Mastering Azure DNS Security Policy: Full Guide for Filtering and Logging DNS Traffic

Introduction

In today’s cloud-first world, DNS traffic is often overlooked—but it’s a vital part of your cloud security surface. Attackers frequently exploit DNS for exfiltration, command and control, or domain hijacking. Microsoft Azure’s DNS Security Policy equips you with the tools to filter DNS queries, block threats, and gain full visibility into DNS behavior within your virtual networks (VNets).

This comprehensive guide walks you through how to filter and log DNS traffic using Azure DNS Security Policy—from core components and setup to advanced monitoring.

What Is Azure DNS Security Policy?

Azure DNS Security Policy allows Azure customers to filter, alert, and log DNS queries within a virtual network. It supports both private and public DNS queries, offering:

Rule-based blocking of malicious or unwanted domains
Detailed logging of DNS traffic
The ability to alert on suspicious queries without disrupting service

This feature is critical for organizations seeking to enforce DNS hygiene, detect DNS tunneling, and enhance overall network observability.

Core Features for DNS Filtering & Logging

Policy Scope & Location

Each DNS Security Policy is region-specific and can only be linked to VNets in the same region. For instance, a policy created in East US can only govern VNets deployed in East US.

📌 Note: You can associate multiple VNets with a single DNS security policy, but each VNet can only be linked to one policy at a time.

DNS Traffic Rules: The Filtering Engine

These rules define how DNS queries should be handled. Each rule includes:

Priority: Integer from 100–65000 (lower numbers = higher priority)
Domain lists: Predefined lists of domains to which the rule applies
Action: One of:
- Allow – Permit and log the query
- Block – Deny and log the query
- Alert – Allow the query but log an alert for analysis
Enable/Disable toggle

Important Rule Behaviors:

DNS rules respect hierarchy. If contoso.com is allowed at higher priority, sub.contoso.com is also allowed even if blocked in a lower-priority rule.
You can apply rules to all domains using the "." wildcard. But be cautious—this could block essential Azure services.

Virtual Network Links: Binding Policy to Traffic

These links bind DNS policies to VNets, enabling policy enforcement.

Each link connects a single DNS policy to one or more VNets
VNets must reside in the same region as the policy
Once linked, all DNS traffic from the VNet is subject to the rules

This enables centralized policy management while keeping per-region compliance intact.

DNS Domain Lists: Organize Your Filters

Domain lists are collections of DNS domain names – like allowlists or blocklists.

🔍 Key Capabilities:

A list can include specific domains (e.g., malicious.contoso.com) or wildcards (e.g., *.example.com)
CNAME “chasing” is supported. If a domain resolves to a blocked target via CNAME, the rule still applies
A domain list must contain at least one domain and can be reused across multiple rules or policies

⚠️ Be cautious with wildcard entries. Blocking "." or "*" may unintentionally prevent legitimate traffic.

Security Use Cases

With Azure DNS Security Policy, you can:

Block known malicious domains from being resolved
Log DNS queries for forensic analysis
Alert on suspicious behavior without disruption
Enforce domain usage policies within VNets (e.g., block social media, torrent trackers, etc.)

📊 Monitoring & Logging

Azure allows extensive telemetry via diagnostic settings. Logs can be sent to:

Azure Monitor / Log Analytics
Storage Accounts (for archiving)
Event Hubs (for integration with SIEM tools)

Each log record includes:

Timestamp
Domain queried
Source IP
Action taken (allow, block, alert)
CNAME resolution path (if applicable)

This visibility allows real-time threat detection and integration with Microsoft Sentinel for custom alerting and automation.

Configuration Limits & Considerations

Component	Limit
Security Policies per Region	1000
DNS Traffic Rules per Policy	10
Domain Lists per Policy	1000
Domains per Domain List	100,000
VNets per Policy	Multiple (same region)
Policies per VNet	1

Step-by-Step: How to Set Up DNS Filtering & Logging

You can configure DNS Security Policy through the Azure Portal or Azure PowerShell.

1. Create the DNS Security Policy

Navigate to Azure DNS > Security Policies
Choose region and resource group

2. Create Domain Lists

Add domains (e.g., blocklist of known threats)
Use wildcards cautiously
Group domains logically (e.g., “malware-domains”, “social-media”, etc.)

3. Define Traffic Rules

Set priority and action (Allow, Block, Alert)
Assign domain lists

4. Link to VNets

Under “Virtual Network Links,” add VNets in the same region

5. Enable Logging

Go to Diagnostics
Forward logs to Log Analytics for analysis

Pro Tips & Best Practices

Start with Alert rules before enforcing Block actions—this helps tune your lists without risk
Use CNAME chasing to catch indirect domain abuse
Build rules using granular domain lists for fine control
Integrate logs into Sentinel to detect patterns like:
- DGA (domain generation algorithm) behaviors
- DNS tunneling
- Beaconing traffic to suspicious domains

Final Thoughts

Azure DNS Security Policy offers a powerful, native method for securing DNS resolution within VNets. By providing policy-driven DNS controls, rich logging, and integration with Azure Monitor, it enables you to protect your cloud workloads against DNS-based attacks – without needing to route traffic through third-party appliances.

Whether you’re a security engineer, cloud architect, or network administrator, adopting DNS security policies helps bolster your zero-trust architecture and improve network observability.

Related Resources

Shailesh Vaja

P.S. Modern AI tool has been used for creating some of the content. Technical validation and proofing are done by the author.