Kubernetes Network Policies: The Gatekeepers of Pod Traffic

Kubernetes Network Policies define how pods talk to each other and to the outside world. They use selectors and rules to allow or block connections at the IP and port level. Without them, your cluster is wide open. With them misconfigured, your services suffocate.

A Network Policy is built on three parts: pod selectors to choose which pods the policy applies to, ingress rules to control inbound traffic, and egress rules to control outbound traffic. Each rule defines allowed connections based on namespace, labels, or IP blocks. By default, if no policies exist, all traffic is allowed. As soon as one policy targets a pod, any traffic not explicitly permitted is denied.

To secure workloads, start with a default deny-all policy for ingress and egress. Then, open only the paths your application needs. For example, allow the front-end namespace to talk to the back-end namespace on specific ports, permit database pods to receive traffic only from authorized services, and block all outbound calls except those to trusted endpoints.

Testing is critical. Deploy changes in staging first. Use kubectl exec and debugging pods to verify which connections work. Monitor network flows with tools like Cilium or Calico, which implement the policies at the CNI level. Watch for DNS resolution issues—Network Policies don’t manage DNS directly, but blocking egress can kill it if you don’t allow UDP/53 or TCP/53 to your DNS service.

Common mistakes include overly broad selectors that unintentionally open traffic, omitting necessary egress rules for API calls, and forgetting that policies are namespace-specific. Keep policies small, explicit, and version-controlled.

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