Tester toolbox 101 – Fiddler


Fiddler is my favorite web debugging proxy. It is Windows-only, but in my opinion it justifies keeping a Windows VM just to be able to use it. What I do is I have a VMware Fusion or Oracle VM VirtualBox running on my Mac, with one of the Windows VMs dedicated to running Fiddler.

See below for some of typical use cases of this tool.

Disclaimer: some steps described below may affect your computer’s or network’s security. So be sure to know what you are doing.

Monitor local applications

This happens right out of the box. You may want to enable the HTTPS inspection as one of the first options after starting the tool:


Fiddler may prompt you to trust the certificate it generated. It is required for HTTPS inspection.

Fiddler2 requires additional steps to monitor Metro-style apps. But with Fiddler4 all should just work automagically.

Monitor remote applications

To monitor the traffic from other computers (like Mac) you need to allow remote computers to connect in Fiddler’s options:


Take a note of the port Fiddler listens on in the same options page (8888 in my case). The only other information you need is the IP address of the system running the proxy. Then set the other computer to use this IP and port as the proxy. Here’s how to do it on Mac. At this point you should see all the remote traffic going through Fiddler.

For HTTPS inspection from remote computers, remember to export the Fiddler’s root certificate and import it as a trusted Root CA on the remote computer. Otherwise you will get security prompts or your applications may refuse to contact their servers.


iOS and Android

The setup here is similar to the monitoring remote applications case – you need to allow remote computers to connect. Next, install the CertMaker for iOS and Android add-on. After that you have to restart Fiddler. Once it comes back online – change your mobile device settings to use the Fiddler machine as proxy. Last, visit the http://<Fiddler.machine.ipv4.address>:8888/ page from your mobile device – and install the root certificate from this website.

Modify the traffic on the fly

FiddlerScript is very powerful. One useful case might be simulating server errors.

When I wanted to test whether my application handles server outages gracefully, I was adding rules to OnBeforeResponse function that would fake service issue:
oSession.oResponse.headers.HTTPResponseCode = 503;


The one very common issue I have seen with Fiddler is that it may leave the proxy enabled on the local system, even if it is not running. It may not break some applications (Firefox maintains its own proxy settings, for example) but will affect others (IE, Chrome etc.). So if you see network-related issues when Fiddler is not running, check your Control Panel > Internet Options and disable the proxy if needed.


The add-ons

The list just keeps going on. My favorites are:

  • CertMaker for iOS and Android – makes capturing mobile traffic easy
  • Syntax-Highlighting Add-Ons
  • Watcher – a Passive Security Auditor – generates security report as you click around your web application

The book

Yes, there is a book. I have not read the “Debugging with Fiddler“, yet. But since The Man wrote it, looks like recommended position. The official documentation is also great.

By the way…

Other similar tools I have used, and may suit your needs better:

  • ZAP – by OWASP, and has a Jenkins plugin
  • Burp – has awesome website mapping, and powerful security scanning built-in
  • Charles Proxy – works on Mac (Yay!), but is not free (Nay)

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