Open Web Interface for .NET (OWIN)

By Steve Smith and Rick Anderson

ASP.NET Core supports the Open Web Interface for .NET (OWIN). OWIN allows web apps to be decoupled from web servers. It defines a standard way for middleware to be used in a pipeline to handle requests and associated responses. ASP.NET Core applications and middleware can interoperate with OWIN-based applications, servers, and middleware.

View or download sample code

Running OWIN middleware in the ASP.NET pipeline

ASP.NET Core’s OWIN support is deployed as part of the Microsoft.AspNetCore.Owin package. You can import OWIN support into your project by adding this package as a dependency in your project.json file:

  "dependencies": {
    "Microsoft.AspNetCore.Server.IISIntegration": "1.0.0",
    "Microsoft.AspNetCore.Server.Kestrel": "1.0.0",
    "Microsoft.AspNetCore.Owin": "1.0.0"

OWIN middleware conforms to the OWIN specification, which requires a Func<IDictionary<string, object>, Task> interface, and specific keys be set (such as owin.ResponseBody). The following simple OWIN middleware displays “Hello World”:

public Task OwinHello(IDictionary<string, object> environment)
    string responseText = "Hello World via OWIN";
    byte[] responseBytes = Encoding.UTF8.GetBytes(responseText);

    // OWIN Environment Keys:
    var responseStream = (Stream)environment["owin.ResponseBody"];
    var responseHeaders = (IDictionary<string, string[]>)environment["owin.ResponseHeaders"];

    responseHeaders["Content-Length"] = new string[] { responseBytes.Length.ToString(CultureInfo.InvariantCulture) };
    responseHeaders["Content-Type"] = new string[] { "text/plain" };

    return responseStream.WriteAsync(responseBytes, 0, responseBytes.Length);

The sample signature returns a Task and accepts an IDictionary<string, object> as required by OWIN.

The following code shows how to add the OwinHello middleware (shown above) to the ASP.NET pipeline with the UseOwin extension method.

public void Configure(IApplicationBuilder app)
    app.UseOwin(pipeline =>
        pipeline(next => OwinHello);

You can configure other actions to take place within the OWIN pipeline.


Response headers should only be modified prior to the first write to the response stream.


Multiple calls to UseOwin is discouraged for performance reasons. OWIN components will operate best if grouped together.

app.UseOwin(pipeline =>
    pipeline(next =>
        // do something before
        return OwinHello;
        // do something after

Using ASP.NET Hosting on an OWIN-based server

OWIN-based servers can host ASP.NET applications. One such server is Nowin, a .NET OWIN web server. In the sample for this article, I’ve included a project that references Nowin and uses it to create an IServer capable of self-hosting ASP.NET Core.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting.Server;
using Microsoft.AspNetCore.Hosting.Server.Features;
using Microsoft.AspNetCore.Http.Features;
using Microsoft.AspNetCore.Owin;
using Microsoft.Extensions.Options;
using Nowin;

namespace NowinSample
    public class NowinServer : IServer
        private INowinServer _nowinServer;
        private ServerBuilder _builder;

        public IFeatureCollection Features { get; } = new FeatureCollection();

        public NowinServer(IOptions<ServerBuilder> options)
            Features.Set<IServerAddressesFeature>(new ServerAddressesFeature());
            _builder = options.Value;

        public void Start<TContext>(IHttpApplication<TContext> application)
            // Note that this example does not take into account of Nowin's "server.OnSendingHeaders" callback.
            // Ideally we should ensure this method is fired before disposing the context. 
            Func<IDictionary<string, object>, Task> appFunc = async env =>
                // The reason for 2 level of wrapping is because the OwinFeatureCollection isn't mutable
                // so features can't be added
                var features = new FeatureCollection(new OwinFeatureCollection(env));

                var context = application.CreateContext(features);
                    await application.ProcessRequestAsync(context);
                catch (Exception ex)
                    application.DisposeContext(context, ex);

                application.DisposeContext(context, null);

            // Add the web socket adapter so we can turn OWIN websockets into ASP.NET Core compatible web sockets.
            // The calling pattern is a bit different
            appFunc = OwinWebSocketAcceptAdapter.AdaptWebSockets(appFunc);

            // Get the server addresses
            var address = Features.Get<IServerAddressesFeature>().Addresses.First();

            var uri = new Uri(address);
            var port = uri.Port;
            IPAddress ip;
            if (!IPAddress.TryParse(uri.Host, out ip))
                ip = IPAddress.Loopback;

            _nowinServer = _builder.SetAddress(ip)

        public void Dispose()

IServer is an interface that requires an Features property and a Start method.

Start is responsible for configuring and starting the server, which in this case is done through a series of fluent API calls that set addresses parsed from the IServerAddressesFeature. Note that the fluent configuration of the _builder variable specifies that requests will be handled by the appFunc defined earlier in the method. This Func is called on each request to process incoming requests.

