Feature/lab4

[Dnau15]

Goal

Complete Lab 4 (OS & Networking): trace a POST /notes request end-to-end at the packet level and debug a broken deploy outside-in, documented on macOS.

Changes

• Added submissions/lab4.md with the full lab writeup (macOS-adapted commands).
• Task 1: annotated tcpdump capture (TCP handshake, HTTP request/response, FIN close), five debugging commands (lsof, netstat -rn, traceroute, dig, app stdout), and a 502-debug
  reflection.
• Task 2: reproduced the bind: address already in use port conflict, walked the outside-in chain (command → output → decision per step), and wrote a blameless postmortem — including the
  go run orphaned-child gotcha.
• Included lab4-trace.txt (decoded packet capture).

Testing

• Verified QuickNotes returns 201 Created on POST /notes and 200 {"notes":7,"status":"ok"} on /health.
• Confirmed the packet capture shows the complete TCP lifecycle (SYN/SYN-ACK/ACK → HTTP → FIN) on IPv6 loopback ::1.
• Reproduced the port-conflict failure and confirmed lsof -nP -iTCP:8080 identifies the real listener.

Checklist

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