DHCP Option 121 (Routes) with Python 3 for pfSense, etc.

dhcp_option_121.py

#!/usr/bin/env python3

from ipaddress import *

routes = [
    # destination       # router
    ('10.0.0.0/24',     '172.16.0.2'),
    ('0.0.0.0/0',       '172.16.0.1')
]

route_bytes = []

for dest, router in routes:
    netmask_cidr = int(dest.split("/")[-1])
    destnet = ip_network(dest)
    dest = list(destnet.network_address.packed)
    netmask = destnet.netmask.packed

    for octet in netmask[::-1]:
        if octet == 0:
            dest = dest[:-1]

    router = list(ip_address(router).packed)

    route_bytes += [netmask_cidr]
    route_bytes += dest
    route_bytes += router

print(':'.join([f'{o:02x}' for o in route_bytes]))