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

Do not consider loopback addresses as martian source or destination while routing. This enables the use of 127/8 for local routing purposes. default FALSE

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