IPV6_UNSPECSRC
if (!(flags & IPV6_UNSPECSRC) &&
ip6_output(mh, &ip6_opts, NULL, ia6 ? 0 : IPV6_UNSPECSRC, &im6o, NULL);
ip6_output(m, NULL, NULL, dad ? IPV6_UNSPECSRC : 0, &im6o, NULL);