_conntrack_max = 1048576

# Allow more pending connections
net.core.somaxconn = 65535

# Reuse TIME-WAIT sockets (careful with this, but necessary for high throughput)
net.ipv4.tcp_tw_reuse = 1

# Increase range of ephemeral ports
net.ipv4.ip_local_port_range = 32768 60999

Ingress vs. Gateway API

For years, the NGINX Ingress Controller was the standard. But as of 2025, the Gateway API has matured enough for production usage. It decouples the role of the infrastructure provider (CoolVDS) from the cluster operator (you).

However, for most teams migrating from legacy setups, NGINX is still the workhorse. The mistake most make is ignoring the buffer sizes. If you are handling large file uploads or heavy API JSON payloads, the defaults will choke.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: main-ingress
  annotations:
    kubernetes.io/ingress.class: "nginx"
    # Essential for preventing 413 Request Entity Too Large
    nginx.ingress.kubernetes.io/proxy-body-size: "50m"
    # Keep-alive optimization for reducing handshake overhead
    nginx.ingress.kubernetes.io/upstream-keepalive-connections: "100"
    nginx.ingress.kubernetes.io/keepalive-timeout: "3600"
spec:
  rules:
  - host: api.coolvds-demo.no
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: backend-service
            port:
              number: 80

The Storage-Network Link: etcd Latency

Here is the dirty secret of Kubernetes networking: it relies entirely on etcd stability. etcd is sensitive to disk latency. If your disk write latency spikes, etcd heartbeats fail, the API server timeouts, and suddenly your networking updates (IP assignments, endpoint slices) stop propagating.

This is where "cheap" VPS providers fail. They put you on shared spinning rust or oversold SSDs with noisy neighbors. When a neighbor runs a backup, your K8s cluster falls apart.

We built CoolVDS on dedicated NVMe arrays specifically to solve this. When we run fio benchmarks, we look for consistent IOPS, not just burst speed. Consistent disk I/O means stable etcd, which means stable networking.

Comparison: Networking Options

Conclusion

Kubernetes networking is unforgiving. It demands precision in configuration and excellence in underlying infrastructure. You can write the best eBPF code in the world, but if the packets drop due to noisy neighbors or geographic latency, it's worthless.

If you are building for the Nordic market, keep your data in Norway and your latency low. Stop fighting with subpar hardware.

Ready to stabilize your cluster? Spin up a CoolVDS NVMe instance in Oslo today. We handle the hardware so you can handle the traffic.