title: HTTP Error 400 running ASP.NET Core 2.1 React application
body: <p>I am developing ASP.NET Core 2.1 React application in Visual Studio 2017. Today I wanted to launch a previously launchable application from Visual Studio, but the error appeared:</p>
  <blockquote>
  <p>Bad Request - Invalid Hostname HTTP Error 400. The request hostname is
  invalid.</p>
  </blockquote>
  <p>After I had switched from IIS Express  to project exe in the launch menu in VS 2017, I observed the following message:</p>
  <blockquote>
  <p>Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]</p>
  <p>The host 'localhost:5001' does not match an allowed host.</p>
  </blockquote>
  <p>The whole log looks like this:</p>
  <pre><code>info: Microsoft.AspNetCore.DataProtection.KeyManagement.XmlKeyManager[0]
        User profile is available. Using 'C:\Users\vdohnal\AppData\Local\ASP.NET\DataProtection-Keys' as key repository and Windows DPAPI to encrypt keys at rest.
  info: Microsoft.AspNetCore.SpaServices[0]
        Starting create-react-app server on port 50222...
  Hosting environment: Development
  Content root path: C:\VSTS\Dixie\Dixie
  Now listening on: https://localhost:5001
  Now listening on: http://localhost:5000
  Application started. Press Ctrl+C to shut down.
  dbug: HttpsConnectionAdapter[1]
        Failed to authenticate HTTPS connection.
  System.IO.IOException: Authentication failed because the remote party has closed the transport stream.
     at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
     at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
     at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
     at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
     at System.Net.Security.SslStream.BeginAuthenticateAsServer(SslServerAuthenticationOptions sslServerAuthenticationOptions, CancellationToken cancellationToken, AsyncCallback asyncCallback, Object asyncState)
     at System.Net.Security.SslStream.&lt;&gt;c.&lt;AuthenticateAsServerAsync&gt;b__51_0(SslServerAuthenticationOptions arg1, CancellationToken arg2, AsyncCallback callback, Object state)
     at System.Threading.Tasks.TaskFactory`1.FromAsyncImpl[TArg1,TArg2](Func`5 beginMethod, Func`2 endFunction, Action`1 endAction, TArg1 arg1, TArg2 arg2, Object state, TaskCreationOptions creationOptions)
     at System.Threading.Tasks.TaskFactory.FromAsync[TArg1,TArg2](Func`5 beginMethod, Action`1 endMethod, TArg1 arg1, TArg2 arg2, Object state, TaskCreationOptions creationOptions)
     at System.Threading.Tasks.TaskFactory.FromAsync[TArg1,TArg2](Func`5 beginMethod, Action`1 endMethod, TArg1 arg1, TArg2 arg2, Object state)
     at System.Net.Security.SslStream.AuthenticateAsServerAsync(SslServerAuthenticationOptions sslServerAuthenticationOptions, CancellationToken cancellationToken)
     at Microsoft.AspNetCore.Server.Kestrel.Https.Internal.HttpsConnectionAdapter.InnerOnConnectionAsync(ConnectionAdapterContext context)
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
        Request starting HTTP/1.1 GET https://localhost:5001/
  info: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
        The host 'localhost:5001' does not match an allowed host.
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
        Request finished in 55.2128ms 400 text/html
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
        Request starting HTTP/1.1 GET https://localhost:5001/favicon.ico
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
        Request starting HTTP/1.1 GET https://localhost:5001/favicon.ico
  info: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
        The host 'localhost:5001' does not match an allowed host.
  info: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
        The host 'localhost:5001' does not match an allowed host.
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
        Request finished in 6.4203ms 400 text/html
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
        Request finished in 8.9463ms 400 text/html
  info: Microsoft.AspNetCore.SpaServices[0]
        &gt; Dixie@0.1.0 start C:\VSTS\Dixie\Dixie\ClientApp
  &gt; rimraf ./build &amp;&amp; react-scripts start
  
  Starting the development server...
  
  info: Microsoft.AspNetCore.SpaServices[0]
  
  Compiled with warnings.
  
  info: Microsoft.AspNetCore.SpaServices[0]
  
  ./src/components/Home.js
  
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
        Request starting HTTP/1.1 GET https://localhost:5001/
  info: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
        The host 'localhost:5001' does not match an allowed host.
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
        Request finished in 3.6957ms 400 text/html
  info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
        Request starting HTTP/1.1 GET https://localhost:5001/
  info: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
        The host 'localhost:5001' does not match an allowed host.
  </code></pre>
  <p>Any ideas what is behind and how to fix it?</p>
  <p>I am suspicious about AVG antivirus making big wreckage lately or some update.</p>