what IS the "DNS" check in xymon?

if we set a hosts.cfg line

10.24.38.132? ? walkie # ssh dns we can expect xymon to check that ssh connections exist for thats erver, and also does some sort of dns check.

what exactly IS this check?

becasue the dns on that server is runjning fine - but the xymon alert shopws red.

Service dns on walkie is not OK : Service unavailable

Name not found

Seconds: 0.007559000

any ideas please/? Ive googled and come up with nothing (though my googling skills are not the greatest)