Network Security for Azure Virtual Networks

Securing your Azure Virtual Network (VNet) is paramount to protecting your cloud resources. Azure provides a comprehensive set of tools and services to implement robust network security at various layers.

Key Network Security Concepts

Network Security Groups (NSGs)

Network Security Groups are fundamental components for filtering network traffic to and from Azure resources in an Azure virtual network. NSGs contain security rules that allow or deny network traffic based on source and destination IP address, port, and protocol.

Inbound Security Rules: Control traffic coming into your resources.
Outbound Security Rules: Control traffic leaving your resources.
Priority: Rules are processed in order of priority (lower number = higher priority).
Association: NSGs can be associated with network interfaces (NICs) or subnets.

Azure Firewall

Azure Firewall is a managed, cloud-native network security service that protects your Azure Virtual Network resources. It's a stateful firewall as a service that offers:

Centralized Policy Management: Apply consistent security policies across your VNets.
Threat Intelligence-based Filtering: Block traffic to known malicious IP addresses and domains.
Network and Application Layer Filtering: Filter traffic based on FQDN tags, application FQDNs, and ports.
High Availability and Scalability: Built-in redundancy and automatic scaling.

Azure DDoS Protection

Azure Distributed Denial of Service (DDoS) Protection provides enhanced DDoS mitigation capabilities to defend your Azure resources from attacks. It offers:

Adaptive Tuning: Automatically optimizes protection based on your traffic patterns.
DDoS Protection Metrics: Provides detailed telemetry and attack alerts.
Mitigation Reporting: Offers insights into mitigated attack traffic.
SLA Guarantees: (With DDoS Protection Standard)

Network Virtual Appliances (NVAs)

For more advanced or specialized network security needs, you can deploy Network Virtual Appliances (NVAs) from third-party vendors. These can include:

Next-generation firewalls (NGFWs)
Intrusion detection/prevention systems (IDPS)
Web application firewalls (WAFs)

NVAs are deployed in your VNet and traffic is routed through them using user-defined routes (UDRs).

Implementing Network Security

Best Practices Summary

Least Privilege: Apply NSG rules to allow only necessary traffic.
Segment Networks: Use subnets and NSGs to isolate different tiers of your application.
Centralize Firewalling: Utilize Azure Firewall for consistent policy enforcement.
Protect Against DDoS: Implement Azure DDoS Protection Standard for critical workloads.
Monitor Traffic: Use Azure Network Watcher and flow logs to monitor traffic patterns and identify suspicious activity.

Example: Securing a Web Application

Consider a typical three-tier web application deployed in Azure:

Web Tier Subnet: Allow inbound HTTP (port 80) and HTTPS (port 443) from the internet. Deny all other inbound traffic. Allow outbound access to the application tier.
Application Tier Subnet: Allow inbound traffic only from the web tier subnet on specific application ports. Allow outbound access to the data tier.
Data Tier Subnet: Allow inbound traffic only from the application tier subnet on the database port. Deny all other inbound traffic. Deny all outbound traffic (or restrict to essential management access).

This segmentation is enforced using NSGs applied to each subnet.

Using Azure Firewall for Centralized Control

To further enhance security and simplify management, you can deploy an Azure Firewall in a dedicated hub VNet or a central security VNet. Traffic from spoke VNets is then routed to the hub VNet's firewall using VNet peering and UDRs.

Azure Firewall Rules:

Network Rules: Filter traffic based on IP address, port, and protocol.
Application Rules: Filter traffic based on FQDNs (fully qualified domain names).

Monitoring and Auditing

Regularly review network security logs and alerts. Azure Monitor, Network Watcher flow logs, and Azure Sentinel provide powerful capabilities for detecting and responding to security threats.