ASP.NET - Accessing a domain share from outside the domain

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ASP.NET

impersonation

I have a Windows Server 2008 domain controller that shares out a particular file folder on the network. I have developed an ASP.NET app that allows outside clients (via the Internet) to browse this folder and download documents (based on permissions handled by the app). In order to bolster security, the web server on which the ASP.NET app runs (and hence is port-forwardable behind the firewall) is outside the domain. My problem is how to access the domain-based share via my ASP.NET application. The account under which my ASP.NET app runs is obviously local to the web server (again, outside the domain). I need to programmatically access the share at runtime. I can't just impersonate a domain account for the entire app because the web server doesn't know anything about that account. Any ideas on what I should do?