Anubis Subdomain enumeration is described as 'Anubis is a subdomain enumeration and information gathering tool. Anubis collates data from a variety of sources, including HackerTarget, DNSDumpster, x509 certs, VirusTotal, Google, Pkey, and NetCraft'. There are three alternatives to Anubis Subdomain enumeration for Linux, Online / Web-based and Self-Hosted solutions. The best alternative is Sublist3r, which is both free and Open Source. Other great apps like Anubis Subdomain enumeration are OWASP Amass (Free, Open Source) and Lepus Subdomain finder (Free, Open Source).
The OWASP Amass Project has developed a tool to help information security professionals perform network mapping of attack surfaces and perform external asset discovery using open source information gathering and active reconnaissance techniques.