Implementing Network Policies with Cilium and eBPF in Kubernetes Clusters

The Challenge of Internal Container Traffic

When running dozens or hundreds of applications inside a Kubernetes cluster (an automated container manager), a primary concern is ensuring that a compromised application cannot access neighboring services. Traditionally, the ecosystem relies on iptables rules, which function like a long queue of instructions that the operating system must read sequentially for every data packet crossing the network. In practice, this means that the more security rules you create, the slower the system becomes, creating an invisible bottleneck that drains server processing power.

To solve this scale and performance dilemma, cloud network engineering shifted its focus toward eBPF, a revolutionary technology that allows small, safe programs to run directly inside the operating system kernel without modifying its source code. In practice, eBPF acts as a set of intelligent sentinels that intercept network traffic upon arrival, applying security filters at machine speed and eliminating the need to pass through dozens of conventional routing tables.

Why Choose Cilium as Your CNI

Cilium is a networking component (known in the ecosystem as a CNI, or Container Network Interface) built from the ground up to harness the full potential of eBPF. While traditional networks need to translate virtual IP addresses into physical addresses through complex tunneling layers, Cilium routes packets directly between cluster nodes with impressive efficiency. In practice, this means the path data takes from one application to another is as short as possible, reducing latency and increasing overall infrastructure stability.

Beyond accelerating traffic, Cilium redefines how we apply security isolation through advanced network policies. Default Kubernetes rules are limited to filtering traffic based on IP addresses and TCP ports, which falls short in modern environments where services constantly change addresses. With Cilium's native support, we can create rules based on application identity, such as allowing only the payment microservice to talk to the database, regardless of which IP those components currently use.

Configuring the Environment and Installing Cilium

Before applying any restrictive rules, we must ensure the Kubernetes cluster runs a recent operating system version with proper eBPF kernel support. Installation is typically performed using Cilium's official command-line tool, cilium-cli, which automatically checks environment compatibility and applies necessary components. In practice, you need administrative cluster permissions and configured access to your Kubernetes context file to begin the process.

The standard procedure for deploying Cilium on a newly created cluster involves running a direct command in your management terminal. Below is a practical example of how to perform this installation using recommended parameters to enable full replacement of the legacy routing system:

cilium install --set ipam.mode=kubernetes --set kubeProxyReplacement=strict

After a few minutes, you can verify that all components started correctly by running the integrated diagnostic command. This command analyzes tunnel health, eBPF load, and inter-node connectivity, ensuring the data plane is ready to receive restrictive security policies.

cilium status --wait

Implementing Restrictive Network Policies in Practice

With Cilium actively operating, we can begin designing default-deny security policies, where all traffic is blocked by default and only strictly necessary communication is allowed. This approach ensures that if an attacker breaches a container, they remain isolated without the capacity to exploit vulnerabilities in neighboring services. In practice, we configure the system to require explicit consent for any pod-to-pod communication.

Below we present a complete YAML manifest defining a restrictive network policy. This example blocks all incoming traffic to the production namespace, exclusively allowing pods labeled as frontend to communicate with the backend service on port 8080:

apiVersion: