How I Built a Login System with Just NGINX, socat, and Bash

Abhishek P
Friday, Jul 25, 2025

Why I Did It

I’ve always configured systems, maintained infrastructure, and debugged networks. But writing web applications? Not really my thing.

Yet one day I asked myself: how does a login system really work, under the hood?

So I decided to build one—not using a language or framework, but with just the tools I know best:

nginx for hosting and routing
socat for TCP forwarding
bash scripts for logic
A flat file for credentials

No libraries. No runtime. No “web stack.” Just Linux.

The Tools I Used

NGINX – to serve static pages and proxy requests
socat – to connect TCP ports to scripts
Bash – to handle POST requests and session logic
login.db – a simple flat file to store userid:hashedpassword (SHA-512)

Here’s how it works, step by step:

A static login.html is served by NGINX. Here’s a minimal version:

<form action="/login" method="POST">
  <input type="text" name="username" />
  <input type="password" name="password" />
  <button type="submit">Login</button>
</form>

2. NGINX Proxies the Request

In my nginx.conf, I configured the /login path like this:

location /login {
    proxy_pass http://127.0.0.1:9000;
}

3. socat Forwards to Bash

socat listens on port 9000 and forwards input to a script:

socat TCP-LISTEN:9000,fork EXEC:/usr/local/bin/login.sh

The login.sh script does the following:

Reads POST data from stdin
Parses username and password
Verifies them against /etc/login.db
On success:
- Generates a random session token
- Saves it in /etc/sessions/<token>
- Sends a Set-Cookie header and a 302 Redirect to /protected

4. Session Validation

Requests to /protected are routed like this:

location /protected {
    proxy_pass http://127.0.0.1:9001;
}

With socat again:

socat TCP-LISTEN:9001,fork EXEC:/usr/local/bin/check_session.sh

The check_session.sh script:

Reads the session cookie
Checks against existing session tokens
If valid: responds with a 302 to /protected/dashboard.html
Else: returns 403 Forbidden

5. Static Dashboard

If the session is valid, NGINX serves the static dashboard.html page.

What I Learned

How HTTP requests and cookies work behind the scenes
Parsing form data with Bash (even if a bit hacky)
Managing sessions with just files and timestamps
Password hashing with openssl passwd -6 (SHA-512 + salt)

Why This Matters

I didn’t use Python, PHP, Go, or Node.js. I didn’t rely on a login plugin, web framework, or ORM.

I just used the tools I’m comfortable with as a Linux admin. It’s far from perfect—but it’s mine, and it works. And most importantly, I learned a lot in the process.

How I Built a Login System with Just NGINX, socat, and Bash

Why I Did It

The Tools I Used

The Login Flow

1. The Login Form