LOGON Triggers in SQL Server

LOGON Triggers in SQL Server

Introduction:

In SQL Server, a logon trigger is a special type of trigger that allows you to execute code automatically when a user session attempts to log in to the SQL Server instance. These triggers can be useful for enforcing security measures, auditing user logins, or performing additional actions during the login process.

Creating a LOGON Trigger:

To create a logon trigger, you must have appropriate permissions in the SQL Server instance. The trigger is written in Transact-SQL (T-SQL) and can be created using the following syntax:

sql
CREATE TRIGGER [Trigger_Name]
ON ALL SERVER
FOR LOGON
AS
BEGIN
    -- Trigger code here
END;

Explanation:

• CREATE TRIGGER: This statement is used to create a new trigger in the SQL Server instance.

• [Trigger_Name]: Replace this with a suitable name for your trigger. Choose a descriptive name that reflects the purpose of the trigger.

• ON ALL SERVER: Specifies that the trigger should be applied to all logon events on the SQL Server instance.

• FOR LOGON: Indicates that the trigger is a logon trigger and will be fired when a user attempts to log in.

• AS BEGIN: The beginning of the trigger code block.

• -- Trigger code here: Write the T-SQL statements that you want to execute when a user logs in. You can include any valid T-SQL statements here.

Example:

Let's create a simple logon trigger that logs the login event and the timestamp into a log table:

sql
CREATE TRIGGER LogonAuditTrigger
ON ALL SERVER
FOR LOGON
AS
BEGIN
    INSERT INTO dbo.LogonAuditTable (Username, LoginTime)
    VALUES (ORIGINAL_LOGIN(), GETDATE());
END;

Explanation:

In this example, we create a logon trigger called LogonAuditTrigger. Whenever a user logs in, the trigger fires and inserts a new row into the LogonAuditTable with the username (retrieved using the ORIGINAL_LOGIN() function) and the current login time (retrieved using the GETDATE() function).

Important Note:

When creating logon triggers, it's essential to ensure that the trigger code is efficient and doesn't introduce performance bottlenecks, as it will be executed during every login attempt. Additionally, always thoroughly test the trigger in a non-production environment before implementing it in a live system.

Remember that logon triggers execute under the security context of the login that is logging in, so ensure that the necessary permissions are granted for the trigger to work correctly. Also, use logon triggers judiciously and only for legitimate use cases to avoid unintended consequences on the SQL Server instance.