Cilium: Next-Gen Networking, Security & Observability for Kubernetes

Kubernetes has become the backbone of container orchestration. But when it comes to networking, Kubernetes only defines how things should work—it doesn’t provide the actual implementation. This is where CNI (Container Network Interface) plugins like Cilium, Flannel, or Calico come in.

Among them, Cilium stands out because it’s powered by eBPF, enabling high-performance networking, advanced security, and observability.

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🌐 Kubernetes Networking Basics

• Pod-to-Pod Communication
  Every pod can talk to every other pod, no matter if they are on the same node, different nodes, or even across data centers—so long as the nodes can reach each other.

• Pods rarely talk directly
  In production, pods usually communicate via Kubernetes Services, not directly with other pods.

• Services managed by kube-proxy
  Services rely on kube-proxy, which uses Linux iptables (or ipvs) rules to route traffic.

• No built-in networking
  Kubernetes doesn’t ship with a network solution. Instead, it defines the CNI framework. Any CNI-compatible solution can be plugged in.

✅ Examples: Cilium, Calico, Flannel → provide pod networking + network policies.

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⚡ What is Cilium?

Cilium is an open-source, cloud-native networking solution built on eBPF. Unlike traditional CNIs, it’s not just about networking—it also provides security and observability.

🔹 Cilium as Networking

• Provides pod networking (CNI plugin).

• Built-in Load Balancer.

• Multi-cluster mesh for hybrid and multi-cloud.

• Ingress/Egress gateways and Gateway API support.

🔹 Cilium as Security

• L3/L4 policies → control which pod/IP can talk to which pod/IP/port.

• L7 policies → via CRDs (e.g., allow only GET requests to a service).

• Encryption → normally pod-to-pod traffic is plain text; Cilium can encrypt it natively, without requiring a service mesh.

🔹 Cilium as Observability

• Hubble (built-in observability platform).

• View network flows in real-time via CLI or GUI.

• Metrics are Prometheus-ready for Grafana dashboards.

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⚙️ How Does Cilium Work?

Cilium uses eBPF (extended Berkeley Packet Filter), which allows programs to run safely inside the Linux kernel.

Key points:

1. Operates at Layer 3–7 (network to application layer).

2. Uses identity-based policies (not just IPs).

3. Can replace kube-proxy (faster service handling).

4. Enables bandwidth control.

5. Provides better scalability because eBPF uses hash lookups instead of linear iptables rules.

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🏗️ Cilium Architecture

1. Cilium Agent

   • Runs as a DaemonSet on every node.

   • Programs eBPF into the kernel.

   • Handles pod networking, load balancing, and L3/L4 policy enforcement.

   • Captures traffic flows for Hubble.

1. Cilium Operator

   • Manages cluster-wide operations like IP address management and multi-cluster support.

2. Envoy Proxy

   • Runs as a DaemonSet.

   • Handles L7 traffic policies (e.g., HTTP filtering).

1. Hubble

   • Hubble Server runs with Cilium Agent.

   • Hubble Relay aggregates data for cluster-wide observability.

2. Kube-Proxy Replacement

   • Instead of iptables (slow, linear rules), Cilium uses eBPF (fast, hash-based).

   • Improves performance in large clusters.

3. Service Mesh (Sidecar-less)

   • Unlike Istio/Linkerd, Cilium supports service mesh without sidecars.

   • eBPF handles L3/L4, and Envoy handles L7 where needed.

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❓ Why Replace Kube-Proxy with eBPF?

• iptables/ipvs are slow → packets checked against rules sequentially.

• Large clusters = bottlenecks → updating rules requires rebuilding chains.

• eBPF is faster → rules processed via per-CPU hash tables.

• Result → low latency, high throughput, and better scalability.

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🎯 Final Thoughts

Cilium is not just another Kubernetes CNI plugin. It’s a comprehensive networking, security, and observability solution for cloud-native environments. With eBPF at its core, Cilium delivers:

✅ Faster networking
✅ Stronger security
✅ Deeper observability