Web Publishing: Paths Root Only

Hey,

I can't quite crack this one.

I need to map any requests to the root, or files in the root to a sub-folder, but not requests directly to a sub-folder.

So, https://domainname.com or https://domainname.com/file.aspx should redirect to https://domainname.com/external and https://domainname.com/external/file.aspx, but https://domainname.com/sub-folder or https://domainname.com/sub-folder/file.aspx should not redirect to the https://domainname.com/external path. Instead they should stay as is.

Is this possible?

Thanks