Surviving the Kubernetes Networking Maze: Flannel, Iptables, and the Schrems I Reality

Let’s be honest. Setting up Kubernetes (K8s) right now feels less like orchestration and more like defusing a bomb. With version 1.1 just landing last month, we finally have some stability, but the networking model remains the single biggest hurdle for anyone moving out of a laptop environment and into production.

If you are still trying to map Docker ports manually, stop. The Kubernetes model assumes a flat IP space where every pod can talk to every other pod without Network Address Translation (NAT). It sounds elegant until you actually have to implement it across multiple hosts. Add the overhead of packet encapsulation, and you have a recipe for latency that can cripple a high-traffic application.

Furthermore, the legal landscape shifted violently this October. The European Court of Justice invalidated the Safe Harbor agreement (Schrems I). If you are piping traffic through US-controlled clouds, you are now operating in a legal grey zone. Latency isn't your only enemy; data sovereignty is.

The Overlay Problem: VXLAN vs. Raw Performance

To achieve the flat IP space across nodes, most of us are using Flannel from CoreOS. It creates an overlay network, usually encapsulating packets in UDP (VXLAN). It works, but it’s not free. Every packet sent between pods on different nodes requires CPU cycles for encapsulation and decapsulation.

I recently audited a cluster for a client in Oslo where their API response times were erratic. The culprit wasn't their Go code; it was a noisy neighbor on a budget VPS stealing CPU cycles, causing the VXLAN processing to choke.

Pro Tip: Never run Kubernetes on OpenVZ or container-based virtualization. You need the kernel isolation of KVM to handle the tun/tap devices and bridge manipulation without fighting the host OS.

Configuring Flannel in Etcd

Before you even start the flanneld daemon, you need to push your network configuration into etcd. If you mess this up, your Docker bridge (docker0) will get an IP range that conflicts with your physical network, and nothing will route.

Once that key is set, your systemd unit for flannel needs to read this, allocate a subnet for the specific host, and write out the environment variables for Docker.

The Shift to Iptables Proxy Mode

In Kubernetes 1.0, kube-proxy ran in "userspace" mode. It was a bottleneck. Traffic had to go from the kernel to the kube-proxy process and back to the kernel. It was slow and consumed memory.

As of Kubernetes 1.1, the iptables mode is graduating to beta/default. This is a massive performance win. kube-proxy simply programs the Linux kernel's iptables NAT table to redirect traffic. The packets stay in kernel-space. This reduces latency significantly, but it makes debugging a nightmare if you don't know your way around netfilter.

Here is what happens when you create a Service. kube-proxy writes rules like this:

# Check your NAT table rules
sudo iptables -t nat -L KUBE-SERVICES -n

Chain KUBE-SERVICES (2 references)
target     prot opt source               destination         
KUBE-SVC-6N4S3G     tcp  --  0.0.0.0/0            10.0.0.148           /* default/my-nginx-service: cluster IP */ tcp dpt:80

If you see the rule there but traffic fails, check your forwarding flags. I've lost hours of my life because net.ipv4.ip_forward was set to 0.

Optimizing the Node for Throughput

Since we are relying on the kernel to do the heavy lifting for both VXLAN (Flannel) and DNAT (Services), the underlying hardware matters more than ever. Standard spinning rust (HDD) is fine for logs, but if your nodes are swapping, your network latency spikes.

We configure our hosts with the following sysctl parameters to handle the connection tracking load generated by K8s services:

# /etc/sysctl.d/k8s.conf
net.ipv4.ip_forward = 1
net.bridge.bridge-nf-call-iptables = 1
net.netfilter.nf_conntrack_max = 131072

If you hit the nf_conntrack_max limit, the kernel will start dropping packets silently. This is often what happens during a DDoS attack or a sudden traffic spike.

Why Hosting Location Matters (Now More Than Ever)

With the Safe Harbor ruling in October, Norwegian businesses are scrambling. Using a US-based provider for data storage is now a compliance risk. You need your data physically located in Norway or the EEA, under a jurisdiction compatible with the Datatilsynet's requirements.

Performance Note: If your users are in Oslo or Bergen, routing traffic to Frankfurt adds 20-30ms of latency. Routing to Virginia adds 90ms+. On CoolVDS, our peering at NIX (Norwegian Internet Exchange) keeps local latency under 5ms.

The CoolVDS Implementation

We don't oversell our nodes. When you deploy a KVM instance on CoolVDS, you get dedicated CPU time. This is critical for Kubernetes. In a shared environment where CPU is stolen (Steal Time > 2%), the context switching kills the performance of the etcd convergence and the flannel encapsulation.

Our infrastructure uses local NVMe storage. In 2015, this is still a luxury for many providers, but for a database-heavy K8s cluster (think MySQL pods with PersistentVolumes), the I/O throughput of NVMe is the difference between a snappy site and a timeout.

Final Thoughts

Kubernetes is the future of infrastructure, but the 1.1 release is still bleeding edge. You have to understand the layers below it. The overlay network adds complexity, and the reliance on iptables demands a robust kernel configuration.

Don't compound the difficulty by running on unstable hardware or legally risky jurisdictions. Control your network, control your data.

Ready to build a compliant, high-performance cluster? Deploy a developer-grade KVM instance on CoolVDS today and get 1GB/s internal networking for your overlay.