Your question doesn't make a lot of sense as Active Directory does not run on Linux if you have some sort of directory/user service running on Linux it would be something like OpenLDAP
], if you are running Active Directory then that would be a Windows server and as you have an ASP.Net web site which would be running under IIS (Windows Web Sever) you could use Integrated Authentication
], this option is best used on intranet sites not internet sites.
Try the Novell LDAP Library
] it can be downloaded from here
], if you want to connect to an LDAP server yourself.