You have a couple of choices here but, as joe said, it would be much easier for us to help you if we could see your code.
First off, a good explination of what you want to do can be found in chapter 12 of: Professional ASP.NET 2.0 Security, Membership, and Role Management (
http://www.wrox.com/WileyCDA/WroxTit...764596985.html ) as chapter 12 covers the ActiveDirectoryMembershipProvider at great length and how to query AD using that provider.
Alternatively you can read that book at
http://wrox.books24x7.com (and alot of the other Wrox publications for that matter) and it only costs $49.99 per 3 months. ^^
In any event, without seeing yoru code, you have a couple of ways to solve this.
First, you could setup Integrated Windows Authentication for your web directory but, by doing this, you make at least 1 assumption: That the windows account accessing the resource has the necessary rights to query Active Directory.
Secondly, you could use impersonation through your web.config so that the your application runs as X Domain Account that has the rights to query Active Directory.
hth.
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Read this if you want to know how to get a correct reply for your question:
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Technical Editor for:
Professional Search Engine Optimization with ASP.NET
Professional IIS 7 and ASP.NET Integrated Programming
Wrox Blox: Introduction to Google Gears
Wrox Blox: Create Amazing Custom User Interfaces with WPF and .NET 3.0
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