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Network Policies

Required knowledge for the CKS certification.

Network Policies in Kubernetes are a fundamental security mechanism used to control traffic flow between pods and external services. They provide network segmentation and help enforce security policies within the cluster.

How Network Policies Work​

Network Policies use selectors and rules to define allowed traffic:

  1. Pod Selector: Specifies the pods the policy applies to.
  2. Ingress Rules: Define allowed incoming traffic to selected pods.
  3. Egress Rules: Define allowed outgoing traffic from selected pods.

By default, Kubernetes allows all traffic between pods unless a Network Policy is defined to restrict it.

1. Enforce Default Deny-All Policies​

To prevent unrestricted communication between pods, enforce a deny-all policy by default.

Secure Default Deny Policy​

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny
  namespace: default
spec:
  podSelector: {}
  policyTypes:
    - Ingress
    - Egress

Why It Matters​

  • Blocks all traffic by default, enforcing a least-privilege model.
  • Requires explicit allow rules for necessary communication.

2. Restrict Ingress Traffic to Specific Pods​

Allow ingress traffic only from trusted sources.

Secure Ingress Policy Example​

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-frontend-to-backend
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
    - Ingress
  ingress:
    - from:
        - podSelector:
            matchLabels:
              app: frontend
      ports:
        - protocol: TCP
          port: 80

Why It Matters​

  • Prevents unauthorized pods from accessing sensitive services.
  • Allows only traffic from the specified frontend application.

3. Restrict Egress Traffic to External Endpoints​

Limit pod communication with external networks to reduce attack exposure.

Secure Egress Policy Example​

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: restrict-egress
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
    - Egress
  egress:
    - to:
        - ipBlock:
            cidr: 192.168.1.0/24
      ports:
        - protocol: TCP
          port: 443

Why It Matters​

  • Prevents unauthorized outbound connections to untrusted networks.
  • Reduces the risk of data exfiltration.

4. Protect the Kubernetes API Server​

Restrict access to the Kubernetes API server to prevent unauthorized requests.

Secure API Server Access Policy​

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: restrict-api-access
  namespace: kube-system
spec:
  podSelector:
    matchLabels:
      component: kube-apiserver
  policyTypes:
    - Ingress
  ingress:
    - from:
        - ipBlock:
            cidr: 10.0.0.0/16

Why It Matters​

  • Limits API access to trusted networks.
  • Prevents external reconnaissance and brute-force attacks.

5. Enforce Namespace Isolation​

Prevent pods in one namespace from communicating with pods in another namespace.

Secure Namespace Isolation Policy​

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: namespace-isolation
  namespace: default
spec:
  podSelector: {}
  policyTypes:
    - Ingress
  ingress: []

Why It Matters​

  • Prevents cross-namespace attacks and lateral movement.
  • Ensures workload segregation for multi-tenant clusters.

Conclusion​

Network Policies provide critical security controls for traffic segmentation, access control, and workload isolation in Kubernetes. By defaulting to deny-all, restricting ingress and egress, securing the API server, and enforcing namespace isolation, administrators can prevent unauthorized access and minimize the attack surface.

References​

This article is based on information from the following official sources:

  1. Network Policies - Kubernetes Documentation
  2. Declare Network Policy - Kubernetes Documentation
  3. CIS Kubernetes Benchmark - Center for Internet Security