--log-warnings option or
log_warnings system variable
controls warning logging to the error log. The default value is
enabled (1). To disable warning logging, set
log_warnings to 0. If the value
is greater than 1, aborted connections are written to the error
log, and access-denied errors for new connection attempts are
written. See Section B.5.2.11, “Communication Errors and Aborted Connections”.