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

Send(router) or accept(host) RFC1620 shared media redirects. Overrides secure_redirects. shared_media for the interface will be enabled if at least one of conf/{all,interface}/shared_media is set to TRUE, it will be disabled otherwise default TRUE

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