RestrictionsBecause the DAC exists solely for diagnosing server problems in rare circumstances, there are some restrictions on the connection:To guarantee that there are resources available for the connection, only one DAC is allowed per instance of SQL Server. If a DAC connection is already active, any new request to connect through the DAC is denied with error 17810.To conserve resources, SQL Server 2005 Express Edition does not listen on the DAC port unless started with a trace flag 7806.The DAC initially attempts to connect to the default database associated with the login. After it is successfully connected, you can connect to the master database. If the default database is offline or otherwise not available, the connection will return error 4060. However, it will succeed if you override the default database to connect to the master database instead using the following command:sqlcmd –A –d master We recommend that you connect to the master database with the DAC because master is guaranteed to be available if the instance of the Database Engine is started.SQL Server prohibits running parallel queries or commands with the DAC. For example, error 3637 is generated if you execute either of the following statements with the DAC:RESTOREBACKUP Only limited resources are guaranteed to be available with the DAC. Do not use the DAC to run resource-intensive queries (for example. a complex join on large table) or queries that may block. This helps prevent the DAC from compounding any existing server problems. To avoid potential blocking scenarios, if you have to run queries that may block, run the query under snapshot-based isolation levels if possible; otherwise, set the transaction isolation level to READ UNCOMMITTED and set the LOCK_TIMEOUT value to a short value such as 2000 milliseconds, or both. This will prevent the DAC session from getting blocked. However, depending on the state that the SQL Server is in, the DAC session might get blocked on a latch. You might be able to terminate the DAC session using CNTRL-C but it is not guaranteed. In that case, your only option may be to restart SQL Server.