DNS Spider is a multi-threaded bruteforcer of subdomains that leverages a wordlist and/or character permutation.
You can download it from the following link: https://packetstormsecurity.com/files/download/146326/dnsspider-0.9.py.txt
Source: https://packetstormsecurity.com/files/146326/DNS-Spider-Multithreaded-Bruteforcer-0.9.html