Separate Application Latency From Network Failure With Hubble

A Kubernetes troubleshooting lab demonstrates how client-side timeouts alone cannot distinguish between a slow dependency, an unreachable service, or packets dropped in transit, using Cilium Hubble for kernel-level flow evidence and proxymock for recording, reproducing, and validating application behavior. The lab deploys a Go service that calls an in-cluster catalog dependency, records a healthy response baseline, then applies an incomplete Cilium egress policy that permits DNS but, through Cilium’s default-deny behavior, silently blocks TCP traffic to the dependency. Requests consequently fail after a consistent five-second timeout with little useful application logging, while Hubble shows DNS queries being forwarded and TCP SYN packets from catalog-api to catalog-fixture on port 8090 being dropped with the POLICY_DENIED reason. The guide stresses that retransmitted dropped packets must not be confused with failed requests, and that evidence should separately support application, dependency, and network conclusions. After adding a narrowly scoped egress rule for the fixture service, functional and load replays confirm that dropped flows disappear, connectivity returns, and the recorded caller-visible response contract remains unchanged. It also notes that Hubble is a discriminator rather than a complete diagnosis tool, with limitations involving ring-buffer retention, platform support, production topology complexity, and the need for traces when flows show the network is healthy.

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