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DNS, TCP, TLS and HTTP from the terminal: debugging the network one layer at a time.
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When every app fails name lookups at the same minute, one shared resolver is down. Find it with resolv.conf, getent vs dig, and one query per hop.
Healthy pods, a Service, and Connection refused. Check the selector (empty endpoints), readiness (ready false) and targetPort - in that order.
A 502 is nginx saying its own connection to the upstream failed. How to read error.log, tell refused from timed out, and test from where nginx stands.
The grey router page means the router could not hand the request to a pod. Wrong host or path, no ready endpoints, or a Route pointing at the wrong port - and the commands that tell them apart.