Ubuntu Security · 4.4

🔥 Ubuntu Firewall (ufw)

Simple, powerful, and not enabled by default⏱ ~2 min

⚠ WarningUbuntu's built-in firewall (ufw) is NOT enabled by default. Enabling it and setting default-deny inbound is one of the most common competition points and takes about 30 seconds.

Basic ufw Commands

bash
# Check firewall status
sudo ufw status
sudo ufw status verbose
 
# Enable the firewall
sudo ufw enable
 
# Disable the firewall
sudo ufw disable
 
# Default rules (set these first!)
sudo ufw default deny incoming    # Block all inbound by default
sudo ufw default allow outgoing   # Allow all outbound by default
 
# Allow specific services/ports
sudo ufw allow ssh                # Allow SSH (port 22)
sudo ufw allow 80/tcp             # Allow HTTP
sudo ufw allow 443/tcp            # Allow HTTPS
 
# Block a specific port
sudo ufw deny 23/tcp              # Block Telnet
sudo ufw deny 3389/tcp            # Block RDP
 
# Delete a rule
sudo ufw delete allow 80/tcp

GUFW — Graphical Interface for ufw

  • •Install: sudo apt install gufw
  • •Run: sudo gufw (or search 'Firewall Configuration' in Ubuntu)
  • •Set Status to ON, incoming to Deny, outgoing to Allow
  • •Add specific rules for services the scenario requires

Deny vs. Reject — What's the Difference?

DENY
  • •Silently drops the packet
  • •Attacker gets no response — doesn't know if port is filtered or just closed
  • •Slightly better for security (less information given to attacker)
REJECT
  • •Sends back a 'connection refused' message
  • •Faster for legitimate clients to detect the port is closed
  • •Slightly more informative for attackers
💡 TipFor competition, use 'deny' as your default and only 'allow' what the scenario explicitly requires. Every unnecessary open port is a potential vulnerability.
🧠Quick Checkfirst try = +5 XP

What's the first ufw command to run in competition?

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