.net core rest api in docker container not running

Question

I am very new to containerization. I have created a new .net core 3.1 rest api in visual studio using default template. I am not changing anything in default template. When I run it using F5, it runs fine in browser with url https://localhost:44332/weatherforecast. Now, I am trying to run it in docker container with the Dockerfile configuration below:

FROM mcr.microsoft.com/dotnet/aspnet:3.1

COPY bin/Release/netcoreapp3.1/publish App/

WORKDIR /App

EXPOSE 5000

EXPOSE 443

ENTRYPOINT ["dotnet", "CoreRestApi.dll"]

I am able to generate the docker image and the container. The container also get's started. But when I try to browse the url's as https://localhost/weatherforecast or http://localhost:5000/weatherforecast to access the api, it displays nothing.

The code for the Main method in application is as below:

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

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>(); 
            });

The docker desktop displays the following:

Am I missing any step over here?

Hans Kilian · Accepted Answer · 2021-08-26 16:54:39Z

7

In the output from your app, you can see that it's listening on port 80 (Now listening on: http://[::]:80).

So when you run your container, you should map port 80 to a free port on your host computer. Let's say we pick port 51234. Then you'd do

docker run -d -p 51234:80 <your-image-name>

Then you'd be able to reach the API on http://localhost:51234/weatherforecast

Your EXPOSE statements in the Dockerfile don't really do anything. You can see them as documentation on what ports you use. The nice thing would of course be to replace the two EXPOSE statements you have with a single EXPOSE 80 statement, but it's not something that will stop your container from working.

answered Aug 26, 2021 at 16:54

Hans Kilian

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3 Comments

Tech Jay Over a year ago

Thank you for the help. Your explanation about EXPOSE does makes sense. IT is the -p (publish) flag in the docker run command that you shared which makes it possible to access the Api from outside of the container. I tried the approach you had suggested and it worked.

Tech Jay Over a year ago

one more question related to your explanation. As we saw in screenshot above that service is running on port 80 and you suggested to use -p flag to map it. When I use the Debug mode based build for dockerization, it suggests that it is listening to port 5000 and I have to map a custom port to 5000. Could you please suggest how we will know whether we have to map to the port 80 or 5000?

Hans Kilian Over a year ago

The reason is that the aspnet runtime docker image from Microsoft has a predefined value for the environment variable ASPNETCORE_URLS of http://+:80. That means that when you use that image as the basis of your Docker image, it'll listen on port 80 by default.

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