DNS Spider is a multithreaded bruteforcer of subdomains that leverages a wordlist and/or character permutation.
You can download it from the following link: https://packetstormsecurity.com/files/download/115635/dnsspider-0.4.py.txt
Source: https://packetstormsecurity.com/files/115635/DNS-Spider-Multithreaded-Bruteforcer-0.4.html