Firewall rules no longer apply when Fiddler is running

[Thald800]

When I open Fiddler Classic, the Firewall rules no longer work. For example, I have a rule blocking all outbound traffic from Google Chrome. When Fiddler is not opened, the rule works and I am unable to access the internet through Chrome, but once I open Fiddler, the Firewall rule stops working and I can browse the internet normally.

I believe this has something to do with Fiddler connecting me to a proxy which bypasses the Firewall. So I tried unchecking “Act as system proxy on startup” under Options → Connections, but this causes Fiddler not to record any traffic at all. I also tried selecting “No Proxy” under the Gateway tab, but this does not seem to have any effect.

How can I fix this issue? Do I have to configure the Firewall settings to apply to the proxy? I am using Windows 10.

[Nick Iliev]

Hello,

 

Your firewall rule likely blocks a specific process, such as the Google Chrome browser, but we can only speculate on the specifics. When Fiddler works as a system proxy, it is an intermediary between the client (e.g., the browser) and the server. This means that any requests made from the browser go through Fiddler first before being sent as outbound traffic to the server. Therefore, the firewall rule is unlikely to be triggered as the actual outbound request is not from the Google Chrome browser but from the Fiddler Classic application.

As a side note, Fiddler has multiple techniques that allow you to block specific domains or filter specific traffic, like using the AutoResponder, the built-in filters, FiddlerScript, etc. Or you can entirely set specific domains to bypass Fiddler through the "Bypass Fiddler Classic for URLs that start with" option in the Connections settings screen.

 

Regards,
Nick Iliev
Progress Telerik

[Thald800]

How can I block traffic from a specific process, e.g. browser.exe?

[Nick Iliev]

You can't block a process through Fiddler, but you can filter traffic based on its process through the Any Process option in the toolbar.

Other options explicitly instruct the specific application not to use the system proxy (when Fiddler acts as a system proxy).

Or you can use FidderScript and the sessions ui-hide flag to hide the traffic from a specific process - this doesn't mean that the application won't use the proxy, but only that Fiddler won't show it.

static function OnPeekAtRequestHeaders(oSession: Session) {
        var sProc = ("" + oSession["x-ProcessInfo"]).ToLower();
        if (!sProc.StartsWith("mylowercaseappname")) oSession["ui-hide"] = "NotMyApp";
}

 

As a side note, the newest version of the Fiddler Everywhere application is currently testing network capturing from the adapter - the features currently only available for macOS and allow you to block a process (as the capturing works on a lower level and not as HTTPS proxy). The team is also working on providing the feature on Windows in the near future.