FTP server with programmable authentication

I'm after an FTP server which will allow me to write the authentication plugin/module. I need to a开发者_开发知识库uthenticate against an exisiting webservice (so ActiveDirectory, LDAP, ODBC, database tables, punched card auth isn't useful).

Doing a bit of research I've found Raiden - which appears to use executables to perform this task.

Preferably want something in the .net arena...

UPDATE: This feature actually exists in IIS7:

• Configuring FTP with .NET Membership Authentication
• Code it yourself against the IFtpAuthenticationProvider Interface

Unfortunately my server box is stuck on Win2003. Currently looking at Virtual users feature of zftpserver.

ANOTHER UPDATE: Ended up going with CompleteFTP. They pretty much added this feature for me, and were very attentive with thier feedback. They also browsed through some wireshark captures to help diagnose an issue that wasn't even related to their product.