Dedicated Admin Connection (DAC) in SQL Server doesn’t need any introduction now that it is in the product for almost 6-7 years i.e from the days of SQL Server 2005. Starting from the early builds of SQL Server 2012, there is an error when connecting to DAC in SQL Server DENALI. This bug exists in SQL Server 2012 RC0 build as well but somehow MSFT can’t reproduce it and has been closed as won’t fix. To some extent I agree that this bug is NOT a big deal in the large scheme of things and am writing this to bring some awareness to the issue in case you happen to stumble upon it when using DAC in a real world scenario.

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The bug is if you connect to a “New Query” window and try to use the same query window to change the connection to use DAC then it throws an error message that failed to connect to DAC. The end result is DAC connection succeeds and works fine but throws an error message which can be confusing. If you are using DAC to solve a real issue on a production box and at that time stumbling on this bug can be really frustrating and this can compound to stress you are already on. Knowing about this bug and the work around, may help to avoid getting into that situation.

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Steps to reproduce:

1. Go to File | New | Database Engine Query | and connect to server with a regular connection.


Step 1 for DAC error in SQL Server 2012 RC0

2. Change Connection | use ADMIN: to connect to DAC
   
   

   Step 2 DAC Error in SQL Server 2012 RC0

3. Shows error message below


DAC error in SQL Server 2012 RC0

4. Click ok and use DAC connection without any issues. How do I know that DAC connection succeeded? One can use the below query to find if DAC connection is being used or NOT. And also there is the information at the bottom of the screen as well.


Step 4 DAC Error in SQL Server 2012 RC0

SELECT
	c.session_id
	, s.login_name
	, c.connect_time
	, c.last_read
	, c.last_write
	, s.host_name
	, c.client_net_address
FROM sys.dm_exec_connections c
    JOIN sys.endpoints e
        ON c.endpoint_id = e.endpoint_id
    JOIN sys.dm_exec_sessions s
        ON s.session_id = c.session_id
WHERE e.is_admin_endpoint = 1



Proof that DAC connection succeeded

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Workaround:

Until the bug is fixed, always use a new query window to connect to DAC directly. Don’t use an existing query window for a DAC connection.

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If you can reproduce the issue, and if you think its important then vote and/or add comments for the bug below.

DAC Connection throws error

Republished from Sankar Reddy [39 clicks].  Read the original version here [32134 clicks].