Resolving Azure SQL Server Connection Issues: A Case Study of Linked Server Failures

Discover how to troubleshoot and solve `connection problems` between on-premises SQL Servers and Azure SQL servers, using real-world examples and best practices.
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Resolving Azure SQL Server Connection Issues: A Case Study of Linked Server Failures

When working with cloud databases like Azure SQL Server, you may encounter connection issues that can be frustrating and challenging to resolve. One such instance is when an on-premises SQL Server cannot connect to an Azure SQL Server, while other machines in the same network can connect just fine. This guide takes a closer look at a specific scenario involving a failed linked server connection and provides insights on how to fix the issue.

The Problem

In this scenario, the user was attempting to create a linked server from an on-premises SQL Server (referred to as BadMachine1) to an Azure SQL Server. The code executed for creating the linked server appeared to be correct, but it resulted in a failure when testing the connection.

Despite successfully establishing connections from two other machines (GoodMachine1 and GoodMachine2), the error persisted on BadMachine1. Examining the error messages revealed a common theme: issues related to network connections and handshake failures.

Key Error Messages

Failed connection message: "Client unable to establish connection due to prelogin failure."

Pre-login handshake failure: "A connection was successfully established with the server, but then an error occurred during the pre-login handshake."

The errors indicated that the network name might not be available or that there was a problem during the initial stages of making the connection.

Investigation & Analysis

After testing various configurations and setups, the user noted a few key differences between the machines involved:

BadMachine1: Old ODBC Driver 11, using Microsoft AS OLE DB Provider for SQL Server 2014, and running SQL Server 2014.

GoodMachine1: ODBC Drivers 11, 13, and 17 installed, with SQL Server 2016.

GoodMachine2: ODBC Driver 13 installed and a recent version of the OLE driver, using SQL Server 2016.

Potential Causes

Old Drivers: The outdated drivers on BadMachine1 were a concern, as they could potentially lead to compatibility issues when trying to connect to Azure services.

Firewall Issues: One of the most common culprits for connection failures is firewall restrictions that might block outbound connections.

The Solution

After thorough investigation, the root cause of the problem was identified: IT had configured specific settings to block outbound connections from BadMachine1. Once these blocks were removed, the connection to the linked server was established successfully.

Steps to Resolve Connection Issues

Check Firewall Settings: Ensure that outbound connections are allowed from the SQL Server machine to the Azure SQL Server endpoints.

Update ODBC Drivers: Install the latest ODBC drivers on the SQL Server to ensure compatibility with Azure.

Monitor Network Configurations: Verify that network settings are appropriately configured and that there are no interruptions in connectivity.

Perform Regular Tests: After making changes, conduct tests to ensure successful connections and diagnose any lingering issues.

Conclusion

Establishing connections between on-premises SQL Servers and Azure SQL servers can sometimes present challenges, as demonstrated in this scenario. However, by systematically addressing potential issues—from firewall settings to driver updates—it is possible to resolve these connection problems effectively. Always ensure that you have the latest tools, configurations, and permissions in place for seamless connectivity and optimal performance.

By following these insights and best practices, you can mitigate similar problems in the future and make the most out of your cloud database services.