Implementing a Robust DNS Security Policy in Azure 💥

 

Introduction

The Domain Name System (DNS) is a fundamental component of modern IT infrastructure, translating domain names into IP addresses. However, it is also a frequent target for cyber threats, including DNS spoofing, cache poisoning, and data exfiltration. Securing DNS in Microsoft Azure is crucial for ensuring the integrity and availability of applications and services.

In this blog, we will explore a detailed DNS security strategy for Azure, covering key aspects such as Microsoft Defender for DNS, Azure Firewall DNS Proxy, private DNS zones, logging, monitoring, and access control. 

Understanding DNS Security in Azure

Azure DNS is a globally distributed and scalable domain name resolution service that supports both public and private DNS zones. Public zones are used for externally accessible domains, while private DNS zones facilitate internal name resolution for Azure resources. Without proper security measures, DNS can become a vector for data exfiltration, command-and-control (C2) attacks, and service disruptions.

Microsoft has outlined several key security recommendations to mitigate these risks:

•     Enable Microsoft Defender for DNS for real-time threat detection.

•     Use Private DNS Zones to protect internal name resolution.

•     Implement Azure Firewall DNS Proxy for centralized security controls.

•     Monitor DNS traffic using Azure Monitor and Log Analytics.

•     Apply least privilege access controls using Microsoft Entra ID.

Now, let's dive into these best practices in details:

• Enabling Microsoft Defender for DNS

Why is it important?

Microsoft Defender for DNS provides advanced threat detection by analyzing DNS traffic for malicious activities, such as:

•     DNS Tunneling: Attackers use DNS queries to exfiltrate sensitive data.
•     Command and Control (C2) Communications: Malware contacts external servers to receive instructions.
•     Domain Generation Algorithms (DGA): Identifies patterns used by malware to generate random domain names.

How to enable Defender for DNS:

• Sign in to the Azure Portal.
• Go to Microsoft Defender for Cloud.
• Navigate to Environment settings and select your subscription.
• Under Defender plans, enable Defender for DNS.
• Monitor alerts in Microsoft Defender for Cloud Security Center.

Defender for DNS Alerts and Threat Analysis:

Once enabled, Defender for DNS provides insights into:

• Unusual spikes in DNS queries
• Attempts to resolve known malicious domains
• Anomalies in DNS resolution patterns

By integrating with Azure Sentinel, you can correlate these alerts with other security signals to detect multi-stage attacks.

• Using Private DNS Zones for Internal Name Resolution

Why use Private DNS Zones?

Private DNS Zones allow you to resolve domain names internally without exposing them to the public internet. This significantly reduces the attack surface and prevents domain hijacking.

Key Best Practices:

•     Always use private DNS zones for internal application communications.

•     Configure conditional forwarding to route queries based on domain.

•     Use Azure Virtual Network (VNet) Integration for private DNS.

•     Implement custom DNS resolvers for hybrid and multi-cloud environments.

• Create a Private DNS Zone

• In the Azure Portal, go to Azure DNS.

• Select Private DNS Zones and click + Create.

• Define a DNS zone (e.g., privatelink.database.windows.net).

•     Go to Virtual Network Link.

• Select the target VNet.

•     Enable auto-registration if needed.

•     Create A, CNAME, and PTR records for internal resources.

•     Use nslookup or dig to verify name resolution inside the VNet.

How to Configure Private DNS Zones:

Link the DNS Zone to a Virtual Network

Define DNS Records

Test Resolution

• Securing DNS Traffic with Azure Firewall DNS Proxy

Why is DNS Proxy Needed?

•     Ensures centralized control over DNS resolution.
•     Blocks access to malicious domains using filtering rules.
•     Provides full visibility into DNS queries for security monitoring.

How to Configure Azure Firewall as a DNS Proxy:

Deploy Azure Firewall

•     In the Azure portal, go to Azure Firewall.
•     Create a firewall in a dedicated hub VNet.

Enable DNS Proxy

•     In the Firewall settings, enable DNS Proxy.
•     Set the Custom DNS Servers to an external trusted resolver (e.g., OpenDNS, Google DNS) or Azure Private Resolver.

Update Virtual Network DNS Settings

•     In the VNet settings, configure DNS servers to point to the firewall’s private IP.

Implement DNS Filtering Rules

•     Create application rules to block traffic to known malicious domains.
• Allow DNS resolution only for approved domains.

• Implementing DNS Logging and Monitoring

Monitoring DNS queries helps detect malicious activities and compliance violations. Azure provides multiple tools for DNS logging and analysis:

Recommended Tools:

•      Azure Monitor & Log Analytics
•     Collects detailed DNS logs and metrics.
• Microsoft Sentinel
•     Correlates DNS logs with security events.
• Azure Firewall Logs
•     Tracks DNS queries routed through the firewall.

How to Enable DNS Query Logging:

1.      Go to Log Analytics Workspace in Azure.
2.      Create a custom query to monitor AzureDiagnostics | where Category == "AzureDnsQueries".
3.      Set alerts for suspicious DNS patterns (e.g., excessive TXT queries, high query rates to unknown domains).
4.      Integrate logs with SIEM systems for real-time analysis.

• Enforcing Least Privilege Access with Microsoft Entra ID

Why is Access Control Important?

DNS configurations are sensitive, and unauthorized changes can lead to domain hijacking, misconfigurations, and security breaches.

Best Practices for Access Management:

•      Use Role-Based Access Control (RBAC) to restrict who can modify DNS settings.
•     Implement Just-in-Time (JIT) access using Privileged Identity Management (PIM).
•     Regularly audit DNS role assignments.
•     Use Multi-Factor Authentication (MFA) for DNS administrators.

Recommended Roles:

Role	Permissions
DNS Reader	View DNS records but cannot modify
DNS Contributor	Modify DNS records but cannot delete zones
DNS Owner	Full control over DNS zones

Conclusion

Securing DNS is essential to protect your Azure workloads from cyber threats. By implementing Microsoft Defender for DNS, Private DNS Zones, Azure Firewall DNS Proxy, and advanced logging mechanisms, organizations can significantly reduce attack surfaces and enhance security visibility.

For more details, check out Microsoft’s official guide on Azure DNS Security Policy