/etc/dhcp3/dhcpd.conf の設定

BIND 
 default-lease-time 600;
 max-lease-time 7200;
 ddns-update-style interim;
 autheritative;
 log-facility local7;
 option domain-name "n9d.no-ip.com";
 option domain-name-servers 192.168.1.30;
 subnet 192.168.1.0 netmask 255.255.255.0 {
   range 192.168.1.40 192.168.1.49;
   option routers 192.168.1.1;
   option broadcast-address 192.168.1.255;
   option subnet-mask 255.255.255.0;
 }

いまいち冗長だけどおいておく

 ddns-update-style interim;
 autheritative;
 log-facility local7;

の部分はDynamicDNSのupdateへの設定