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

Accept packets with source address 0.b.c.d destined not to this host as local ones. It is supposed, that BOOTP relay daemon will catch and forward such packets. conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay for the interface default FALSE Not Implemented Yet.

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