Android: Direct connection not possible when connected via smartphone's hotspot

[lgrahl]

Expected Behavior

When the smartphone is being used as a hotspot (Bluetooth or WiFi) and the notebook is connected to the internet via that hotspot, a direct connection should be established (well, at least if the browser uses WebRTC mode <= 2).

Current Behavior

The connection is established via a relay (smartphone)/prflx (notebook) candidate pair.

Possible Solution

Looking at about:webrtc reveals that no UDP host candidates are being gathered (only TCP active) for the hotspot interface. I don't know why that is the case at the moment... It may be a limitation of the interface or a limitation/bug of/in Google's WebRTC library.

Steps to Reproduce (for bugs)

1. Start a hotspot (Bluetooth or WiFi) on your smartphone.
2. Connect to the hotspot with your notebook.
3. Open Threema Web on the notebook.
4. Check the selected candidate pair in about:webrtc.

Your Environment

• Threema Web version: 1.8.2
• Threema for Android version: 3.41
• Threema for Android source: Pirate Bay (obviously)
• Browser name and version: Firefox 59.0.2