Fierce Domain Scanner is a Perl tool written in Perl that quickly scans domains. It uses a variety of techniques to locate undocumented, internal or hard-to-find resources via the DNS system. Version 2 of the scanner is now available to download and use for the first time in a few minutes, assuming no network lag. Fierce is a reconnaissance tool that quickly scanned domains (usually in just a few. minutes) Using a variety. of techniques, Fierce can also locate undocumented or hard to find resources via DNS.
Source: https://thehackernews.com/2010/11/fierce-domain-scanner-v2-new-version.html