YARP container fails to resolve named endpoint via service discovery after upgrade to Aspire.Hosting.Yarp 13.2.1

[paulomorgado]

YARP container fails to resolve named endpoint via service discovery after upgrade to Aspire.Hosting.Yarp 13.2.1

Description

After upgrading from Aspire.Hosting.Yarp 13.1.2 to 13.2.1 (and Aspire.AppHost.Sdk 13.1.2 → 13.2.1), a YARP gateway container fails to resolve named endpoints configured via WithConfiguration / AddRoute. The YARP reverse proxy falls through to DNS hostname resolution instead of using the Aspire service discovery mechanism.

This worked correctly with version 13.1.2.

Reproduction

AppHost setup:

var builder = DistributedApplication.CreateBuilder(args);

var api = builder.AddProject<Projects.MyApi>("myapi",
    options =>
    {
        options.LaunchProfileName = null;
        options.ExcludeLaunchProfile = true;
        options.ExcludeKestrelEndpoints = true;
    })
    .WithHttpEndpoint(env: "PORT")
    .WithEndpoint(name: "prometheus", scheme: "http", env: "PROMETHEUS_PORT");

var gateway = builder.AddYarp("gateway");

builder.Eventing.Subscribe<BeforeStartEvent>((evt, ct) =>
{
    var prometheusEndpoint = api.GetEndpoint("prometheus");
    gateway.WithConfiguration(yarp =>
    {
        yarp
            .AddRoute("/prometheus/myapi/metrics", prometheusEndpoint)
            .WithTransformPathRemovePrefix("/prometheus/myapi");
    });

    return Task.CompletedTask;
});

builder.Build().Run();

Expected behavior

The YARP gateway container should resolve http://_prometheus.myapi via Aspire's service discovery, using the services__myapi__prometheus__0 environment variable injected into the container.

Actual behavior

The YARP container receives these environment variables:

# Service discovery - only 'http' endpoint, no 'prometheus' endpoint
services__myapi__http__0 = http://aspire.dev.internal:33945

# Named endpoint reference (non-service-discovery format)
MYAPI_PROMETHEUS = http://aspire.dev.internal:33945

# YARP destination using service discovery format
REVERSEPROXY__CLUSTERS__cluster_myapi__DESTINATIONS__destination1__ADDRESS = http://_prometheus.myapi

Note that services__myapi__prometheus__0 is missing. The MYAPI_PROMETHEUS env var contains the correct URL but is not in the services__ format that Aspire's service discovery resolver uses.

As a result, when Prometheus scrapes /prometheus/myapi/metrics through the gateway, YARP logs:

info: Yarp.ReverseProxy.Forwarder.HttpForwarder[9]
      Proxying to http://myapi/metrics HTTP/2 RequestVersionOrLower

warn: Yarp.ReverseProxy.Forwarder.HttpForwarder[48]
      Request: An error was encountered before receiving a response.
      System.Net.Http.HttpRequestException: Name or service not known (myapi:80)
       ---> System.Net.Sockets.SocketException: Name or service not known

The service discovery strips the _prometheus. prefix but cannot resolve myapi via the services__ env vars (only http is present, not prometheus), so it falls through to DNS resolution which fails because myapi is a project resource running on the host, not a container in the Docker network.

Versions

• Aspire.AppHost.Sdk: 13.2.1 (worked with 13.1.2)
• Aspire.Hosting.Yarp: 13.2.1 (worked with 13.1.2)
• OS: Windows 11
• .NET: 10.0
• Container runtime: Docker Desktop

Additional context

• The project resources (myapi) are .NET project resources, not containers.
• The prometheus endpoint is a dynamically allocated port added via .WithEndpoint(name: "prometheus", scheme: "http", env: "PROMETHEUS_PORT").
• The gateway is a container resource (mcr.microsoft.com/dotnet/nightly/yarp:2.3-preview).
• The AddRoute API receives the endpoint reference (api.GetEndpoint("prometheus")), which should cause Aspire to inject the corresponding services__myapi__prometheus__0 environment variable into the YARP container, but only services__myapi__http__0 is injected.

[paulomorgado]

For this to work, I had to add the old style environment variables (services__myservice__myendpoint__0 = https://localhost:5001).