Mastering Azure Network Security: Best Practices for Subnetting and Terraform

Delving into the intricacies of Azure application network security reveals that designing a secure, resilient infrastructure isn’t just about spinning up resources—it’s about structuring those resources with care and precision. When applications communicate behind the scenes over the Azure backbone, ensuring that each component resides in its own well-defined space becomes paramount. One recent blog post exemplifies these best practices by walking through the process of segmenting applications with subnets, service delegations, private endpoints, and even leveraging Terraform to deploy standardized network security policies.

Understanding Azure Network Segmentation​

In today’s cloud era, segregating application components is more than a best practice—it’s a necessity. The approach of dividing a Virtual Network (vNet) into multiple subnets allows for granular control over communication, resource isolation, and ultimately, security. In our highlighted scenario, two Azure Function Apps are deployed, each residing in its own subnet within the same vNet. This separation means that, even as these apps communicate with each other, unnecessary exposure is minimized, directly aligning with Azure’s security blueprint.
Key points include:

• Separating applications into dedicated subnets to isolate traffic.
• Minimizing the attack surface by controlling which services can communicate.
• Ensuring that Azure’s built-in security controls (and routing policies) function as intended.

The Role of Subnet Delegation​

A critical technical detail discussed in the post is subnet delegation. When you delegate a subnet, you’re essentially instructing Azure to enforce a set of service-specific rules and policies directly on that network segment. This is particularly important for services like Azure Function Apps that have known networking behaviors.
Delegation helps by:

• Automatically managing IP allocation.
• Enforcing service-specific routing policies.
• Preventing configuration or security conflicts between on-premises and cloud components.
• Ensuring that network policies tailored for dedicated services are automatically applied.

A common stumbling block encountered is the error generated when trying to add a private endpoint to a subnet already delegated to a service (e.g., Microsoft.Web/serverFarms). This error is a security mechanism because delegated subnets are strictly reserved for their designated resources. The solution? Create a separate, non-delegated subnet to host private endpoints.

Private Endpoints and the Azure Private Link​

Private endpoints are the unsung heroes of secure Azure networking. Instead of exposing resources to the public Internet, private endpoints provide direct, secure, in-network IP addresses. In our example, even after successfully deploying two apps in their delegate-backed subnets, attempts to attach private endpoints to them result in errors—an intentional consequence of Azure’s segregation rules.
Imagine a private endpoint as a bridge with controlled access:

• It represents a private IP from your vNet.
• It’s akin to a network interface controller (NIC) created automatically, forwarding secure traffic.
• It underpins the concept of Azure Private Link, connecting your private endpoint to the app in such a way that communication traverses the secure Azure backbone.

The key learning? When using delegated subnets, it’s crucial to have a dedicated subnet (such as a “subnet3”) solely for private endpoints. This separation not only prevents configuration errors but also aligns with the principle of least privilege, ensuring that each network component has a singular, well-defined role.

Embracing Terraform for Consistent Infrastructure Deployments​

Infrastructure as Code (IaC) tools like Terraform revolutionize the deployment process by allowing you to codify your entire network configuration. In the blog post, Terraform scripts showcase how to:

• Create and configure storage accounts, service plans, and function apps.
• Structure a Virtual Network with multiple subnets.
• Update subnet configurations to include delegation for services like Azure App Service.
• Properly separate private endpoints onto their own subnet to avert conflicts with delegated subnets.

Using Terraform not only speeds up deployment but also ensures that environments are reproducible and less prone to human error. The code snippets in the blog serve as both a tutorial and a working example of how to balance network segmentation with secure endpoint connectivity.
Consider this excerpt from the Terraform code that sets up the virtual network and delegated subnets:

---

resource "azurerm_virtual_network" "vnet" {
name = "dns-vnet"
location = azurerm_resource_group.rg.location
resource_group_name = azurerm_resource_group.rg.name
address_space = [ "10.0.0.0/16" ]
}

resource "azurerm_subnet" "subnet1" {
name = "subnet1"
resource_group_name = azurerm_resource_group.rg.name
virtual_network_name = azurerm_virtual_network.vnet.name
address_prefixes = [ "10.0.1.0/24" ]
delegation {
name = "delegation"
service_delegation {
name = "Microsoft.Web/serverFarms"
actions = [ "Microsoft.Network/virtualNetworks/subnets/join/action", "Microsoft.Network/virtualNetworks/subnets/prepareNetworkPolicies/action" ]
}
}
}​

This snippet highlights how delegation is configured, ensuring that each subnet can automatically enforce the correct routing and service rules. It also hints at the broader implications of network security—by codifying these configurations, you reduce misconfigurations and potential vulnerabilities.

