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/133741/dnsspider-0.7.py.txt
Source: https://packetstormsecurity.com/files/133741/DNS-Spider-Multithreaded-Bruteforcer-0.7.html