file: /proc/sys/net/ipv4/tcp_l3mdev_accept
variable: net.ipv4.tcp_l3mdev_accept
Official reference

Enables child sockets to inherit the L3 master device index. Enabling this option allows a “global” listen socket to work across L3 master domains (e.g., VRFs) with connected sockets derived from the listen socket to be bound to the L3 domain in which the packets originated. Only valid when the kernel was compiled with CONFIG_NET_L3_MASTER_DEV.