Dnscan is described as 'python wordlist-based DNS subdomain scanner'. There are three alternatives to Dnscan 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 Dnscan 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.