Azure Firewall is a managed, cloud-native network security service designed to protect your Azure Virtual Network resources. It is a stateful firewall-as-a-service (FWaaS) offering built into the Azure platform, providing inherent high availability and unrestricted cloud scalability. Think of it as a highly intelligent and controllable gatekeeper for your Azure network, inspecting all incoming and outgoing traffic and deciding whether to allow or block it based on a set of defined rules that you control. It simplifies network security management by centralizing policy administration and enforcement across multiple subscriptions and virtual networks.

Core Features of Azure Firewall

Key Concepts: What is a Virtual Network (VNet)?

Before diving deeper into the firewall, it’s crucial to understand an Azure Virtual Network (VNet). A VNet is a fundamental building block for your private network in Azure. It is logically isolated from other virtual networks and allows Azure resources like virtual machines (VMs), web apps, and databases to securely communicate with each other, the internet, and on-premises networks. You can think of a VNet as your own private section of the Azure cloud, complete with its own IP address range, subnets, and security policies. Azure Firewall is deployed inside a dedicated subnet within this VNet to inspect and govern all traffic flowing through it.

How Azure Firewall Works: The Central Hub

A common and highly effective design pattern is the Hub-and-Spoke network model. In this architecture, the “hub” is a central virtual network that contains shared services, like Azure Firewall, that need to be consumed by multiple workloads. The “spokes” are separate virtual networks that host your different applications and workloads (e.g., production, development, testing). All traffic between the spoke networks, and from the spokes to the internet or on-premises datacenters, is forced to flow through the central hub VNet. This process is called forced tunneling. Azure Firewall, sitting in the hub, then acts as a central checkpoint, inspecting and applying security rules to all this cross-network traffic, ensuring consistent security policy enforcement across your entire organization.

The Heart of Azure Firewall: Understanding Rules

Rules are the fundamental logic that tells the firewall what to do with each piece of network traffic. Azure Firewall uses a structured hierarchy of rule collections to process traffic logically. The firewall is a stateful service, which is a critical concept. Statefulness means the firewall remembers the state of active connections. For example, if an internal server sends a request out to the internet, the firewall automatically allows the corresponding response back in because it knows it’s part of an established, ongoing conversation. This eliminates the need to write a separate rule to allow the return traffic, simplifying management significantly.

Rule Collection Hierarchy: The Order of Operations

Azure Firewall processes rules in a specific, prioritized order. The first level of organization is the Rule Collection. There are two main types of rule collections: Network Rule Collections and Application Rule Collections. A third type, NAT Rule Collections, handles inbound connections. These collections are grouped further into a hierarchy where each collection has a priority number. A lower priority number (e.g., 100) means the collection is processed before a collection with a higher number (e.g., 200). This is vital because the first matching rule found for a packet determines its fate. Once a match is found, the rule is applied, and the firewall stops processing further rules for that packet.

Deep Dive: Network Rule Collections

Network Rule Collections are used to filter traffic based on fundamental network properties. These rules are about the “who,” “where,” and “what port.” They define allow or deny decisions based on:

• Source IP address: Where is the traffic coming from? (e.g., a specific VM’s IP or a range of IPs).
• Destination IP address: Where is the traffic trying to go? (e.g., a database server’s IP or an external web server’s IP).
• Destination Port: Which door is it trying to use? (e.g., port 80 for HTTP web traffic, port 443 for HTTPS, port 3389 for RDP).
• Protocol: What language is it speaking? (TCP or UDP).

For instance, a network rule could be: “Allow any source IP within the 10.1.2.0/24 subnet to access the destination IP 8.8.8.8 on port 53 (DNS) using the UDP protocol.” This is a very basic, low-level form of filtering, similar to traditional firewalls.

Deep Dive: Application Rule Collections

Application Rule Collections provide a much smarter, more granular level of filtering. They operate at a higher level of the network stack and are used to allow or deny outbound access to fully qualified domain names (FQDNs) and URLs. This means you can control access based on the application being used, not just its raw IP address and port. A key feature here is the use of FQDN Tags and Web Categories.

An FQDN Tag represents a group of FQDNs associated with well-known Azure services. For example, you can create a rule that allows outbound traffic for the WindowsUpdate tag. This automatically allows your VMs to communicate with all Microsoft Windows Update endpoints without you having to know and manually list all the complex and changing URLs. Similarly, you can create a rule to allow traffic for the AzureBackup tag to enable backup operations.

Web Categories allow administrators to allow or deny access to entire classes of websites based on their content, such as “Social Media,” “Gambling,” or “News.” This is extremely powerful for enforcing corporate acceptable use policies directly from the cloud firewall.

An example of an application rule is: “Allow all source addresses in the ‘Finance’ subnet to access the FQDN finance-application.corporate.com but deny the same subnet access to any URL categorized as ‘Social Media’.”

