I'm a bind beginner and I set up a bind9 master public server.
This seem to work fine according to my tests and those of some dns web
checker, but now in my query log file, amongst the 'good' queries, I get
hundreds of line like those :

client query: . IN NS

What does this mean ?
Is this normal, or are they misconfigured dns servers ?
This does not seem to trouble my dns server, just inflate the logs.
Is there a way to avoid that ?

Nicolas Ecarnot