Network › Module 1 › Lesson 3
Default Deny and Allowlists
Allowlists beat surprise ports — DEMO GATE, then write YOUR intended flows.
Visual · default_deny_allowlist
Default deny: name the flows you want, refuse the rest. http://192.168.0.1/ YOUR lab.
Opening
Allowlists are how forgotten defaults stop being an open door.
Lessons 1–2 named policy and where it lives. This lesson is the posture: default deny. You list the flows that must work (HTTPS out, maybe SSH from YOUR admin station to YOUR lab VM). Everything else is refused until someone writes a ticket. Allowlists feel slower than “open then lock down later.” Later rarely comes. Original Cyberlium. You will write an allowlist for YOUR teaching listener, not punch a hole to 0.0.0.0/0. Next: Lab — Host Firewall Check on Your PC.
1. Default deny is a list of yeses, not a vibe
A real allowlist is specific: source, destination, protocol, port, and why. “Allow web” is not a list. “This laptop may connect to for Cyberlium teaching” is a list row. Cloud security groups that say 0.0.0.0/0 on SSH fail this lesson even if the exam brand on the slide was expensive.
2. 0.0.0.0 is not a lab convenience
Binding a teaching server to 0.0.0.0 advertises it on every interface, including café Wi-Fi. Cyberlium labs bind 127.0.0.1. If a tutorial says 0.0.0.0 “so your phone can join,” that phone lab is a different, authorized network — not this course's default. Refuse it here.
Command guide
0.0.0.0 is not a lab convenience
═══ INSTALL ═══
Linux (Debian/Ubuntu):
Command — copy this
sudo apt install python3
macOS:
Command — copy this
brew install python3
Windows: Download https://python.org/downloads/
═══ COMMANDS ═══
Command — copy this
python3 - << 'PY'
HOST = "127.0.0.1"
if HOST != "127.0.0.1":
raise SystemExit("refusing non-loopback bind")
print("bind policy ok:", HOST)
print("0.0.0.0 is not a Cyberlium teaching bind")
PY3. Inbound vs outbound allowlists
Inbound: who may start a conversation with a service on YOUR host. Outbound: where YOUR host may start conversations. Malware loves outbound that nobody listed. You will not build an enterprise proxy in this lesson. You will write three outbound rows you actually need (updates, DNS you chose, class site) and notice how many apps assume “any.”
4. What you ship: a written allowlist before you open a port
No new inbound on YOUR host without a CSV/note row. Teaching bind 127.0.0.1 only. SSH from the world is not a beginner lab. DEMO identify / STOP still stands.
5. What you record before the next lesson
Allowlist file t11-m01-allowlist.csv chmod 600. Three yeses. 0.0.0.0 refused. Next lab checks the live host firewall.
6. Wrong vs right: stranger networks vs literacy on systems you own
Worked failure — same network word, opposite target. Right never needs a café or campus LAN.
Wrong
ufw allow 22 from anywhere “for SSH practice.” python -m http.server --bind 0.0.0.0. Open 3389 because remote desktop is convenient.
Right
Write the CSV. Keep the teaching listener on Next: Lab — Host Firewall Check on Your PC.
Mission: write YOUR allowlist before the lab
Create t11-m01-allowlist.csv with at least the loopback teaching row. Do not add 0.0.0.0/0. or STOP.
Stuck? Ask Cyberlium AI Mentor
Ask Mentor why 127.0.0.1 is the teaching bind — not how to expose the lab to a phone on café Wi-Fi.
Knowledge Check
APPLY: A blog says bind the lab to 0.0.0.0 so classmates can join. Cyberlium says:
Multiple choice
Knowledge Check
APPLY: True or False: “Allow web” is a complete allowlist row.
True or False
Knowledge Check
APPLY: Default deny without a written list usually becomes:
Multiple choice