深入剖析ASPNET Core2.1 部署模型，你会大吃一惊

//------------- 节选自Microsoft.AspNetCore.Hosting.WebHostBuilderIISExtensions---------------------
   public static class WebHostBuilderIISExtensions
    {
        // These are defined as ASPNETCORE_ environment variables by IIS's AspNetCoreModule.
        private static readonly string ServerPort = "PORT";
        private static readonly string ServerPath = "APPL_PATH";
        private static readonly string PairingToken = "TOKEN";
        private static readonly string IISAuth = "IIS_HTTPAUTH";
        private static readonly string IISWebSockets = "IIS_WEBSOCKETS_SUPPORTED";

        /// <summary>
        /// Configures the port and base path the server should listen on when running behind AspNetCoreModule.
        /// The app will also be configured to capture startup errors.
        /// </summary>
        /// <param name="hostBuilder"></param>
        /// <returns></returns>
        public static IWebHostBuilder UseIISIntegration(this IWebHostBuilder hostBuilder)
        {
            if (hostBuilder == null)
            {
                throw new ArgumentNullException(nameof(hostBuilder));
            }

            // Check if `UseIISIntegration` was called already
            if (hostBuilder.GetSetting(nameof(UseIISIntegration)) != null)
            {
                return hostBuilder;
            }

            var port = hostBuilder.GetSetting(ServerPort) ?? Environment.GetEnvironmentVariable($"ASPNETCORE_{ServerPort}");
            var path = hostBuilder.GetSetting(ServerPath) ?? Environment.GetEnvironmentVariable($"ASPNETCORE_{ServerPath}");
            var pairingToken = hostBuilder.GetSetting(PairingToken) ?? Environment.GetEnvironmentVariable($"ASPNETCORE_{PairingToken}");
            var iisAuth = hostBuilder.GetSetting(IISAuth) ?? Environment.GetEnvironmentVariable($"ASPNETCORE_{IISAuth}");
            var websocketsSupported = hostBuilder.GetSetting(IISWebSockets) ?? Environment.GetEnvironmentVariable($"ASPNETCORE_{IISWebSockets}");

            bool isWebSocketsSupported;
            if (!bool.TryParse(websocketsSupported, out isWebSocketsSupported))
            {
                // If the websocket support variable is not set, we will always fallback to assuming websockets are enabled.
                isWebSocketsSupported = (Environment.OSVersion.Version >= new Version(6, 2));
            }

            if (!string.IsNullOrEmpty(port) && !string.IsNullOrEmpty(path) && !string.IsNullOrEmpty(pairingToken))
            {
                // Set flag to prevent double service configuration
                hostBuilder.UseSetting(nameof(UseIISIntegration), true.ToString());

                var enableAuth = false;
                if (string.IsNullOrEmpty(iisAuth))
                {
                    // back compat with older ANCM versions
                    enableAuth = true;
                }
                else
                {
                    // Lightup a new ANCM variable that tells us if auth is enabled.
                    foreach (var authType in iisAuth.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries))
                    {
                        if (!string.Equals(authType, "anonymous", StringComparison.OrdinalIgnoreCase))
                        {
                            enableAuth = true;
                            break;
                        }
                    }
                }

                var address = "http://127.0.0.1:" + port;
                hostBuilder.CaptureStartupErrors(true);

                hostBuilder.ConfigureServices(services =>
                {
                    // Delay register the url so users don't accidently overwrite it.
                    hostBuilder.UseSetting(WebHostDefaults.ServerUrlsKey, address);
                    hostBuilder.PreferHostingUrls(true);
                    services.AddSingleton<IStartupFilter>(new IISSetupFilter(pairingToken, new PathString(path), isWebSocketsSupported));
                    services.Configure<ForwardedHeadersOptions>(options =>
                    {
                        options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
                    });
                    services.Configure<IISOptions>(options =>
                    {
                        options.ForwardWindowsAuthentication = enableAuth;
                    });
                    services.AddAuthenticationCore();
                });
            }

            return hostBuilder;
        }
    }

//---------------------------------节选自Microsoft.AspNetCore.Hosting.Internal.WebHost------------------------------------    
  private RequestDelegate BuildApplication()
 {
      try
      {
           _applicationServicesException?.Throw();
           EnsureServer();

          var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>();
          var builder = builderFactory.CreateBuilder(Server.Features);
          builder.ApplicationServices = _applicationServices;

          var startupFilters = _applicationServices.GetService<IEnumerable<IStartupFilter>>();
          Action<IApplicationBuilder> configure = _startup.Configure;
          foreach (var filter in startupFilters.Reverse())
          {
               configure = filter.Configure(configure);        // 挨个启动功能
          }

          configure(builder);

          return builder.Build();
       }
       ......
}

//---------------------------------节选自Microsoft.AspNetCore.Server.IISIntegration.IISSetupFilter------------------------------------    
namespace Microsoft.AspNetCore.Server.IISIntegration
{
    internal class IISSetupFilter : IStartupFilter
    {
        private readonly string _pairingToken;
        private readonly PathString _pathBase;
        private readonly bool _isWebsocketsSupported;

        internal IISSetupFilter(string pairingToken, PathString pathBase, bool isWebsocketsSupported)
        {
            _pairingToken = pairingToken;
            _pathBase = pathBase;
            _isWebsocketsSupported = isWebsocketsSupported;
        }

        public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
        {
            return app =>
            {
                app.UsePathBase(_pathBase);
                app.UseForwardedHeaders();                                           //  转发时保持原始请求，放在header里面传给kestrel
                app.UseMiddleware<IISMiddleware>(_pairingToken, _isWebsocketsSupported);  //  阻止非aspnetcore module转发的请求  
                next(app);
            };
        }
    }
}