Application Gateway: Network Isolation

by HubSite 365 about John Savill's [MVP]

Principal Cloud Solutions Architect

Azure DataCenter Networking Learning Selection

Microsoft expert on Azure Application Gateway network isolation, private deployment, control plane and WAF

Key insights

Network Isolation: Azure Application Gateway v2 can run with private frontend IPs only, removing the need for a public endpoint and reducing Internet exposure.
It keeps both data and management traffic inside your virtual network when configured for private deployment.
Deployment Flag / Switch: Microsoft exposes a feature flag that turns on the new network-isolated architecture for new gateways.
The flag only affects instances created after it is set; existing gateways do not automatically convert.
Control Plane Traffic: Management traffic to the Gateway Manager can use private paths and VNet DNS instead of the public Internet.
This lets you avoid opening inbound or broad outbound routes to the Internet for control-plane communication.
Frontend Options: You can choose private-only frontends or keep public frontends as needed, giving flexible trade-offs between isolation and external access.
Web Application Firewall (WAF) support exists in the new model, but check current feature coverage before rolling out production workloads.
NSG and Outbound Restrictions: Apply strict Network Security Group rules, including deny-all outbound, to limit egress and prevent data exfiltration.
The new model also allows blocking Gateway Manager service tag inbound rules that were previously required.
Migration and Limitations: Migration from v1 to v2 and adoption of the isolated model require planning—some features and behaviors differ and some controls still have limits.
Ensure subnet delegation for application gateways and validate feature support for your workloads before switching to private-only deployments.

John Savill's [MVP] recent video outlines a major update to Azure's Application Gateway Network Isolation, explaining how the service can now operate without a public endpoint. The presentation walks viewers through design changes, deployment options, and practical implications for security and operations. Consequently, the video serves as both a technical guide and a planning resource for teams evaluating private-only Application Gateway deployments.

Overview of the new architecture

To begin, the update lets the Application Gateway run entirely with private IP addresses inside a virtual network, removing the previous need for a public frontend. Moreover, control plane and data plane communication can now be constrained within Azure networking boundaries, which improves privacy and reduces Internet exposure. As a result, organizations can adopt stricter network controls while retaining core load‑balancing and Layer 7 features.

John Savill maps this change to the evolution from App GW v1 to v2 and highlights the feature flag that enables the new behavior for new instances. Importantly, the flag affects only newly created gateways and does not retroactively change existing instances, so planning matters. Therefore, teams must treat this as a deployment-time decision rather than a simple configuration flip.

How the private-only deployment works

First, the gateway requires placement in a delegated subnet and relies on private DNS resolution and VNet routing to reach Azure management endpoints. Then, Network Security Groups (NSGs) can apply strict deny-all outbound rules to reduce egress risk while still allowing required control plane interactions. Consequently, operators gain tighter control but must also ensure the VNet provides correct name resolution and routing for management traffic.

Furthermore, the video explains that the control plane’s traffic path changes when the option is enabled; management communication no longer forces a public IP path. However, teams must still allow specific Azure service communications from within the VNet, and they may need to adjust route tables or private DNS zones. Thus, while isolation increases, so does the need for deliberate network configuration.

Benefits and security tradeoffs

The primary benefit is a reduced attack surface because no public frontend IP is required and inbound Internet access can be eliminated. Moreover, denying outbound Internet egress helps prevent accidental or malicious data exfiltration and aligns with zero‑trust principles. At the same time, the tighter posture may complicate tasks like remote troubleshooting or third‑party integrations that expect public endpoints.

In addition, the model supports stronger compliance postures and better integration with private services such as internal App Service Environments or private endpoints. Nevertheless, teams trade convenience for control: some automated updates, diagnostics, or third‑party monitoring tools might need network adjustments or private connectivity to continue functioning. Therefore, the security benefits are clear, but they require additional operational effort to maintain functionality.

Challenges and operational considerations

One notable challenge is migration and lifecycle management because the flag only applies to new instance creation and cannot convert existing gateways in place. Consequently, organizations that run production gateways must plan blue/green migrations or redeployments to adopt the new architecture. In practice, this can introduce temporary complexity, potential downtime windows, and extra testing requirements.

Another challenge involves observability and support. Since the gateway no longer exposes a public path, teams must ensure their logging, alerting, and support connections work across private paths. Additionally, the feature imposes current limitations that John flags in the video, such as scope constraints and WAF compatibility nuances that require careful validation. Thus, operations teams should conduct thorough preproduction trials before rolling out broadly.

Adoption guidance and best practices

For successful adoption, the video recommends preparing the VNet with proper subnet delegation, private DNS zones, and tailored NSG rules that permit necessary Azure management calls while denying unwanted egress. Moreover, transitional steps such as staging new instances, running canary traffic, and validating WAF behavior help reduce risk. Therefore, a phased rollout combined with automation and configuration-as-code pays off.

Finally, teams should weigh the tradeoffs between enhanced isolation and added complexity. While the update delivers meaningful security gains and tighter compliance controls, it also demands stronger network engineering and change management practices. In short, organizations that invest in planning and testing will likely benefit most from the Network Isolation option highlighted by John Savill's [MVP] video.