Causes for Application Gateway Connection Timeout

Question

Greeetings,

I have the following setup

Application Gateway -- Private Endpoints -- App Services

The application gateway is in its own resource group along with the virtual network in the 1.0.0.0 subnet.

The private endpoints and app services have a resource group per resource. So for me that would be 5 Services plus the main group with the AG.

I created all elements via the Portal.

After setting up the backend pool (for starters just for one service) and using the basic settings on all the elements (listeners, rules) I wanted to connect to the gateway public ip or dns name, however all I receive is a timeout, without any hint whatsoever in the monitoring as to what could cause the problem.

The application gateway does not even register a request.

Does anyone know what could be the cause of this? Could the Public IP be faulty for some reason? I even tried disabling the private endpoint on one of the services for debugging purposes but to no avail, seems like that is not the cause.

Any help is appreciated :)

Answer 1

Connection Timeout means that one or more than one server could not complete your request within some period and does not receive a timely response from another one that works as a gateway or proxy i.e; did not complete within alloted time.

Initially try to reload the page in different browser or even on different devices.Clean the site from spam and cookies.

Please check if any of the below possible causes:

1. (main cause)REQUEST TIMEOUT : The number of seconds that the application gateway will wait to receive a response from the backend pool before it returns a “connection timed out” error message.

   When a user request is received, the application gateway applies the configured rules to the request and routes it to a back-end pool instance. It waits for a configurable interval of time for a response from the back-end instance. By default, this interval is 20 seconds.

   Solution: Reference >> Try Setting request-timeout :Application Gateway allows you to configure this setting via the BackendHttpSetting,

   Ex:

    New-AzApplicationGatewayBackendHttpSettings -Name 'Setting01' -Port 80 -Protocol Http -CookieBasedAffinity Enabled -RequestTimeout 60
   

   Also see App gateway -troubleshoot-app-service-redirection-app-service-url.

   NOTE: If your connections are getting dropped at less than the request time out seconds set, then we need to find which connection is trigged. And may need to Contact azure support

In addition to server timeouts, there are other causes

2. See if it due to default health check probe:like 1) Back-end VMs or instances of virtual machine scale set are not responding to the default health probe. 2) Invalid or improper configuration of custom health probes. 3) Azure Application Gateway's back-end pool is not configured or empty.

   Troubleshoot problems-with-default-health-probe and custom healtH probe : Application gateway automatically configures a default health probe using properties of the BackendHttpSetting but Custom health probes allow additional flexibility to the default probing behavior where you can configure the probe interval, the URL, the path to test, and how many failed responses to accept before marking the back-end pool instance as unhealthy.

   Also check the app service time outs : see appgw-timeouts and app service time out setting

Other causes to check

1. Slow server, Problems with the firewall,network connection
2. If NSG, UDR or Custom DNS blocking access to backend pool members.
3. If VMs or instances in virtual machine scale set are healthy.

For those : Check the logs and DNS records and try by disabling the proxy or temporarily disabling the CDN

References:

1. Azure application gateway throws 502 when application sends 401 - Stack Overflow
2. Azure Application Gateway error 502 when using application gateway -Stack Overflow