Knock is a python script, written by Gianni 'guelfoweb' Amato, designed to enumerate subdomains on a target domain through a wordlist.
For more information I have posted a documentation page. If you want to see how it works, you can see this sample output:
New: Knock is now available on BackBox 2, Italian distribution for Penetration Test.
In version 1.5 is enabled support to wildcard bypass.
Scan subdomains DNS request for zone transfer DNS resolver Wildcard testing Wildcard bypass
Knock 1.5 for all operating system