file: /proc/sys/net/ipv4/conf/interface/drop_unicast_in_l2_multicast
variable: net.ipv4.conf.interface.drop_unicast_in_l2_multicast
Official reference

Drop any unicast IP packets that are received in link-layer multicast (or broadcast) frames. This behavior (for multicast) is actually a SHOULD in RFC 1122, but is disabled by default for compatibility reasons. Default: off (0)

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.