Lab 4: Trace a request and debug a broken QuickNotes deploy

[danielpancake]

Goal

Trace one POST /notes end-to-end at the packet level and walk an outside-in debug of a deliberately broken QuickNotes deploy.

Changes

• submissions/lab4.md: Task 1 (annotated packet trace + five debugging commands + 502 reflection) and Task 2 (broken-deploy reproduction, outside-in chain, root cause, blameless postmortem).
• submissions/attachments/lab4/lab4-trace.txt: full tcpdump decode of the captured request.

Testing

Captured live in WSL2 (Ubuntu 24.04); QuickNotes was cross-compiled to a linux/amd64 binary and run inside WSL so loopback traffic was visible to tcpdump -i lo.

• Task 1: one POST /notes captured on the IPv6 loopback; the decode shows the TCP handshake ([S]/[S.]/[.]), the POST /notes HTTP/1.1 request + JSON body, the HTTP/1.1 201 Created response + JSON, and the [F.] close. All five debug commands (ss, ip route, mtr, dig, journalctl) returned sensible output.
• Task 2: ran two instances on :8080; the second failed with listen tcp :8080: bind: address already in use. Outside-in chain (ps -> ss -> curl -> firewall -> dig) showed an all-green path with a stale instance holding the port; killing it and restarting restored /health to 200.

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/lab4.md updated