How does ShareDrop (a website) get my LAN IP address?

http, ip

Solution

Go here and view the source - http://net.ipcalf.com/ They parse the metadata from the SDP while creating a WebRTC connection. The code is commented with links to the relevant RFCs.

This is probably how ShareDrop does it too.

Edit: ShareDrop is open source and they do this exactly the same way as mentioned above, see https://github.com/cowbell/sharedrop/blob/master/app/scripts/app/controllers/index_controller.js

Problem

I just went to https://www.sharedrop.io/ and it says my private LAN IP address 192.168.0.3, which is correct. How can it know this? This information isn't sent out via the browser. Or is it? I'm using Chrome, no extensions installed.

Original source