Does Fiddler Everywhere proxy server modify any data by default?

[P]

I would like to use Fiddler Everywhere for remote support and trouble shooting in real time, Is there anyway for the Server to know that the Client is using Fiddler Everywhere as the proxy server if it doesn't modify any data, headers, or flags?

Thanks!

[Nick Iliev]

Hello,

 

By default, Fiddler Everywhere is a local proxy that only works on the Fiddler host machine. However, when the option "Allow remote computers to connect" is enabled (from Settings > Connections), then Fiddler will open its proxy port (by default, the port is 8866). Note that while Fiddler opens the port on the host machine (the client that runs Fiddler Everywhere), most uncommon ports used for user services (ports from 1025 to 65535) are, by default, closed for inbound connections. That said, the possibility of accessing a custom port like 8866 from an external network is relatively small. Additionally, most hosts are not using directly the external IP but instead are using a local IP address, which means that any external calls to port 8866 must be port-forwarded to a local IP address within the internal network.

All of the above considered, one way to check if Fiddler Everywhere is running on a remote host is to check if the proxy port is in use. However, this corner scenario assumes that the client has explicitly configured FIddler for remote connections and that the default port is not changed from the Fiddler application.

That said, there is no other reliable way to detect if Fiddler is running on a remote client apart from the client using a specific custom header/body for each HTTP request (e.g., added through a custom rule).

Regards,
Nick Iliev
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