Performing a DNS lookup with dig results in this answer:

What might be wrong in the zone definition?

A.
Nothing. All seems to be good.

B.
There’s no “.” after linuserv.example.net in the PTR record in the forward lookup zone file.

C.
There’s no “.” after linuserv in the PTR record in the forward lookup zone file.

D.
There’s no “.” after linuserv.example.net in the PTR record in the reverse lookup zone file.

E.
The “.” in the NS definition in reverse lookup zone has to be removed.