nat64_extract_ip4
ip.ip_src.s_addr = nat64_extract_ip4(&ip6i->ip6_src, cfg->plat_plen);
ip.ip_dst.s_addr = nat64_extract_ip4(&ip6->ip6_dst, cfg->plat_plen);
in_addr_t nat64_extract_ip4(const struct in6_addr *ip6, int plen);
aaddr = nat64_extract_ip4(&ip6->ip6_src, cfg->base.plat_plen);
daddr = nat64_extract_ip4(&ip6i->ip6_dst, cfg->base.clat_plen);
addr = nat64_extract_ip4(&f_id->dst_ip6, cfg->base.plat_plen);