Server App Blank Page via Traffic Manager + App Gateway – Suspected Asset Blocking or WebSocket Handling

Question

Server App Blank Page via Traffic Manager + App Gateway – Suspected Asset Blocking or WebSocket Handling

Mohammad Khan 20

We have configured a application (.NET 9) hosted on Linux App Service (Basic B2), fronted by an Azure Application Gateway (Standard V2). Traffic is routed via Azure Traffic Manager, and the Traffic Manager profile URL is added as a CNAME in our DNS hosting provider.

The application loads successfully via App Service default URL, but when accessed through the custom domain routing via Traffic Manager + App Gateway, we observe a blank white page after loading for sometime.

Observations

WebSocket handshake (wss://dev.mindzillion.com/_blazor?...) returns HTTP 101, confirming upgrade.
App loads perfectly when bypassing Traffic Manager and App Gateway.
When accessed via Traffic Manager + App Gateway path, browser dev tools show:
- 403 Forbidden errors on api.raygun.io assets and telemetry calls.
  - 404 error for BlazorTable.min.js – critical for rendering.
    - No explicit JavaScript exceptions, but UI fails to load.
    - WebSocket was confirmed enabled via az webapp config show.
No caching or DNS issues confirmed — cache cleared, tested across networks/browsers.

Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-06T14:22:39.81+00:00

Hello Mohammad Khan

I understand that you're facing an issue with your .NET application hosted on Azure, specifically involving Traffic Manager and Application Gateway.

Based on the information you've provided, here are some steps and considerations that might help you troubleshoot the blank page issue you're experiencing:

Since your application uses WebSockets and the handshake returns HTTP 101, it confirms that the upgrade is successful. However, ensure that the Azure Application Gateway is properly configured to support WebSockets. You can check this documentation for specific configuration requirements.

The 404 error for BlazorTable.min.js suggests that your application may not be able to locate this resource when accessed via the Application Gateway. Please verify the routing rules set up on your Application Gateway to ensure that assets are correctly directed to their respective resources.

Since you are using services that might be cross-origin (Traffic Manager and other APIs), make sure that CORS (Cross-Origin Resource Sharing) settings are configured correctly on your app service to allow the necessary origins.

Consider enabling Application Insights or using Application Gateway access log to capture more detailed error logs which may provide insights into what exactly goes wrong when the page fails to render.

If using WAF, disable it briefly to check if security rules are blocking assets.

I see that your custom domain **"https://dev.mindzillion.com" still fails to render the page over application gateway.

dev.mindzillion.com
Non-authoritative answer:
Name: testag.centralindia.cloudapp.azure.com
Address: 20.197.9.71

Aliases: dev.mindzillion.com
mindzillionhealthdev.trafficmanager.net

By default, the Application Gateway sends the same HTTP host header to the backend as it receives from the client. If your backend application/service requires a specific host value, you can override it as required.

We recommend that you preserve the original HTTP host name when you use a reverse proxy in front of a web application. Having a different host name at the reverse proxy than the one that's provided to the back-end application server can lead to cookies or URLs that don't work properly.

Since you confirmed that directly accessing the app service works fine, so can you please share the HAR trace for the working scenario (App service)?

Please let me know if you have any further questions or issues.
Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-07T17:10:18.4366667+00:00

Hello Mohammad Khan

Just checking in to see if above information was helpful. If you have any further updates on this issue, please let me know. Thank you
Mohammad Khan 20 Reputation points

2025-05-08T09:14:18.4933333+00:00

Hello @Shravan Addagatla rest all the settings seems fine. I tried updating CORS settings but it didn't worked and showing me the blank page.
Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-08T17:23:02.2033333+00:00

Hi Mohammad Khan

Can you please check the private message and provide the details for further assistance.

Thanks,
Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-09T19:50:26.5133333+00:00

Hi Mohammad Khan

I just wanted to check if you had a chance to gather the details requested in the private messages, so we can assist you better.
Venkat V 2,545 Reputation points Microsoft External Staff Moderator

2025-05-14T05:25:42.68+00:00

Hi @Mohammad Khan

I just wanted to follow up and see if you had a chance to review my response to your question. Please let us know if it was helpful, and feel free to reach out if you have any further queries.

