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Chapter 2. Connection to MySQL Server Failing on Windows

When you're running a MySQL server on Windows with many TCP/IP connections to it, and you're experiencing that quite often your clients get a Can't connect to MySQL server error, the reason might be that Windows doesn't allow for enough ephemeral (short-lived) ports to serve those connections.

The purpose of TIME_WAIT is to keep a connection accepting packets even after the connection has been closed. This is because Internet routing can cause a packet to take a slow route to its destination and it may arrive after both sides have agreed to close. If the port is in use for a new connection, that packet from the old connection could break the protocol or compromise personal information from the original connection. The TIME_WAIT delay prevents this by ensuring that the port cannot be reused until after some time has been allowed for those delayed packets to arrive.

It is safe to reduce TIME_WAIT greatly on LAN connections because there is little chance of packets arriving at very long delays, as they could via the Internet with its comparatively large distances and latencies.

By default, Windows allows 5000 ephemeral (short-lived) TCP ports to the user. After any port is closed it will remain in a TIME_WAIT status for 120 seconds. The port will not be available again until this time expires.

With a small stack of available TCP ports (5000) and a high number of TCP ports being open and closed over a short period of time along with the TIME_WAIT status you have a good chance for running out of ports. There are two ways to address this problem:

  • Reduce the number of TCP ports consumed quickly by investigating connection pooling or persistent connections where possible

  • Tune some settings in the Windows registry (see below)

IMPORTANT: The following procedure involves modifying the Windows registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, view the following article in the Microsoft Knowledge Base: http://support.microsoft.com/kb/256986/EN-US/.

  1. Start Registry Editor (Regedt32.exe).

  2. Locate the following key in the registry: 

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

  3. On the Edit menu, click Add Value, and then add the following registry value: 

    Value Name: MaxUserPort
Data Type: REG_DWORD
Value: 65534

    This sets the number of ephemeral ports available to any user. The valid range is between 5000 and 65534 (decimal). The default value is 0x1388 (5000 decimal).

  4. On the Edit menu, click Add Value, and then add the following registry value: 

    Value Name: TcpTimedWaitDelay
Data Type: REG_DWORD
Value: 30

    This sets the number of seconds to hold a TCP port connection in TIME_WAIT state before closing. The valid range is between 0 (zero) and 300 (decimal). The default value is 0x78 (120 decimal).

  5. Quit Registry Editor.

  6. Reboot the machine.

Note: Undoing the above should be as simple as deleting the registry entries you've created.