Subfinder
Fast passive subdomain discovery tool that enumerates valid subdomains for a domain.
Subfinder is an open-source utility designed for passive subdomain enumeration, allowing users to identify valid subdomains associated with a specific target domain. It operates by querying various public data sources without sending direct requests to the target infrastructure. This approach makes it useful for security researchers and investigators mapping the external attack surface of an organization during reconnaissance phases.
Official site: https://github.com/projectdiscovery/subfinder