Accepted answer

0 additional answers

Your answer

Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-07T17:10:18.4366667+00:00

Hello Mohammad Khan

Just checking in to see if above information was helpful. If you have any further updates on this issue, please let me know. Thank you
Mohammad Khan 20 Reputation points

2025-05-08T09:14:18.4933333+00:00

Hello @Shravan Addagatla rest all the settings seems fine. I tried updating CORS settings but it didn't worked and showing me the blank page.
Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-08T17:23:02.2033333+00:00

Hi Mohammad Khan

Can you please check the private message and provide the details for further assistance.

Thanks,
Shravan Addagatla 1,530 Reputation points Microsoft External Staff Moderator

2025-05-09T19:50:26.5133333+00:00

Hi Mohammad Khan

I just wanted to check if you had a chance to gather the details requested in the private messages, so we can assist you better.
Venkat V 2,545 Reputation points Microsoft External Staff Moderator

2025-05-14T05:25:42.68+00:00

Hi @Mohammad Khan

I just wanted to follow up and see if you had a chance to review my response to your question. Please let us know if it was helpful, and feel free to reach out if you have any further queries.

Answer 1

Venkat V 2,545 Microsoft External Staff Moderator

Hi @Mohammad Khan

I understand you’ve configured a WebSocket-based .NET 9 app hosted on a Linux App Service (Basic B2), with traffic routed through Azure Traffic Manager and Application Gateway.

To validate this setup, I deployed a sample WebSocket app to App Service, fronted it with Application Gateway, and registered the App Gateway’s public IP as an endpoint in Traffic Manager.

App Service URL (Direct): https://venkat-app-ecehcjfmctc0cuhj.canadacentral-01.azurewebsites.net
**Output:
**
Application Gateway IP: http://130.107.8.41/

Gateway backend Settings
User's image

**Backend health
**
User's image

Output:
User's image

Traffic Manager Endpoint: [http://test-trafficmanager.trafficmanager.net/]

Traffic Manager endpoint User's image

Output:

User's image

The application works as expected when accessed via Traffic Manager, and the WebSocket connection remains functional.

If it's not working on your end, please verify:

Whether the app is accessible directly from the Application Gateway’s public IP (without Traffic Manager).
That WebSockets are enabled in App Service.
That your App Gateway’s HTTP settings have WebSocket support enabled (this must be toggled on explicitly in Standard V2).
Check browser console and network tab for wss:// connection errors or CORS issues.

Reference: WebSocket support in Azure Application Gateway

I hope this helps to resolve your issue. Please feel free to ask any questions if the solution provided isn't helpful.

Please don’t forget to close the thread by clicking "Accept the answer" wherever the information provided helps you, as this can be beneficial to other community members

Venkat V 2,545 Reputation points Microsoft External Staff Moderator

2025-05-15T04:32:51.6233333+00:00

Hi @Mohammad Khan

I just wanted to follow up and see if you had a chance to review my response to your question. Please let us know if it was helpful, and feel free to reach out if you have any further queries.
Mohammad Khan 20 Reputation points

2025-05-21T05:19:19.4666667+00:00

Hi, I followed and rechecked the steps shared by you but still the issue persists, and I am receiving blank page.
Venkat V 2,545 Reputation points Microsoft External Staff Moderator

2025-05-21T05:26:01.8566667+00:00

Hi @mohammad khan

Can you please confirm whether you are able to access the application through the application gateway?
Mohammad Khan 20 Reputation points

2025-05-21T05:34:05.7933333+00:00

When I am trying to access it directly from app service url i am able to access but whenever i am trying to access from App Gateway after loading it is showing me the blank page
Venkat V 2,545 Reputation points Microsoft External Staff Moderator

2025-05-21T06:52:44.9733333+00:00

Can you please share the requested details in a private chat so we can connect offline?
Mohammad Khan 20 Reputation points

2025-05-21T07:16:18.9833333+00:00

Hi I have shared the mail id in Private chat

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Server App Blank Page via Traffic Manager + App Gateway – Suspected Asset Blocking or WebSocket Handling

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