Merge pull request #229389 from jaesoni/restart-for-custom-dns

Restart after VNet DNS changes

Author: Maggiemouse1

articles/application-gateway/application-gateway-faq.yml

@@ -186,7 +186,10 @@ sections:
       - question: Can Application Gateway communicate with instances outside of its virtual network or outside of its subscription?
         answer: |
           As long as you have IP connectivity, Application Gateway can communicate with instances outside of the virtual network that it's in. Application Gateway can also communicate with instances outside of the subscription it's in. If you plan to use internal IPs as backend pool members, use [virtual network peering](../virtual-network/virtual-network-peering-overview.md) or [Azure VPN Gateway](../vpn-gateway/vpn-gateway-about-vpngateways.md).
-          
+       
+      - question: Why am I seeing 502 errors or unhealthy backend servers after I changed the DNS servers for the virtual network?
+        answer: The instances of your application gateway use the virtual network's DNS configuration for name resolution. After changing any DNS server configuration, you need to restart (Stop and Start) the application gateway for the new DNS servers to get assigned. Until then, FQDN-based name resolutions for outbound connectivity could fail.
+       
       - question: Can I deploy anything else in the application gateway subnet?
         answer: No. But you can deploy other application gateways in the subnet.
 

articles/application-gateway/configuration-infrastructure.md

@@ -5,7 +5,7 @@ services: application-gateway
 author: greg-lindsay
 ms.service: application-gateway
 ms.topic: conceptual
-ms.date: 02/23/2023
+ms.date: 03/03/2023
 ms.author: greglin
 ---
 
@@ -54,6 +54,9 @@ Subnet Size /24 = 256 IP addresses - 5 reserved from the platform = 251 availabl
 > [!TIP]
 > It is possible to change the subnet of an existing Application Gateway within the same virtual network. You can do this using Azure PowerShell or Azure CLI. For more information, see [Frequently asked questions about Application Gateway](application-gateway-faq.yml#can-i-change-the-virtual-network-or-subnet-for-an-existing-application-gateway)
 
+### DNS Servers for name resolution
+The virtual network resource supports [DNS server](../virtual-network/manage-virtual-network.md#view-virtual-networks-and-settings-using-the-azure-portal) configuration, allowing you to choose between Azure-provided default or Custom DNS servers. The instances of your application gateway also honor this DNS configuration for any name resolution. Thus, after you change this setting, you must restart ([Stop](/powershell/module/az.network/Stop-AzApplicationGateway) and [Start](/powershell/module/az.network/start-azapplicationgateway)) your application gateway for these changes to take effect on the instances.
+
 ### Virtual network permission 
 Since the application gateway resource is deployed inside a virtual network, we also perform a check to verify the permission on the provided virtual network resource. This validation is performed during both creation and management operations. You should check your [Azure role-based access control](../role-based-access-control/role-assignments-list-portal.md) to verify the users or service principals that operate application gateways also have at least **Microsoft.Network/virtualNetworks/subnets/join/action** permission on the Virtual Network or Subnet.