This article describes how to set up an Azure App Service web app in a network environment that enforces strict policies for inbound and outbound network flows. In such cases, the web app can't be directly exposed to the internet. Instead, all traffic needs to go through an Azure Firewall or a third-party network virtual appliance (NVA).
Potential use cases
These use cases have similar design patterns:
- Connect from a web app or an Azure Function to any platform as a service (PaaS) offering that supports Azure Private Link-based private endpoints for inbound network flows. Examples include Azure Storage, Azure Cosmos DB, and other web apps.
- Connect from a web app or an Azure Function to a virtual machine (VM) in the cloud or on premises by using virtual private networks (VPNs) or Azure ExpressRoute.
Download a Visio file of this architecture.
Cross-reference the following 10 steps with the annotated architecture diagram shown previously while we describe the solution's dataflow:
An Azure Front Door instance provides Azure Web Application Firewall (WAF) features and terminates TLS/SSL connections from clients. End-to-end TLS ensures the network traffic is re-encrypted before it's forwarded to Azure Firewall.
A custom fully qualified domain name (FQDN) is chosen to represent the back-end web app and is mapped through CNAME or A DNS resource records to the public IP address of an Azure firewall or third-party NVA.
A private endpoint for the web app is created in a virtual network subnet (subnet-privatelink-1 in the example).
The Azure Firewall or third-party NVA is deployed to its own reserved subnet in a virtual network (Hub Virtual Network in the example). The appliance is configured to perform destination network address translation (DNAT) of incoming requests to the private IP address or the private endpoint associated with the web app. Azure Firewall can also handle source NAT (SNAT) for Internet-outbound egress traffic from your Azure VMs.
The web app is assigned the custom FQDN through the domain verification ID property of the web app. This allows the custom FQDN already mapped to the public IP address of the Azure Firewall or third-party NVA to be reused with the web app without altering Domain Name System (DNS) name resolution and network flows.
The web app connects to a virtual network subnet (subnet-webapp in the example) through regional VNet integration. The WEBSITE_VNET_ROUTE_ALL application setting is enabled, which forces all outbound traffic from the web app into the virtual network and allows the web app to inherit the virtual network's DNS resolution configuration, including custom DNS servers and integration with Azure Private DNS zones used for private endpoint name resolution.
A custom route table that's attached to the web app subnet (subnet-webapp in the example) forces all outbound traffic that comes from the web app to go to the Azure Firewall or third-party NVA.
Azure Route Server(/azure/route-server/overview) is an alternative to manually maintained custom route tables that uses Border Gateway Protocol (BGP) to automate route propagation to your Azure virtual network subnets.
One or more private DNS zones link to the virtual network that contains the web app (Spoke Virtual Network 1 in the example) to allow DNS resolution of PaaS resources deployed with private endpoints.
A private endpoint for an Azure SQL Database virtual server is created in a virtual network subnet (subnet-privatelink-2 in the example). A corresponding DNS record is created on the matching Azure Private DNS zone.
The web app can now be accessed only through Azure Front Door and Azure Firewall. It can also establish a connection to the Azure SQL Database virtual server through the private endpoint, securing the communication over private IP addresses only.
- Azure Virtual Networks form the underlying network traffic segmentation vehicle for this solution. Consider implementing Azure Virtual Network Manager (AVNM) to apply consistent administration and network security group policies to multiple VNets simultaneously.
- Azure Front Door provides Azure WAF features and terminates TLS/SSL connections from clients.
- Azure Firewall provides security to the web app for both ingress and egress.
- Azure App Service allows you to create web apps and deploy them in a cloud infrastructure.
- Azure Private Link private endpoints allow you to connect privately and with improved security to Azure services.
- Azure SQL connects to the web app via a private endpoint.
- You can deploy the web app to an internal, single-tenant App Service Environment to provide isolation from the public Internet. This example uses an Azure App Service web app to reduce operating costs.
- You can replace Azure Front Door with an Azure Application Gateway if you also need to deploy the WAF component of the solution behind a firewall or within a virtual network.
These considerations implement the pillars of the Azure Well-Architected Framework, a set of guiding tenets that can be used to improve the quality of a workload. For more information, see Microsoft Azure Well-Architected Framework.
Reliability ensures your application can meet the commitments you make to your customers. For more information, see Overview of the reliability pillar.
Azure Front Door is a global service with built-in availability and redundancy and a high service level agreement (SLA).
Azure Firewall features built-in availability and a high SLA. You can deploy it to span multiple availability zones to increase the SLA. If you use a third-party or custom NVA, you can achieve the same SLA targets by configuring your deployment to use availability sets or availability zones.
Azure web apps support built-in availability. You can deploy them across multiple availability zones.
