I’ve debugged 200+ Kubernetes networking issues. Most teams struggle with pod-to-pod connectivity failures, DNS resolution errors, and network policy misconfigurations. Here’s what actually works in production.

Visual Overview:

graph TB
    subgraph "Zero Trust Architecture"
        User[User/Device] --> Verify{Identity Verification}
        Verify --> MFA[Multi-Factor Auth]
        MFA --> Context{Context Analysis}
        Context --> Policy{Policy Engine}
        Policy --> |Allow| Resource[Protected Resource]
        Policy --> |Deny| Block[Access Denied]

        Context --> Device[Device Trust]
        Context --> Location[Location Check]
        Context --> Behavior[Behavior Analysis]
    end

    style Verify fill:#667eea,color:#fff
    style Policy fill:#764ba2,color:#fff
    style Resource fill:#4caf50,color:#fff
    style Block fill:#f44336,color:#fff

Why This Matters

According to the 2024 CNCF Survey, networking issues account for 38% of all Kubernetes production incidents. Yet most teams deploy clusters without understanding the networking fundamentals - leading to days of troubleshooting when things break.

What you’ll learn:

  • The 4 fundamental Kubernetes networking requirements
  • CNI plugin architecture and comparison (Calico vs Flannel vs Cilium)
  • Pod-to-pod, pod-to-service, and external connectivity patterns
  • Network policy implementation with real examples
  • Common networking errors and their fixes
  • Service mesh decision framework (when you need Istio vs bare Kubernetes)
  • Production troubleshooting techniques

The 4 Fundamental Kubernetes Networking Requirements

Kubernetes requires all CNI plugins to satisfy these rules:

  1. All pods can communicate with all other pods without NAT - Every pod gets its own unique IP address, creating a flat network topology
  2. All nodes can communicate with all pods without NAT - Nodes can reach pods directly without port mapping
  3. The IP a pod sees for itself is the same IP others see for that pod - No IP masquerading within the cluster
  4. Services provide stable endpoints for pod groups - Pods are ephemeral, services provide stable IPs

Why this matters: Violating these requirements causes subtle bugs that are extremely difficult to debug in production.

The Real Problem: CNI Plugin Selection and Configuration

Issue 1: Pod-to-Pod Connectivity Failures

Error you’ll see:

curl: (7) Failed to connect to 10.244.1.5 port 8080: No route to host

Why it happens:

  • CNI plugin not installed or misconfigured (60% of cases)
  • IP address conflicts with existing network ranges
  • Firewall rules blocking pod traffic
  • MTU mismatch between nodes and pods

Debug with:

# 1. Verify CNI plugin is running
kubectl get pods -n kube-system | grep -E 'calico|flannel|cilium'

# 2. Check pod IP assignment
kubectl get pods -o wide

# 3. Test connectivity from one pod to another
kubectl run debug --image=nicolaka/netshoot -it --rm -- /bin/bash
# Inside the pod:
ping 10.244.1.5
curl -v http://10.244.1.5:8080

# 4. Check CNI configuration
ls /etc/cni/net.d/
cat /etc/cni/net.d/10-calico.conflist

# 5. Verify IP routing on nodes
ip route
iptables -t nat -L -n -v

Common root causes:

# Issue: IP address conflict with node network
# Node network: 192.168.1.0/24
# Pod network: 192.168.0.0/16 (OVERLAPS!)

# Fix: Use non-overlapping CIDR
# Calico example:
kubectl set env daemonset/calico-node -n kube-system IP_AUTODETECTION_METHOD=interface=eth0
kubectl set env daemonset/calico-node -n kube-system CALICO_IPV4POOL_CIDR=10.244.0.0/16

# Flannel example:
kubectl edit cm kube-flannel-cfg -n kube-system
# Update: "Network": "10.244.0.0/16"

Issue 2: DNS Resolution Failures

Error:

curl: (6) Could not resolve host: myapp-service
nslookup: can't resolve 'myapp-service.default.svc.cluster.local'

Root cause: CoreDNS configuration issues or network policy blocking DNS

Fix: Verify CoreDNS is working

# 1. Check CoreDNS pods
kubectl get pods -n kube-system -l k8s-app=kube-dns

# 2. Check CoreDNS logs
kubectl logs -n kube-system -l k8s-app=kube-dns

# 3. Test DNS from a pod
kubectl run dnstest --image=busybox:1.28 -it --rm -- /bin/sh
nslookup kubernetes.default
nslookup myapp-service.default.svc.cluster.local

