bindv6only

file: /proc/sys/net/ipv6/bindv6only
variable: net.ipv6.bindv6only
Official reference

Default value for IPV6_V6ONLY socket option, which restricts use of the IPv6 socket to IPv6 communication only. TRUE: disable IPv4-mapped address feature FALSE: enable IPv4-mapped address feature

Default: FALSE (as specified in RFC3493)

source