Fiddler Beginner Guide

Fiddler is a web debugging tool that allows you to monitor and modify HTTP and HTTPS traffic between your computer and the internet. It’s an invaluable tool for developers, QA engineers, and anyone working with web applications, helping to troubleshoot network issues and analyze traffic.

What is Fiddler?

Fiddler acts as a “man-in-the-middle” between your computer and the Internet. When you install Fiddler, it captures HTTP/HTTPS traffic from your computer and allows you to inspect the requests and responses. It can be used to:

• Debug issues with web applications.
• Inspect and modify HTTP requests and responses.
• Test APIs and endpoints.
• View and analyze web traffic.

Getting Started with Fiddler

To download and install Fiddler, follow the instructions listed here.

Basic Actions in Fiddler

• Capture Traffic: By default, Fiddler will capture all traffic. You can start or stop traffic capture from the File > Capture Traffic menu.
• Inspect Traffic: When you make a request (like visiting a website or calling an API), Fiddler will capture it and display it in the Sessions list. Click on a session to view details like the request URL, HTTP method, and status code.

Modifying Requests and Responses

You can modify requests and responses using Fiddler:

• Edit Requests: Right-click on a request and choose “Composer” to modify headers or the request body before sending it.
• Edit Responses: Right-click on the request in the Sessions list, select “Reissue and Edit,” and modify the response as needed.

Working with HTTPS Traffic

Since most websites use HTTPS, Fiddler needs to decrypt the traffic to inspect it. Here’s how to enable HTTPS decryption:

• Go to Tools > Options > HTTPS.
• Check the box for Decrypt HTTPS traffic.
• Follow the steps to install the Fiddler Root Certificate on your system.

Filtering Traffic

You can filter out irrelevant traffic to focus on the requests you care about. Use the Filters Tab in Fiddler to set up filters by host, method, status code, and more.

Using the Composer

The Composer tab lets you send custom HTTP requests to any endpoint. You can manually compose requests like GET, POST, PUT, DELETE, etc., and specify headers, query parameters, and request bodies.

Saving and Analyzing Sessions

You can save the captured traffic for later analysis. To do so, go to File > Save > All Sessions or File > Save > Selected Sessions.

Common Troubleshooting Tips

• Traffic is not captured: Make sure that Capture Traffic is enabled in Fiddler.
• HTTPS traffic is not decrypted: Ensure the Decrypt HTTPS traffic option is enabled and the Fiddler root certificate is installed.
• Fiddler is not working with some apps: Some apps bypass system proxies. You may need to configure them to use Fiddler.

Common Use Cases

• Debugging Web Applications: Inspect network traffic to troubleshoot issues with requests or responses.
• API Testing: Send custom API requests and analyze the responses.
• Modifying Requests for Testing: Modify traffic to see how apps behave with different data.
• Performance Analysis: Measure response times and optimize performance.

Fiddler is an incredibly powerful tool for inspecting, debugging, and manipulating HTTP and HTTPS traffic. As a beginner, you’ll mostly use Fiddler to inspect requests and responses, modify data for testing, and troubleshoot issues with web applications and APIs.