"502 Bad Gateway" on Application Gateway when "Override with new host name" is disabled, redirect issues when enabled

Question

Hello,

I am creating an Application Gateway to forward traffic to an Azure App Service with the domain: aladwaa360.azurewebsites.net. However, I’m facing two issues with the host name override feature:

502 Bad Gateway with host name override disabled: When I access the Application Gateway's public IP, I receive a "502 Bad Gateway" error if the "Override with new host name" option is set to No in the backend settings.

Redirection issue with host name override enabled: When I set the "Override with new host name" option to Yes, the gateway works, and I can access the app service through my custom domain (staging.example.com). However, I encounter a new issue related to redirection.

• I have a login form on the app service. Upon successful login, it should redirect me to the main app at staging.example.com. However, what happens instead is:

  - After logging in, the URL briefly changes to `staging.example.com` but then quickly redirects back to the default App Service URL (`aladwaa360.azurewebsites.net`).
  

Configuration Details:

• I’ve set up a custom domain for the Application Gateway: staging.example.com.
• The login form sends me to another application (hosted on staging.account.example.com) and is supposed to redirect back to staging.example.com after successful login.

Goal: I need to ensure that, after logging in, the browser's URL remains as staging.example.com (instead of reverting to the default aladwaa360.azurewebsites.net). Additionally, I plan to route traffic based on the URL path (e.g., /qrcode should route to a different backend service).

Questions:

• Why does this redirection back to the default app service URL happen?
• How can I ensure that the custom domain (staging.example.com) remains in the browser's address bar, even after logging in?
• What is the best way to set up the Application Gateway to handle routing traffic to different backends based on the path?

Answer 1

@Muhammad Elzedy ,

Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.

I understand that you are facing issues with HostName while using a reverse proxy such as Application Gateway.

Your case is a classic and is explained here : Preserve the original HTTP host name between a reverse proxy and its back-end web application.

• This is more of a design issue rather than Application Gateway or Backend App Service's configuration issue.
• The exact issue is : Incorrect redirect URLs
• 

This is why you are being redirected to the App Serice instead of the App Gateway (reverse proxy).

Now to answer your queries,

1 . Why does this redirection back to the default app service URL happen?

• I believe the document I shared above explains this issue.

2 . How can I ensure that the custom domain (staging.example.com) remains in the browser's address bar, even after logging in?

• The recommendation is to have a single/same domain in both the App Gateway and the backend App Service.
• i.e.,
  • Add a custom domain to AppService : Map custom DNS name to Azure App Service
  • And make the custom domain resolve to the IP of the App gateway
• This way, both the service and the App Gateway will have the same domain "staging.example.com" and there is no redirection in the first place.
  • i.e., all URLs become same
• Make sure to update your Identity providers with the URL of the App Service as "staging.example.com" and not the default "<AppServiceName>.azurewebsites.net"

See : Implementation guidance for common Azure services

If you host your web application in App Service, you can attach a custom domain name to the web app and avoid using the default azurewebsites.net host name toward the back end. You don't need to change your DNS resolution when you attach a custom domain to the web app: you can verify the domain by using a TXT record without affecting your regular CNAME or A records. (These records will still resolve to the IP address of the reverse proxy.)

3 . What is the best way to set up the Application Gateway to handle routing traffic to different backends based on the path?

• See : URL Path Based Routing for concepts
• To create path-based routing rules, refer here

Hope this clarifies.

Thanks,

Kapil

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