My Mandrake 9.2 box works properly in this respect. Seems you can move up or find out what is wrong with it.
My linux (Mandrake 6.1 Helios) box gets net info via dhcpcd from a Netgear router. It receives an ip address and is told about a dns server running on the router, but it isn't telling the router its hostname. A WinXp system on the LAN does tell its hostname to the router.