Managing DNS and NSGs for Enhanced Security​

While subnets, endpoints, and private links form the backbone of secure network designs, no discussion would be complete without addressing Domain Name System (DNS) configurations and Network Security Groups (NSGs). Both play pivotal roles:

• Private DNS: When connecting private endpoints to your applications, associating them with a Private DNS zone can streamline the routing process. It ensures that DNS resolution within the vNet directs traffic to the correct private IP addresses.
• NSGs: These control network traffic to and from Azure resources. By combining NSGs with dedicated subnets, you enforce granular rules that filter both inbound and outbound traffic.

The blog touches on how these elements integrate into the overall architecture. When misconfigured, you risk leaving entry points open or inadvertently blocking traffic between trusted services. The solution lies in a balanced approach: leveraging NSGs to enforce policy while utilizing Private DNS to maintain connectivity among segregated services.

Balancing Security with Functionality​

An underlying theme throughout the discussion is the balancing act between security and functionality. It’s one thing to secure your application, but it’s another to ensure that these security measures do not hinder the core operations of your services. Delegated subnets and private endpoints are designed precisely with this balance in mind. However, when these features collide—as seen when trying to mix delegated subnets and private endpoints—the solution is to enforce proper architectural boundaries.
The recommended approach is simple:

1. Use service-specific subnets (with delegation) for resources like Azure Function Apps.
2. Configure a separate subnet exclusively for private endpoints to facilitate secure connections.
3. Integrate Terraform to codify these best practices, ensuring consistency across deployments.

It begs the question: Have you ever tried patching together network security features only to find them conflicting? The answer is a resounding yes for many administrators during the early days of Azure adoption. By learning from these documented practices, you can avoid common pitfalls and ensure that security measures do not inadvertently become a roadblock.

Real-World Implications and Best Practices​

Deploying apps securely in Azure is not just about the initial configuration—it has lasting impacts on operational efficiency and security posture. A well-segmented network means:

• Reduced attack surfaces: If one segment is compromised, the blast radius is contained.
• Simplified management: Isolated subnets mean troubleshooting and maintenance are more straightforward.
• Compliance readiness: Many regulatory frameworks require strict network segmentation for sensitive data.

Furthermore, when applying these configurations using Terraform, you benefit from version control and reproducibility. This is critical when rolling out updates or scaling out resources.
A few salient best practices drawn from the discussion include:

• Always segregate delegated resources from private endpoints by using separate subnets.
• Use Private DNS and NSGs to manage intra-network traffic effectively.
• Validate your configurations with small-scale tests before large-scale deployments.
• Keep abreast of Azure’s evolving documentation and updates, as Microsoft frequently enhances security features and guidelines.

Conclusion​

In a cloud-first world, the subtle art of securing application networks in Azure demands a thoughtful synthesis of multiple components. From segregating services in dedicated subnets and configuring delegation to deploying private endpoints and enforcing policies with NSGs, every decision contributes to a robust security model. Leveraging tools like Terraform not only simplifies these processes but also ensures that your infrastructure is built on a foundation of consistency and best practices.
As illustrated in the detailed blog post, a security-conscious design requires planning for every scenario—from preventing IP conflicts to ensuring that network rules do not inadvertently block legitimate traffic. If you’re looking to bolster your Azure architecture, consider revisiting your network segmentation policies, reevaluating your subnet delegations, and ensuring that your private endpoints are isolated appropriately.
In the realm of Azure network security, every detail counts. Have you rethought your Azure networking strategy lately? With evolving threats on the horizon and new Azure features being introduced regularly, now is always the right time to assess, adapt, and secure your cloud infrastructure.

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Source: Medium