问题
I have set up a new application Gateway following the MS document. I have configured the back end pool .it is connected to an App Service via FQDN. Somehow I receive 502 immediately every time I browse to the application gateway. Browsing to the website directly works with no issues .
回答1:
I have resolved this issue. Here is the solution.
If you have web apps in your backend pools you CAN'T set up the health probes via Azure portal.
You need to set them up via resource templates or Powershell. The key is that you need to leave the host field empty and set -PickHostNameFromBackendAddress property.
The two lines below did the magic to make the 502s go away
# Create a probe with the PickHostNameFromBackendHttpSettings switch for web apps
$probeconfig = New-AzureRmApplicationGatewayProbeConfig -name webappprobe -Protocol Http -Path / -Interval 30 -Timeout 120 -UnhealthyThreshold 3 -PickHostNameFromBackendHttpSettings
# Define the backend http settings
$poolSetting = New-AzureRmApplicationGatewayBackendHttpSettings -Name appGatewayBackendHttpSettings -Port 80 -Protocol Http -CookieBasedAffinity Disabled -RequestTimeout 120 -PickHostNameFromBackendAddress -Probe $probeconfig
来源：https://stackoverflow.com/questions/46046056/application-gateway-502-error