Fiddler tutorial – How to use Fiddler

Revision History
Revision 1.0 01.15.2013 Admin Fiddler tutorial
Revision 1.1 – 1.3 09.09.2013 – 10.04.2013 Admin bugfixes and enhancements

Fiddler is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software, communications protocol development and education. Fiddler captures HTTP and HTTPS traffic data between browsers and servers. These data are extremely valuable for troubleshooting, performance turning and system monitoring. This article is a step by step tutorial to guide how to use Fiddler and capture HTTP Traffic.

Capturing of HTTP and/or HTTPS traffic may include private or confidential information, such as usernames, passwords or bank info. Use caution when entering in sensitive data while Fiddler is configured to capture traffic. If possible, use test or dummy data instead of confidential data while using Fiddler.
If you are a web developer, Fiddler is the must one in your tool kits.

On 12 September 2012 Eric Lawrence announced that Fiddler was acquired by Telerik and he would join the company to work on Fiddler on a full-time basis.

1. Fiddler Overview

Fiddler sits between HTTP client and server and listened on a port. As the finger below, it acts as €œman-in-the-middle€ proxy, when using fiddler, the requests is being sent directly to Fiddler proxy bridge, Fiddler will forward the request to real server, Likewise, the response from server is also sent to the fiddler, fiddler will forward it to the browser. To the client browser, Fiddler claims to be the web server, and to the web server, Fiddler mimics the web browser.  In this process, Fiddler intercepts and records all the incoming and outgoing messages.

Besides Fiddler, FireBug is an alternative one and most popular and powerful web development tool, it can Inspect HTML and modify style and layout in real-time.

2. Download Fiddler

Fiddler is a Windows Forms application, available for Windows XP and later, go to download page, there have two versions fiddler2 and fiddler4 available, fiddler2 was written in .Net 2, fiddler4 was higher, please choose either the one to download.

3. Install Fiddler on your client machine

After downloaded, double-click the exe file and agree with the installation license, follow the step-by-step wizard to complete the installation, during installing, you may be asked to close all browsers.

4. Start Fiddler

Lunch the shortcut of Fiddler from from Start > All Program, you will see the below main Fiddler UI, when starting Fiddler, it will automatically check the latest version against the Fiddler server and ask you upgrade if there have new version available.

5. Use Fiddler to Capture HTTP or HTTPS traffic

5.1 Check overall Web sessions

Use Internet Explorer or Mozilla FireFox, open a web page that sends a request via the explorer, we will instantly see a list of the web requests in the web sessions panel, while, if you visit more web pages, the recorded requests  will continue growing and keep in order.

The web sessions show all HTTP and HTTPS sessions captured by Fiddler, the messages include certain key information:

  • # – An ID# of the request generated by Fiddler for your convenience
  • Result – The Result code from the HTTP Response
  • Protocol – The Protocol (HTTP/HTTPS/FTP) used by this session
  • Host – The hostname of the server to which the request was sent
  • URL – The path and file requested from the server
  • Body – The number of bytes in the Response body
  • Caching – Values from the Response’s Expires or Cache-Control headers
  • Process – The local Windows Process from which the traffic originated
  • Content-Type – The Content-Type header from the Response
  • Custom – A text field you can set via scripting
  • Comments – A text field you can set from scripting or the session’s context menu

The each type of item have an own icon on the left hand, the below is the list of the all icons.

Of course, It’s just an overall summary list, it give a generic sense that we can choose what we want.

5.2  Check Fiddler Inspectors

If you double click on a HTTP session, the Inspectors tab on right hand will be displayed, it visualize requests or response content in meaningful ways. In the top half of the right hand side is the request what was sent to server, there have multiple tabs of view where show format message and data(Headers, Text View, web forms, HexView, Auth, Cookies, Raw, JSON and XML). The first one is header, it logged cache, Client, Cookies/Login, Miscellaneous and Transport, these infos are particular helpful to troubleshoot.

Request Inspectors
[RW] Headers Shows request headers and status.
[RW] TextView Shows the request body in a text box.
[RW] HexView Shows the request body in a hexadecimal view.
[RO] XML Shows the request body as an XML DOM in a tree view.
Response Inspectors
[RW] Transformer Removes GZip, DEFLATE, and CHUNKED encodings for easier debugging.
[RW] Headers Shows response headers and status.
[RW] TextView Shows the response body in a text box.
[RW] HexView Shows the response body in a hexadecimal view.
[RO] ImageView Shows the response body as an Image. Supports all .NET image formats.
[RO] XML Shows the response body as an XML DOM in a tree view.
[RO] Privacy Explains the P3P statement in the response headers, if present.

The bottom half is the response tab, this is the entire response being sent back to the client browser, it may possible be HTML page, image, JSON string, Cascading Style Sheet (CSS) and other resources, we can click textview, syntaxview, Imageview and other clickable tab to view it in different format, the below example demos we syntax view the html source page of this site.

5.3  Check statistics of the request

Click on statistics tab, it will show the estimated performance statistics for the selected HTTP sessions. It gives us a straight-forward insight for receive status and chart from high level. It’s good for performance turning.

For instance, the below is the analytic data which retrieved the home page of this site.

Request Count: 1
Bytes Sent: 1,178 (headers:1,178; body:0)
Bytes Received: 148 (headers:148; body:0)ACTUAL PERFORMANCE
ClientConnected: 21:44:54.604
ClientBeginRequest: 21:44:54.650
GotRequestHeaders: 21:44:54.650
ClientDoneRequest: 21:44:54.650
Determine Gateway: 47ms
DNS Lookup: 0ms
TCP/IP Connect: 452ms
HTTPS Handshake: 0ms
ServerConnected: 21:44:55.141
FiddlerBeginRequest: 21:44:55.141
ServerGotRequest: 21:44:55.142
ServerBeginResponse: 21:44:56.012
GotResponseHeaders: 21:44:56.012
ServerDoneResponse: 21:44:56.012
ClientBeginResponse: 21:44:56.012
ClientDoneResponse: 21:44:56.012Overall Elapsed: 0:00:01.362RESPONSE BYTES (by Content-Type)
~headers~: 148
The following are VERY rough estimates of download times when hitting servers based in WA, USA.US West Coast (Modem – 6KB/sec)
RTT: 0.10s
Elapsed: 0.10sJapan / Northern Europe (Modem)
RTT: 0.15s
Elapsed: 0.15sChina (Modem)
RTT: 0.45s
Elapsed: 0.45sUS West Coast (DSL – 30KB/sec)
RTT: 0.10s
Elapsed: 0.10sJapan / Northern Europe (DSL)
RTT: 0.15s
Elapsed: 0.15sChina (DSL)
RTT: 0.45s
Elapsed: 0.45s

6. This Fiddler tutorial is done, leave a question or comment to us if you have


In this tutorial introduces the basic features and examples of Fiddler, Fiddler is a very good debug tool. You may take a try if you have not tried it.

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