How to use firewall and iptables basics on Linux

Linux Firewall Controls What Traffic Gets Through

Linux uses iptables (or the newer nftables) to filter network traffic via chains of rules. For most servers, ufw (Uncomplicated Firewall) wraps iptables in a simpler interface. Know both.

ufw — Simple Firewall Management

# Check firewall status
sudo ufw status verbose

# Enable the firewall
sudo ufw enable

# Allow SSH (do this BEFORE enabling ufw or you lock yourself out)
sudo ufw allow ssh
sudo ufw allow 22

# Allow a specific port
sudo ufw allow 80
sudo ufw allow 443
sudo ufw allow 8080

# Allow from specific IP only
sudo ufw allow from 192.168.1.100 to any port 5432

# Deny a port
sudo ufw deny 23

# Delete a rule
sudo ufw delete allow 8080

Direct iptables Commands

# List all rules with line numbers
sudo iptables -L -n -v --line-numbers

# Block an IP address
sudo iptables -A INPUT -s 203.0.113.5 -j DROP

# Allow port 80 for TCP
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Save rules so they persist after reboot (Ubuntu)
sudo netfilter-persistent save

Always allow SSH before enabling a firewall on a remote server. Blocking port 22 locks you out permanently unless you have console access.