Lab — Port & Connection Check

ss -tuln is still the opening move: TCP/UDP, listening, numeric. Before you start extra processes, snapshot what is already there. Name three listeners if you can: port, protocol, bind address, likely service. 127.0.0.53:53 on Ubuntu is often the local stub, not "a public DNS server you should test from the café." *:22 or 0.0.0.0:22 is SSH on all IPv4 NICs. 127.0.0.1:anything is local-only. nc -zv 127.0.0.1 PORT asks your own stack whether that port completes a TCP connect. "succeeded" means a local listener accepted. "Connection refused" means nothing is listening on that address/port — a normal result, not a broken lab. nc -zv against a LAN IP, a classmate, or a printer is out of scope even if the command "just works."

You will then start a tiny HTTP server bound only to 127.0.0.1 on port 8000, curl it, watch it appear in ss, then stop it. Binding 127.0.0.1 is the whole point: a forgotten python http.server on 0.0.0.0:8000 is the 8080 story from the last lesson. When the server is gone, ss should no longer show 8000. Cleanup is part of the lab, not extra credit.

Use two terminals if you can: one holds the server in the foreground so you remember to kill it; the other runs ss/curl/nc. If python3 is missing, skip the server and still complete ss plus nc -zv 127.0.0.1 on a port that already exists (or a port that refuses). Refused is data. Do not install random "port scanners" to fill the screenshot.

mkdir -p "$HOME/cyberlium-lab"
NOTES="$HOME/cyberlium-lab/port-check.txt"

# 1) Snapshot YOUR listeners
{
  echo '=== ss -tuln (before) ==='
  ss -tuln
} > "$NOTES"

# 2) Optional: nc against loopback only (refused is OK)
#    Do NOT change 127.0.0.1 to a LAN address.
{
  echo
  echo '=== nc -zv 127.0.0.1 (examples) ==='
  nc -zv 127.0.0.1 22 || true
  nc -zv 127.0.0.1 80 || true
} >> "$NOTES" 2>&1

# 3) Temporary HTTP door bound ONLY to loopback
# Two-terminal version (do NOT paste the next line into a full-script run):
#   python3 -m http.server 8000 --bind 127.0.0.1
# Other terminal while it runs: ss -tuln | grep 8000; curl -I http://127.0.0.1:8000/; nc -zv 127.0.0.1 8000
# Then Ctrl+C the server.

# One-shot version (background, curl, kill) — still 127.0.0.1
python3 -m http.server 8000 --bind 127.0.0.1 >/tmp/cyberlium-http8000.log 2>&1 &
HTTP_PID=$!
sleep 1
{
  echo
  echo '=== ss during python http.server 8000 --bind 127.0.0.1 ==='
  ss -tuln | grep -E '8000|Local' || ss -tuln
  echo
  echo '=== curl -I http://127.0.0.1:8000/ ==='
  curl -I http://127.0.0.1:8000/ || true
  echo
  echo '=== nc -zv 127.0.0.1 8000 ==='
  nc -zv 127.0.0.1 8000 || true
} >> "$NOTES" 2>&1
kill "$HTTP_PID" 2>/dev/null || true
wait "$HTTP_PID" 2>/dev/null || true

{
  echo
  echo '=== ss -tuln (after stop) — 8000 should be gone ==='
  ss -tuln
} >> "$NOTES"
chmod 600 "$NOTES"
cat "$NOTES"

# NEVER: nmap the LAN, school, work, or neighbors
# NEVER: python -m http.server 8000  without --bind 127.0.0.1 on a shared network
# NEVER: nc -zv 192.168.x.x or other people's IPs "for comparison"

You inventoried your own sockets, proved a loopback TCP door with curl/nc, and destroyed the door so it would not follow you to a café. That is the entire ethical port lab. Next — Quiz — Transport & Ports — APPLY TCP vs UDP, the short port list, bind addresses, and this localhost-only discipline, then Topic 3 continues with NAT, Firewalls & Routers.