Network policies control the permissible traffic flows to Pods in your cluster. Each NetworkPolicy object targets a set of Pods and defines the IP address ranges, Kubernetes namespaces, and other Pods that the set can communicate with.

Pods that aren’t covered by a policy have no networking restrictions imposed. This is a security issue because it unnecessarily increases your attack surface. A compromised neighboring container could direct malicious traffic to sensitive Pods without being subject to any filtering.

Including all Pods in at least one NetworkPolicy is a simple but effective layer of extra protection. Policies are easy to create, too – here’s an example where only Pods labeled app-component: api can communicate with those labeled app-component: database:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: database-policy
spec:
  podSelector:
    matchLabels:
      app-component: database
  policyTypes:
    - Ingress
    - Egress
  ingress:
    - from:
        - podSelector:
            matchLabels:
              app-component: api
  egress:
    - to:
        - podSelector:
            matchLabels:
              app-component: api

Why Network Policies Are Crucial

Reducing the Attack Surface

Without network policies, Pods in your Kubernetes cluster are free to communicate with any other Pod or external IP, unless specifically restricted. This lack of control can significantly increase your attack surface. If an attacker gains control of one Pod, they could easily attempt to exploit other Pods within the cluster, especially those handling sensitive data.

Isolating Sensitive Workloads

Network policies allow you to isolate sensitive workloads from other parts of your cluster. By restricting communication between Pods based on labels, namespaces, or IP ranges, you can ensure that only authorized Pods can communicate with your critical services. This reduces the risk of unauthorized access and data breaches.

Enforcing Least Privilege

Applying the principle of least privilege to your network traffic is crucial for maintaining a secure environment. Network policies let you enforce this by only allowing necessary communication paths. Pods should only be able to communicate with other Pods that are essential for their function, and nothing more.

How to Implement Network Policies

Implementing network policies in Kubernetes is straightforward. Here’s how you can define a policy that restricts traffic to and from specific Pods:

Defining the Policy

The example provided earlier shows how to create a NetworkPolicy that only allows communication between Pods labeled app-component: api and app-component: database. Here’s a breakdown of the key components:

  • podSelector: This selects the Pods that the policy applies to. In the example, it targets Pods labeled app-component: database.

  • policyTypes: This defines the direction of traffic that the policy applies to. Ingress controls incoming traffic to the selected Pods, while Egress controls outgoing traffic from them.

  • ingress: This section specifies the rules for incoming traffic. The example allows ingress traffic only from Pods labeled app-component: api.

  • egress: This section specifies the rules for outgoing traffic. The example allows egress traffic only to Pods labeled app-component: api.

Applying the Policy

Once you’ve defined your NetworkPolicy, apply it to your cluster using the following command:

kubectl apply -f database-policy.yaml

This will enforce the network restrictions defined in the policy, ensuring that only the specified Pods can communicate with each other.

Best Practices for Network Policies

To effectively secure your Kubernetes cluster using Network Policies, consider the following best practices:

  • Apply Policies to All Pods: Ensure that every Pod in your cluster is covered by at least one NetworkPolicy. This prevents any Pod from being left open to unrestricted network access.

  • Start with a Default Deny Policy: Begin by implementing a default deny-all policy, and then explicitly allow only the necessary traffic. This ensures that all communication is intentional and controlled.

  • Regularly Review and Update Policies: As your application evolves, so should your network policies. Regularly review and update your policies to reflect changes in your workloads and their communication needs.

  • Use Namespaces for Isolation: Leverage namespaces to isolate different environments or application components. Apply Network Policies at the namespace level to control traffic between them.

Conclusion

Network Policies are a critical component of securing your Kubernetes cluster. By controlling which Pods can communicate with each other, you reduce the risk of unauthorized access and potential security breaches. Implementing Network Policies is straightforward and should be a standard practice in any Kubernetes deployment.

Don’t overlook the importance of network security in your cluster. By applying Network Policies, you add a robust layer of protection that helps safeguard your workloads from potential threats.