Padma Achanta's Insights on Database Technologies

A Secure Repository for Unlocking Database Knowledge

Unplanned Failover Due to DNS Registration Failure of Availability Group Listener

In this post, we will discuss a specific issue that can lead to an unplanned failover in a SQL Server Always on Availability Group (AG). This problem occurs when the Availability Group Listener fails to register its Virtual Network Name (VNN) in DNS, which can cause client applications to lose connectivity to the primary replica, eventually triggering a failover.

Issue

In a SQL Server Always On Availability Group setup, the AG Listener is responsible for routing client connections to the correct primary replica. However, if the Listener is unable to register its DNS name properly due to permission issues or network configuration, client applications will not be able to resolve the Listener’s IP address.

In this scenario, an unplanned failover occurred because the SQL Server cluster service account did not have the necessary permissions to update the DNS record. After repeated failures to resolve the primary replica, the failover cluster initiated a failover process to maintain high availability.

Error Messages

You may find errors related to DNS registration failure in the SQL Server Error Logs and Windows Event Logs. Below are sample error messages that indicate the issue:

2024-12-2 14:35:12.34 Server      The Availability Group Listener ‘AGListener1’ failed to register its Virtual Network Name (VNN) in DNS. The error code is 4201. Ensure that the network configuration allows the proper DNS registration, and the cluster service account has permissions to register DNS names.

2024-12-2 14:35:15.78 Server      SQL Server detected a DNS lookup failure while attempting to connect to the primary replica of availability group ‘AG1’. Error code: 11004.

2024-12-2 14:35:20.22 Server      Failed to connect to the primary replica ‘PrimaryReplicaNode’ for availability group ‘AG1’. SQL Server will retry.

The DNS registration failure prevents the Listener from resolving the primary replica’s IP address, leading to connection issues and failover.

Solution

To resolve this issue and prevent future occurrences, follow these steps:

Step 1: Verify DNS Registration Failure

First, confirm the failure by reviewing the SQL Server Error Logs or Windows Event Viewer. The error log entries (as shown above) will provide more details about the registration failure.

Step 2: Grant DNS Registration Permissions

The primary cause of this issue is insufficient permissions for the SQL Server cluster service account to update the DNS entry. To resolve this, you need to adjust the DNS permissions:

  1. Open DNS Manager on your domain controller.
  2. Locate the DNS record for the Availability Group Listener (e.g., ‘AGListener1’).
  3. Right-click the record and choose Properties.
  4. Go to the Security tab.
  5. Ensure that the cluster computer object has permissions to create and delete child objects (e.g., Create All Child Objects and Delete All Child Objects).

This will allow the AG Listener to update its DNS record when failovers occur or the primary replica changes.

Step 3: Manually Register the DNS Record (only If needed)

In case the automatic DNS registration fails even after updating permissions, you can manually register the Listener’s DNS record:

  1. Open Command Prompt on a cluster node.
  2. Run the following command to force DNS registration:

ipconfig /registerdns

This command will manually update the DNS record for the AG Listener and ensure clients can resolve the correct primary replica.

Step 4: Verify the Fix

After making the necessary adjustments, verify the fix:

  1. Check Failover Cluster Manager to ensure the AG Listener is online.
  2. Test client connections to the Availability Group Listener using tools like SQL Server Management Studio (SSMS).
  3. Confirm that no further DNS errors appear in the SQL Server Error Logs.

Conclusion

The unplanned failover in this case was triggered by the AG Listener’s inability to register its DNS record due to insufficient permissions. By adjusting DNS registration permissions for the cluster service account and manually registering the DNS record if needed, you can ensure smooth failovers and avoid future connectivity issues.

Happy Learning 🙂