Restrictions
Because 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:
RESTORE
BACKUP 

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.

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