How to: Use the Dedicated Administrator Connection with SQL Server Management Studio
Microsoft SQL Server provides a dedicated administrator connection (DAC). The DAC allows an administrator to access a running instance of SQL Server Database Engine to troubleshoot problems on the server—even when the server is unresponsive to other client connections. The DAC is available through the sqlcmd utility and SQL Server Management Studio. The connection is only allowed from a client running on the server. No network connections are permitted.
To use SQL Server Management Studio with the DAC, connect to an instance of the SQL Server Database Engine with Query Editor by typing ADMIN: before the server name. Object Explorer cannot connect using the DAC.
To connect to a server using the DAC
In SQL Server Management Studio, with no other DACs open, on the toolbar, click Database Engine Query.
In the Connect to Database Engine dialog box, in the Server name box, type ADMIN: followed by the name of the server instance. For example, to connect to a server instance named ACCT\PAYABLE, type ADMIN:ACCT\PAYABLE.
Complete the Authentication section, providing credentials for a member of the sysadmin group, and then click Connect.
The connection is made.
If the DAC is already in use, the connection will fail with an error indicating it cannot connect.