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

Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a multicast routing daemon is required. conf/all/mc_forwarding must also be set to TRUE to enable multicast routing for the interface

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