There are at least six different user actions that can lead to MongoDB not running on the provided host and port.  Some other error messages are less overloaded, but possibly confusing in the absence of understanding the underlying exception that caused them.

Recommendation

Add a small link or button, labeled +ADVANCED, to the Compass connection error display when a connection fails or closes unexpectedly. This link/button presents a dialog with addition information for troubleshooting the problem, including:

• the underlying exception text
• success/failure of DNS
• state of the replica set, if known
• state of the TCP socket, if known
• state of the SSH connection, if applicable,
• the last command sent to the server
• the last reply received from the server.