Build Your Own Nginx+Lua Web Application Firewall: Step‑by‑Step Guide

Introduction

The author, a non‑security professional, shares the challenges faced by small‑to‑medium enterprises that lack dedicated security engineers and explains why ops engineers can and should take security into their own hands.

What Is a Web Application Firewall (WAF)?

A WAF operates at the application layer (OSI layer 7) to detect and block attacks such as SQL injection, XSS, and malicious file uploads that traditional network firewalls cannot handle.

Pain Points of Using Plain Nginx for Security

No whitelist support: Difficult to exempt specific IPs or URLs.

Complex configuration: Writing extensive

if‑else

blocks is cumbersome and logging is limited.

Limited syntax: Nginx’s simple directives are insufficient for advanced logic.

Why Build Your Own WAF?

Existing solutions like ModSecurity were tested but caused significant performance loss on Nginx, and third‑party projects such as

ngx_lua_waf

inspired a custom implementation using Nginx+Lua (OpenResty).

Design Overview

A functional WAF can be broken down into four essential modules plus a configuration module:

Request parsing: Use OpenResty’s rich API to decode HTTP requests.

Rule detection: Apply a rule set to identify malicious patterns.

Defense actions: Decide whether to block, redirect, or return a custom response.

Logging: Record defense events in JSON for ELK‑Stack analysis.

Configuration: Control switches, rate limits, and feature toggles.

Installation of Nginx+Lua (OpenResty)

Install required packages, download and compile OpenResty, then verify the installation.

<code>https://github.com/loveshell/</code>

Test the installation with a simple "Hello World" page.