I recently encountered a frustrating scenario where my local machine suddenly couldn't connect to a SQL Server instance that had worked perfectly for months. The error message was particularly vexing:

A connection was successfully established with the server, 
but then an error occurred during the pre-login handshake. 
(provider: SSL Provider, error: 0 - The wait operation timed out.) 
(.Net SqlClient Data Provider)

SQL Server connections go through several phases before authentication:

1. TCP connection establishment
2. Pre-login handshake (TDS protocol negotiation)
3. SSL negotiation (if configured)
4. Actual authentication

The error occurs specifically during phase 2, after TCP connection succeeds but before SSL negotiation completes.

Based on my troubleshooting experience, these are the most likely causes:

1. SSL/TLS configuration mismatch
2. Firewall/network changes blocking handshake packets
3. Certificate validation issues
4. Changes to SQL Server network configuration
5. IIS installation potentially intercepting traffic

Here's what ultimately worked for me, after trying numerous approaches:

1. Verify basic connectivity:

telnet sqlserver.example.com 1433

2. Check SSL settings in SQL Server Configuration Manager:

-- Verify encryption setting
SELECT session_id, encrypt_option 
FROM sys.dm_exec_connections

3. Modify connection string to explicitly control encryption:

string connectionString = "Server=myServerAddress;Database=myDataBase;
User Id=myUsername;Password=myPassword;
Encrypt=Optional;TrustServerCertificate=True;Connect Timeout=30;"

4. Reset network components:

-- On client machine
netsh int ip reset
netsh winsock reset

If the issue persists, consider these more advanced steps:

-- Check for certificate issues
DBCC TRACEON (4032)
DBCC TRACESTATUS(4032)

-- Network trace with Wireshark or Microsoft Message Analyzer
-- Look for dropped packets during handshake phase

To avoid future occurrences:

• Document all network configuration changes
• Test connections after any system updates
• Consider using AlwaysOn or connection retry logic in your apps

// Example retry logic in C#
int retryCount = 0;
while(retryCount < 3)
{
    try
    {
        using (SqlConnection conn = new SqlConnection(connectionString))
        {
            conn.Open();
            break;
        }
    }
    catch (SqlException)
    {
        retryCount++;
        Thread.Sleep(1000 * retryCount);
    }
}

---

This frustrating error occurs when your client machine establishes a connection with the SQL Server but fails during the SSL/TLS handshake phase. The complete error message typically looks like:

A connection was successfully established with the server, but then an error occurred during the pre-login handshake. 
(provider: SSL Provider, error: 0 - The wait operation timed out.) (.Net SqlClient Data Provider)

From my experience troubleshooting similar issues, here are the most common culprits:

• Network configuration changes (especially after IIS installation)
• SSL/TLS protocol mismatch between client and server
• Firewall interference with encrypted connections
• SQL Server configuration issues

Before attempting fixes, let's verify some key points:

-- Check if TCP/IP is enabled on SQL Server
USE master
GO
EXEC xp_readerrorlog 0, 1, 'Server is listening on'
GO

-- Check SSL configuration
SELECT * FROM sys.dm_exec_connections WHERE encrypt_option = 'TRUE'

1. Adjust Network Protocols

First, ensure SQL Server Configuration Manager has TCP/IP enabled:

1. Open SQL Server Configuration Manager
2. Navigate to SQL Server Network Configuration
3. Enable TCP/IP protocol
4. Restart SQL Server service

2. Modify Connection String

Try adding these parameters to your connection string:

"Server=your_server;Database=your_db;User Id=your_user;Password=your_pwd;
Encrypt=False;TrustServerCertificate=True;Connection Timeout=30;"

3. Update SSL/TLS Settings

For .NET applications, you might need to force specific protocols:

// Add this before your first connection attempt
System.Net.ServicePointManager.SecurityProtocol = 
    SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;

4. Firewall Configuration

Ensure these ports are open in your firewall:

• SQL Server default port (usually 1433)
• SQL Browser service port (1434 UDP)
• Ephemeral ports for encrypted communication

If the issue persists, try these advanced steps:

-- Check for certificate issues
DBCC TRACEON(4032)
DBCC TRACESTATUS(4032)

-- Monitor handshake process
EXEC sp_readerrorlog 0, 1, 'SSL', 'server'

Remember to document any changes you make so you can revert them if needed. The key is to methodically test each potential solution while monitoring the connection behavior.