Compute strings for DHCP Option 121, Classless Routes

dhcproute.py

import ipaddress
import math
import sys


def pack_address(addr):
    """Pack an IPv4 Address into colon-delimited format."""
    return ':'.join('{:02x}'.format(c) for c in addr.packed)


def pack_subnet(subnet):
    """Pack the subnet."""
    sig = math.ceil(subnet.prefixlen / 8)
    packed_sig = ':'.join(
            '{:02x}'.format(c) for c in subnet.network_address.packed[:sig])
    return '{:02x}:{}'.format(subnet.prefixlen, packed_sig)


def parse_pair(subnet, router):
    """Parse a pair into a DHCP route."""
    subnet = ipaddress.ip_network(subnet)
    router = ipaddress.ip_address(router)
    return pack_subnet(subnet) + ':' + pack_address(router)


def main(argv):
    pairs = argv[1:]
    assert(len(pairs) > 1)
    assert(len(pairs) % 2 == 0)
    packed = [
            parse_pair(pairs[i], pairs[i+1]) for i in range(0, len(pairs), 2)]
    print(':'.join(packed))


if __name__ == '__main__':
    main(sys.argv)