route-tool.py

#!/usr/bin/env python3

from ipaddress import IPv4Network, collapse_addresses
from argparse import ArgumentParser, RawDescriptionHelpFormatter
from typing import List


def main():
    parser = ArgumentParser(description='Create excluding IPv4 routing tables, and simplify IPv4 routing tables.',
                            formatter_class=RawDescriptionHelpFormatter,
                            epilog='Examples:\n'
                                   '198.51.100.0/25, and 198.51.100.128/25 will be merged to 198.51.100.0/24:\n'
                                   '\troute-tool -s 198.51.100.0/25 198.51.100.128/25\n'
                                   '\n'
                                   'Create a set of routes which are nearly default, but exclude two subnets:\n'
                                   '\troute-tool -e 198.51.100.0/24 203.0.113.0/24\n'
                                   '\n'
                                   'Route your network through a VPN, but exclude the VPN server at 192.0.2.62:\n'
                                   '\troute-tool -i 192.0.2.0/24 -e 192.0.2.62')
    parser.add_argument('--include', '-i', type=IPv4Network, nargs='*', default=[IPv4Network('0.0.0.0/0')],
                        help='The positive routing table. Defaults to 0.0.0.0/0.')
    parser.add_argument('--exclude', '-e', type=IPv4Network, nargs='*', default=[],
                        help='The negative routing table, i. e. the list of networks to exclude from the positive '
                             'routing table. Defaults to empty list.')
    parser.add_argument('--simplify', '-s', action='store_const', default=False, const=True,
                        help='Simplify the resulting routing table by merging adjacent networks.')

    args = parser.parse_args()

    result: List[IPv4Network] = []
    include: IPv4Network
    for include in args.include:
        to_add = [include]

        exclude: IPv4Network

        something_changed = True
        while something_changed:
            something_changed = False

            for exclude in args.exclude:
                for net in to_add:
                    if net.subnet_of(exclude):
                        to_add = []
                        something_changed = True
                        break
                    elif exclude.subnet_of(net):
                        to_add.remove(net)
                        to_add.extend(net.address_exclude(exclude))
                        something_changed = True
                        break

                if something_changed:
                    break

        result.extend(to_add)

    if args.simplify:
        result = list(collapse_addresses(result))

    for i in result:
        print(f'{i.network_address}/{i.prefixlen}')


if __name__ == '__main__':
    main()