How to restrict interaction between applications running in different namespaces?

Network Policies: Define which sets of pods are allowed to interact with each other and with external endpoints. They are applied at the Namespace, Pod, or Egress level.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: deny-cross-namespace
  namespace: default # Namespace to which the policy applies
spec:
  podSelector: {} # Apply to all pods in the 'default' Namespace
  policyTypes:
    - Ingress
    - Egress
  ingress:
    - from:
        - podSelector: {} # Allow incoming traffic from pods in the same Namespace
  // No egress section, which by default denies all outgoing traffic
  // Except for cases explicitly allowed.

Service Meshes (e.g., Istio, Linkerd): Introduce sidecar containers to pods and manage network traffic at the application level. They enable implementing security policies, including restricting interactions between Namespaces.

• Mutual TLS (mTLS): Encrypts and authenticates all traffic between services.
• Authorization Policies: Define which services can interact with each other based on identity.

Namespace-Specific SecurityContextConstraints (SCC) in OpenShift: Limit the capabilities of pods in specific Namespaces, affecting network capabilities.

Separate Cluster for Different Security Zones: The most strict method, but also the most costly. Fully isolates applications running in different clusters.

Firewall Rules: Configuration of rules at the network infrastructure level (e.g., AWS Security Groups, GCP Firewall Rules) that allow or deny traffic between subnets where Worker Nodes of different Namespaces are located.

Network Segmentation at Hypervisor Level: Used when working on virtual machines, where each Namespace or group of Namespaces is placed in a separate subnet with corresponding routing and firewall rules.