Fiddler to capture request from Firefox

Fiddler to capture requests from Firefox you need to do a small configuration in Firefox.

In Firefox 3.0.8 (one I am using) follow the steps bellow

• Go to Tools > Options > Advanced > Settings
• Set the “Automatic proxy configuration URL” to “C:\Users\<user name>\Documents\Fiddler2\Scripts\BrowserPAC.js”.(NOTE : change the BrowserPAC.js path accordingly)
• Click Ok
• Close the Fiddler and open if you have already opened the fiddler.

Reference : http://www.fiddlertool.com/Fiddler/help/hookup.asp

Behind the scene :

What happens here is that , when you open the fiddler, the function “FindProxyForURL” in the “BrowserPAC.js” will change to this

function FindProxyForURL(url, host){
return 'PROXY 127.0.0.1:8888';
}

When you closes the fiddler it will change to

function FindProxyForURL(url, host){
return 'DIRECT';
}

Since you have configured the Firefox to look at this file, it will pick the proxy configuration automatically.

Problems I encountered :

After closing the Fiddler I wanted to browse some sites in Firefox but it didn’t work.It says “Connection Interrupted”.

Then I changed the proxy configuration to “No proxy” and it works.

Conclusion :

After using the fiddler make sure to revert the proxy settings back to the original in Firefox.

Worth looking at how IE handled this scenario.