Lab 4: TCP trace, 5 debug commands, and broken deploy postmortem

[infernaltiger]

Goal

Complete Lab 4 by capturing and analyzing TCP/HTTP traffic to QuickNotes and debugging a deliberately-broken deployment using the outside-in diagnostic chain.

Changes

• Added submissions/lab4.md with complete Lab 4 documentation
• Task 1: Annotated packet capture showing TCP three-way handshake, HTTP request/response, and connection close; executed 5 debugging commands (ss, ip route, mtr, dig, journalctl) with interpretations; wrote 502-debug reflection paragraph
• Task 2: Reproduced broken deploy with port conflict; walked outside-in debugging chain (process check -> listening check -> reachability -> firewall -> DNS); identified root cause (bind: address already in use); wrote blameless mini-postmortem (<=200 words) with systemic analysis and prevention tooling recommendations

Testing

• Captured live traffic with tcpdump -i lo while running QuickNotes via go run .
• Verified packet capture contains TCP handshake (SYN/SYN-ACK/ACK), HTTP POST request with JSON body, HTTP 201 Created response, and FIN-based connection close
• Executed all 5 debugging commands and confirmed sensible output (process listening on :8080, routes configured, localhost reachable, DNS resolving correctly)
• Reproduced broken deploy by launching two instances on same port; verified second instance fails with "address already in use" error
• Walked complete outside-in debugging chain and documented decision at each step
• Verified mini-postmortem word count <= 200 words

Checklist

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