Container -> host communication

[MeikelLP]

Im running Keycloak and want it to write traces to the Aspire dashboard. I tried it with Jaeger (in a container) and it worked fine.

var keycloak = builder.AddKeycloak("keycloak", 5677, adminPassword: keycloakAdminPassword)
    .WithLifetime(ContainerLifetime.Persistent)
    .WithContainerRuntimeArgs("--add-host=host.docker.internal:host-gateway") // -- required on linux for host.docker.internal to work
    .WithEnvironment("KC_TRACING_ENABLED", "true")
    .WithEnvironment("KC_TRACING_ENDPOINT", "https://host.docker.internal:21092");

Sadly Keycloak does not understand (read/load?) values set by WithOtlpExporter thus I have to connect this manually.

However I get the following error

2025-08-21 11:51:57,296 traceId=, parentId=, spanId=, sampled= WARNING [io.quarkus.opentelemetry.runtime.exporter.otlp.sender.VertxGrpcSender] (vert.x-eventloop-thread-1) Failed to export TraceRequestMarshalers. The request could not be executed. Full error message: Connection refused: host.docker.internal/10.250.248.1:21092

The hostname is resolved properly but nothing is availabe on that port. It works on my host https://localhost:21092

[davidfowl]

I'm guessing you are not using docker desktop.

[davidfowl]

Clever @KennethHoff !

[davidfowl]

Maybe you want to mark all endpoints?

[KennethHoff]

Maybe you want to mark all endpoints?

What do you mean by this?

[davidfowl]

builder.Eventing.Subscribe<InitializeResourceEvent>((@event, ct) =>
{
    if (@event.Resource.TryGetAnnotationsOfType<EndpointAnnotation>(out var annotations))
    {
        foreach (var annotation in annotations)
        {
            annotation.TargetHost = "0.0.0.0";
        }
    }

    return Task.CompletedTask;
});

[KennethHoff]

Oh right. That makes sense. Didn't consider the fact that there can be multiples of the same annotation. Thanks!

By the way, isn't it like 4AM where you live now? :s