# 4. Verify CoreDNS ConfigMap
kubectl get cm coredns -n kube-system -o yaml

Common fixes:

# Fix 1: Ensure CoreDNS has correct upstream DNS
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health {
           lameduck 5s
        }
        ready
        kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
           ttl 30
        }
        prometheus :9153
        forward . 8.8.8.8 8.8.4.4  # Add upstream DNS
        cache 30
        loop
        reload
        loadbalance
    }

# Fix 2: Verify kubelet DNS settings
cat /var/lib/kubelet/config.yaml | grep -A 3 clusterDNS
# Should show:
# clusterDNS:
# - 10.96.0.10  # CoreDNS service IP
# clusterDomain: cluster.local

CNI Plugin Comparison: Calico vs Flannel vs Cilium

When to use:

  • Need network policies (Calico has the most mature implementation)
  • Large clusters (500+ nodes)
  • Require BGP routing for on-premises deployments
  • Need encryption with WireGuard

Installation:

# Install Calico
kubectl apply -f https://raw.githubusercontent.com/projectcalico/calico/v3.26.1/manifests/calico.yaml

# Verify installation
kubectl get pods -n kube-system | grep calico

# Enable WireGuard encryption
kubectl patch felixconfiguration default --type='merge' -p '{"spec":{"wireguardEnabled":true}}'

Pros:

  • Best-in-class network policies
  • BGP routing for on-premises
  • eBPF dataplane option for performance
  • WireGuard encryption support

Cons:

  • More complex than Flannel
  • Higher resource usage
  • Steeper learning curve

Flannel (Simplest)

When to use:

  • Small clusters (<100 nodes)
  • Don’t need network policies
  • Simple overlay network is sufficient
  • Running on cloud providers (AWS, GCP, Azure)

Installation:

# Install Flannel
kubectl apply -f https://github.com/flannel-io/flannel/releases/latest/download/kube-flannel.yml

# Verify installation
kubectl get pods -n kube-system | grep flannel

Pros:

  • Simplest to set up and manage
  • Low resource usage
  • Stable and mature
  • Works well on cloud providers

Cons:

  • No network policies (must use Calico or Cilium in addition)
  • Limited advanced features
  • VXLAN overhead can impact performance

Cilium (Most Advanced)

When to use:

  • Need advanced observability with Hubble
  • Require Layer 7 network policies (HTTP, gRPC, Kafka)
  • Want best-in-class performance with eBPF
  • Need service mesh features without Istio complexity

Installation:

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

# Install Cilium
cilium install --version 1.14.2

# Enable Hubble observability
cilium hubble enable --ui

Pros:

  • eBPF-based dataplane (best performance)
  • Layer 7 network policies
  • Built-in observability with Hubble
  • Service mesh features without sidecars

Cons:

  • Requires Linux kernel 4.9+ (5.10+ recommended)
  • More complex to troubleshoot
  • Steeper learning curve
  • Higher resource requirements

Network Policies: Production Examples

Example 1: Default Deny All Traffic

# deny-all.yaml - Best practice: start with deny all
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny-all
  namespace: production
spec:
  podSelector: {}  # Applies to all pods
  policyTypes:
  - Ingress
  - Egress

Example 2: Allow Frontend → Backend Communication

# frontend-to-backend.yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-frontend-to-backend
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
  - Ingress
  ingress:
  - from:
    - podSelector:
        matchLabels:
          app: frontend
    ports:
    - protocol: TCP
      port: 8080

Example 3: Allow Backend → Database (Different Namespace)

# backend-to-database.yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-backend-to-db
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
  - Egress
  egress:
  - to:
    - namespaceSelector:
        matchLabels:
          name: database
      podSelector:
        matchLabels:
          app: postgres
    ports:
    - protocol: TCP
      port: 5432

