Enable forwarded headers middleware when publishing projects by default

Fixes #290

src/Aspire.Hosting/ApplicationModel/DisableForwardedHeadersAnnotation.cs

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+
+namespace Aspire.Hosting.ApplicationModel;
+
+[DebuggerDisplay("Type = {GetType().Name,nq}")]
+internal sealed class DisableForwardedHeadersAnnotation : IResourceAnnotation
+{
+
+}

src/Aspire.Hosting/Extensions/ProjectResourceBuilderExtensions.cs

@@ -13,6 +13,8 @@ namespace Aspire.Hosting;
 /// </summary>
 public static class ProjectResourceBuilderExtensions
 {
+    private const string AspNetCoreForwaredHeadersEnabledVariableName = "ASPNETCORE_FORWARDEDHEADERS_ENABLED";
+
     /// <summary>
     /// Adds a .NET project to the application model. By default, this will exist in a Projects namespace. e.g. Projects.MyProject.
     /// If the project is not in a Projects namespace, make sure a project reference is added from the AppHost project to the target project.
@@ -92,6 +94,19 @@ private static IResourceBuilder<ProjectResource> WithProjectDefaults(this IResou
         builder.WithOtlpExporter();
         builder.ConfigureConsoleLogs();
 
+        var projectResource = builder.Resource;
+
+        if (builder.ApplicationBuilder.ExecutionContext.IsPublishMode)
+        {
+            builder.WithEnvironment(context =>
+            {
+                if (!projectResource.Annotations.OfType<DisableForwardedHeadersAnnotation>().Any())
+                {
+                    context.EnvironmentVariables[AspNetCoreForwaredHeadersEnabledVariableName] = "true";
+                }
+            });
+        }
+
         if (excludeLaunchProfile)
         {
             builder.WithAnnotation(new ExcludeLaunchProfileAnnotation());
@@ -103,8 +118,6 @@ private static IResourceBuilder<ProjectResource> WithProjectDefaults(this IResou
             builder.WithAnnotation(new LaunchProfileAnnotation(launchProfileName));
         }
 
-        var projectResource = builder.Resource;
-
         if (builder.ApplicationBuilder.ExecutionContext.IsRunMode)
         {
             // Automatically add EndpointAnnotation to project resources based on ApplicationUrl set in the launch profile.
@@ -208,6 +221,17 @@ public static IResourceBuilder<ProjectResource> ExcludeLaunchProfile(this IResou
         throw new InvalidOperationException("This API is replaced by the AddProject overload that accepts a launchProfileName. Null means exclude launch profile. Method will be removed by GA.");
     }
 
+    /// <summary>
+    /// Configures the project to disable forwarded headers when being published.
+    /// </summary>
+    /// <param name="builder">The project resource builder.</param>
+    /// <returns>A reference to the <see cref="IResourceBuilder{T}"/>.</returns>
+    public static IResourceBuilder<ProjectResource> DisableForwadedHeadersOnPublish(this IResourceBuilder<ProjectResource> builder)
+    {
+        builder.WithAnnotation<DisableForwardedHeadersAnnotation>(ResourceAnnotationMutationBehavior.Replace);
+        return builder;
+    }
+
     private static bool IsKestrelHttp2ConfigurationPresent(ProjectResource projectResource)
     {
         var projectMetadata = projectResource.GetProjectMetadata();

tests/Aspire.Hosting.Tests/ManifestGenerationTests.cs

@@ -136,6 +136,50 @@ public void ExcludeLaunchProfileOmitsBindings()
             "Service has no bindings because they weren't populated from the launch profile.");
     }
 
+    [Fact]
+    public void ProjectResourceEmitsForwardedHeadersEnvironmentVariableByDefault()
+    {
+        var appBuilder = DistributedApplication.CreateBuilder(new DistributedApplicationOptions
+        { Args = GetManifestArgs(), DisableDashboard = true, AssemblyName = typeof(ManifestGenerationTests).Assembly.FullName });
+
+        appBuilder.AddProject<Projects.ServiceA>("servicea", launchProfileName: null);
+
+        appBuilder.Services.AddKeyedSingleton<IDistributedApplicationPublisher, JsonDocumentManifestPublisher>("manifest");
+
+        var program = appBuilder.Build();
+        var publisher = program.Services.GetManifestPublisher();
+
+        program.Run();
+
+        var resources = publisher.ManifestDocument.RootElement.GetProperty("resources");
+
+        Assert.Equal("true",
+            resources.GetProperty("servicea").GetProperty("env").GetProperty("ASPNETCORE_FORWARDEDHEADERS_ENABLED").GetString());
+    }
+
+    [Fact]
+    public void DisableForwardedHeadersOmitsEnvironmentVariable()
+    {
+        var appBuilder = DistributedApplication.CreateBuilder(new DistributedApplicationOptions
+        { Args = GetManifestArgs(), DisableDashboard = true, AssemblyName = typeof(ManifestGenerationTests).Assembly.FullName });
+
+        appBuilder.AddProject<Projects.ServiceA>("servicea", launchProfileName: null)
+            .DisableForwadedHeadersOnPublish();
+
+        appBuilder.Services.AddKeyedSingleton<IDistributedApplicationPublisher, JsonDocumentManifestPublisher>("manifest");
+
+        var program = appBuilder.Build();
+        var publisher = program.Services.GetManifestPublisher();
+
+        program.Run();
+
+        var resources = publisher.ManifestDocument.RootElement.GetProperty("resources");
+
+        Assert.False(
+            resources.GetProperty("servicea").GetProperty("env").TryGetProperty("ASPNETCORE_FORWARDEDHEADERS_ENABLED", out _),
+            "Service has no ASPNETCORE_FORWARDEDHEADERS_ENABLED environment variable because it was disabled.");
+    }
+
     [Fact]
     public void EnsureContainerWithEndpointsEmitsContainerPort()
     {

tests/Aspire.Hosting.Tests/ProjectResourceTests.cs

@@ -199,7 +199,8 @@ public async Task VerifyManifest()
               "path": "net8.0/another-path",
               "env": {
                 "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EXCEPTION_LOG_ATTRIBUTES": "true",
-                "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EVENT_LOG_ATTRIBUTES": "true"
+                "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EVENT_LOG_ATTRIBUTES": "true",
+                "ASPNETCORE_FORWARDEDHEADERS_ENABLED": "true"
               },
               "bindings": {
                 "http": {