Just a quick glance at the domain_analyzer script from Sebastián García and Verónica Valeros. “Domain analyzer is a security analysis tool which automatically discovers and reports information about the given domain. Its main purpose is to analyze domains in an unattended way.” Nice one. If you’re running your own DNS servers you should check e.g. whether your firewall rules are correct (scanned with Nmap) or whether you’re not allowing zone transfer, etc.