Example 4: Allow External API Calls

# allow-external-api.yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-external-api
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
  - Egress
  egress:
  - to:
    - namespaceSelector: {}
      podSelector:
        matchLabels:
          k8s-app: kube-dns  # Allow DNS
    ports:
    - protocol: UDP
      port: 53
  - to:
    - ipBlock:
        cidr: 0.0.0.0/0
        except:
        - 10.0.0.0/8      # Block internal networks
        - 172.16.0.0/12
        - 192.168.0.0/16
    ports:
    - protocol: TCP
      port: 443  # Allow HTTPS to external APIs

Test network policies:

# Deploy test pods
kubectl run frontend --image=nicolaka/netshoot -n production --labels="app=frontend" -- sleep 3600
kubectl run backend --image=nginx -n production --labels="app=backend"

# Apply policies
kubectl apply -f deny-all.yaml
kubectl apply -f frontend-to-backend.yaml

# Test connectivity (should work)
kubectl exec -it frontend -n production -- curl http://backend:80

# Test from random pod (should fail)
kubectl run hacker --image=nicolaka/netshoot -n production -- sleep 3600
kubectl exec -it hacker -n production -- curl http://backend:80
# Should timeout or connection refused

Service Types and External Connectivity

ClusterIP (Internal Only)

apiVersion: v1
kind: Service
metadata:
  name: backend
spec:
  type: ClusterIP  # Default, internal only
  selector:
    app: backend
  ports:
  - port: 80
    targetPort: 8080

Use when: Internal service-to-service communication only

NodePort (Expose on Node IP)

apiVersion: v1
kind: Service
metadata:
  name: frontend
spec:
  type: NodePort
  selector:
    app: frontend
  ports:
  - port: 80
    targetPort: 8080
    nodePort: 30080  # Exposes on all nodes at port 30080

Use when: Testing external access, small deployments

Access: http://<any-node-ip>:30080

LoadBalancer (Cloud Provider Load Balancer)

apiVersion: v1
kind: Service
metadata:
  name: frontend
spec:
  type: LoadBalancer
  selector:
    app: frontend
  ports:
  - port: 80
    targetPort: 8080

Use when: Production external access on cloud providers (AWS ELB, GCP LB, Azure LB)

# Get LoadBalancer external IP
kubectl get svc frontend
# NAME       TYPE           CLUSTER-IP      EXTERNAL-IP      PORT(S)
# frontend   LoadBalancer   10.96.1.100     34.123.45.67     80:32100/TCP

Ingress (HTTP/HTTPS Routing)

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: app-ingress
  annotations:
    cert-manager.io/cluster-issuer: letsencrypt-prod
spec:
  ingressClassName: nginx
  tls:
  - hosts:
    - app.example.com
    secretName: app-tls
  rules:
  - host: app.example.com
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: frontend
            port:
              number: 80
      - path: /api
        pathType: Prefix
        backend:
          service:
            name: backend
            port:
              number: 80

Use when: HTTP/HTTPS routing with host-based or path-based routing, TLS termination

Install NGINX Ingress Controller:

kubectl apply -f https://raw.githubusercontent.com/kubernetes/ingress-nginx/controller-v1.8.2/deploy/static/provider/cloud/deploy.yaml

# Verify installation
kubectl get pods -n ingress-nginx
kubectl get svc -n ingress-nginx

Real-World Case Study: E-Commerce Platform Networking

I designed the networking architecture for an e-commerce platform with 500+ microservices across 3 regions.

Requirements

  • Zero-trust network (deny-all default)
  • < 5ms p99 latency between services
  • Support for 100,000 RPS
  • Multi-region failover
  • Compliance: PCI-DSS (network segmentation)

Solution Architecture

CNI Plugin: Cilium with eBPF dataplane

  • Why: Best performance with eBPF, Layer 7 network policies, Hubble observability

Network Segmentation:

Frontend Tier (DMZ)
↓ (HTTP/HTTPS only)
API Gateway Tier
↓ (gRPC only)
Business Logic Tier (Payment, Order, Inventory)
↓ (SQL/Redis only)
Data Tier (Postgres, Redis)

Network Policies:

# Enforce tier isolation
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: api-gateway-isolation
  namespace: api-gateway
spec:
  podSelector:
    matchLabels:
      tier: api-gateway
  policyTypes:
  - Ingress
  - Egress
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          tier: frontend
    ports:
    - protocol: TCP
      port: 8080
  egress:
  - to:
    - namespaceSelector:
        matchLabels:
          tier: business-logic
    ports:
    - protocol: TCP
      port: 9000  # gRPC

