libdnet: distinguish more correctly between Ethernet and non-Ethernet interfaces

[dmiller-nmap]

Libdnet has a function, _intf_set_type, that is used to determine the interface type:

/* XXX - this is total crap. how to do this without walking ifnet? */
static void
_intf_set_type(struct intf_entry *entry)
{
  if ((entry->intf_flags & INTF_FLAG_LOOPBACK) != 0)
    entry->intf_type = INTF_TYPE_LOOPBACK;
  else if ((entry->intf_flags & INTF_FLAG_BROADCAST) != 0)
    entry->intf_type = INTF_TYPE_ETH;
  else if ((entry->intf_flags & INTF_FLAG_POINTOPOINT) != 0)
    entry->intf_type = INTF_TYPE_TUN;
  else
    entry->intf_type = INTF_TYPE_OTHER;
}

The primary problem that Nmap has with this method is that it detects some types of interfaces (IEEE1394 network, Infiniband, etc) as Ethernet based on the broadcast flag, when they are NOT Ethernet. IEEE1394, for instance, uses a different type of ARP protocol. It should be a fatal error to try to send an Ethernet frame on these interface types, but it should work just fine to do most scanning. Right now, the workaround is for the user to specify --send-ip for those interfaces. Better detection at this point in libdnet would make it "just work."