L10. Firewall Fundamentals: UFW and iptables

What Does a Firewall Do at the OS Level?

A firewall inspects network packets as they enter, leave, or pass through your system and decides whether to allow, drop, or reject each one. On Linux, the kernel's netfilter framework handles this filtering at the network stack level, which means it processes packets before they ever reach an application.

The two most common tools for managing netfilter rules are iptables (the traditional interface) and UFW (Uncomplicated Firewall, a user-friendly frontend). Both configure the same underlying kernel framework.

iptables: The Foundation

Understanding Chains

iptables organizes rules into chains. Each chain handles traffic at a different point:

When a packet arrives, the kernel walks through the matching chain from top to bottom. The first matching rule wins. If no rule matches, the chain's default policy applies.

Basic iptables Commands

# View current rules with line numbers
sudo iptables -L -n --line-numbers
# Allow incoming SSH on port 22
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
# Allow established and related connections (critical for responses)
sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
# Drop all other incoming traffic
sudo iptables -A INPUT -j DROP

Rule Order Matters

Rules are evaluated top to bottom. If you place a DROP rule before an ACCEPT rule, the ACCEPT rule never fires. This is the most common mistake when writing iptables rules manually.

# Delete a specific rule by line number
sudo iptables -D INPUT 3
# Insert a rule at a specific position
sudo iptables -I INPUT 1 -p tcp --dport 443 -j ACCEPT

Making Rules Persistent

iptables rules exist only in memory by default. They vanish after a reboot. To persist them:

# Debian/Ubuntu
sudo apt install iptables-persistent
sudo netfilter-persistent save
# RHEL/CentOS
sudo service iptables save

UFW: The Friendly Frontend

UFW wraps iptables with a simpler command syntax. It is installed by default on Ubuntu and available on most distributions.

Enabling UFW

# Set default policies first
sudo ufw default deny incoming
sudo ufw default allow outgoing
# Allow SSH before enabling (or you will lock yourself out)
sudo ufw allow 22/tcp
# Enable the firewall
sudo ufw enable

Common UFW Commands

# Allow HTTPS
sudo ufw allow 443/tcp
# Allow a specific IP to reach port 3306 (MySQL)
sudo ufw allow from 10.0.1.50 to any port 3306
# Deny traffic from a specific subnet
sudo ufw deny from 192.168.100.0/24
# View all rules with numbering
sudo ufw status numbered
# Delete a rule by number
sudo ufw delete 3
# Check the firewall status
sudo ufw status verbose

Default Deny: The Most Important Policy

A default-deny policy means the firewall drops all incoming traffic unless a rule explicitly allows it. This is the single most impactful firewall decision you can make.

Without default deny, every new service you install automatically becomes reachable from the network. With default deny, you must consciously open each port. This aligns with the principle of least privilege applied to network access.

# iptables default deny
sudo iptables -P INPUT DROP
# UFW default deny
sudo ufw default deny incoming

When to Use iptables vs UFW

For most single-server setups, UFW is sufficient and much harder to misconfigure. As your infrastructure grows, learning raw iptables (or its successor, nftables) becomes necessary.

Verifying Your Firewall

After configuring rules, always verify from outside the machine:

# From another machine, scan common ports
nmap -sT target-server
# Check if a specific port is reachable
nc -zv target-server 22

A firewall you never test is a firewall you cannot trust.