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  • 4/7/2015



HTTP proxies

Last updated 4 years ago



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Frequently when you experience a site problem, it's difficult to see exactly what is causing the issue. Ideally you could attach a debugger and step through the code, but often this is not possible. As a good starting point to diagnosing an issue, it's good to see what's going on "under the covers". To do this, you could load a copy of the websites log files and step through to ensure that the error is not being caused by missing files, incorrect permissions, a path or url request that you were not expected (or didn't even know happened), but the easier way is to use a HTTP Proxy.

HTTP proxies are very useful in allowing us to see all details of the data that get's passed to and from a website. One of the most popular (and free) for the windows platform is HTTP Fiddler, and their website offers the following description:

"Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more."

Installing and using

To install HTTP Fiddler , visit and download the latest version. Once it's installed it can be started via the program menu, or else via the integrated browser version. If using Internet Explorer, click the "Tools" menu item and select "Fiddler2".

Once fiddler is open , if a trace begins running, you'll want to uncheck "File -> Capture traffic" or press F12. Clear the entries by using "Edit -> Remove -> All Sessions". Now browse to the area of the site causing issues, and then press F12 to start a trace, visit the exact problem location, and then press F12 to finish. You can now inspect the session details and use it to follow the exact sequence of page requests (including any "invisible" response.redirects or server.transfers) as well as spot common errors such as :
•Missing files (shows as 404 errors)
•lost authentication (click on a page name, and then click inspectors in the right tab and examine if all cookies are passed as expected)
•access denied errors (showing up as 401 errors - the sub number indicates the exact failure e.g. 401.3 is an "access denied by ACL on resource" error. This means that you have not granted sufficent permissions to access that file.

More reading

A full examination of this tool is beyond the scope of this wiki. We recommend you take a look at the following resources.
* Video tutorials

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