Dhcpd

If you want to use a sniffer to watch dhcp requests, replace rl0 with your NIC:

# tcpdump -i rl0 -lenX -s 1500 port bootps or port bootpc

Sample dhcpd.conf

ddns-update-style none;
default-lease-time 60000;
max-lease-time 72000;
local-address 192.168.1.1;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.3.255;
option routers 192.168.1.1;
# This will become the /etc/resolv.conf nameserver line(s):
option domain-name-servers 192.168.1.1, 206.248.154.22, 65.39.192.130;
# This will become the /etc/resolv.conf search line:
option domain-name "home.lan example.com";
subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.2 192.168.1.15;
}

BSD

vi /usr/local/etc/dhcpd.conf

Perhaps using the above example. In your /etc/rc.conf you will want to add:

dhcpd_enable="YES"
dhcpd_flags="-q rl0"
dhcpd_chroot_enable="YES"

And if all looks well, you will want to start the daemon:

/usr/local/etc/rc.d/isc-dhcpd.sh start

and then make sure it's running on the right ports.