Clarification of sshd ClientAlive* settings

ClientAliveCountMax

Sets the number of client alive messages which may be sent without sshd(8) receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session.

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The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. Setting a zero ClientAliveCountMax disables connection termination.

Referring to the above settings, the connection is terminated 45 seconds AFTER sending the first client alive message that receives no response.  However, since the first client alive message is sent after ClientAliveInterval seconds of client inactivity, this results in:

ClientAliveInterval + (ClientAliveInterval * ClientAliveCountMax) seconds since the last successful communication.

Applying the above settings, this effectively means that the connection is terminated 15 + (15 * 3) seconds = 60 seconds.

It should also be emphasized that saying "the last client message or activtity" is not equivalent to saying "the last user message or activity".  Client Alive packets get answered silently by the ssh client code.  Even if the user is idle, the client code can be responsive, and can answer Client Alive packets.  In other words, the Client Alive mechanism is not intended to check on user idleness.  It is a check on successful communication between the client and server.  The Client Alive mechanism essentially detects if the client has died or if the network between server and client is no longer delivering packets.