Thank you for your response Ray,
So I do have to put a full Domain name (like server.domain.com) pointing to the desired iBays in the domain panel. And then I guess I should also delete the auto-created hostnames in the "hostname panel".
But this solution force me to declare those different fully qualified domains to my referal and make them point to the right ip. Time consumming when doing it with a lot of domain but a least it works.
Any other idea are welcome.
Jerome