DNS Discovery is a multi-threaded DNS sub-domain brute-forcing utility. It is meant to be used by pen-testers during initial stages.
You can download it from the following link: https://packetstormsecurity.com/files/download/105244/dns-discovery.tar.bz2
Source: https://packetstormsecurity.com/files/105244/DNS-Discovery-Sub-Domain-Brute-Forcer.html