How do network connectivity or firewall rules contribute to Error 18456 with Windows Authentication, and how can they be troubleshooted?

Network connectivity issues or firewall rules can prevent the client from establishing a connection to the SQL Server or prevent the SQL Server from reaching a domain controller for authentication, leading to Error 18456. **Causes:** 1. **Firewall Blocking Port:** Client-side, server-side, or network firewalls might block the SQL Server's listening port (default 1433 for default instances, dynamic ports for named instances), or UDP port 1434 for the SQL Server Browser service. 2. **DNS Resolution Failure:** The client cannot resolve the SQL Server instance name to its correct IP address. 3. **Domain Controller Unreachable:** The SQL Server machine cannot communicate with a domain controller to authenticate the Windows user, often due to network issues or firewall rules. **Troubleshooting:** 1. **Ping/Test-NetConnection:** From the client machine, `ping ` to check basic connectivity. Use `Test-NetConnection -ComputerName -Port 1433` (or the specific port) to check if the port is open and reachable. 2. **Telnet:** From the client, `telnet 1433` (or the correct port). A blank screen indicates a successful connection to the port. 3. **Firewall Rules:** Check Windows Firewall on the SQL Server machine to ensure inbound rules allow connections on the SQL Server port(s) (TCP 1433, and UDP 1434 for SQL Browser if using named instances) for the appropriate network profiles (Domain, Private). 4. **SQL Server Browser Service:** For named instances, ensure the SQL Server Browser service is running on the SQL Server and UDP port 1434 is open in the firewall. 5. **Event Logs:** Check the System and Security event logs on both the client and SQL Server for network-related errors or authentication failures.

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Answered on 28 Agustus 2025