What is the difference between active and passive DNS?

The Domain Name System (DNS) is essential for the operation of the Internet. It enables the assignment of human-readable hostnames to machine-readable IP addresses and vice-versa.

Active DNS data are the current DNS records available also in the Domain Name System upon name server queries.

Passive DNS data are collected with sensors from the DNS traffic; they contain DNS records that were actually transmitted along with the timestamp of their observation. Thus passive DNS data have partial coverage only but reveal the history of DNS records that is missing from active DNS.

Two typical forms of passive DNS data access include passive DNS lookups and complete passive DNS databases. A sample of passive DNS data can be downloaded for free here.