subfinder
Fast passive subdomain enumeration tool
...Its modular architecture allows users to enable dozens of data providers through API keys, expanding coverage as needed. Subfinder integrates easily into automation pipelines and CI workflows thanks to its clean command-line design and structured output formats. The tool is intentionally specialized, doing one job extremely well rather than attempting to be an all-in-one scanner. In practice, Subfinder serves as a foundational building block for modern reconnaissance stacks.