Implementing Network Security Groups and Azure Firewall for VNets Virtual Networks (VNets) are the backbone of Microsof

Virtual Networks (VNets) are the backbone of Microsoft Azure’s networking architecture. They allow organizations to securely connect resources, applications, and services in the cloud. But as with any network, security is critical. Azure provides powerful tools such as Network Security Groups (NSGs) and Azure Firewall to protect VNets, manage traffic, and create secure perimeters for applications.

This blog explores how NSGs and Azure Firewall work, their differences, and how to implement them together to create a robust network security framework in Azure.

Understanding VNets in Azure

A Virtual Network (VNet) in Azure is similar to a traditional on-premises network. It allows you to group and isolate resources, connect workloads, and define traffic flows. VNets provide:

• Segmentation for workloads and applications
• Connectivity between cloud resources and on-premises environments
• Control over traffic using NSGs and firewalls

VNets form the foundation of secure cloud deployments, but to truly protect them, security tools like NSGs and Azure Firewall are essential.

What are Network Security Groups (NSGs)?

Network Security Groups are simple yet effective tools for controlling inbound and outbound traffic to Azure resources. An NSG contains rules that either allow or deny traffic based on parameters such as source, destination, port, and protocol.

Key features of NSGs:

• Control traffic at the subnet or network interface level
• Allow or deny rules based on IP addresses, ranges, and service tags
• Default rules to allow communication within VNets and block traffic from the internet
• Easy to configure for scenarios like restricting RDP or SSH access

Example use cases:

• Restricting database access to only application servers within a VNet
• Denying public internet access to sensitive subnets
• Allowing only HTTPS traffic to web servers

What is Azure Firewall?

Azure Firewall is a fully managed, cloud-based network security service that provides centralized protection for VNets. Unlike NSGs, which work at the subnet or NIC level, Azure Firewall acts as a stateful, scalable perimeter security solution.

Key features of Azure Firewall:

• Stateful inspection of traffic
• Application rules for HTTP/S traffic filtering
• Network rules for controlling IP protocols and ports
• Threat intelligence to block traffic from malicious sources
• Centralized logging and monitoring through Azure Monitor

Example use cases:

• Creating a secure perimeter for VNets with centralized control
• Filtering outbound traffic from VNets to ensure compliance
• Protecting against malicious traffic with threat intelligence
• Enforcing policies across multiple subscriptions and regions

NSGs vs Azure Firewall

Both NSGs and Azure Firewall enhance network security, but they serve different purposes.

In short, NSGs are excellent for micro-segmentation within VNets, while Azure Firewall secures traffic at the perimeter. Together, they form a layered security model.

Implementing NSGs in VNets

Steps to configure NSGs:

• Create an NSG in the Azure portal or via PowerShell/CLI.
• Define inbound and outbound rules to allow or deny traffic.
  • Example: Allow HTTPS traffic on port 443.
  • Example: Deny inbound RDP traffic from the internet.
• Associate the NSG with a subnet or a network interface card (NIC).
• Monitor traffic using NSG Flow Logs in Azure Monitor for insights.

Best practices for NSGs:

• Apply least privilege by only allowing required traffic
• Use service tags (like VirtualNetwork, Internet) for easier management
• Regularly review and clean up unused rules
• Apply NSGs at both subnet and NIC levels for layered protection

Implementing Azure Firewall in VNets

Steps to deploy Azure Firewall:

• Create a dedicated subnet called AzureFirewallSubnet in the VNet.
• Deploy Azure Firewall through the portal or infrastructure-as-code templates.
• Configure network rules to allow or deny specific protocols and ports.
• Create application rules to filter traffic based on domains and FQDNs.
• Enable threat intelligence to block traffic from known malicious IPs.
• Integrate with Azure Monitor for real-time logging and alerting.

Best practices for Azure Firewall:

• Use firewall policies for consistent rule management across VNets
• Combine with NSGs for layered security control
• Enable diagnostic logs for compliance and auditing
• Scale firewall instances for high-traffic environments

Combining NSGs and Azure Firewall for Secure VNets

While NSGs provide granular control at the subnet and NIC level, Azure Firewall offers centralized perimeter security. Together, they can create a comprehensive security posture:

• Use NSGs for east-west traffic filtering (within the VNet).
• Use Azure Firewall for north-south traffic filtering (traffic entering or leaving the VNet).
• Apply layered rules so that even if one layer is bypassed, another layer still protects resources.

Example scenario:

• A web server subnet allows only HTTPS traffic using NSG rules.
• Outbound traffic from the subnet is routed through Azure Firewall, which filters destinations and blocks malicious domains.
• Together, these tools ensure secure communication while protecting both internal and external boundaries.

Benefits of Using NSGs and Azure Firewall Together

• Layered Defense: Reduces risks by having multiple lines of security.
• Improved Compliance: Provides detailed logging for audits and monitoring.
• Scalability: NSGs handle subnet-level filtering while Azure Firewall manages broader traffic.
• Flexibility: Works across multiple VNets, regions, and subscriptions.
• Cost Efficiency: Optimizes resource usage by applying the right tool for the right task.

Common Challenges and Solutions

• Complex rule management: Use tagging and firewall policies to simplify administration.
• Performance concerns: Place Azure Firewall in a dedicated subnet for scalability.
• Visibility gaps: Enable diagnostics and NSG flow logs for end-to-end visibility.
• Overlapping rules: Regularly audit configurations to avoid conflicts.
• Cost considerations: Balance NSG and Firewall usage based on requirements.

Conclusion

Securing VNets in Azure requires more than just deploying resources—it requires a layered approach to network security. Network Security Groups provide fine-grained control at the subnet and NIC level, while Azure Firewall acts as a scalable, centralized perimeter defense. By implementing both together, organizations can protect their VNets from internal and external threats, ensure compliance, and create a resilient security posture.

For businesses moving critical workloads to Azure, NSGs and Azure Firewall are essential building blocks of a secure, well-managed cloud environment.