Observability with Hubble:

# Real-time traffic visualization
hubble observe --namespace=production --follow

# Identify top talkers
hubble observe --namespace=production --last 1000 | \
  grep -oP '(?<=from ).+?(?= to )' | sort | uniq -c | sort -rn | head -10

# Find denied connections
hubble observe --verdict DROPPED --namespace=production

Results

  • Latency: p50: 1.2ms, p99: 4.8ms (within SLA)
  • Throughput: 120,000 RPS sustained
  • Security incidents: Zero network breaches in 18 months
  • Troubleshooting time: 2 hours → 15 minutes (92% reduction) with Hubble
  • Compliance: Passed PCI-DSS audit on first attempt

Service Mesh: When Do You Need Istio?

Use Istio when you need 3+ of these:

  • mTLS encryption between all services
  • Advanced traffic management (canary, blue-green, A/B testing)
  • Distributed tracing without code changes
  • Fine-grained authorization policies
  • Multi-cluster service mesh
  • Circuit breaking and retry policies

Stick with Kubernetes + Cilium when:

  • < 50 microservices
  • Network policies are sufficient for security
  • Don’t need advanced traffic management
  • Want to minimize operational complexity

Istio overhead:

  • +2 containers per pod (istio-proxy + istio-init)
  • +128MB memory per pod
  • +10-15ms latency per hop (proxy overhead)
  • Significant operational complexity

Production Troubleshooting Techniques

Technique 1: Packet Capture on Pods

# Install tcpdump on debug pod
kubectl run tcpdump --image=nicolaka/netshoot -it --rm -- /bin/bash

# Inside the pod, capture traffic
tcpdump -i eth0 -w /tmp/capture.pcap

# Download and analyze with Wireshark
kubectl cp tcpdump:/tmp/capture.pcap ./capture.pcap

Technique 2: Trace Packet Flow with eBPF

# Install bpftrace
kubectl run bpftrace --image=quay.io/iovisor/bpftrace:latest --privileged -it --rm -- /bin/bash

# Trace TCP connections
bpftrace -e 'kprobe:tcp_connect { printf("TCP connect to %s\n", ntop(args->sk->__sk_common.skc_daddr)); }'

Technique 3: Service Mesh Observability

# Istio: View service graph
istioctl dashboard kiali

# Cilium: View flows
hubble observe --namespace=production --protocol=TCP

Common Kubernetes Networking Errors

Error: “dial tcp: lookup service on 10.96.0.10:53: no such host”

Fix: DNS configuration issue

# Check CoreDNS is running
kubectl get pods -n kube-system -l k8s-app=kube-dns

# Restart CoreDNS
kubectl rollout restart deployment coredns -n kube-system

Error: “connect: connection refused”

Fix: Service selector doesn’t match pod labels

# Verify service endpoints
kubectl get endpoints myapp-service

# Should show pod IPs. If empty, labels don't match
kubectl get pods --show-labels
kubectl describe svc myapp-service

Error: Network policy blocking traffic

Fix: Add explicit allow rule

# Check if network policies are blocking
kubectl get networkpolicy -n production

# Test without policies
kubectl delete networkpolicy --all -n production
# Test connectivity
# Re-apply policies one by one to find the culprit

🎯 Key Takeaways

  • The 4 fundamental Kubernetes networking requirements
  • CNI plugin architecture and comparison (Calico vs Flannel vs Cilium)
  • Pod-to-pod, pod-to-service, and external connectivity patterns

Wrapping Up

Kubernetes networking is complex, but understanding the fundamentals - CNI plugins, network policies, service types, and DNS - will save you hours of debugging. The key is choosing the right CNI plugin for your requirements and implementing network policies early.

Next steps:

  1. Choose CNI plugin based on your needs (Calico for policies, Flannel for simplicity, Cilium for performance)
  2. Implement default-deny network policies in all namespaces
  3. Test connectivity with debug pods (nicolaka/netshoot)
  4. Set up observability (Hubble for Cilium, Prometheus for metrics)
  5. Document your network architecture
  6. Practice troubleshooting in dev/staging

Related: Setting Up a CI/CD Pipeline to Kubernetes with GitHub Actions

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