ASP.NET Membership for one website but multiple and potentially unknown sql server instance?

This is an interesting problem. Seems like you are attempting to use a different connection string for the database, based on the URL. This might be a bit less elegant, but how about getting the code for the sql membership provider and modifying it so that the connection string which it uses is based on the users URL.

I think this could be a good work around if you are truly only using one asp.net application.

I think I have the solution but it's a bit strange.

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