We’ll also add an IWebHostBuilder extension to make it easy to add and configure the Nowin server.

using System;
using Microsoft.AspNetCore.Hosting.Server;
using Microsoft.Extensions.DependencyInjection;
using Nowin;
using NowinSample;

namespace Microsoft.AspNetCore.Hosting
    public static class NowinWebHostBuilderExtensions
        public static IWebHostBuilder UseNowin(this IWebHostBuilder builder)
            return builder.ConfigureServices(services =>
                services.AddSingleton<IServer, NowinServer>();

        public static IWebHostBuilder UseNowin(this IWebHostBuilder builder, Action<ServerBuilder> configure)
            builder.ConfigureServices(services =>
            return builder.UseNowin();

With this in place, all that’s required to run an ASP.NET application using this custom server to call the extension in Program.cs:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;

namespace NowinSample
    public class Program
        public static void Main(string[] args)
            var host = new WebHostBuilder()


Learn more about ASP.NET Servers.

Run ASP.NET Core on an OWIN-based server and use its WebSockets support

Another example of how OWIN-based servers’ features can be leveraged by ASP.NET Core is access to features like WebSockets. The .NET OWIN web server used in the previous example has support for Web Sockets built in, which can be leveraged by an ASP.NET Core application. The example below shows a simple web app that supports Web Sockets and echoes back everything sent to the server through WebSockets.

    public class Startup
        public void Configure(IApplicationBuilder app)
            app.Use(async (context, next) =>
                if (context.WebSockets.IsWebSocketRequest)
                    WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
                    await EchoWebSocket(webSocket);
                    await next();

            app.Run(context =>
                return context.Response.WriteAsync("Hello World");

        private async Task EchoWebSocket(WebSocket webSocket)
            byte[] buffer = new byte[1024];
            WebSocketReceiveResult received = await webSocket.ReceiveAsync(
                new ArraySegment<byte>(buffer), CancellationToken.None);

            while (!webSocket.CloseStatus.HasValue)
                // Echo anything we receive
                await webSocket.SendAsync(new ArraySegment<byte>(buffer, 0, received.Count), 
                    received.MessageType, received.EndOfMessage, CancellationToken.None);

                received = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), 

            await webSocket.CloseAsync(webSocket.CloseStatus.Value, 
                webSocket.CloseStatusDescription, CancellationToken.None);

This sample is configured using the same NowinServer as the previous one - the only difference is in how the application is configured in its Configure method. A test using a simple websocket client demonstrates the application:


OWIN keys

OWIN depends on an IDictionary<string,object> object to communicate information throughout an HTTP Request/Response exchange. ASP.NET Core implements the keys listed below. See the primary specification, extensions, and OWIN Key Guidelines and Common Keys.

Request Data (OWIN v1.0.0)

Key Value (type) Description
owin.RequestScheme String  
owin.RequestMethod String  
owin.RequestPathBase String  
owin.RequestPath String  
owin.RequestQueryString String  
owin.RequestProtocol String  
owin.RequestHeaders IDictionary<string,string[]>  
owin.RequestBody Stream  

Request Data (OWIN v1.1.0)

Key Value (type) Description
owin.RequestId String Optional

Response Data (OWIN v1.0.0)

Key Value (type) Description
owin.ResponseStatusCode int Optional
owin.ResponseReasonPhrase String Optional
owin.ResponseHeaders IDictionary<string,string[]>  
owin.ResponseBody Stream  

Other Data (OWIN v1.0.0)

Key Value (type) Description
owin.CallCancelled CancellationToken  
owin.Version String  

Common Keys

Key Value (type) Description
ssl.ClientCertificate X509Certificate  
ssl.LoadClientCertAsync Func<Task>  
server.RemoteIpAddress String  
server.RemotePort String  
server.LocalIpAddress String  
server.LocalPort String  
server.IsLocal bool  
server.OnSendingHeaders Action<Action<object>,object>  

SendFiles v0.3.0

Key Value (type) Description
sendfile.SendAsync See delegate signature Per Request

Opaque v0.3.0

Key Value (type) Description
opaque.Version String  
opaque.Upgrade OpaqueUpgrade See delegate signature
opaque.Stream Stream  
opaque.CallCancelled CancellationToken  

WebSocket v0.3.0

Key Value (type) Description
websocket.Version String  
websocket.Accept WebSocketAccept See delegate signature.
websocket.AcceptAlt   Non-spec
websocket.SubProtocol String See RFC6455 Section 4.2.2 Step 5.5
websocket.SendAsync WebSocketSendAsync See delegate signature.
websocket.ReceiveAsync WebSocketReceiveAsync See delegate signature.
websocket.CloseAsync WebSocketCloseAsync See delegate signature.
websocket.CallCancelled CancellationToken  
websocket.ClientCloseStatus int Optional
websocket.ClientCloseDescription String Optional