Network Troubleshooting

"It's not connecting" describes a symptom, not a cause, and the cause could be any of six or seven layers between the client and the application. The single biggest time-waster in network troubleshooting is guessing at the application layer first because it's the most familiar, when the fault is actually two layers down in DNS or routing. Working through the stack in order — resolution, then reachability, then the port, then the protocol, then the application — turns a guessing session into a bounded diagnosis.

Isolate the layer before you fix anything; a fix aimed at the wrong layer just adds noise to the next person's investigation.

For a symptom-driven command cookbook (dig, curl, traceroute, ss, tcpdump), read references/diagnostic-commands.md.

1. Confirm DNS resolves to what you expect before touching anything else