CloudFlare sends out the users IP as "REMOTE_ADDR" to the webserver.
Also, http://community.mybb.com/thread-104837.html will help you more.
MyBB Support
Connected via ""CDN SERVER"
Submitted by Oxygen, 14-03-2015, 05:53 PM, Thread ID: 1853
Thread Closed
RE: Connected via ""CDN SERVER" 14-03-2015, 05:56 PM #2
14-03-2015, 05:56 PM
#2