You can further increase the availability of the solution by spreading it across multiple Azure regions. You can accomplish this by deploying new instances of all components (except Azure Front Door) to other Azure regions and then configuring the original Azure Front Door instance with multiple back-end targets. If you use Azure SQL as your data store, you can then join multiple servers to an auto-failover group to enable transparent and coordinated failover of multiple databases.
See these reference architectures to learn about deploying highly available web applications in Azure and setting up multi-region SQL Server instances to work with private endpoints:
- Highly available multi-region web application
- Multi-region web app with private connectivity to a database
Security provides assurances against deliberate attacks and the abuse of your valuable data and systems. For more information, see Overview of the security pillar.
Specifically, this solution deploys an Azure Front Door instance, which terminates TLS/SSL connections from clients and provides a rich set of WAF configurations. We recommend that you further lock down your applications to accept traffic coming only from your Azure Front Door instance. You can do this in several ways, depending on the NVA you're using and your application configuration.
Some security options to consider integrating into your solution include:
- Placing Azure Front Door Premium on a private endpoint. An Azure Front Door Premium private endpoint secures your origin to a virtual network, ensuring consumers interact with your solution using non-Internet routable private IP addresses only.
- Associating network security groups (NSGs) to each VNet subnet. NSGs protect ingress and egress network traffic at Open Systems Interconnection (OSI) Layer 4 (Transport Layer). Private Endpoint support for NSGs (preview) enables you to implement advanced security controls on VNet egress traffic to a private endpoint.
- Configuring your Azure Firewall or NVA to accept traffic only from the AzureFrontDoor.Backend Azure IP ranges.
- Configuring your NVA to integrate with Azure service tags.
- Configuring your application to accept traffic only from your Azure Front Door instance by validating request headers.
- Defending against threats with Security Information and Event Management (SIEM) plus eXtended Detection and Response (XDR). Solutions within this category include Microsoft Sentinel, Microsoft Defender for Identity, and Microsoft Defender for Cloud.
- Enabling enterprise-scale Distributed Denial of Service (DDoS) protection with Azure DDoS Network Protection. This solution protects your Front Door and Azure Firewall public IP addresses against abuse and provides customers with deep insights into Microsoft's automatic mitigation processes. -Considering Microsoft Purview to establish unified data governance not only for your Azure SQL data, but potentially all data in your hybrid cloud, multicloud enterprise
For more information, see How do I lock down the access to my backend to only Azure Front Door?.
The solution also uses an Azure SQL Database virtual server that accepts traffic only through a private endpoint, locking down traffic that comes from external sources. The web app used in the solution is configured to ensure proper DNS resolution of private endpoints and allow secure communication with the SQL Server instance.
When you deploy resources that use private endpoints in your environments, it's important to configure your DNS infrastructure properly. For more information, see Azure private endpoint DNS configuration.
Cost optimization is about looking at ways to reduce unnecessary expenses and improve operational efficiencies. For more information, see Overview of the cost optimization pillar.
The example scenario features a deployment within a hardened network environment. So an Azure Firewall or third-party NVA most likely already exists in the target infrastructure.
The main consideration for the remaining infrastructure is the stock keeping unit (SKU) of the App Service plan that hosts the web app. Private endpoints for web apps are available only in the Premium App Service plan SKUs.
Use the Azure pricing calculator to estimate your costs. Here are two possible pricing estimates:
Operational excellence covers the operations processes that deploy an application and keep it running in production. For more information, see Overview of the operational excellence pillar.
You can use Azure Monitor to monitor all components of this solution. You can use Log Analytics to monitor logs related to the WAF and network inspection rules of Azure Front Door and Azure Firewall. You can use Application Insights to monitor performance and availability and gain insights into your use of web applications.
Performance efficiency is the ability of your workload to scale to meet the demands placed on it by users in an efficient manner. For more information, see Performance efficiency pillar overview.
All components of the solution either provide transparent built-in scalability or expose a rich set of features, like Azure web app autoscale, for scaling the number of available instances.
Deploy this scenario
- An Azure account. If you don't have an Azure subscription, create a free account before you start.
- A publicly routable domain. Additionally, you must have permissions to create two DNS records in your public DNS zone.
- A valid TLS/SSL certificate to use for your web app. For instructions on how to do this, see Add a TLS/SSL certificate in Azure App Service.
This article is maintained by Microsoft. It was originally written by the following contributors:
- Davide Maccarrone | Senior Consultant
- Tim Warner | Senior Content Developer
- Mikey Lombardi | Senior Content Developer
- Azure Web Apps deployment best practices
- Azure private endpoint DNS configuration
- Azure Web Application Firewall on Azure Front Door