Linux iptables Guide

Linux iptables Guide

Complete beginner-friendly guide to iptables on Linux, covering Arch Linux, CachyOS, and other distributions including rules, chains, and firewall configuration.

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Table of Contents

1. Understanding iptables
2. iptables Installation
3. Basic Rules
4. Advanced Configuration
5. Saving Rules
6. Troubleshooting

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Understanding iptables

What is iptables?

iptables is packet filtering firewall for Linux.

Components:

• Tables: filter, nat, mangle, raw
• Chains: INPUT, OUTPUT, FORWARD
• Rules: Match and target actions

Common tables:

• filter: Packet filtering (default)
• nat: Network address translation
• mangle: Packet modification

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iptables Installation

Install iptables

Arch/CachyOS:

# Install iptables
sudo pacman -S iptables

# Install nftables (modern replacement)
sudo pacman -S nftables

Debian/Ubuntu:

sudo apt install iptables

Fedora:

sudo dnf install iptables-services

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Basic Rules

Allow SSH

Allow SSH access:

# Allow SSH (port 22)
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Allow established connections
sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Allow Loopback

Allow localhost:

# Allow loopback
sudo iptables -A INPUT -i lo -j ACCEPT

Default Policy

Set default policy:

# Deny all incoming
sudo iptables -P INPUT DROP

# Allow all outgoing
sudo iptables -P OUTPUT ACCEPT

# Deny forwarding
sudo iptables -P FORWARD DROP

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Advanced Configuration

Allow HTTP/HTTPS

Web server rules:

# Allow HTTP
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Allow HTTPS
sudo iptables -A INPUT -p tcp --dport 443 -j ACCEPT

Rate Limiting

Limit connections:

# Limit SSH connections
sudo iptables -A INPUT -p tcp --dport 22 -m connlimit --connlimit-above 3 -j REJECT

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Saving Rules

Save Rules

Arch/CachyOS:

# Save rules
sudo iptables-save > /etc/iptables/iptables.rules

# Restore rules
sudo iptables-restore < /etc/iptables/iptables.rules

# Auto-restore on boot
sudo systemctl enable iptables

Debian/Ubuntu:

sudo netfilter-persistent save

Fedora:

sudo service iptables save

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Troubleshooting

Check Rules

List rules:

# List all rules
sudo iptables -L -v -n

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

# List specific chain
sudo iptables -L INPUT -v -n

Delete Rules

Remove rules:

# Delete by line number
sudo iptables -D INPUT 1

# Delete specific rule
sudo iptables -D INPUT -p tcp --dport 22 -j ACCEPT

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Summary

This guide covered iptables configuration for Arch Linux, CachyOS, and other distributions, including basic and advanced rules.

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Next Steps

• Firewall Configuration - Firewall setup
• Security Configuration - Security
• ArchWiki iptables: https://wiki.archlinux.org/title/iptables

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This guide covers Arch Linux, CachyOS, and other Linux distributions. For distribution-specific details, refer to your distribution's documentation.