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

Do proxy arp. proxy_arp for the interface will be enabled if at least one of conf/{all,interface}/proxy_arp is set to TRUE, it will be disabled otherwise

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