Why do I get 502 ERROR_INTERNET_CONNECTION_ABORTED on App Service only when accessed via Application Gateway?

Question

I setup a Sitecore infrastructure on azure (I created the same before and it worked) and when I connect to the web apps directly, they all work. I configured access to two of the apps via Application Gateway - while one works, the other one gives 502 on the health status. I checked on the application and there it shows me that the error is "ERROR_INTERNET_CONNECTION_ABORTED".

When I completely deactivate Sitecore (rename default.aspx and web.config) and put an index.html page, the application gateway can access it.

As mentioned - I have a running deployment that I did with the same ARM template. I also deleted everything and redeployed - same issue. As mentioned - direct access to the web app works perfectly. I have NO network restrictions on the web app yet.

Answer 1

It might be the case for Sitecore initial startup to take ages (more than health probe from service gate).

Considering Service Gate does not receive response from WebApp, it assumes application to be unhealthy, and might cache the 502 code.

What does your web app diagnostics say?

Answer 2

I finally found the issue. It was not the startup time and not an issue with the Application Gateway:

<ipSecurity allowUnlisted="false" denyAction="AbortRequest">
  <clear />
  <add ipAddress="0.0.0.0" subnetMask="0.0.0.0" allowed="true" />
</ipSecurity>

this is in the web.config for newer Sitecore Versions. No idea why it worked for the other installation - but removing this solved the issue for me.