Kubernetes Network Policies: What They Are and Why They Matter

4 min readMar 12, 2023

Kubernetes is a widely used container orchestration platform that provides a lot of features to manage containerized applications. One important aspect of Kubernetes is network policies, which is an important security feature that allows you to control the traffic flow between pods in a Kubernetes cluster. In this article, we’ll explore network policies in Kubernetes, including supported CNI plugins, their comparison with firewalls, their importance in a good Kubernetes architecture, practical use cases, and versions that support them.

What are Network Policies in Kubernetes?

Network policies are a Kubernetes feature that allows you to control the network traffic between pods in a cluster. A network policy is a specification of how groups of pods are allowed to communicate with each other and other network endpoints. This specification defines the rules for ingress and egress traffic, such as allowing or denying traffic based on IP addresses, ports, protocols, and labels.

By default, all pods in a Kubernetes cluster can communicate with each other, which can create security concerns. Network policies provide a way to restrict pod-to-pod communication, reducing the risk of attacks, and enabling a more secure environment.

Why are Kubernetes Network Policies Important?

In a Kubernetes cluster, every pod has a unique IP address and can communicate with every other pod in the same namespace by default. However, this can lead to security and performance issues if not properly controlled. Network policies provide a way to segment your network and control traffic flow between pods.

Network policies are especially important in a multi-tenant Kubernetes environment, where multiple users or teams share the same cluster. Network policies help ensure that pods are isolated and can’t access resources or services they shouldn’t.

Network policies are also essential for compliance and regulatory requirements. For example, if you’re processing sensitive data, you may need to restrict access to certain pods or services.

CNI Plugins that Support Network Policies

There are several CNI plugins that currently support network policies in Kubernetes. These plugins include:

Calico

Calico is an open-source CNI plugin that provides advanced network policies, security, and connectivity for Kubernetes clusters. It uses eBPF (extended Berkeley Packet Filter) to enforce network policies and provides a distributed, scalable network architecture.

Cilium

Cilium is another open-source CNI plugin that provides network policies and security for Kubernetes clusters. It uses eBPF to enforce policies and provides advanced visibility and troubleshooting capabilities.

Flannel

Flannel is a popular CNI plugin that supports network policies in Kubernetes. It uses a simple overlay network to provide connectivity between pods and supports several backends, including etcd and Kubernetes API server.

Weave Net

Weave Net is a CNI plugin that provides network policies, connectivity, and management for Kubernetes clusters. It uses a simple overlay network and provides visibility and troubleshooting capabilities.

Network Policies vs. Firewalls

Network policies in Kubernetes can be compared to traditional firewalls, which provide traffic control between network endpoints. However, there are some key differences between network policies and firewalls.

Firewalls operate at the network layer, while network policies operate at the pod level. This means that network policies are more granular and can provide more precise control over traffic flow.

Another key difference is that firewalls are typically deployed as separate infrastructure components, while network policies are built into the Kubernetes platform. This means that network policies are easier to manage and can be integrated with other Kubernetes features.

Importance of Network Policies in Kubernetes

Network policies are an important aspect of a good Kubernetes architecture because they provide a way to enforce security and reduce the risk of attacks. By restricting pod-to-pod communication, you can ensure that only authorized pods can communicate with each other, reducing the attack surface.

Network policies also enable a more efficient use of network resources by allowing you to control the traffic flow between pods. By limiting the traffic flow, you can reduce network congestion and improve application performance.

Practical Use Cases

Network policies in Kubernetes can be used in a variety of practical scenarios, including:

Enforcing security policies between pods
Restricting access to sensitive data
Enabling multi-tenancy by isolating pods within a cluster
Reducing network congestion by controlling traffic flow
Enabling compliance with regulatory requirements

Versions that Support Network Policies

Network policies were introduced in Kubernetes 1.3 and have been available since then. However, support for network policies may vary depending on the Kubernetes distribution and the CNI plugin being used.

Comparing Kubernetes Network Policies to a Firewall

Kubernetes network policies are similar to traditional firewalls in that they control network traffic. However, there are some key differences between the two.

Firewalls typically operate at the network layer, while Kubernetes network policies operate at the pod and namespace level. Kubernetes network policies allow you to define rules based on labels, while firewalls typically use IP addresses and ports.

Another key difference is that Kubernetes network policies are implemented as part of the cluster infrastructure, while firewalls are typically a separate network appliance. This makes it easier to manage and automate network policies in a Kubernetes environment.

How to Use Kubernetes Network Policies

To use Kubernetes network policies, you need to have a network plugin installed in your cluster that supports them. Once you have a network plugin installed, you can define network policies using YAML files.

Here’s an example of a network policy that restricts traffic to a specific pod:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: db-policy
spec:
  podSelector:
    matchLabels:
      app: db
  ingress:
  - from:
    - podSelector:
        matchLabels:
          app: web
    ports:
    - protocol: TCP
      port: 3306

This network policy restricts incoming traffic to the pod with the label app: db. The ingress section specifies that traffic is only allowed from pods with the label app: web on port 3306.

Kubernetes network policies are available in Kubernetes version 1.7 and later. However, support for network policies varies depending on the network plugin used by your cluster. Some plugins, such as Calico, provide more advanced features and granular control over network policies.

Conclusion

Kubernetes network policies provide a way to control network traffic between pods in a secure and predictable way. They are essential for isolating workloads, ensuring compliance, and improving performance in a multi-tenant Kubernetes environment. By using network policies, you