I've been told new OpenBSD stuff should use this license:

/*
 * Copyright (c) 2025 Tomasz Kramkowski <tomasz@kramkow.ski>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

What follows is the output of the following command:

$ head -n10000 /etc/rc.d/zoneregend /usr/local/sbin/zoneregend /usr/local/sbin/zoneregen /usr/local/sbin/leases2zones /etc/zoneregen

==> /etc/rc.d/zoneregend <==
#!/bin/ksh

daemon="/usr/local/sbin/zoneregend"
daemon_logger="daemon.info"

. /etc/rc.d/rc.subr

pexp="bash $pexp"

rc_bg=YES

rc_cmd $1

==> /usr/local/sbin/zoneregend <==
#!/usr/bin/env bash

shopt -s lastpipe

LEASES=/var/db/dhcpd.leases

[[ -r /etc/zoneregen ]] && . /etc/zoneregen

do_regen() {
	printf 'Running zoneregen\n'
	/usr/local/sbin/zoneregen
}

trap 'do_regen' HUP
trap 'kill 0' EXIT

inotifywait -e modify -m "$LEASES" --format '' 2>&1 |
while read -r; do
	case $REPLY in
	'Watches established.'|'') do_regen ;;
	esac
done

==> /usr/local/sbin/zoneregen <==
#!/usr/bin/env bash

LEASES=/var/db/dhcpd.leases
FWD_PRE=/dev/null
FWD_POST=/dev/null
REV_PRE=/dev/null
REV_POST=/dev/null

. /etc/zoneregen || exit

trap 'rm -f -- "$fwd_part" "$rev_part" "$full"' EXIT
fwd_part=$(mktemp) || exit
rev_part=$(mktemp) || exit
full=$(mktemp) || exit

leases2zones "$FWD_DOMAIN" "$fwd_part" "$rev_part" <"$LEASES" || exit

zone_serial() {
	ldns-read-zone -c -E SOA -- "$1" | \
		awk '{ serial = $7; exit } END { printf "%d\n", serial }'
}

regen_zone() {
	local output=$1 domain=$2 pre=$3 post=$4 part=$5
	local serial=$(zone_serial "$output")
	ldns-read-zone -zS "$serial" -- \
		<(cat "$pre" "$part" "$post") >"$full" || exit
	wait "$!" || exit
	cmp -s -- "$output" "$full" && return
	ldns-read-zone -zS "$((serial + 1))" -- "$full" >"$output" || exit
	nsd-control reload "$domain" || exit
}

regen_zone "$FWD" "$FWD_DOMAIN" "$FWD_PRE" "$FWD_POST" "$fwd_part"
regen_zone "$REV" "$REV_DOMAIN" "$REV_PRE" "$REV_POST" "$rev_part"

==> /usr/local/sbin/leases2zones <==
#!/usr/bin/env python3
from dataclasses import dataclass
from datetime import datetime
from ipaddress import IPv4Address, IPv6Address, ip_address
from sys import argv, stdin, stderr
from typing import Union, Optional, TextIO

IPAddress = Union[IPv4Address, IPv6Address]

@dataclass
class Lease:
    ip: IPAddress
    mac: str
    hostname: Optional[str]

@dataclass
class Record:
    name: str
    type: str
    target: str
    def __str__(self):
        return f'{self.name} {self.type} {self.target}'

def parse_leases(f: TextIO) -> list[Lease]:
    now = datetime.now()
    leases: dict[IPAddress, Lease] = {}
    lease = None
    for line in f:
        line = line.strip()
        if line.startswith('lease'):
            lease = dict()
            _, lease['ip'], _ = line.split(' ')
        if not lease:
            continue
        if line.startswith('starts'):
            _, dt = line[:-1].split(' ', maxsplit=1)
            lease['starts'] = datetime.strptime(dt, '%w %Y/%m/%d %H:%M:%S %Z')
        if line.startswith('ends'):
            _, dt = line[:-1].split(' ', maxsplit=1)
            lease['ends'] = datetime.strptime(dt, '%w %Y/%m/%d %H:%M:%S %Z')
        if line.startswith('hardware ethernet'):
            _, _, lease['mac'] = line[:-1].split(' ')
        if line.startswith('client-hostname'):
            _, lease['hostname'], _ = line.split('"')
        if line.startswith('abandoned'):
            lease['abandoned'] = True
        if line == '}':
            if lease and not lease.get('abandoned') and \
                    lease.get('mac') and \
                    lease.get('starts', now) <= now and \
                    lease.get('ends', now) > now:
                ip = ip_address(lease['ip'])
                leases[ip] = Lease(ip, lease['mac'],
                        lease.get('hostname'))
            lease = None
    return list(leases.values())

def to_zones(leases: list[Lease], domain: str) -> tuple[list[Record], list[Record]]:
    forward: list[Record] = list()
    reverse: list[Record] = list()
    for l in leases:
        typ = 'A' if isinstance(l.ip, IPv4Address) else 'AAAA'
        rev = l.ip.reverse_pointer + '.' # 3.5
        if l.hostname:
            base, *_ = l.hostname.split('.')
            hostname_fqdn = base + '.' + domain
            forward.append(Record(hostname_fqdn, typ, str(l.ip)))
            reverse.append(Record(rev, 'PTR', hostname_fqdn))
        mac_fqdn = l.mac.replace(':', '_') + '.' + domain
        forward.append(Record(mac_fqdn, typ, str(l.ip)))
        if not l.hostname:
            reverse.append(Record(rev, 'PTR', mac_fqdn))
    return forward, reverse

def main() -> None:
    if len(argv) != 4:
        argv0 = argv[0] if len(argv) >= 1 else 'leases2zones'
        print(f'Usage: {argv0} domain forward-filename reverse-filename',
                file=stderr)
        exit(1)
    domain, forward_filename, reverse_filename = argv[1:]
    domain = '.'.join(domain.split('.')) + '.'
    forward, reverse = to_zones(parse_leases(stdin), domain)
    with open(forward_filename, 'w') as f:
        for record in forward:
            print(str(record), file=f)
    with open(reverse_filename, 'w') as f:
        for record in reverse:
            print(str(record), file=f)

if __name__ == '__main__':
    main()

==> /etc/zoneregen <==
ZONE_DIR=/var/nsd/zones/master
FWD_DOMAIN=your.local.domain
FWD=$ZONE_DIR/$FWD_DOMAIN.zone
FWD_PRE=$FWD.pre
REV_DOMAIN=168.192.in-addr.arpa
REV=$ZONE_DIR/$REV_DOMAIN.zone
REV_PRE=$REV.pre