SubFinder is a subdomain discovery tool that discovers valid subdomains for
websites by using passive online sources. It has a simple modular architecture
and has been aimed as a successor to sublist3r project. SubFinder uses Passive
Sources, Search Engines, Pastebins, Internet Archives, etc to find subdomains
and then it uses a permutation module inspired by altdns to generate
permutations and resolve them quickly using a powerful bruteforcing engine. It
can also perform plain bruteforce if needed. The tool is highly customizable,
and the code is built with a modular approach in mind making it easy to add
functionalities and remove errors.

WWW: https://github.com/projectdiscovery/subfinder