Fierce is an open source active information gathering tool to identify subdomains of a target website. This tool was written by Robert Hansen. It’s a PERL script that quickly scans domains (usually in just a few minutes, assuming no network lag) using several tactics. The tool comes pre-installed by default in Kali Linux from Offensive-Security. The Perl tool uses techniques such as zone transfer and wordlist brute-forcing to find subdomains of the target domain.
How to open it?
Application → Kali Linux → Information gathering → DNS Analysis → Fierce
Open the terminal and write the fierce –h command to open the tool with the help option.