undermybelt 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
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