Understanding Loopback URLs and Their Purpose

If you have ever scratched your head in confusion when you came across the term “loopback URL”, you’re not alone. Honestly, researching it made me dizzy🥴 so I decided to write a simple explanation…maybe simply as a reminder to myself 🙈.

A loopback URL works like a secure handshake between your application and an online service, confirming each other’s identity without sharing sensitive details. It’s important for secure logins, especially when your app (like MS Access) needs to connect safely to online services.

In this article we will quickly answer What, Why and How.

Table of Contents:

• What is a Loopback URL?
• Why is a Loopback URL Important?
• How to Set Up a Local Loopback Redirect Server
• Summary
• What’s Next?

What is a Loopback URL?

A loopback URL is just a way of addressing your own computer. For example, both:

http://127.0.0.1:8080/

and

http://localhost:8080/

refer to the same computer—you can use either one!

These URLs never send data outside your computer. Instead, they let a local program—like an Access VBA script—handle its own network requests. This is especially handy when your application needs to verify your identity or access secure data without using a public URL.

But if the loopback URL doesn’t send data out, how does it get data from an external service? It works as the destination for responses sent by those external services.

For instance, during OAuth authentication, an external service (like Google) sends an authentication token back to a loopback address (such as http://localhost:8080/). Even though the initial request is made from your computer, the service sends its response to that same local address, and your application—listening on that address—captures the data securely, all without exposing a public URL.

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Why is a Loopback URL Important?

When accessing data from external online services, even if your application is hosted on a server, a loopback URL is often crucial. It plays a central role in the OAuth process, ensuring secure authentication and identity verification.

A loopback URL allows you to simulate a web server on your own machine, providing a safe endpoint for receiving authentication tokens from services. This is especially critical during OAuth authentication, where your application must securely receive and handle these tokens.

Included at the end of this article is an example video showing how loopback URLs work in action—demonstrating everything from setting up the loopback server to seamlessly integrating data from external services into your system.

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How to Set Up a Local Loopback Redirect Server

Follow these simple steps to create a loopback server on your computer:

1. Install Python

The loopback server runs on Python. If you haven’t installed Python yet, download the latest version from the official Python website.

2. Start the Server

Open your Command Prompt (or terminal) and run the following command:

python -m http.server 8080

This command starts a simple local web server on http://localhost:8080/.

3. Verify the Server

Open a web browser and navigate to:

http://localhost:8080/

If you see a directory listing or a blank page, your server is running correctly.

4. Keep the Server Running

Leave the Command Prompt open (or minimize it) while you work. To stop the server, simply press Ctrl + C.

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Summary

A loopback URL is a powerful tool that bridges the gap between your local application and external online services. It acts as a secure endpoint in the OAuth process, ensuring that your application safely receives authentication tokens without exposing sensitive data.

In this guide, we have covered:

• What? A secure handshake between your application and an online service.
• Why? It keeps your data safe and makes sure your app and the service trust each other—and sometimes external guidelines even require it.
• How? 4-Step process to set-up a Loopback Server.

Whether you’re developing a desktop application or a locally hosted web app, understanding loopback URLs is key to enhancing your system’s security and functionality when interacting with online services.

For a deeper dive into how this process fits into the larger picture of connecting Google Forms to Microsoft Access, check out our detailed 10-Step Guide video: https://youtu.be/equTtWnKEWY

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