Fierce Domain Scan - DNS Name discovery

From: http://ha.ckers.org

Fierce domain scan was born out of personal frustration after performing a web application security audit. It is traditionally very difficult to discover large swaths of a corporate network that is non-contiguous. It's terribly easy to run a scanner against an IP range, but if the IP ranges are nowhere near one another you can miss huge chunks of networks.

First what fierce is not. Fierce is not an IP scanner, it is not a DDoS tool, it is not designed to scan the whole internet or perform any un-targeted attacks. It is meant specifically to locate likely targets both inside and outside a corporate network. Only those targets are listed. No exploitation is performed. Fierce is a reconnaissance tool. Fierce is a perl script that quickly scans domains (usually in just a few minutes, assuming no network lag) using several tactics.

See more at: http://ha.ckers.org/fierce/