SQL Server Dedicated Admin Connection (DAC)

The Dedicated Admin Connection (or DAC for short - not to be confused with DAC packages) is exactly that - a dedicated connection that's available to SQL Server sysadmins, for use when a SQL Server might have become unresponsive. This backdoor for the sysadmins has reserved resources at all times, and allows a means for connecting to SQL Server when other ways of connecting may not be working due to performance (or other) issues.

By default, connection to SQL Server via the DAC is only available locally from the server - i.e. you'd need to remote onto the server then use SQLCMD or SSMS to use the DAC. If the host is unresponsive due to performance issues, this could prove problematic. The solution is to enable remote admin connections - this allows connecting via the DAC remotely, and can be switched on by using the following SQL:

EXEC sp_configure 'remote admin connections', 1;

GO

RECONFIGURE

GO

Connecting to the DAC remotely via SSMS is then as simple as prefixing your server\instance name with "admin:"

Kendra Little has an excellent blog post and video explaining more, including how to tell who is using the DAC (if you're unable to use it), how to enable on clusters, and more:

https://www.brentozar.com/archive/2011/08/dedicated-admin-connection-why-want-when-need-how-tell-whos-using/