HTTP 503 error when behind ZScaler corporate proxy and trying to access web app

Question

A couple of days ago our web app started showing a 503 error when trying to access our web app. It works fine when ZScaler is off. This is an old app running .NET MVC 5 on a Windows Server 2012 machine. No changes were made from our side when the issue started happening, so I was suspecting that the.networking team had done something. However, they cannot seem to locate the issue.

When trying to access the page through Chrome, I get a HTTP 503 error, but on Firefox it shows a blank page and says that the connection is not secure. I can access the web app through the server hostname URL fine. However, when trying to use the registered domain name, it is displaying this error. I have checked the logs on the server, but it isn't showing anything specific to this error.

Does anyone know what this could be? Is there anything in IIS that I should look at specifically?

I have checked the logs on the server, but it isn't showing anything specific to this error.

Answer 1

An HTTP 503 error usually indicates that the server is currently not able to handle the request due to maintenance or capacity issues. It could be caused due to many factors, like a server outage, an issue with the.network, or a problem with the application itself.

Here are a few things you can try to troubleshoot the issue:

1. Check the IIS logs, IIS logs are located in the C:\.netpub\logs\LogFiles folder by default.
2. Check the event logs, event logs can be accessed through the Event Viewer (eventvwr.msc).
3. Try accessing the web app using the server's IP address instead of the registered domain name. This can help determine if the issue is related to the domain name or the server itself.
4. Make sure that the application pool for the web app is running and that the application pool identity has the necessary permissions to access the web app's files and resources.
5. Check the.network configuration and make sure that the server is properly connected to the.network and that there are no issues with the.network connection.
6. Check the web.config to make sure that it is properly set up and that there are no issues with the web.config file.
7. Check the ZScaler configuration to make sure that it is not blocking access to the web app. You may need to work with the.networking team to determine if there are any issues with the ZScaler configuration.