An alpha build of Fiddler for Mono is available and enables you to run Fiddler directly on the Mac. Unfortunately Mac’s Mono WinForms framework isn’t very robust and frequently crashes due to bugs not present in the Linux Mono framework or the Microsoft .NET Framework on Windows. Therefore, Mac users often get the best results by running Fiddler inside a virtual machine running either Linux or Windows.
In the documentation, I wrote about how to run Fiddler inside Parallels, but some users prefer to run the free and open-source VirtualBox client. Running Fiddler inside VirtualBox is simple.
- Install VirtualBox and create a Windows or Linux virtual machine.
- In VirtualBox Manager, select the VM and click the Settings icon at the top of the window.
- Click the Network icon at the top of the window and change Attached to: to Bridged Adapter. Click Ok.
- Start the Virtual Machine and install Fiddler.
- Start Fiddler and click Tools > Fiddler Options > Connections. Tick the “Allow remote computers to connect” box.
- Restart Fiddler.
- Note the IPv4 address of the Fiddler instance by hovering over the Online icon in Fiddler’s toolbar, or run ifconfig eth0 in the terminal (Linux) or ipconfig (Windows).
- On the Mac, click the Apple Menu > System Preferences > Network.
- Click the Advanced button.
- On the Proxies tab, Enable the Web Proxy (HTTP) and Secure Web Proxy (HTTPS) options to point to the IPv4 address of the virtual machine (found in Step #7) using port 8888.
- To decrypt HTTPS traffic, you’ll need to install the HTTPS certificate in the Mac’s keychain. You can download this certificate by visiting the http://IPAddress:8888.
Until a new build of Fiddler for the Mac (based on a more mature UI Framework) is available, I hope these instructions enable you to use Fiddler effectively on your Mac!