Odd DNS requests

[Ian Smith]

Hi,

recently we've had a Mac notebook of some sort on our LAN, that likes to 
make these DNS queries from time to time, to no avail, as noticed on a 
filtering bridge between the LAN and the router+DNS at 192.168.0.1:

16:13:05.020397 192.168.0.59.53207 > 192.168.0.1.53:  63162+ PTR? b._dns-sd._udp.0.0.168.192.in-addr.arpa. (57) [tos 0x18]
16:13:05.021093 192.168.0.1.53 > 192.168.0.59.53207:  63162 NXDomain* 0/1/0 (128) (DF)
16:13:05.215790 192.168.0.59.64633 > 192.168.0.1.53:  61059+ PTR? db._dns-sd._udp.0.0.168.192.in-addr.arpa. (58) [tos 0x18]
16:13:05.216469 192.168.0.1.53 > 192.168.0.59.64633:  61059 NXDomain* 0/1/0 (129) (DF)
16:13:05.226242 192.168.0.59.61635 > 192.168.0.1.53:  6749+ PTR? r._dns-sd._udp.0.0.168.192.in-addr.arpa. (57) [tos 0x18]
16:13:05.226789 192.168.0.1.53 > 192.168.0.59.61635:  6749 NXDomain* 0/1/0 (128) (DF)
16:13:05.237319 192.168.0.59.56300 > 192.168.0.1.53:  21450+ PTR? dr._dns-sd._udp.0.0.168.192.in-addr.arpa. (58) [tos 0x18]
16:13:05.237842 192.168.0.1.53 > 192.168.0.59.56300:  21450 NXDomain* 0/1/0 (129) (DF)
16:13:05.248440 192.168.0.59.60806 > 192.168.0.1.53:  10032+ PTR? lb._dns-sd._udp.0.0.168.192.in-addr.arpa. (58) [tos 0x18]
16:13:05.249252 192.168.0.1.53 > 192.168.0.59.60806:  10032 NXDomain* 0/1/0 (129) (DF)

What exactly are these hoping to discover, and what needs turning off in 
the Mac's setup (OSX, most likely a recent version) to quell them?

cheers, Ian  (please cc me, I'm subscribed to the -digest)