https://code.launchpad.net/~smoser/ubuntu/+source/initramfs-tools/+git/initramfs-tools/+ref/lp-1631010
there i added 'ip6=only' and 'ip6=only-S' (we can change the syntax), but '-S' means to pass '-S' to 'dhclient -6'.
https:/ /code.launchpad .net/~smoser/ ubuntu/ +source/ initramfs- tools/+ git/initramfs- tools/+ ref/lp- 1631010
there i added 'ip6=only' and 'ip6=only-S' (we can change the syntax), but '-S' means to pass '-S' to 'dhclient -6'.