Introduction to Kestrel web server implementation in ASP.NET Core

By Tom Dykstra, Chris Ross, and Stephen Halter

Kestrel is a cross-platform web server for ASP.NET Core based on libuv, a cross-platform asynchronous I/O library. Kestrel is the web server that is included by default in ASP.NET Core new project templates.

Kestrel supports the following features:

  • Opaque upgrade used to enable WebSockets
  • Unix sockets for high performance behind Nginx

Kestrel is supported on all platforms and versions that .NET Core supports.

View or download sample code

When to use Kestrel with a reverse proxy

If your application accepts requests only from an internal network, you can use Kestrel by itself.

Kestrel to internal network

If you expose your application to the Internet, you must use IIS, Nginx, or Apache as a reverse proxy server. A reverse proxy server receives HTTP requests from the Internet and forwards them to Kestrel after some preliminary handling.

Kestrel to Internet

A reverse proxy is required for edge deployments (exposed to traffic from the Internet) for security reasons. Kestrel is relatively new and does not yet have a full complement of defenses against attacks. This includes but isn't limited to appropriate timeouts, size limits, and concurrent connection limits.

Another scenario that requires a reverse proxy is when you have multiple applications that share the same port running on a single server. That doesn't work with Kestrel directly because Kestrel doesn't support sharing a port between multiple processes. When you configure Kestrel to listen on a port, it handles all traffic for that port regardless of host header. A reverse proxy that can share ports must then forward to Kestrel on a unique port.

Even if a reverse proxy server isn't required, using one can simplify load balancing and SSL set-up -- only your reverse proxy server requires an SSL certificate, and that server can communicate with your application servers on the internal network using plain HTTP.

How to use Kestrel in ASP.NET Core apps

Install the Microsoft.AspNetCore.Server.Kestrel NuGet package.

Call the UseKestrel extension method on WebHostBuilder in your Main method, specifying any Kestrel options that you need, as shown in the following example:

public static int Main(string[] args)
    Console.WriteLine("Running demo with Kestrel.");

    var config = new ConfigurationBuilder()

    var builder = new WebHostBuilder()
        .UseKestrel(options =>
            if (config["threadCount"] != null)
                options.ThreadCount = int.Parse(config["threadCount"]);

    var host = builder.Build();

    return 0;

URL prefixes

By default ASP.NET Core binds to http://localhost:5000. You can configure URL prefixes and ports for Kestrel to listen on by using the UseUrls extension method, the urls command-line argument, or the ASP.NET Core configuration system. For more information about these methods, see Hosting. For information about how URL binding works when you use IIS as a reverse proxy, see ASP.NET Core Module.

URL prefixes for Kestrel can be in any of the following formats.

  • IPv4 address with port number
 is a special case that binds to all IPv4 addresses.

  • IPv6 address with port number


    [::] is the IPv6 equivalent of IPv4

  • Host name with port number

    Host names, *, and +, are not special. Anything that is not a recognized IP address or "localhost" will bind to all IPv4 and IPv6 IPs. If you need to bind different host names to different ASP.NET Core applications on the same port, use WebListener or a reverse proxy server such as IIS, Nginx, or Apache.

  • "Localhost" name with port number or loopback IP with port number


    When localhost is specified, Kestrel tries to bind to both IPv4 and IPv6 loopback interfaces. If the requested port is in use by another service on either loopback interface, Kestrel fails to start. If either loopback interface is unavailable for any other reason (most commonly because IPv6 is not supported), Kestrel logs a warning.

  • Unix socket


If you specify port number 0, Kestrel dynamically binds to an available port. Binding to port 0 is allowed for any host name or IP except for localhost name.

When you specify port 0, you can use IServerAddressesFeature to determine which port Kestrel actually bound to at runtime. The following example gets the bound port and displays it on the console.

public void Configure(IApplicationBuilder app, ILoggerFactory loggerFactory)

    var serverAddressesFeature = app.ServerFeatures.Get<IServerAddressesFeature>();


    app.Run(async (context) =>
        context.Response.ContentType = "text/html";
        await context.Response
            .WriteAsync("<p>Hosted by Kestrel</p>");

        if (serverAddressesFeature != null)
            await context.Response
                .WriteAsync("<p>Listening on the following addresses: " +
                    string.Join(", ", serverAddressesFeature.Addresses) +

        await context.Response.WriteAsync($"<p>Request URL: {context.Request.GetDisplayUrl()}<p>");

URL prefixes for SSL

Be sure to include URL prefixes with https: if you call the UseHttps extension method, as shown below.

var host = new WebHostBuilder() 
    .UseKestrel(options => 
        options.UseHttps("testCert.pfx", "testPassword"); 
   .UseUrls("http://localhost:5000", "https://localhost:5001") 

HTTPS and HTTP cannot be hosted on the same port.

Next steps

For more information, see the following resources: