Azure DNS Issue

Summary

The issue at hand involves resolving a Conditional Forwarding loop between On-Premises DNS and Azure’s non-routable IP 168.63.129.16. This problem arises when implementing a Zero Trust architecture with Azure Private DNS Zones and Private Endpoints. The goal is to enable on-premises developers to connect to an Azure SQL Database without manually editing hosts files on every laptop.

Root Cause

The root cause of this issue lies in the way Azure Private DNS Zones handle name resolution for Private Endpoints. When an on-premises user attempts to connect to the Azure SQL Database, their DNS query is forwarded to Azure’s non-routable IP 168.63.129.16, which then tries to resolve the Private Endpoint IP. However, this process can create a Conditional Forwarding loop, causing the SQL connection string to resolve to the Public IP instead of the Private Endpoint IP.

Why This Happens in Real Systems

This issue occurs in real systems because of the complexities involved in integrating on-premises DNS infrastructure with Azure’s Private DNS Zones. The Conditional Forwarding loop is created when the on-premises DNS server forwards the DNS query to Azure’s non-routable IP, which then forwards it back to the on-premises DNS server, causing a loop.

Real-World Impact

The real-world impact of this issue is significant, as it prevents on-premises developers from connecting to the Azure SQL Database, hindering productivity and development. Moreover, manually editing hosts files on every developer’s laptop is unmanageable and not a scalable solution.

Example or Code

To illustrate the solution, consider the following Azure CLI command to create a conditional forwarder for the Private DNS Zone:

az network dns zone create -g MyResourceGroup -n MyPrivateDNSZone \
  --zone-type Private \
  --registration-vnets MyVNet \
  --conditional-forwarders 168.63.129.16=myprivatelink.database.windows.net

This command creates a conditional forwarder for the Private DNS Zone, which forwards DNS queries for the Private Endpoint to Azure’s non-routable IP.

How Senior Engineers Fix It

Senior engineers fix this issue by implementing a Conditional Forwarding solution that avoids the loop between On-Premises DNS and Azure’s non-routable IP. This involves configuring the on-premises DNS server to forward DNS queries for the Private Endpoint to a designated DNS proxy server, which then resolves the Private Endpoint IP. Additionally, they ensure that the DNS proxy server is configured to handle the Conditional Forwarding loop and avoid the “Double-Hop” latency.

Why Juniors Miss It

Junior engineers may miss this issue because they may not fully understand the complexities of integrating on-premises DNS infrastructure with Azure’s Private DNS Zones. They may not be aware of the Conditional Forwarding loop and its impact on name resolution for Private Endpoints. Moreover, they may not have the experience to design and implement a scalable solution that avoids the “Double-Hop” latency and handles multiple Private DNS Zones.