on [<modes>]server_lost [<serial#>] [-|^]<match> { <action> }

This hook is triggered whenever the client is disconnected from any server for any reason.

This hook is thrown when the server is in the CLOSING state, but before it switches to the CLOSED state. Whether or not the socket to the server is still up, or usable, is undefined behavior. If you need defined behavior, catch on server_status for the switch to CLOSING.