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/152193/dnsspider-1.1.py.txt
Source: https://packetstormsecurity.com/files/152193/DNS-Spider-Multithreaded-Bruteforcer-1.1.html