IPtables: is there a minimum UDP packet size for a DNS lookup?

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http://serverfault.com – IPtables for a virtual dedicated server. I would like to block UDP scans and I was wondering whether there's a minimum packet size for a DNS lookup? Nmap sends 0-byte UDP packets (source : http://nmap.org/bennieston-tutorial/ ), but there're probably tools available that allow you to add a few bytes. Also, I don't quite understand how nmap's UDP packets can be 0 bytes. (HowTos)