I found the problem: I enabled IP filtering.. Whitout It all goes at the right place (but It is a weird thing, anyway!!)
PS> Mandrake tools for network are very poor... I have to set up a firewall/router for a network with one single NIC (with 2 aliases) so, I have to edit manually my iptables rules (yes, I have other rules in my tables) because, for some reasons, the graphic tools (guard dog, firestarter) don't seem to cover this topic..