Kubernetes Networking Deep Dive: Stop Guessing, Start Tracing

Kubernetes networking is magic, until it isn't. One minute your pods are talking happily; the next, you're staring at a CrashLoopBackOff caused by a DNS timeout that only happens intermittently under load. I've spent more nights than I care to admit debugging packet drops inside overlay networks. Most tutorials gloss over the messy reality of VXLAN encapsulation, MTU mismatches, and the sheer IOPS required to keep etcd happy.

If you are running a cluster in 2023 without understanding what happens to a packet when it leaves a pod interface, you are just renting time until your next outage. Let's tear apart the abstraction layers.

The CNI Battlefield: Calico vs. Cilium (and eBPF)

The Container Network Interface (CNI) isn't just a plugin; it's the nervous system of your cluster. For years, Calico has been the reliable workhorse using standard Linux iptables. However, in 2023, if you aren't looking at Cilium and eBPF (Extended Berkeley Packet Filter), you are leaving performance on the table. iptables is a linked list; it doesn't scale well when you have thousands of services. eBPF allows the kernel to process packets without the overhead of traversing the entire netfilter stack.

Here is the reality check: running eBPF requires a kernel that supports it (5.10+ recommended), and more importantly, it requires virtualization that doesn't strip kernel flags. This is where cheap VPS providers fail. They give you a containerized OS where you can't load the BPF modules.

To check if your current node handles eBPF correctly for Cilium:

bpftool feature probe | grep "Program types"

If you see a wall of "NOT supported," you need to migrate. At CoolVDS, our KVM instances run full kernels. We don't artificially restrict your syscalls.

The Silent Killer: MTU Fragmentation

I recently audited a setup for a client in Oslo. Their API latency was inconsistent. The application logs showed nothing. The culprit? MTU (Maximum Transmission Unit).

Standard Ethernet MTU is 1500 bytes. If you use an overlay network (like VXLAN or IPIP), the CNI wraps your packet in another packet. This adds headers (usually 50 bytes for VXLAN). If your pod sends a 1500-byte packet, and the outer interface also has an MTU of 1500, the packet must be fragmented or dropped. This kills performance.

The fix: You must lower the MTU inside the CNI configuration to account for the overhead. For VXLAN, you typically want 1450 bytes.

# Example: Patching Calico Config to fix MTU
kubectl patch installation default --type=merge -p '{"spec": {"calicoNetwork": {"mtu": 1450}}}'

Pro Tip: Don't guess. Use tracepath from inside a busybox pod to see where packets are dying. If tracepath stops at a specific hop with "Message too long," you found your issue.

Storage IOPS = Network Stability

You might ask: "Why are you talking about storage in a networking article?" Because Kubernetes uses etcd as its source of truth. Every network state change, every pod IP assignment, goes into etcd. If your disk write latency spikes, etcd heartbeats fail. If etcd fails, the API server hangs. If the API server hangs, kube-proxy stops updating iptables rules.

This is the chain reaction of doom. Magnetic spinning disks or shared SATA SSDs are often too slow for production K8s clusters. You need NVMe.

Check your etcd disk WAL (Write Ahead Log) duration metrics. If you see high wal_fsync_duration_seconds, your network is about to flake out.

# Check disk latency with fio (replicates etcd write pattern)
fio --rw=write --ioengine=sync --fdatasync=1 --directory=test-data --size=22m --bs=2300 --name=mytest

On our CoolVDS NVMe instances, we consistently see fsync durations well below the 10ms danger zone recommended by the Kubernetes documentation. Cheap hosting often hits 40ms+ during peak hours.

Ingress: The Norwegian Context

If your target audience is in Norway, routing matters. You want your traffic hitting the Norwegian Internet Exchange (NIX) in Oslo as fast as possible. Hosting in Frankfurt or Amsterdam adds 20-30ms of round-trip time (RTT). For a chat app or a financial trading platform, that's an eternity.

When configuring NGINX Ingress Controller, ensure you are preserving the client source IP. By default, K8s might SNAT (Source Network Address Translation) the traffic, making all requests look like they come from the worker node. This breaks geo-blocking and logging required for GDPR compliance.

Set externalTrafficPolicy: Local in your Service definition to preserve the true client IP, but beware: this can cause imbalanced load if your external load balancer isn't smart enough.

apiVersion: v1
kind: Service
metadata:
  name: ingress-nginx-controller
  namespace: ingress-nginx
spec:
  type: LoadBalancer
  externalTrafficPolicy: Local
  ports:
  - name: http
    port: 80
    targetPort: http
  selector:
    app.kubernetes.io/name: ingress-nginx

Data Sovereignty and Schrems II

Since the Schrems II ruling, transferring personal data outside the EEA is a legal minefield. Using US-owned cloud providers for core infrastructure puts you at risk of violating Datatilsynet (Norwegian Data Protection Authority) guidelines. Running your own Kubernetes cluster on Norwegian VPS infrastructure isn't just a technical decision; it's a compliance strategy.

CoolVDS is purely European. Your data stays on our hardware in Oslo. No hidden replications to Virginia.

Conclusion: Own Your Stack

Kubernetes reduces the friction of deployment, but it increases the complexity of debugging. You can't rely on "managed" magic when the network degrades. You need visibility (eBPF), you need configuration accuracy (MTU), and you need raw I/O speed (NVMe).

If you are tired of fighting with "noisy neighbors" stealing your CPU cycles or opaque network policies blocking your CNI, it is time to build on a solid foundation.

Stop debugging phantom packet drops. Deploy a test cluster on a high-performance CoolVDS instance today and see the difference raw NVMe power makes.