Host filtering with ASP.NET Core Kestrel web server

While Kestrel supports configuration based on prefixes such as http://example.com:5000, Kestrel largely ignores the host name. Host localhost is a special case used for binding to loopback addresses. Any host other than an explicit IP address binds to all public IP addresses. Host headers aren't validated.

As a workaround, use Host Filtering Middleware. The middleware is added by CreateDefaultBuilder, which calls AddHostFiltering:

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

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

Host Filtering Middleware is disabled by default. To enable the middleware, define an AllowedHosts key in appsettings.json/appsettings.<EnvironmentName>.json. The value is a semicolon-delimited list of host names without port numbers:

appsettings.json:

{
  "AllowedHosts": "example.com;localhost"
}

Note

Forwarded Headers Middleware also has an AllowedHosts option. Forwarded Headers Middleware and Host Filtering Middleware have similar functionality for different scenarios. Setting AllowedHosts with Forwarded Headers Middleware is appropriate when the Host header isn't preserved while forwarding requests with a reverse proxy server or load balancer. Setting AllowedHosts with Host Filtering Middleware is appropriate when Kestrel is used as a public-facing edge server or when the Host header is directly forwarded.

For more information on Forwarded Headers Middleware, see Configure ASP.NET Core to work with proxy servers and load balancers.