DHCPD config

[fbsd_user]

I use dhcp client to get the info needed from my ISP at boot time.
I also run dhcpd for issuing ip address to my LAN.

In the dhcpd.conf file I have option statement for the
ISP's dns ip address like this:

option domain-name-servers xx.168.xxx.6, xx.168.xxx.7;

the xx are just for this post.

How can I change this so dhcpd will automatically use the
dns ip address the dhcp client gets so when the ISP changes
the ip address of the dns to use the change will also effect dhcpd?