How to Fix SQL Server Connection Errors (e.g., Error 18456)?

 

Connectivity issues rank high in searches, stemming from auth failures, network problems, or config errors.

Scenario: A web app can't connect to SQL Server after a server migration, showing "Login failed for user" errors.

Detailed Solution with Code Examples:

• Step 1: Check Error Log. View via SSMS (Object Explorer > SQL Server Logs) or:
  sql

  EXEC xp_readerrorlog 0, 1, N'Login failed';
• Step 2: Verify Authentication. For SQL auth, reset password:
  sql

  ALTER LOGIN YourLogin WITH PASSWORD = 'NewPassword';
• Step 3: Enable Protocols. Use SQL Server Configuration Manager to enable TCP/IP, restart service.
• Step 4: Test Connection. Use PowerShell:
  powershell

  $conn = New-Object System.Data.SqlClient.SqlConnection

  $conn.ConnectionString = "Server=YourServer;Database=YourDB;Integrated Security=True;"

  try { $conn.Open(); Write-Host "Connected!"; } catch { Write-Host $_.Exception.Message; }
• Step 5: Fix Firewall/Port Issues. Ensure port 1433 is open; for dynamic ports, query:
  sql

  SELECT DISTINCT local_tcp_port FROM sys.dm_tcp_listener_states;

Best Practices:

• Use Windows auth over SQL for security.
• Implement connection pooling in apps to reduce overhead.
• Monitor failed logins with audits: CREATE SERVER AUDIT FailedLogins TO FILE (FILEPATH = 'C:\Audits\');.
• In Azure, check NSG rules.
• Test connections post-updates; use aliases for flexibility.

This resolves 80% of issues quickly, ensuring reliable access.

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