• ASP.NET Core apps configure and launch a host.
• The host is responsible for app startup and lifetime management.
• At a minimum, the host configures a server and a request processing pipeline.
• Create a host using an instance of WebHostBuilder. This is typically performed in the app's entry point, the Main method.

public class Program
{
    public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

    public static IWebHost BuildWebHost(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .Build();
}

• CreateDefaultBuilder performs the following tasks:

1. Configures Kestrel as the web server.
2. Sets the content root to the path returned by Directory.GetCurrentDirectory.
3. Loads optional configuration from:
   • appsettings.json.
   • appsettings.{Environment}.json.
   • User secrets when the app runs in the Development environment.
   • Environment variables.
   • Command-line arguments.
4. Configures logging for console and debug output.
   • Logging includes log filtering rules specified in a Logging configuration section of an appsettings.json or appsettings.{Environment}.json file.
5. When running behind IIS, enables IIS integration.
   • Configures the base path and port the server listens on when using the ASP.NET Core Module.
   • The module creates a reverse proxy between IIS and Kestrel.
   • Also configures the app to capture startup errors.
6. Sets ServiceProviderOptions.ValidateScopes to true if the app's environment is Development.

• The content root determines where the host searches for content files, such as MVC view files.
  • When the app is started from the project's root folder, the project's root folder is used as the content root.
  • This is the default used in Visual Studio and the dotnet new templates.

Web Server Implementations

• An ASP.NET Core app runs with an in-process HTTP server implementation.
• The server implementation listens for HTTP requests and surfaces them to the app as sets of request features composed into an HttpContext.
• ASP.NET Core ships two server implementations:
  • Kestrel is a cross-platform HTTP server based on libuv, a cross-platform asynchronous I/O library.
  • HTTP.sys is a Windows-only HTTP server based on the HTTP.sys kernel driver and HTTP Server API.

Kestrel

• Kestrel is the web server that's included by default in ASP.NET Core new-project templates.
• Kestrel can be used by itself or with a reverse proxy server, such as IIS, Nginx, or Apache.