Strange, I search for 184.108.40.206 in Google and it returns me a thread from an HK forum, which teaches people how to set DD-WRT to use bogus-nxdomain. Coincidentally, the thread is started 7 days ago. You probably don't read Chinese, but here is the page anyway. They mention DNSMasq and bogus-nxdomain=220.127.116.11: http://www.hkepc.com/forum/viewthread.php?tid=1446475
I think I know why they use 18.104.22.168.
It seems to be an address belonging to PCCW, possibly part of their backbone or something. So there are PCCW customers unhappy with PCCW hijacking failed DNS requests towards Yellow Pages HK, and so decide to configure routers to hammer a PCCW server instead for every failed DNS request.