* Idea to think about: --salt 3b5b91c5a2 XOR client addresses for some more privacy
the salt can be added e.g. when restarting router/iptables or may be generated for
a particular host once. Purpose: hide MAC addresses behind the address mapping
gateway 

* one map16 function for all cases?

* Implment bit-offset for arbitrary prefix length

* Check compat with tcpdump. Linux2.4 needs to copy skb() for that -> oops

* Check ipv6_skip_exthdr(). Correct handling of skb.len? Otherwise do this:

unsigned int ptr = skb_network_offset(skb) + sizeof(struct ipv6hdr);
u_int8_t c = ipv6hdr.nexthdr;
while (c != NEXTHDR_NONE && ip6t_ext_hdr(currenthdr)) {
  const struct ipv6_opt_hdr *hp = skb_header_pointer(skb, ptr, sizeof(_hdr), &_hdr);
  switch(c) {
    case IPPROTO_FRAGMENT:
      hdrlen = 8;
      break;
    case IPPROTO_DSTOPTS:
    case IPPROTO_ROUTING:
    case IPPROTO_HOPOPTS:
      hdrlen = ipv6_optlen(hp);
      break;
    case IPPROTO_AH:
      hdrlen = (hp->hdrlen+2)<<2;
      break;
    default:
      return;
  }
  c = hp->nexthdr;
  ptr += hdrlen;
}