dhcpd.conf in /etc/dhcp
for a typical lan setup you can use something like this
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.15 192.168.1.35;
option routers 192.168.1.1;
}
option domain-name-servers 208.67.222.222, 208.67.220.220;