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What is Fiddler?
Fiddler is an HTTP Debugging Proxy by Bayden Systems (Fiddler 2.2 and below). There is also a version from Microsoft (Fiddler 22.214.171.124) and an updated version (Fiddler 126.96.36.199) from Eric Lawrence of Microsoft.
A Web Debugger freeware program that is used for tagging HTTP requests. In fact, it sees all HTTP sessions, requests, and responses before being returned to the client application. (Note: For applications to run with no user interface, FiddlerCore (a version of Fiddler by Microsoft) will run applications as a library.)
Current version is Fiddler 2.x (File size: 658 KB) by Eric Lawrence. Unlike the previous version, Fiddler 2 supports debugging HTTPS traffic. "By default, Fiddler2 accepts SSLv2 SSLv3 and TLSv1 from the client, and offers SSLv3 and TLSv1 to the server." 
How it works:
- It works automatically with Internet Explorer, but any browser or application can be configured to route traffic through Fiddler.
- It uses the proxy address 127.0.0.1 and port 8888.
- Every Web request that is routed via the proxy is seen as a separate node in the web sessions list.
- It inspects all HTTP Traffic that is incoming or outgoing data.
- It can log and profile HTTP/HTTPS traffic that has been sent, received, and redirected over the network.
- It can be used as a shared proxy server running on one PC that can store a full registry of all the HTTP traffic.
- It can be configured to run in reverse proxy.
- It is able to eliminate specific scripts that slow down system performance.
- It will provide specific data about all the Internet traffic that goes through to the PC.
- Is compatible with the most important Web browsers: Internet Explorer, Mozilla Firefox, and others.
- Its user interface is known to make it easy to monitor HTTP requests.
- It's extensible using the .NET Framework.
- It supports JScript .NET (an event-based scripting subsystem).
- Has problems when working with an authenticating upstream proxy server.
- It is not able to monitor requests if using a localhost or 127.0.0.1 address (as local addresses typically bypass any proxies).
- Fiddler 2 requires version 2.0 of the .NET Framework. And, Fiddler v2.x loads only .NET CLR v2.0 assemblies.
- Fiddler 1.3 is used for .NET Framework version 1.1 users
For Windows platforms: Windows 2000 / XP / 2003 / Vista with Microsoft .NET Framework v1.1 (4322) or later10 megabytes disk space / 800Mhz Pentium/Athlon processor (Screams at 2ghz)128 megabytes RAM (512 or more highly recommended). Also compatible with Server 2007/8.
What's new: The latest update of Fiddler 188.8.131.52 includes:
- Tweaks for Firefox 5
- Assorted bugfixes
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Running Fiddler captures requests and responses on the proxy server. It will trace the traffic between the client and server.
Fiddler can be used for...
- Debugging traffic from any application. Use the "Fiddler Web Debugger" (a web development tool) to profile HTTP traffic.
- Troubleshooting and tracing a network.
- Performance testing Web applications. As well, to test the performance script result.
- Logging all HTTP(S) traffic between a computer and the Internet.
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Install and Setup
The setup command line: FiddlerSetup.exe /S
When Fiddler is run, the program registers itself as the system proxy. (Note: Fiddler can launch on system startup.)
To check the Proxy Settings or manually enter in the proxy address, click Tools, click Internet Options, click Connections, click LAN Setting, and finally click Advanced.
When closing Fiddler, it unregisters itself as the system proxy.
Note: In order to run Fiddler as a service, go to MyComputer > Manage > Services and Applications > Services. Find the FiddlerService in the services list and double click it. Log in and out as FiddlerUser. There wil be a FiddlerUser password that needs to be entered by the user prior to seeing the service run.
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Fiddler can be used to change your browser network configuration to act as a proxy. All it requires is to change the proxy settings by running Fiddlers configuration script. Users can also configure the settings manually to...
- To add power to Fiddler, update its JScript.NET CustomRules.js file.
- To change the JScript editor, do it from the Rules menu. Click Tools | Fiddler Options and edit the Editor string. Option: Download FiddlerScript Editor to help edit script rules for Fiddler.
- To add .NET assemblies in a script, click Tools | Fiddler Options and edit the References list on the Extensions tab. (Note: Users can increase Fiddler2 options via both script and .NET code.)
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 HTTPS Protocol Support - http://www.fiddler2.com/Fiddler/dev/FiddlerArchitecture.asp
Get Fiddler! - http://www.fiddler2.com/fiddler2/version.asp
Image: FiddlerScript Editor