How to Prevent Docker from Resolving localhost to host.docker.internal?

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Asked By TechWizard42 On

I've been working on adding .NET Aspire to my solution which includes an API application, a Hangfire application, and a React frontend. Everything is going smoothly except for one issue: the API address set in my React app's environment variables is supposed to be `http://localhost:56731/`. However, when this is resolved within Docker, it changes to `http://host.docker.internal:56731/`, which is not what I want. I need the client on the host machine to connect to the API at localhost, but Docker seems to handle it differently.

I've already explored various configuration options within Aspire, but nothing seems to address this issue. Is this a default behavior in Docker? If so, how can I specify that I want to connect to localhost from the client's browser on the host? Here's a snippet of my Aspire Apphost configuration to give you context on how I've set it up:

```csharp
var builder = DistributedApplication.CreateBuilder(args);
var frontendPath = Environment.GetEnvironmentVariable("FRONTENDPATH");
var webApi = builder.AddProject("WebApi")
.WithExternalHttpEndpoints()
.WithReference(sqlDatabase)
.WaitFor(sqlDatabase)
.WaitFor(migrations);

var frontend = builder.AddDockerfile("frontend", frontendPath)
.WithEnvironment((ecc) =>
{
var apiEndpoint = webApi.GetEndpoint("http");
ecc.EnvironmentVariables.Add("PUBLIC_DISMANTLING_API_HOST", apiEndpoint);
})
.WithBuildArg("NODE_ENV_FILE", "local")
.WithReference(webApi)
.WaitFor(webApi)
.WithHttpEndpoint(port: 3000, targetPort: 3000)
.WithExternalHttpEndpoints();

builder.Build().Run();
```

Any advice on how to approach this problem would be greatly appreciated!

2 Answers

Answered By DockerGuru21 On

It sounds like your frontend runs in a Docker container while your backend runs on the host. The environment variables generated by Aspire could point to the localhost address, which is exactly how Docker works. For local development, you might have to replace `localhost` with your host's IP address. When you deploy it to a service like Kubernetes or Azure, those environment variables will usually point to the right locations. Sadly, in your current setup, `localhost` may not work as desired.

Answered By CodeNinja99 On

If you need the API to be accessed locally from within the container, try using `127.0.0.1`. But for access outside the container, using `0.0.0.0` would be the way to go. The idea is that `localhost` in Docker resolves to the container's own loopback address, which can cause this behavior you're seeing.

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