I've used this module twice now to write two different kinds of FTP servers:
- Once that was a normal server, and I just overrode the authentication mechanisms.
- Once that was a completely virtual server, virtual filesystem, etc.
Though not always straightforward, this module makes it much easier to write an FTP server than normally would be possible. It takes care of all the FTP protocol stuff and let's you get down to the business of writing your custom functionality. Great module!