Feature/lab4

[BearAx]

Goal

Complete Lab 4 by tracing a QuickNotes request end-to-end, analyzing TCP and HTTP traffic, debugging a broken deploy outside-in, and documenting the TLS bonus task.

Changes

• Added submissions/lab4.md
• Captured a real POST /notes request with tcpdump
• Annotated the TCP three-way handshake
• Annotated the HTTP request, JSON body, HTTP response, and connection close
• Added outputs for the required debugging commands:
  • ss -tlnp
  • ip route show
  • mtr -rwc 5 localhost
  • dig +short example.com @1.1.1.1
  • journalctl --user -u quicknotes -n 20
• Reproduced a broken deploy caused by a port conflict
• Documented the outside-in debugging chain
• Identified the root cause: bind: address already in use
• Repaired the broken instance and re-verified /health
• Added a blameless mini-postmortem
• Completed the bonus TLS handshake investigation with Caddy, tcpdump, curl, tshark, and certificate-chain evidence

Testing

• Ran QuickNotes locally on :8080
• Sent a POST /notes request with curl -v
• Captured and decoded packets from loopback traffic
• Verified QuickNotes was listening with ss
• Verified local reachability with curl and mtr
• Verified DNS behavior with dig
• Simulated a failed second deploy on the same port
• Confirmed the failed process produced bind: address already in use
• Restarted QuickNotes cleanly and confirmed /health returned {"notes":4,"status":"ok"}
• Ran HTTPS through Caddy on localhost:8443
• Captured and decoded the TLS handshake
• Verified TLS 1.3 negotiation and certificate-chain output

Checklist

• Title is a clear sentence
• Commits are signed
• submissions/lab4.md updated
• TCP handshake documented
• HTTP request and response documented
• Connection close documented
• Five debugging command outputs included
• 502-debug reflection included
• Broken deploy reproduced
• Root cause identified
• Outside-in debugging chain documented
• Repair and re-verification included
• Blameless mini-postmortem included
• Bonus TLS handshake evidence included