Malicious DNS request?

Hi, In past days I noticed the nxdomain statistics in named.stats keeps increasing.( I run it every 5 min) By tcpdump, it's found a remote computer keep asking address for record like 999d38e693b9e6293b450.0existence.com, 60d38e693b9e6293b450.0be6c1xfa.net. is that a virus affacted computer? How could such request be filtered or minimize its affaction on DNS server? regards Joe __________________________________________________ Do You Yahoo!? Log on to Messenger with your mobile phone! http://sg.messenger.yahoo.com