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Install an App Service with a service endpoint with outbound traffic routed through a Firewall and inbound traffic coming through an App Gateway.

UPDATE: as of April 2020 the custom DNS server is not needed, an alternate version of this repo can be found here https://github.com/fortunkam/webapp-serviceendpoint

UPDATE: May 2020 - the Azure CLI version of this script will no longer be updated and has been moved the /archive folder

How to use the Terraform script

In the /terraform folder are a selection of tf files. You will need to have terraform installed (I am running v0.12.24 on Powershell Core 7). run terraform init in a powershell prompt and then run the setup powershell script here, this script runs the terraform apply and then deploys the sample app to the website. The resource prefix and the location are defined as variables and you should provide new values (particularly for the prefix). The resources can be cleaned up by running terraform destroy

What is the script doing?

What gets deployed! The script will create

  • 3 resource groups
  • 2 virtual networks (peered)
  • 5 subnets
  • A storage account with table storage accessible via a private endpoint
  • A private DNS zone to allow resolution of the storage private endpoint
  • An App Service (Website + plan) running a simple node application, locked down using access restrictions and service endpoints
  • A Firewall that all outbound network traffic is routing through
  • An App Gateway that all inbound traffic to the app service is routed through.
  • A VM Scale Set hosting a custom DNS Forwarder, with a load balancer (A commented out version of the script exists for a single VM running the same scripts ).

How does the inbound traffic get routed to my website

Inbound traffic routing

  1. A request comes into the Application gateway public ip address. The App Gateway forwards the request onto the website.
  2. The website uses access restrictions to prevent access on the public endpoint to only the ip address of the Application Gateway and the user that ran the script.

How are internet calls from my website routed?

outbound internet traffic routing

The application that gets deployed makes calls to http://httpbin.org/ip to retrieve the outbound ip of the server making the call. By default a website with a vnet intergration will always go direct for outbound internet calls (using the app service shared outbound ips). By adding the WEBSITE_VNET_ROUTE_ALL=1 setting it will use the UDR applied to the subnet.

  1. The website has a VNET Integration to the web subnet.
  2. The web subnet contains a UDR that routes all traffic to the Azure Firewall.
  3. The firewall contains application rules that allow traffic to httpbin.org (for the running application), github and npm (for the application build).

The /ip route on the website shows the call to httpbin.org/ip is successful and should also return the IP address of the firewall showing the request has been routed.

How are internet calls to my table storage routed (private endpoint)?

outbound private endpoint routing

NOTE: At the current point, App Service Plans with VNET Integration are unable to resolve private endpoint (internal) addresses. The solution is to deploy a custom DNS with a conditional forwarding zone to allow it to resolve correctly. In the near future steps 2-4 will not be required.

  1. Website requests a MYSTORAGE.table.core.windows.net address. By default this would resolve to the public ip address (which is locked down)
  2. The vnet is configured to point it's DNS requests at our custom DNS Server.
  3. The DNS server contains a Conditional Forwarder Zone for MYSTORAGE.table.core.windows.net which points at the Azure DNS address (168.63.129.16) https://docs.microsoft.com/en-us/azure/virtual-network/what-is-ip-address-168-63-129-16
  4. The Azure DNS is aware of our Private DNS Zone so forwards the request there.
  5. The private DNS zone is configured to resolve the privatelink address to an ip address on the vnet.
  6. The web app can now communicate directly with the storage account over the private endpoint ip address.