Lepus Subdomain finder is described as 'Online Subdomain finder and subdomain Enumerating tools using Sublist3r, DNscan, Nmap, Anubis and Amass. Discover subdomains of target domain. 8 subdomain tools hosted online'. There are four alternatives to Lepus Subdomain finder 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 Lepus Subdomain finder are OWASP Amass (Free, Open Source), Anubis Subdomain enumeration (Free, Open Source) and Dnscan (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.
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.
Dnscan is a python wordlist-based DNS subdomain scanner. The script will first try to perform a zone transfer using each of the target domain's nameservers. If this fails, it will lookup TXT and MX records for the domain.