Getting DHCP to update DNS

Greetings, My DNS doesn't get updated immediatly when DHCP assigns an IP to a sysem, Is this possible or is it a manual process? What I would favour is that the dhcp assigns an IP to a system, and then updates dns with a system name linked to the IP. When the system is diconnected the lease is freed and the dns entry removed. I have tried the two options in the dhcpd.conf file that I know of but niether is working... they are ddns-update-style interim ddns-update-style ad-hoc Thanks