add info to faq about TURN ports on Chrome

Author: lukehb

FAQ.md

@@ -30,6 +30,7 @@ This list is to help solve a variety of issues that you may stumble across. Some
  - [How do I test my STUN/TURN Server?](#how-do-i-test-my-stunturn-server)
  - [STUN](#stun)
  - [TURN](#turn)
+ - [TURN/relay candidates are not generated in Chrome?](#turnrelay-candidates-are-not-generated-in-chrome)
 
 
 
@@ -230,4 +231,13 @@ This will force all connections to use the TURN relay.
 
 4. Try and connect to your application in a new tab. If the TURN server is working properly, it should connect. If it does not, it will likely get stuck and fail to connect.
 
-Alternatively, you can see our page on doing a STUN/TURN Debug using pion/turn [here](ICE%20Debugging.md).
+Alternatively, you can see our page on doing a STUN/TURN Debug using pion/turn [here](ICE%20Debugging.md).
+
+## TURN/relay candidates are not generated in Chrome?
+Chrome will only let you use TURN if you are running TURN on port 53, 80, 443, 1024, or any port above 1024 [see WebRTC source](https://chromium.googlesource.com/external/webrtc/+/refs/heads/master/p2p/base/turn_port.cc#937). We typically recommend you run TURN on UDP port 19303. However, if you desperately need to run TURN on a port below 1024 you can do so by launching Chrome with a special field trial flag, like so:
+
+**MacOS**
+`/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --force-fieldtrials="WebRTC-Turn-AllowSystemPorts/Enabled/"`
+
+**Windows**
+`Chrome.exe --force-fieldtrials="WebRTC-Turn-AllowSystemPorts/Enabled/"`