Container Network Interface (CNI) is a specification and set of libraries for configuring network interfaces in Linux containers. This guide explores different CNI plugins, their architectures, and how to choose the right one for your environment.

What is CNI?

CNI (Container Network Interface) is:

A specification for configuring network interfaces in Linux containers
A set of plugins that implement the specification
A library for writing plugins
A simple contract between container runtime and network implementation

Basic CNI Flow

Container Runtime -> CNI Plugin -> Network Configuration

Popular CNI Plugins

1. Calico

Overview

Layer 3 networking
Network policy enforcement
BGP routing support
High performance

Installation

# Install Calico operator
kubectl create -f https://raw.githubusercontent.com/projectcalico/calico/v3.25.0/manifests/tigera-operator.yaml

# Install Calico custom resources
kubectl create -f https://raw.githubusercontent.com/projectcalico/calico/v3.25.0/manifests/custom-resources.yaml

Key Features

BGP routing
Network policies
IPAM
Cross-subnet overlay

2. Flannel

Overview

Simple overlay network
Easy to set up
VXLAN encapsulation
Focused on networking only

Installation

kubectl apply -f https://raw.githubusercontent.com/flannel-io/flannel/master/Documentation/kube-flannel.yml

Key Features

Layer 2 overlay
Multiple backend support
Simple architecture
Low overhead

3. Cilium

Overview

eBPF-based networking
Advanced security features
High performance
Observability

Installation

# Install Cilium CLI
curl -L --remote-name-all https://github.com/cilium/cilium-cli/releases/latest/download/cilium-linux-amd64.tar.gz
sudo tar xzvfC cilium-linux-amd64.tar.gz /usr/local/bin

# Install Cilium
cilium install

Key Features

eBPF-based networking
Layer 7 policy enforcement
Kubernetes services implementation
Advanced observability

4. Weave Net

Overview

Multi-host networking
Service discovery
Network policy support
Encryption support

Installation

kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 | tr -d '\n')"

Key Features

Automatic network configuration
Encryption
Multicast support
Service discovery

CNI Comparison

Feature Matrix

Feature	Calico	Flannel	Cilium	Weave Net
Network Policy	✓	-	✓	✓
Encryption	✓	-	✓	✓
BGP Support	✓	-	-	-
Layer 7 Policy	-	-	✓	-
Performance	High	Medium	Very High	Medium
Complexity	Medium	Low	High	Medium

Performance Characteristics

Network Latency

Cilium (eBPF) < Calico (Native) < Weave < Flannel (VXLAN)

Memory Usage

Flannel < Calico < Weave < Cilium

Deployment Scenarios

1. On-Premises Deployment

Best choices:

Calico: For BGP integration
Cilium: For high performance

2. Cloud Provider

Best choices:

Flannel: For simplicity
Calico: For network policy

3. Edge/IoT

Best choices:

Flannel: For resource constraints
Calico: For security requirements

Advanced Configuration

Calico BGP Configuration

apiVersion: projectcalico.org/v3
kind: BGPConfiguration
metadata:
  name: default
spec:
  logSeverityScreen: Info
  nodeToNodeMeshEnabled: true
  asNumber: 63400

Cilium Network Policy

apiVersion: "cilium.io/v2"
kind: CiliumNetworkPolicy
metadata:
  name: "http-policy"
spec:
  endpointSelector:
    matchLabels:
      app: myapp
  ingress:
  - fromEndpoints:
    - matchLabels:
        role: frontend
    toPorts:
    - ports:
      - port: "80"
        protocol: TCP
      rules:
        http:
        - method: GET
          path: "/api/v1"

Flannel Backend Configuration

apiVersion: v1
kind: ConfigMap
metadata:
  name: kube-flannel-cfg
data:
  net-conf.json: |
    {
      "Network": "10.244.0.0/16",
      "Backend": {
        "Type": "vxlan"
      }
    }

Troubleshooting

Common Issues

1. Pod Network Connectivity

# Check CNI pods
kubectl get pods -n kube-system -l k8s-app=calico-node

# Check CNI logs
kubectl logs -n kube-system calico-node-xxxxx -c calico-node

# Test pod connectivity
kubectl exec -it pod-name -- ping other-pod-ip

2. Network Policy Issues

# Verify policy
kubectl describe networkpolicy policy-name

# Check CNI policy logs
kubectl logs -n kube-system calico-node-xxxxx -c calico-node | grep policy

3. Performance Issues

# Check CNI metrics
kubectl -n kube-system exec -it cilium-xxxxx -- cilium metrics

Best Practices

Selection Criteria
- Network requirements
- Security needs
- Performance demands
- Operational complexity
Deployment
- Use operator patterns
- Configure IPAM properly
- Enable monitoring
- Regular updates
Maintenance
- Monitor performance
- Regular updates
- Backup configurations
- Document customizations

Conclusion

Choosing the right CNI plugin depends on your specific requirements:

Use Calico for strong network policy and BGP support
Use Cilium for high performance and advanced security
Use Flannel for simplicity and basic networking
Use Weave Net for encryption and multicast needs

For more information, check out: