Network segmentation has long been a cornerstone of enterprise security, but traditional approaches struggle in containerized environments. Kubernetes Network Policies represent a paradigm shift in how organizations implement zero-trust principles at the container orchestration layer.
The Challenge of Container Networking
Kubernetes abstracts underlying infrastructure through an overlay network, creating both opportunities and challenges for security teams. By default, all pods can communicate with all other pods within a cluster—a permissive posture that violates zero-trust principles. Organizations deploying Kubernetes must implement explicit deny-by-default policies, allowing only necessary traffic between workloads.
Network Policies Explained
A Kubernetes NetworkPolicy is a cluster-scoped resource that defines ingress and egress rules for pod-to-pod communication. Unlike traditional firewall rules operating at the IP layer, Network Policies work at the pod level, leveraging Kubernetes labels for dynamic rule matching.
Key Components:
- Pod Selector: Identifies which pods the policy applies to using label selectors
- Policy Types: Specifies whether the policy controls ingress, egress, or both
- Rules: Defines allowed traffic patterns with source/destination selectors and ports
Implementation Patterns
Deny-All Default Policy
The foundation of zero-trust is explicit denial. Deploy a default deny policy to your namespace:
yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
name: default-deny-ingress
spec:
podSelector: {}
policyTypes:
- Ingress
This single policy denies all inbound traffic to all pods. Subsequently, you whitelist only necessary connections.
Microservice-to-Microservice Communication
For applications using service-to-service communication, allow only required connections:
yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
name: allow-api-calls
spec:
podSelector:
matchLabels:
app: backend-service
policyTypes:
- Ingress
ingress:
- from:
- podSelector:
matchLabels:
app: frontend-service
ports:
- protocol: TCP
port: 8080
Performance and Observability Considerations
Network Policies add computational overhead, particularly on nodes with thousands of pods. Container network interface (CNI) implementations vary in their efficiency:
- Calico: Uses eBPF for kernel-level enforcement (lowest latency)
- Cilium: Native eBPF implementation with advanced observability
- Weave: Software-based filtering (higher CPU overhead)
Choose your CNI based on performance requirements and observability needs. Cilium, in particular, provides Hubble—native Network Policy visualization and troubleshooting.
Common Pitfalls
1. Forgetting Egress Rules
Many teams implement strict ingress policies but leave egress unrestricted. This allows compromised containers to exfiltrate data or perform reconnaissance. Always apply both ingress and egress controls.
2. DNS Traffic Exceptions
Applications need DNS resolution. Ensure your policies allow UDP port 53 to kube-dns or your external DNS:
yaml
ingress:
- to:
- namespaceSelector:
matchLabels:
name: kube-system
ports:
- protocol: UDP
port: 53
3. Insufficient Label Consistency
Network Policies depend on accurate labeling. Inconsistent or missing labels lead to rules that don't match intended targets. Enforce labeling standards through admission controllers.
Monitoring and Validation
Network Policies are complex—validation is critical:
- Use tools like Cilium CLI or Calico's calicoctl to test policies
- Enable audit logging for denied connections
- Implement network monitoring agents (Cilium Hubble, Calico Enterprise) for real-time visibility
Future Directions
Kubernetes is evolving its network security model. Kubernetes 1.30+ introduces enhanced policy semantics, including:
- Cluster-scoped policies for cross-namespace enforcement
- Traffic policy matching based on protocol-specific attributes
- Integration with service mesh for application-layer policies
Sources
Kubernetes Official: Network Policies