Dedicated Administrator Connection (DAC)

• SQL Server 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.
• This dedicated administrator connection (DAC) supports encryption and other security features of SQL Server.
• The DAC only allows changing the user context to another admin user.
• SQL Server makes every attempt to make DAC connect successfully, but under extreme situations it may not be successful.
• By default, the connection is only allowed from a client running on the server. Network connections are not permitted unless they are configured by using the sp_configure stored procedure with the remote admin connections option.
• 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). For more information about using sqlcmd, see Using sqlcmd with Scripting Variables. You can also connect prefixing admin: to the instance name in the format sqlcmd -Sadmin:<instance_name>. You can also initiate a DAC from a SQL Server Management Studio Query Editor by connecting to admin:<instance_name>.
  Example

  In this example, an administrator notices that server URAN123 is not responding and wants to diagnose the problem. To do this, the user activates the sqlcmd command prompt utility and connects to server URAN123 using -A to indicate the DAC.

  sqlcmd -S URAN123 -U sa -P <xxx> –A