#!/usr/bin/env python3
#
# Copyright (C) 2020 VyOS maintainers and contributors
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 2 or later as
# published by the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

import json
import os

from copy import deepcopy
from sys import exit
from netifaces import interfaces

from vyos.config import Config
from vyos.template import render
from vyos.util import call, cmd
from vyos import ConfigError

default_config_data = {
    'prerouting_ct_helper': '',
    'prerouting_ct_conntrack': '',
    'output_ct_helper': '',
    'output_ct_conntrack': '',
    'destination': [],
    'source': []
}

iptables_nat_config = '/tmp/vyos-nat-rules.nft'

def _check_kmod():
    """ load required Kernel modules """
    modules = ['nft_nat', 'nft_chain_nat_ipv4']
    for module in modules:
        if not os.path.exists(f'/sys/module/{module}'):
            if call(f'modprobe {module}') != 0:
                raise ConfigError(f'Loading Kernel module {module} failed')


def get_handler(chain, target):
    """ Get handler number of given chain/target combination. Handler is
        required when adding NAT/Conntrack helper targets """
    tmp = json.loads(cmd('nft -j list table raw'))
    for rule in tmp.get('nftables'):
        # We're only interested in rules - not chains
        if not 'rule' in rule.keys():
            continue

        # Search for chain of interest
        if rule['rule']['chain'] == chain:
            for expr in rule['rule']['expr']:
                # We're only interested in jump targets
                if not 'jump' in expr.keys():
                    continue

                # Search for target of interest
                if expr['jump']['target'] == target:
                    return rule['rule']['handle']

    return None


def parse_source_destination(conf, source_dest):
    """ Common wrapper to read in both NAT source and destination CLI """
    tmp = []
    base_level = ['nat', source_dest]
    conf.set_level(base_level)
    for number in conf.list_nodes(['rule']):
        rule = {
            'description': '',
            'dest_address': '',
            'dest_port': '',
            'disable': False,
            'exclude': False,
            'interface_in': '',
            'interface_out': '',
            'log': False,
            'protocol': '',
            'number': number,
            'source_address': '',
            'source_port': '',
            'translation_address': '',
            'translation_port': ''
        }
        conf.set_level(base_level + ['rule', number])

        if conf.exists(['description']):
            rule['description'] = conf.return_value(['description'])

        if conf.exists(['destination', 'address']):
            rule['dest_address'] = conf.return_value(['destination', 'address'])

        if conf.exists(['destination', 'port']):
            rule['dest_port'] = conf.return_value(['destination', 'port'])

        if conf.exists(['disable']):
            rule['disable'] = True

        if conf.exists(['exclude']):
            rule['exclude'] = True

        if conf.exists(['inbound-interface']):
            rule['interface_in'] = conf.return_value(['inbound-interface'])

        if conf.exists(['outbound-interface']):
            rule['interface_out'] = conf.return_value(['outbound-interface'])

        if conf.exists(['log']):
            rule['log'] = True

        if conf.exists(['protocol']):
            rule['protocol'] = conf.return_value(['protocol'])

        if conf.exists(['source', 'address']):
            rule['source_address'] = conf.return_value(['source', 'address'])

        if conf.exists(['source', 'port']):
            rule['source_port'] = conf.return_value(['source', 'port'])

        if conf.exists(['translation', 'address']):
            rule['translation_address'] = conf.return_value(['translation', 'address'])

        if conf.exists(['translation', 'port']):
            rule['translation_port'] = conf.return_value(['translation', 'port'])

        tmp.append(rule)

    return tmp

def get_config():
    nat = deepcopy(default_config_data)
    conf = Config()
    if not conf.exists(['nat']):
        return None
    else:
        conf.set_level(['nat'])

    nat['pre_ct_ignore'] = get_handler('PREROUTING', 'VYATTA_CT_IGNORE')
    nat['pre_ct_conntrack'] = get_handler('PREROUTING', 'VYATTA_CT_PREROUTING_HOOK')
    nat['out_ct_ignore'] = get_handler('OUTPUT', 'VYATTA_CT_IGNORE')
    nat['out_ct_conntrack'] = get_handler('OUTPUT', 'VYATTA_CT_OUTPUT_HOOK')

    # use a common wrapper function to read in the source / destination
    # tree from the config - thus we do not need to replicate almost the
    # same code :-)
    for tgt in ['source', 'destination']:
        nat[tgt] = parse_source_destination(conf, tgt)

    return nat

def verify(nat):
    if not nat:
        return None

    if not (nat['pre_ct_ignore'] or nat['pre_ct_conntrack'] or nat['out_ct_ignore'] or nat['out_ct_conntrack']):
        raise Exception('could not determine nftable ruleset handlers')

    for rule in nat['source']:
        interface = rule['interface_out']
        if interface and interface not in interfaces():
            print(f'NAT configuration warning: interface {interface} does not exist on this system')

    return None

def generate(nat):
    if not nat:
        return None

    render(iptables_nat_config, 'firewall/nftables-nat.tmpl', nat, trim_blocks=True, permission=0o755)

    return None

def apply(nat):
    if not nat:
        return None

    call(f'{iptables_nat_config}')
    return None

if __name__ == '__main__':
    try:
        _check_kmod()
        c = get_config()
        verify(c)
        generate(c)
        apply(c)
    except ConfigError as e:
        print(e)
        exit(1)