Dedicated Administrator Connection (DAC)

SQL Server 2005 provides a special diagnostic connection for administrators when standard connections to the server are not possible. This diagnostic connection allows an administrator to access SQL Server to execute diagnostic queries and troubleshoot problems even when SQL Server is not responding to standard connection requests.
Only members of the SQL Server sysadmin role can connect using the DAC.
The DAC is available and supported through the sqlcmd command-prompt utility using a special administrator switch (-A). You can also connect prefixing admin: to the instance name in the format sqlcmd -Sadmin:. You can also initiate a DAC from a SQL Server Management Studio Query Editor by connecting to admin:.