# Guided Exercise: Categorize the Failure Before You Fix It

This is a paper exercise — commit to a category before checking it in the Lab.

## Your task

For each symptom, name which of the four failure categories (DNS / routing-connection /
TCP-hangs / HTTP-application) it belongs to:

1. `curl: (6) Could not resolve host: api.internal`
2. `curl: (7) Failed to connect to api.internal port 443: Connection refused`
3. A request eventually returns, but with HTTP 502 Bad Gateway
4. A request just hangs for 30 seconds with no response, then finally times out

## A worked shape to check yourself against

1. DNS — the name never resolved.
2. Routing/connection — DNS worked, but nothing accepted the connection on that port.
3. HTTP/application — a full HTTP response came back (502 is a real, complete response), so
   DNS/routing/TCP all worked; the failure is in the application layer (often a proxy unable to
   reach ITS OWN backend, which is itself worth investigating further).
4. TCP-hangs / ambiguous-timeout — this is the least specific signature; it could mean
   unreachable, silently dropped by a firewall, or an application that accepted the connection
   but never responds — worth narrowing further with `ss`/`ip route` before concluding.

## Instructor Notes

**Facilitation:** Item 3 (502) is a good one to slow down on — many students initially assume
any 5xx code means "the network is broken," when it actually proves the OPPOSITE for every
layer below the application.

**Approximate timing:** 10-12 minutes live discussion; 8 minutes self-paced.