Forcing dhcp setting on the network using openbsd

You can force workstation to using dhcp configuration at their network setting by using standard dhcpd and pf. First you can set dhcpd.conf like normally. Then edit /etc/rc.conf.local at

dhcpd_flags=""

with

dhcpd_flags="-L leased_ips -A abandoned_ips"

the at pf.conf you can block or/and pass the table like ordinary ip table.
further info please see dhcpd manual