Solving `Empty reply from server` in DotNet Core

tldr;

If you’re getting curl: (52) Empty reply from server check that you’ve configured Kestrel correctly.

Long version

I’ve started playing with Dotnet Core and Docker and have had a few issues getting started. I’ve had a little bit of experience with Docker, though only a little. I’ve worked with DotNet previously but never with Core.

Environment

The setup I’m using is a Windows laptop running Vagrant. Via Vagrant I’m running an Ubuntu Virtual Machine with the Docker run time installed. Feels a litle like inception.

Key Problem

The key problem that I’ve had getting started was having my Docker container not responding correctly to my curl commands. I get this:
curl: (56) Recv failure: Connection reset by peer
curl: (52) Empty reply from server

Given that I’m not all that familliar with Docker or Dotnet Core I decided to remove one of the variables and set up a simple Express app in a different docker container. This validated that the commmand I was using to run the container (docker run -v /vagrant/dotnetcoretest/src:/src -p 5000: 5000 -d dnctest dotnet run) was binding the ports correctly.

This left a problem with the Dotnet Core web api app. I was surprised that this would be the problem given that I had created the app with the dotnet new webapigenerator.

Searching around I found that the problem was that by default Kestrel listens on localhost:5000. To fix this add .UseUrls("http://*:5000") to the WebHostBuilder in Program.cs. It should now look like

public class Program
{
    public static void Main(string[] args)
    {
        var host = new WebHostBuilder()
            .UseKestrel()
            .UseContentRoot(Directory.GetCurrentDirectory())
            .UseUrls("http://*:5000")
            .UseIISIntegration()
            .UseStartup<Startup>()
            .Build();

        host.Run();
    }
}

Rebuild the app and run curl localhost:5000/api/values and you should get ["value1","value2"].