Kubernetes NetworkPolicy configurations often fail to account for traffic paths involving hostNetwork, DNS, and infrastructure components. Standard testing methods using simple curl commands frequently overlook these bypasses, as they only verify pre-defined routes rather than actual kernel-level traffic behavior.
To ensure effective isolation, engineers should utilize eBPF-based tools like Cilium Hubble and Tetragon. These tools allow for monitoring traffic events directly from the kernel, identifying whether packets are forwarded or dropped, and tracking specific process identities, which standard UI filters often hide by default.
Common security gaps include NodeLocal DNSCache and hostNetwork pods. Because Cilium treats NodeLocal traffic as a reserved host identity, standard pod-level policies often fail to restrict it. Furthermore, L4-level policies can inadvertently override L7-level DNS filtering, rendering domain-based access controls ineffective if both rule types coexist.
Testing on a cluster with 11 nodes and Cilium 1.20.4 revealed that hostNetwork pods can bypass network restrictions to access local endpoints. This occurs because the local node stack allows traffic from hostNetwork processes by default to support kubelet readiness probes, creating a significant security blind spot for internal services.