Deep Dive: NAT Rule Collections

NAT Rule Collections (Destination Network Address Translation or DNAT rules) are used to handle inbound internet traffic. They allow you to redirect unsolicited incoming traffic from the internet to a specific private IP address and port inside your virtual network. This is how you securely expose services, like a web server or Remote Desktop session, hosted on an Azure VM to the internet.

The rule translates the public IP address of the Azure Firewall to the private IP address of the internal server. For example, you can create a rule that says: “Any incoming internet traffic arriving at the firewall’s public IP address on port 443 should be translated and forwarded to the internal web server at the private IP 10.1.1.10 on port 443.” This hides your internal server’s real IP and provides a single, secure entry point.

Threat Intelligence: Proactive Protection

Threat Intelligence is a powerful, proactive filtering feature. When enabled, Azure Firewall can automatically alert and deny traffic flowing to or from known malicious IP addresses and domains. This intelligence is powered by the Microsoft Threat Intelligence feed, which is continuously updated with data from Microsoft’s vast global footprint. This provides an essential layer of defense against bots, malware, and other cyber threats without requiring you to manually maintain blocklists for bad actors. You can configure it in either “Alert” mode, which just logs the event, or “Deny” mode, which actively blocks the malicious traffic.

Azure Firewall vs. Network Security Groups (NSGs)

It is very important to understand that Azure Firewall and Network Security Groups (NSGs) are complementary services, not direct replacements for each other. They operate at different layers and scopes. An NSG is a basic, stateless packet filter that operates at the level of a subnet or a individual network interface (NIC) of a VM. It is a distributed firewall, meaning the rules are applied directly at the resource level. NSGs are best for providing micro-segmentation within a subnet, like controlling traffic between tiers of an application (e.g., web tier to data tier).

 

 

 

Difference between NSG and Azure Firewall

Aspect	Network Security Group (NSG)	Azure Firewall
Purpose	Basic network traffic filtering at network layer	Advanced, fully stateful firewall as a service
OSI Layer	Layer 3 & 4 (Network & Transport layers)	Layer 3 to 7 (Network to Application layers)
Statefulness	Stateless (each rule must be explicitly defined)	Stateful (remembers state of connections)
Application Filtering	No application-level filtering	Yes, FQDN and URL filtering capabilities
Threat Intelligence	Not available	Integrated threat intelligence feeds
Deployment Scope	NIC, subnet, or both	Virtual Network (dedicated subnet)
Management	Distributed management	Centralized management and logging
Pricing Model	Free (no additional cost)	Per hour deployment fee + data processing cost
Use Case	Micro-segmentation within a network	Perimeter protection and hub-spoke architectures

In contrast, Azure Firewall is a centralized, stateful, and feature-rich network firewall service. It is designed to govern traffic between networks (e.g., VNet-to-VNet, VNet-to-Internet) and is application-aware. A common best practice is to use both: use Azure Firewall as the central internet gateway and control point between different environments (hub-and-spoke), and use NSGs for fine-grained east-west traffic control within a single subnet or VNet. This is known as a defense-in-depth strategy.

Logging and Monitoring: Gaining Visibility

A critical part of any security system is visibility. Azure Firewall integrates seamlessly with Azure Monitor and Azure Monitor Logs (specifically a feature called Log Analytics). Every decision the firewall makes—whether to allow or deny traffic—can be logged and sent to these services. You can then run complex queries against these logs to analyze traffic flows, investigate potential incidents, audit rule effectiveness, and create visual dashboards. This data is invaluable for troubleshooting connectivity issues and understanding your network’s security posture.

Pricing Model: What to Expect

Azure Firewall operates on a pay-as-you-go pricing model. The cost is primarily based on two factors: the deployment type you choose and the amount of data processed. There are different SKUs (stock-keeping units) like Standard, Premium, and Basic, each offering different levels of performance and features (e.g., the Premium SKU includes advanced features like IDPS – Intrusion Detection and Prevention System). You are billed a fixed hourly fee for the deployment itself and a variable fee for every gigabyte of data processed by the firewall. It’s important to estimate your expected traffic to forecast costs accurately.

Summary and Use Cases

In summary, Azure Firewall is a powerful, scalable, and intelligent central gatekeeper for Azure networks. Its core strength lies in its stateful inspection, application-aware filtering through FQDN and URL control, integrated threat intelligence, and centralized management, especially within a hub-and-spoke architecture.

Typical use cases include: providing secure outbound internet connectivity for all workloads in a VNet (e.g., allowing only specific browsers to access specific websites); creating a Demilitarized Zone (DMZ) for inbound internet traffic using NAT rules; securely connecting hybrid networks (Azure and on-premises) by acting as a central firewall; and achieving regulatory compliance by enforcing consistent and auditable network security policies across all cloud deployments. By mastering Azure Firewall and its rule concepts, you gain a critical tool for building secure and well-architected solutions in Microsoft Azure.