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.

The tool 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.

Some of the features are:

• Simple and modular code base making it easy to contribute.
• Fast And Powerful Bruteforcing Module
• Powerful Permutation generation engine. (In Development)
• Many Passive Data Sources (31 At Present)
• Multiple Output formats
• Embeddable Project
• Raspberry Pi Support

You can read more and download this tool over here: https://github.com/subfinder/subfinder