Dns queries question

RB serving as gateway for a hundred smartphone clients
RB is DNS/DHCP server

In few hours I end up with many hundreds DNS cache entries like these :

AP are WiFi routers indeed, and I continuously have dns queries from their ip address to RB in 5-10 pkt/sec rate

I’m not aware of wifi routers configuration , they get each a dhcp lease from RB

Is this normal behavior or something to be worried about ?

http://forum.mikrotik.com/t/suspicious-dns-cache-entries/134219/1

So, it seems common and safe…