ASP.NET Core Hosting

In a classic ASP.NET application everything is hosted inside of an IIS Worker Process (w3wp.exe) which is the IIS Application Pool. The AppPool hosts your ASP.NET application and your application is instantiated by the built-in ASP.NET hosting features in IIS. The native runtime manager instantiates the .NET Runtime on your application's behalf and brings up the HttpRuntime object which is then used to fire requests through the ASP.NET application pipeline as requests come in from the native http.sys driver. Requests come in from http.sys and are dispatched to the appropriate site that is mapped to the Application Pool and the HttpRuntime instance hosted there.

ASP.NET Core with IIS

Things are quite different with ASP.NET Core which doesn't run in-process to the IIS worker process, but rather runs as a separate, out of process Console application that runs its own Web server using the Kestrel component. Kestrel is a .NET Web Server implementation that has been heavily optimized for throughput performance. It's fast and functional in getting network requests into your application, but it's 'just' a raw Web server. It does not include Web management services as a full featured server like IIS does.

If you run on Windows you will likely want to run Kestrel behind IIS to gain infrastructure features like port 80/443 forwarding via Host Headers, process lifetime management and certificate management to name a few.

Here's what it looks like when you run your ASP.NET Core application behind an IIS Web front:

ASP.NET Core applications are standalone Console applications invoked through the dotnet runtime command. They are not loaded into an IIS worker process, but rather loaded through a native IIS module called AspNetCoreModule that executes the external Console application.

The AspNetCoreModule's job is to ensure that your application gets loaded when the first request comes in and that the process stays loaded if for some reason the application crashes. You essentially get the same behavior as classic ASP.NET applications that are managed by WAS (Windows Activation Service).

Once running, incoming Http requests are handled by this module and then routed to your ASP.NET Core application.

So, requests come in from the Web and int the kernel mode http.sys driver which routes into IIS on the primary port (80) or SSL port (443). The request is then forwarded to your ASP.NET Core application on the HTTP port configured for your application which is not port 80/443. In essence, IIS acts a reverse proxy simply forwarding requests to your ASP.NET Core Web running the Kestrel Web server on a different port.

Kestrel picks up the request and pushes it into the ASP.NET Core middleware pipeline which then handles your request and passes it on to your application logic. The resulting HTTP output is then passed back to IIS which then pushes it back out over the Internet to the HTTP client that initiated the request - a browser, mobile client or application.

The AspNetCoreModule is configured via the web.config file found in the application's root, which points a the startup command (dotnet) and argument (your application's main dll) which are used to launch the .NET Core application. The configuration in the web.config file points the module at your application's root folder and the startup DLL that needs to be launched.

Here's what the web.config looks like:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <!--
    Configure your application settings in appsettings.json. Learn more at http://go.microsoft.com/fwlink/?LinkId=786380
  -->
  <system.webServer>
    <handlers>
      <add name="aspNetCore" path="*" verb="*"
        modules="AspNetCoreModule" resourceType="Unspecified" />
    </handlers>
    <aspNetCore processPath="dotnet"
                arguments=".\AlbumViewerNetCore.dll" 
                stdoutLogEnabled="false" 
                stdoutLogFile=".\logs\stdout" 
                forwardWindowsAuthToken="false" />
  </system.webServer>
</configuration>