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

Controls the fwmark of kernel-generated IPv6 reply packets that are not associated with a socket for example, TCP RSTs or ICMPv6 echo replies). If unset, these packets have a fwmark of zero. If set, they have the fwmark of the packet they are replying to. Default: 0