Logon triggers in SQL Server

 Logon triggers in SQL Server are a type of trigger that executes in response to a LOGON event. These triggers can be used to control and manage logon sessions, enforce security policies, and audit logon activities.

Key Concepts

• LOGON Event: The event that triggers the execution of the logon trigger, which occurs when a user connects to the SQL Server instance.
• Context: Logon triggers can be used to capture and respond to connection details such as the login name, client host, and other session-related information.

Creating Logon Triggers

Logon triggers are created using the CREATE TRIGGER statement at the server level and can be used to execute specific actions when a user logs on to the SQL Server instance.

Syntax

CREATE TRIGGER trigger_name
ON ALL SERVER
FOR LOGON
AS
BEGIN
    -- Trigger logic goes here
END;

Example Scenarios

1. Auditing Logon Events

You can use logon triggers to log information about user logons to a custom table for auditing purposes.

-- Create a table to log logon events
CREATE TABLE LogonLog (
    LogID INT IDENTITY(1,1) PRIMARY KEY,
    LogonDateTime DATETIME DEFAULT GETDATE(),
    LoginName NVARCHAR(256),
    ClientHost NVARCHAR(256)
);

-- Create the logon trigger
CREATE TRIGGER trgLogonAudit
ON ALL SERVER
FOR LOGON
AS
BEGIN
    INSERT INTO LogonLog (LoginName, ClientHost)
    VALUES (ORIGINAL_LOGIN(), HOST_NAME());
END;

2. Enforcing Security Policies

You can use logon triggers to enforce security policies, such as restricting connections based on the time of day or disallowing certain logins.

-- Create the logon trigger to enforce login policies
CREATE TRIGGER trgRestrictLogon
ON ALL SERVER
FOR LOGON
AS
BEGIN
    DECLARE @currentTime TIME = CONVERT(TIME, GETDATE());

    -- Restrict logins between 10 PM and 6 AM
    IF @currentTime BETWEEN '22:00:00' AND '06:00:00'
    BEGIN
        -- Optionally log the attempt
        INSERT INTO LogonLog (LoginName, ClientHost)
        VALUES (ORIGINAL_LOGIN(), HOST_NAME());

        -- Raise an error to prevent the logon
        RAISERROR('Logins are restricted between 10 PM and 6 AM.', 16, 1);
        ROLLBACK;
    END
END;

3. Redirecting Logins

You can use logon triggers to redirect users to a specific database or perform other custom actions based on the login.

-- Create the logon trigger to redirect logins
CREATE TRIGGER trgRedirectLogon
ON ALL SERVER
FOR LOGON
AS
BEGIN
    DECLARE @loginName NVARCHAR(256) = ORIGINAL_LOGIN();
    
    -- Redirect specific users to a particular database
    IF @loginName = 'SpecialUser'
    BEGIN
        -- Example action: Change the default database for the user
        EXEC sp_defaultdb @loginame = @loginName, @defdb = 'SpecialDatabase';
    END
END;

Best Practices

1. Performance: Be cautious with the complexity of logon triggers, as they execute every time a user logs on. Avoid complex logic that could impact performance or delay logon times.
2. Error Handling: Include proper error handling within logon triggers to ensure that issues are managed gracefully and do not disrupt user logons.
3. Security: Ensure that logon triggers do not expose sensitive information and are used in accordance with your security policies.
4. Testing: Thoroughly test logon triggers in a development environment before deploying them to production to ensure they work as intended.
5. Documentation: Document the purpose and logic of each logon trigger to facilitate maintenance and understanding.

Logon triggers provide a powerful mechanism for managing and controlling user logons to SQL Server, allowing you to enforce security policies, audit logon activities, and implement custom behaviors based on user connections.