How to fix Error 9651 on Windows Server 2025 / Windows 11

by

Troubleshooting Guide: Error 9651 – System Call Failed During Service Broker or Database Mirroring Operation

Why This Error Happens

Error 9651 occurs when the underlying Windows networking stack rejects or fails a system call that SQL Server 2025 issues to establish or maintain a Service Broker or Database Mirroring endpoint connection. This is typically caused by a blocked or conflicting TCP port, insufficient permissions on the SQL Server service account to bind to the network socket, or corrupted/missing network provider components (such as the Winsock catalog) on Windows Server 2025 or Windows 11. It can also surface when a firewall, antivirus, or third-party network filter driver intercepts the socket operation before SQL Server can complete it.

Step-by-Step Resolution Guide

Step 1: Identify the Exact System Error Returned

  • Open the SQL Server Error Log via SQL Server Management Studio (SSMS) or C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG.
  • Locate the full text of Error 9651 — the %ls placeholder will contain a Windows system error code (e.g., “10048” for address already in use, or “10013” for permission denied).
  • Cross-reference the embedded Windows error code using net helpmsg <code> in an elevated Command Prompt to get a plain-language description.

Step 2: Check for Port Conflicts on the Mirroring/Broker Endpoint

  • Run netstat -ano | findstr <port_number> (commonly 5022 for mirroring or 4022 for Service Broker) to see if another process is already bound to that port.
  • If a conflict exists, either stop the competing process or reassign the endpoint port using:
  ALTER ENDPOINT Mirroring STATE = STOPPED;
  ALTER ENDPOINT Mirroring AS TCP (LISTENER_PORT = 5023);
  ALTER ENDPOINT Mirroring STATE = STARTED;
  • Update firewall rules accordingly via wf.msc or netsh advfirewall firewall add rule name="SQL Mirroring" dir=in action=allow protocol=TCP localport=5023.

Step 3: Verify Service Account Permissions and Socket Binding Rights

  • Confirm the SQL Server service account (visible in SQL Server Configuration Manager) has “Log on as a service” rights and is not restricted by Group Policy.
  • Check that the account is not blocked by Network access: Restrict clients allowed to make remote calls to SAM policies under secpol.msc.
  • Restart the SQL Server service after any permission changes: net stop MSSQLSERVER && net start MSSQLSERVER.

Step 4: Repair the Winsock Catalog and Network Stack

  • If the error code references a Winsock failure (e.g., 10050, 10051, 10060), reset the TCP/IP stack:
  netsh winsock reset
  netsh int ip reset
  • Reboot the server after running these commands, as a stale or corrupted Winsock catalog frequently causes 9651 on freshly patched Windows Server 2025 builds.

Step 5: Disable Interfering Security Software Temporarily

  • Temporarily disable third-party antivirus or endpoint protection network filters and retest the mirroring/broker session.
  • If the issue resolves, add explicit exclusions for sqlservr.exe and the relevant TCP ports in the security software’s configuration.

Alternative or Verification Steps

  • After applying fixes, re-establish the mirroring session or Service Broker conversation and confirm no 9651 entries appear in the Error Log for at least one full sync cycle.
  • As an alternative, use sys.dm_tcp_listener_states to verify the endpoint state is “Ready” and check sys.service_broker_endpoints to ensure the endpoint configuration matches expected port and state values.