Option to reduce Probe Response frame responses during max STA

Option to reduce Probe Response frame responses during max STA

The new hostapd configuration parameter no_probe_resp_if_max_sta=1 can
be used to request hostapd not to reply to broadcast Probe Request
frames from unassociated STA if there is no room for additional stations
(max_num_sta). This can be used to discourage a STA from trying to
associate with this AP if the association would be rejected due to
maximum STA limit.

Signed-off-by: Jouni Malinen <j@w1.fi>