Implementing Network Policies For Kubernetes

Implementing Network Policies for Kubernetes

Overview

Kubernetes NetworkPolicies provide pod-level network segmentation by defining ingress and egress rules that control traffic flow between pods, namespaces, and external endpoints. Combined with CNI plugins like Calico or Cilium, network policies enforce zero-trust microsegmentation to prevent lateral movement within the cluster.

When to Use

• When deploying or configuring implementing network policies for kubernetes capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Common Misconfigurations & Verification

• No default-deny: NetworkPolicy is default-allow and additive. Without a podSelector: {} deny-all for Ingress and Egress per namespace, the allow rules below don't actually segment anything.
• CNI doesn't implement NetworkPolicy: plain flannel ignores NetworkPolicy objects, so kubectl apply succeeds but nothing is enforced. Confirm the CNI supports it (Calico, Cilium, Antrea) and that policies take effect.
• Egress deny without DNS allow: dropping egress without the kube-system UDP/TCP 53 rule breaks name resolution; operators then strip the egress policy, losing all egress control.
• Cloud metadata not blocked: an egress policy that doesn't except 169.254.169.254/32 (AWS/GCP) and 169.254.169.254/100.100.100.200 (Azure) leaves the SSRF→IMDS credential path open.
• Selector scope mistakes: podSelector: {} applies to the whole namespace (often intended for deny-all); a labelled selector with a typo silently matches no pods.
• Verify enforcement, not just creation: kubectl get netpol -n production, then a blocked test: kubectl run t --image=busybox --restart=Never -n production -- wget -qO- --timeout=2 http://db-svc:5432 must time out, while a correctly-labelled app=frontend pod reaches the backend.

Prerequisites

• Kubernetes cluster with NetworkPolicy-supporting CNI (Calico, Cilium, Antrea)
• kubectl configured with admin access
• Understanding of pod labels and selectors

Workflow

Step 1: Default Deny All Traffic

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

Step 2: Allow DNS Egress (Required for Service Discovery)

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-dns
  namespace: production
spec:
  podSelector: {}
  policyTypes:
    - Egress
  egress:
    - to:
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: kube-system
      ports:
        - protocol: UDP
          port: 53
        - protocol: TCP
          port: 53

Step 3: Application-Specific Policies

# Allow frontend to reach backend only
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: backend-allow-frontend
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
    - Ingress
  ingress:
    - from:
        - podSelector:
            matchLabels:
              app: frontend
      ports:
        - protocol: TCP
          port: 8080
---
# Allow backend to reach database only
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: database-allow-backend
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: database
  policyTypes:
    - Ingress
  ingress:
    - from:
        - podSelector:
            matchLabels:
              app: backend
      ports:
        - protocol: TCP
          port: 5432

Step 4: Cross-Namespace Policies

# Allow monitoring namespace to scrape metrics
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-monitoring-scrape
  namespace: production
spec:
  podSelector: {}
  policyTypes:
    - Ingress
  ingress:
    - from:
        - namespaceSelector:
            matchLabels:
              purpose: monitoring
      ports:
        - protocol: TCP
          port: 9090  # Prometheus metrics port

Step 5: Egress Restrictions

# Restrict egress to specific external services
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: restrict-egress
  namespace: production
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
    - Egress
  egress:
    - to:
        - podSelector:
            matchLabels:
              app: database
      ports:
        - protocol: TCP
          port: 5432
    - to:  # Allow external API
        - ipBlock:
            cidr: 203.0.113.0/24
      ports:
        - protocol: TCP
          port: 443
    - to:  # DNS
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: kube-system
      ports:
        - protocol: UDP
          port: 53

Step 6: Block Cloud Metadata Access

# Prevent SSRF to cloud metadata service
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: block-metadata
  namespace: production
spec:
  podSelector: {}
  policyTypes:
    - Egress
  egress:
    - to:
        - ipBlock:
            cidr: 0.0.0.0/0
            except:
              - 169.254.169.254/32  # AWS/GCP metadata
              - 100.100.100.200/32  # Azure metadata

Validation Commands

# Verify policies are applied
kubectl get networkpolicies -n production

# Test connectivity (should be blocked)
kubectl run test-pod --image=busybox --restart=Never -n production -- wget -qO- --timeout=2 http://database-service:5432
# Expected: timeout (blocked by policy)

# Test allowed traffic
kubectl run frontend-test --image=busybox --labels=app=frontend --restart=Never -n production -- wget -qO- --timeout=2 http://backend-service:8080
# Expected: connection succeeds

References

• Kubernetes Network Policies
• Calico Network Policies
• Cilium Network Policies
• Network Policy Editor