Microsoft SQL server error 233 triggers while users login to an existing database. It happens due to a number of reasons ranging from exceeding the connection limit to firewall restrictions.

As a part of our Server Management Services, we help our Customers to fix database-related errors regularly.

Let us today discuss the possible causes and fixes for this error.

What causes the SQL server error 233?

Users often experience that they are unable to connect to an existing database even after providing the correct login credentials. The error message that they notice may resemble the one given below:

The common reasons for the Microsoft SQL server error 233 include;

1. SQL server not set for Mixed mode
2. TCP/IP protocol disabled
3. Shared Memory Protocol disabled
4. Named Pipes Protocol disabled
5. Virtual Interface Adapter disabled
6. View Port in Windows Firewall
7. Check for Remote Connection
8. Exceeding the number of Connections

Let us now look into each of these reasons in detail and their fixes.

SQL Server is not set to Mixed Mode

Mixed Mode (SQL Authentication Mode) provides a System Administrator account using a separate user name and password that can also be used to connect to the SQL server in addition to the Windows account. The 233 error can trigger if it is disabled. You can verify the cause of this error by trying to log in to SQL Server Management Studio (SSMS) using the System Administrator account.

1. Log in to SSMS using Windows Authentication.
2. Right-click on the main entry for the server itself and select Properties.
3. In the Properties screen, select Security on the left.
4. Change the Server Authentication setting from Windows Authentication to 'SQL Server and Windows Authentication mode'. Click OK.
5. To apply these settings, restart the SQL Server services by right-clicking on the server's main entry and selecting Restart.

TCP/IP protocol disabled

For SQL Server to be connected properly, TCP/IP should be enabled. It can be enabled using the steps given below:

1. Open MS SQL Server in your system
2. Go to configuration tools, and then open SQL Server Configuration Manager in your system >> SQL Server Network Configuration.
3. Choose TCP/IP option and enable it with a right-click

Finally, restart the SQL Server for the changes to take effect.

Shared Memory Protocol disabled

SQL Server Shared Memory protocol is used by clients to connect to the SQL Server instance that is running on the same machine. Sometimes, the user faces an authentication error, when the Shared Memory protocol is disabled. It can be easily enabled with the steps discussed below:

1. Go to Start → All Programs → MS SQL Server → Configuration Tools → SQL Server Configuration Manager.
2. Choose SQL Server Network Configuration.
3. Then, right-click on the option Shared Memory → Choose Enable.

Named Pipes Protocol disabled

Named Pipes protocols are established for local area networks. In it, a part of memory is used by one procedure to pass the info to another process. It can be on the same system or on a networked system. The user faces this error when the Named Pipes are disabled.The steps to enable it includes:

1. Open the SQL Server Configuration Manager and Navigate to SQL Server Network Configuration
2. From the SQL Server Network Configuration go to  Named Pipes Protocol and Enable it.

Virtual Interface Adapter disabled

VIA or Virtual Interface Adapter works with VIA hardware. Users face an SQL error when VIA is not enabled in the system. Thus. to remove an error issue, enable the VIA option from the SQL Server Network Configuration.

View Port in Windows Firewall

SQL Server ports running on the system, are added to exception, and firewall filters all traffic from those ports. SQL Server runs on 1433 port by default but if the default port is changed, then the new port is added to the exception. It can be done with the steps below:

1. First, open Control Panel in your system.
2. Now select Windows Firewall → Change Settings.
3. Click on Exceptions, and then select Add Port.
4. A window will appear, now fill the port number, and click on Ok button.
5. Then, enable the SQL.

Check for Remote Connection

Remote connection is important to connect to the SQL server remotely. Thus, it may trigger the error 233 error on a remote connection. To enable it, follow the steps below:

1. Click on Server node → choose Properties.
2. At left Tab select connection → check the option “allow remote connections to this server”

Exceeding the number of Connections

If the number of connections exceeds the limit, we won’t be able to connect until another connection becomes available. In such cases, it triggers the 233 error. This can be fixed with the following measures:

1. Set the user connections option to 0 to enable the maximum number of allowed connection (which is 32767)
2. Increase the value of user connections so that SQL Server can accept the new connections.
3. You can also close your existing connections so that new connections can be accepted by SQL Server. But this is not a permanent solution.

The changes to the maximum number of allowed connection can be performed from SQL Server Management Studio (SSMS) by following the below steps:

1. First, connect to SQL Server Instance in SSMS.
2. Next, right-click on SQL Server Instance in SSMS and choose properties. The Server Properties window will appear to make server-level changes.
3. Click on the “Connections” tab from the left side pane. Set this value to 0 to accept the maximum number of allowed connections or increase the value to fix the error.
4. Finally, restart the server to apply this change into effect.

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Conclusion

In short, Microsoft SQL server error 233 triggers while login to an existing database. It happens due to a number of reasons ranging from exceeding the connection limit to firewall restriction. Today, we saw how our Support Engineers fix this error.