CoreDNS

5 NetworkPolicy Mistakes That Leave Kubernetes Clusters Completely Unprotected

A detailed analysis from OTUS highlights five frequent configuration errors in Kubernetes NetworkPolicy that allow unrestricted traffic despite policies appearing active in the cluster. The article explains that Kubernetes itself does not enforce NetworkPolicy objects; enforcement depends entirely on the CNI plugin such as Calico, Cilium, or Weave Net, while Flannel and kubenet ignore them entirely. Common pitfalls include blocking DNS resolution when applying default-deny egress rules, incorrect use of namespaceSelector versus podSelector leading to overly permissive access, and confusing the top-level podSelector with selectors inside ingress or egress blocks. Additional issues arise when policies fail to account for ingress controllers, hostNetwork pods, or inter-container localhost communication within the same pod. The piece stresses the importance of traffic-based validation rather than relying on kubectl output and recommends testing with deny-all policies plus continuous monitoring during rollout.