Public Resource

Search engines (Google, Bing, Yahoo, Baidu)

https://virustotal.com/ – If you search for "domain:target.com," virustotal will provide you a wealth of information in addition to its list of all known subdomains, known as Observed subdomains.

https://dnsdumpster.com – The name explains everything. To profit, enter the desired domain and press search.

https://crt.sh/?q=%25target.com – SSL can occasionally be a veritable information gold mine. Use this website to search for "%target.com"; you will receive results with subdomains. Simple victory.

https://censys.io – Not excellent, but occasionally has some helpful information.

http://searchdns.netcraft.com/ – One more to watch out for.

https://www.shodan.io – Designed primarily for security professionals, Shodan is an infrastructure-based spider that comes with a database for caching information. It contains information on a wide range of internet servers, both historical and current, about things like server versioning and seen-subdomains.