webrtc leak addon
About the WebRTC Leak Addon
"As Daniel Roesler showed in January 2015, browsers with WebRTC implementation allow requests to STUN servers which will return the provider assigned IP address for the user even if he is connected via VPN. For more details on this, read on and also see Daniel's GitHub page."
Web RTC Leak Addon (Firefox)
WebRTC Leak Addon (Chrome)