Fiddler: a free web debugging tool

The Fiddler tool helps you debug web applications by capturing network traffic between the Internet and test computers. The tool enables you to inspect incoming and outgoing data to monitor and modify requests and responses before the browser receives them. Fiddler also includes a powerful event-based scripting subsystem, which you can extend by using any .NET Framework language.

Fiddler and the HTTP replay options can help you troubleshoot client-side issues with web applications by making an offline copy of the test site. With these tools, you can create offline images of the browsing experience and then package and analyze the results to obtain more detailed debug information.

The proxy basically sits between the Windows WinINET interface and the internet. WinInet is high-level API to work with HTTP, FTP, and Gopher protocols.

Current versions of Internet Explorer, Google Chrome, Apple Safari, and Opera all automatically use Fiddler when Fiddler is configured to Capture Traffic.

You can download Fiddler at here.

HTTP Compression can dramatically decrease the number of bytes that are transmitted between the server and the client: savings of over 50 percent for HTML, XML, CSS, and JavaScript are common. Using Fiddler’s Transformer tool on the Sessions tab, you can decompress the response or simulate HTTP compression by checking "Simulate GZIP Compression" on the Fiddler Rules menu.

In addition, Fiddler helps you decrypt HTTPs traffic and display and modify the requests that would otherwise be inscrutable to observers on the network using a man-in-the-middle decryption technique.

Configure Fiddler to Decrypt HTTPS Traffic

To enable HTTPS traffic decryption:

1. Click Tools > Fiddler Options > HTTPS.

2. Click the Decrypt HTTPS Traffic box.

To skip traffic decryption for a specific host

1. Click Tools > Fiddler Options > HTTPS.

2. Type the hostname in the Skip Decryption.