Passive DNS replication is a technology which constructs zone replicas without cooperation from zone administrators, based on captured name server responses. The main idea behind passive DNS is as follows:
Inter-server DNS messages are captured by sensors and forwarded to a collection point for analysis.
After being processed, individual DNS records are stored in a database where they can be indexed and queried.
As such, passive DNS can help in answering the following questions:
Where did this domain name point to in the past?
What domain names are hosted by a given nameserver?
What domain names point into a given IP network?
What subdomains exist below a certain domain name?
read more..........http://blog.virustotal.com/2013/04/virustotal-passive-dns-replication.html