DNS enum:

http://whois.domaintools.com/
https://toolbar.netcraft.com/site_report
https://www.robtex.com/
bing.com     ip:127.0.0.1
https://github.com/guelfoweb/knock

lbd - load balancing detector 0.4 - Checks if a given domain uses load-balancing. Written by Stefan Behte (http://ge.mine.nu)
dnsmap 0.30 - DNS Network Mapper by pagvac (gnucitizen.org) searching (sub)domains using built-in wordlist
amass enum --passive -d intellexa.com

https://github.com/subfinder/subfinder
https://github.com/TheRook/subbrute
https://github.com/aboul3la/Sublist3r
https://pentester.land/cheatsheets/2018/11/14/subdomains-enumeration-cheatsheet.html

https://censys.io/domain?q=
https://securitytrails.com/domain/
https://viewdns.info
http://www.crimeflare.org:82
https://dnsdumpster.com/