I have created a reverse lookup zone in my DNS server and fill the zone file manually in order to make my PRTG resolve IP addresses to host names. The prtg server is able to make reverse lookup successfully.(ping and nslookup can resolve IP addresses to host names) When netflow sensor starts receiving data, I can see successful lookups on the web page of the toplists. But after a few days the ip addresses have been failed to be resolved even the operating system (ping, nslookup) of the server is still able to resolve. I reset the DNS server but no success.