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. 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. The DAC is available and supported through the sqlcmd command-prompt utility using a special administrator switch -A.
Connecting with DAC
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Speak guys!! However, SQL Browser is not used to allow the use of the connection via remote admin connections? If so, would it be a mistake to keep him active? As you know, the connection Dedicated Admin Connection DAC Allows SQL Server to reserve a connection slot so that in extreme cases, such as all connections being used or logon triggering preventing connections, you can still log in to the instance and try to fix the problem without restarting the SQL service.
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How to enable DAC for remote connections
The Dedicated Admin Connection was built to help you connect and run basic troubleshooting queries in cases of serious performance problems. The DAC uses a special reserved scheduler which has one thread for processing requests. This essentially means that SQL Server is keeping a backdoor open to processor resources available just for you. Two percent of you will be tempted to use this for your maintenance jobs on busy systems. The DAC was not designed for high performance. By default the DAC is only enabled for accounts logged onto the local machine. For production servers, that means it only works for remote desktop sessions to non-clustered SQL Server instances.
You would be able to see the connection name at the bottom of the query window as shown in the snippet below. Then, you would end up receiving below mentioned error message. Once you have established a successful DAC connection you can execute troubleshooting queries to identify the bottleneck and take action accordingly.