TKIT_dev

Accessing localhost .Net Core WebAPI from a Docker container

March 07, 2020

In days of .Net Framework, I used to run .Net web apps using IIS. Create a new website in IIS, point it to the web project folder, set the app pool, add an entry to hosts file and you’re good. Others could also access it over the local network.

With .Net Core I started using IIS Express, and it served me well until I tried accessing it from a Docker container. I kept getting HTTP 400 - Bad Request even though I used the special DNS host.docker.internal from inside the container.

Turns out IIS Express doesn’t support access from outside the box (which is the situation we have with the Docker container, I should have known this in advance!) without applying some additional modifications to your OS and Visual Studio.

I personally sorted this out by running the WebAPI using the Kestrel server instead of IIS Express. It is the simpler solution for now and I have no issues with running the app using Kestrel. The setup in my case was very simple. I added the following two entries to my launchSettings.json:

{
  "Kestrel": {
    "commandName": "Project",
    "launchBrowser": true,
    "applicationUrl": "https://localhost:44313;http://localhost:26628",
    "environmentVariables": {
      "ASPNETCORE_ENVIRONMENT": "Development"
    }
  },
  "Kestrel Watch": {
    "executablePath": "dotnet.exe",
    "workingDirectory": "$(ProjectDir)",
    "commandLineArgs": "watch run",
    "launchBrowser": true,
    "applicationUrl": "https://localhost:44313;http://localhost:26628",
    "environmentVariables": {
      "ASPNETCORE_ENVIRONMENT": "Development"
    }
  },
}

And that was it, I was able to access the WebAPI from the Docker container after that.

And here are 2 extra videos which explain in and out of process hosting in .Net Core WebAPI. They are related and you might find them useful:


Written by Tihomir Kit, a senior software engineer @Dovetail.
🖖 Stay curious. @GitHub