Blazor Server + YARP - Static Files (CSS/JS) 404 & Circuit Failed to Initialize

[mundas26]

Hello community,

I am currently implementing YARP (Yet Another Reverse Proxy) in my Blazor Server application to proxy multiple existing projects. However, I am encountering the following issues:

1. Static files (CSS, JS, etc.) are not loading (404 errors)

• The browser console shows multiple Failed to load resource: the server responded with a status of 404 errors for CSS and JS files.

2. Blazor Circuit Initialization Failure

- After the WebSocket connection is established, I get the following error:

[Error] The circuit failed to initialize.
Connection disconnected.

### Project Setup & Code Snippets
Program.cs Configuration

builder.Services.AddReverseProxy();
builder.Services.AddSingleton<IProxyConfigProvider, DynamicProxyConfigProvider>();



var app = builder.Build();

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error", createScopeForErrors: true);
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles(); 


app.UseRouting();

app.UseAntiforgery();

app.UseAuthentication();
app.UseAuthorization();


app.MapReverseProxy();

app.MapRazorComponents<App>().AddInteractiveServerRenderMode();

app.Run();

DynamicProxyConfigProvider.cs (Route & Cluster Configuration)

routes.Add(new RouteConfig
{
    RouteId = routeId,
    ClusterId = clusterId,
    Match = new RouteMatch { Path = $"{proxiedUrl}/{{**catch-all}}" },
    Transforms = new List<Dictionary<string, string>>
    {
        new Dictionary<string, string> { { "PathRemovePrefix", proxiedUrl } }
    }
});

clusters.Add(new ClusterConfig
{
    ClusterId = clusterId,
    Destinations = new Dictionary<string, DestinationConfig>
    {
        { "destination", new DestinationConfig { Address = item.Url } }
    }
});

Index Page (Navigation to Proxy URL)

string sanitizedTitle = Uri.EscapeDataString(item.Title.Trim().Replace(" ", "-").ToLower());

string proxiedUrl = $"/system/{sanitizedTitle}-{item.Id}";

Navigation.NavigateTo(proxiedUrl, true);

Steps to Reproduce:

1. Set up a Blazor Server application with YARP.
2. Proxy an external Blazor app via YARP.
3. Open the browser and check the console (F12).
4. Observe the 404 errors for static files.
5. Attempt to initialize a Blazor session and note the circuit failure.

What I Have Tried:
-Ensured Static Files Exist in wwwroot.
-Added UseStaticFiles() before UseRouting().
-Checked _content/ Paths for Blazor resources.
-Enabled WebSockets in YARP.
-Modified appsettings.json to match YARP paths.

Expected Behavior:
-Static files should load correctly.
-Blazor Circuit should initialize without failure.

Environment:

• .NET Version: net9.0
• YARP Version: 2.3.0
• Blazor Server Framework Version: net9.0
• Hosting Environment: Localhost (https://localhost:7287)
• Browser: Edge

### Questions:

1. How do I ensure YARP properly serves static files?
2. How can I debug Blazor Circuit Initialization Failure through YARP?

[mundas26]

here is the result of all the logs.

dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-24'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-24' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-23'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-23' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-22'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-22' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-21'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-21' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-20'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-20' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-19'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-19' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-18'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-18' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-17'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-17' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-16'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-16' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[24] Destination 'destination' has been added. dbug: Yarp.ReverseProxy.Forwarder.ForwarderHttpClientFactory[45] New client created for cluster 'cluster-15'. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[21] Cluster 'cluster-15' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-24' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-23' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-22' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-21' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-20' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-19' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-18' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-17' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-16' has been added. dbug: Yarp.ReverseProxy.Management.ProxyConfigManager[27] Route 'system-15' has been added. dbug: Microsoft.Extensions.Hosting.Internal.Host[1] Hosting starting info: Microsoft.Hosting.Lifetime[14] Now listening on: https://localhost:7287 info: Microsoft.Hosting.Lifetime[14] Now listening on: http://localhost:5059 info: Microsoft.Hosting.Lifetime[0] Application started. Press Ctrl+C to shut down. info: Microsoft.Hosting.Lifetime[0] Hosting environment: Development info: Microsoft.Hosting.Lifetime[0] Content root path: C:\DEVOPS\Service Portal Application (ADPIA)\SPA.Web dbug: Microsoft.Extensions.Hosting.Internal.Host[2] Hosting started dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '/ (/)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint '/ (/)' dbug: Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware[1] Response markup is scheduled to include Browser Link script injection. info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint '/ (/)' dbug: Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware[2] Response markup was updated to include Browser Link script injection. dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_framework/blazor.web.js' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint 'Blazor web static files' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Blazor web static files' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Blazor web static files' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_blazor/negotiate' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '(null)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_blazor' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '(null)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' dbug: Microsoft.AspNetCore.Routing.Tree.TreeRouter[1] Request successfully matched the route with name '(null)' and template '/' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/system/why-not-space-16' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1005] Endpoint 'system-16' with route pattern '/system/why-not-space-16/{**catch-all}' is valid for the request path '/system/why-not-space-16' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint 'system-16' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'system-16' info: Yarp.ReverseProxy.Forwarder.HttpForwarder[9] Proxying to http://whynot.adpia-itsolutions.com/ HTTP/2 RequestVersionOrLower info: Yarp.ReverseProxy.Forwarder.HttpForwarder[56] Received HTTP/1.1 response 200. dbug: Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware[1] Response markup is scheduled to include Browser Link script injection. info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'system-16' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_blazor/disconnect' dbug: Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware[2] Response markup was updated to include Browser Link script injection. dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '(null)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/css/print.css' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/css/app.css' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/_content/Syncfusion.Blazor.Themes/bootstrap5.css' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/CLA.Web.styles.css' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_framework/blazor.web.js' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint 'Blazor web static files' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Blazor web static files' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Blazor web static files' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/_content/Syncfusion.Blazor.Core/scripts/syncfusion-blazor.min.js' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1000] No candidates found for the request path '/js/rfidListener.js' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[2] Request did not match any endpoints info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_blazor/negotiate' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '(null)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' dbug: Microsoft.AspNetCore.Routing.Matching.DfaMatcher[1001] 1 candidate(s) found for the request path '/_blazor' dbug: Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware[1] Request matched endpoint '(null)' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint' info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'