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alok-38

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Backend Foundations Lab — Full Walkthrough

#backend #virtualmachine #ubuntu

How I Finally gained a surface level intuition of how the web works (By Building a primitive Backend)

When people say “learn how the web works,” it usually sounds abstract — DNS, client–server model, HTTP requests, ports — a bunch of terms that don’t really click until something breaks. Instead of just reading about it, I decided to set up a real Ubuntu virtual machine, run a backend server on it, and connect to it from my own computer. Along the way, I hit firewall issues, port forwarding problems, and confusing network errors — and that’s exactly when everything started to make sense. This post is a hands-on walkthrough of how I finally understood how the web actually works by building and debugging a real client–server setup.

Setup Overview

To understand how the web actually works in practice, I created a small, local client–server environment that mirrors how real backend systems operate in production — just on my own machine. The setup looked like this:

  • Host machine (Client):
    My Windows laptop, acting as the client making HTTP requests using curl and SSH.

  • Virtual machine (Server):
    An Ubuntu VM running inside VirtualBox, acting as the backend server.

  • Networking:
    The VM was configured using NAT with port forwarding, so requests from my Windows machine could reach services running inside the VM (similar to how traffic reaches servers behind NAT or load balancers in the cloud).

  • Backend service:
    A simple Python HTTP server running on the VM to simulate a backend API/server.

  • Access method:

    • SSH into the VM to manage the server
    • HTTP requests from Windows to the VM to simulate real client–server communication 
Client (Windows)
      |
      |  HTTP request (curl / browser)
      v
VirtualBox NAT (port forwarding)
      |
      v
Ubuntu VM (Python backend server)
      |
      v
HTTP response back to client
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🎯 Goal

Understand How the web works (DNS + client–server model) by:

  • running a server on a Ubuntu VM

  • connecting to it from Windows

- debugging real network + firewall issues

Connected to Ubuntu VM over SSH (Client → Server)

Command (from Windows) 

ssh alok@localhost -p 2222
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📌 What this means

  • Windows = client

  • Ubuntu VM = server

  • Port forwarding:

Windows:2222 → VM:22
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🧠 New thing I noticed 

Last login: ... from 10.0.2.2
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✅ Why that happened

  • VirtualBox NAT hides your real IP

  • VM sees the NAT gateway (10.0.2.2)

  • This is how real servers behave behind NAT/load balancers (like on AWS)

2️⃣ Started a web server on the VM (Become the Server)

✅ Command (on VM) 

python3 -m http.server 8000 --bind 0.0.0.0
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🧠 What this did

  • Started an HTTP server

  • Listening on:

0.0.0.0:8000
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- 0.0.0.0 = accept connections from outside the VM

3️⃣ First attempt to access from Windows failed

❌ Command (on Windows) 

curl http://localhost:8000
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❌ Error 

Unable to connect to the remote server
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🧠 Root cause

Even though:

  • Server was running ✅

  • Port forwarding was set ✅

The VM firewall (ufw) was blocking port 8000

4️⃣ Checked firewall (Found the real problem)

✅ Command (on VM) 

sudo ufw status
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📌 Output (important part) 

Status: active
OpenSSH ALLOW
22 ALLOW
3999 ALLOW
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🧠 What this means

  • Port 8000 was NOT allowed

- So Windows → VM:8000 was blocked

5️⃣ Fixed firewall (Opened port 8000)

✅ Commands (on VM) 

sudo ufw allow 8000/tcp
sudo ufw reload
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✅ Verified 

sudo ufw status
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Now shows:

8000/tcp ALLOW Anywhere
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6️⃣Verified server was running (Correct diagnosis)

✅ Command 

sudo ss -tulnp | grep 8000
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Output 

tcp LISTEN 0.0.0.0:8000 users:(("python3",pid=1989))
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Meaning

  • Python server is alive

  • Listening on port 8000

  • Accepting external connections

- No need to restart it

7️⃣Final success: Windows → VM HTTP request worked 🎉

✅ Command (on Windows) 

curl.exe http://localhost:8000
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✅ Output 

<h1>Hello from my backend server 👋</h1>
<p>This response came from my Ubuntu VM.</p>
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🗺 Final working network flow 

Windows (Client)
   ↓ curl http://localhost:8000
VirtualBox NAT (port forwarding)
   ↓
Ubuntu VM (Server)
   ↓
Python HTTP Server
   ↓
HTML response back to Windows
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🧠 Core Concepts I Actually Learned (not just memorized)

Concept I experienced it
Client Windows using curl
Server Ubuntu VM running Python
HTTP Requests + responses
Ports 22 (SSH), 8000 (web server)
DNS Earlier with dig google.com
NAT 10.0.2.2 hiding your real IP
Firewall ufw blocking then allowing 8000
Binding 0.0.0.0 vs localhost
Infra debugging App was fine, network blocked
Real-world behavior Same as cloud servers on AWS

🏁 Big takeaway

I didn’t just “learn how the web works.”
I debugged real backend infrastructure problems:

  • Server running but unreachable

  • Firewall blocking traffic

  • NAT hiding client IP

  • Port already in use

  • Binding to correct network interface

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