Understanding Network Security Groups (NSGs) in Azure

Introduction

Network Security Groups (NSGs) in Azure play a critical role in safeguarding network communications. This blog post aims to demystify NSGs, explore their use cases, and provide practical scenarios to help us understand their importance in Azure’s networking infrastructure.

What Are Network Security Groups?

Network Security Groups are a type of Azure resource that allows us to control network traffic to and from Azure resources in an Azure Virtual Network (VNet). An NSG contains a list of security rules that filter traffic based on several parameters like protocol, source and destination IP address, port number, and direction (inbound or outbound).

Key Features of NSGs

• Fine-Grained Control: NSGs offer precise control over traffic, enabling us to define security policies at the subnet or individual resource level.
• Directional Traffic Management: Separate rules for inbound and outbound traffic ensure granular network flow management.
• Integration with Azure Resources: NSGs can be associated with VMs (through its NIC), subnets, and standalone NICs within VNets.
• Default and Custom Rules: Azure provides default rules, but we can create custom rules to meet specific security requirements.

Use Cases and Scenarios

1. Securing Virtual Machines: By applying NSGs to VMs, we can control which ports are exposed to the internet, thereby reducing the attack surface.

   Example Scenario: Imagine we have a VM hosting a web server. We can configure an NSG to allow only HTTP (port 80) and HTTPS (port 443) traffic, blocking all other inbound traffic to the VM.

2. Subnet Isolation: NSGs can be used to isolate subnets within a VNet, ensuring that different segments of our network have specific access policies.

   Example Scenario: In a multi-tier application, we can allow the front-end servers to be accessible from the internet, whereas database servers are accessible only from the front-end servers.

3. Hybrid Cloud Connectivity: NSGs help manage and secure traffic flow when integrating with on-premises networks.

   Example Scenario: For a hybrid setup with an on-premises data centre, we can use NSGs to ensure that only specific traffic from the data centre can reach the Azure resources.

Where Can NSGs be Configured and Attached?

NSGs can be associated with the following Azure resources:

• Subnets: Applying an NSG to a subnet affects all resources within that subnet.
• Network Interfaces (NICs): Associating an NSG with an NIC impacts the traffic to and from that particular VM or resource.
• Application Gateway (indirectly): NSGs cannot be directly applied to Azure Application Gateways. However, we can indirectly secure an Application Gateway by applying NSGs to the subnet that hosts it or the backend resources it connects to.

Best Practices and Considerations

• Rule Prioritization: Understand the rule processing order. Rules are processed based on priority, with lower numbers processed first.
• Default Rules: Be aware of the default rules provided by Azure and how they interact with our custom rules.
• Testing: Test our NSG rules in a non-production environment to ensure they behave as expected.
• Monitoring and Logging: Utilize Azure’s monitoring tools to log and review the effectiveness of our NSG rules.

Conclusion

Network Security Groups are a vital component of Azure's security framework. By effectively utilizing NSGs, we can create a robust and secure network environment tailored to our organization's needs. Whether we’re securing a single VM or an entire subnet, NSGs provide the tools necessary to protect our Azure resources.

See also

• Configuring Inbound and Outbound Rules in Azure NSGs