How to combine IIS , Active directory and SQL

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ASP.NET

SQL

IIS

Here is the scenario:

I am using ASP.NET C# to make a web application.I am using Forms authentication and it's integrated with Active directory (Only The users in active directory can use the application).BUT not all users are on the same domain,that means i cannot use KERBEROS to achieve my goals.So back to the point.How can i LOG IN with AD user credentials using forms authentication (this thing i managed it to work) and the credentials of the logged in user to be "passed to the connection string" to the database.