file: /proc/sys/net/ipv6/conf/interface/use_oif_addrs_only
variable: net.ipv6.conf.interface.use_oif_addrs_only
Official reference

When enabled, the candidate source addresses for destinations routed via this interface are restricted to the set of addresses configured on this interface (vis. RFC 6724, section 4).

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.