Subfinder is a tool for discovering subdomains for a given domain.
It can discover both known and unknown subdomains, which can be useful for:
⏺discovering hidden resources or services;
⏺determining network boundaries and perimeters;
⏺vulnerability testing and vulnerability detection;
⏺monitoring and controlling network security.
— To discover subdomains, Subfinder uses various sources, including DNS resolvers, WHOIS databases, internet archives, and others.