#!/usr/bin/env python3
#
# Copyright (C) 2018 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 sys
import os
import jinja2

from vyos.config import Config
from vyos import ConfigError

config_file = r'/etc/default/isc-dhcp-relay'

# Please be careful if you edit the template.
config_tmpl = """
### Autogenerated by dhcp_relay.py ###

# Defaults for isc-dhcp-relay initscript
# sourced by /etc/init.d/isc-dhcp-relay

#
# This is a POSIX shell fragment
#

# What servers should the DHCP relay forward requests to?
SERVERS="{{ server | join(' ') }}"

# On what interfaces should the DHCP relay (dhrelay) serve DHCP requests?
INTERFACES="{{ interface | join(' ') }}"

# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-4 {% if port -%} -p {{ port }}{%- endif %} {{ options | join(' ') }}"
"""

default_config_data = {
    'interface': [],
    'server': [],
    'options': [],
    'port': '',
    'hop_count': '10',
    'relay_agent_packets': 'forward'
}

def get_config():
    relay = default_config_data
    conf = Config()
    if not conf.exists('service dhcp-relay'):
        return None
    else:
        conf.set_level('service dhcp-relay')

    # Network interfaces to listen on
    if conf.exists('interface'):
        relay['interface'] = conf.return_values('interface')

    # Servers equal to the address of the DHCP server(s)
    if conf.exists('server'):
        relay['server'] = conf.return_values('server')

    conf.set_level('service dhcp-relay relay-options')

    if conf.exists('hop-count'):
        count = '-c ' + conf.return_value('hop-count')
        relay['options'].append(count)

    # Specify the maximum packet size to send to a DHCPv4/BOOTP server.
    # This might be done to allow sufficient space for addition of relay agent
    # options while still fitting into the Ethernet MTU size.
    #
    # Available in DHCPv4 mode only:
    if conf.exists('max-size'):
        size = '-A ' + conf.return_value('max-size')
        relay['options'].append(size)

    # Listen and transmit on port <xy>. This is mostly useful for debugging
    # purposes. Default is port 67 for DHCPv4/BOOTP, or port 547 for DHCPv6.
    if conf.exists('port'):
        relay['port'] = conf.return_value('port')

    # Control the handling of incoming DHCPv4 packets which already contain
    # relay agent options. If such a packet does not have giaddr set in its
    # header, the DHCP standard requires that the packet be discarded. However,
    # if giaddr is set, the relay agent may handle the situation in four ways:
    # It may append its own set of relay options to the packet, leaving the
    # supplied option field intact; it may replace the existing agent option
    # field; it may forward the packet unchanged; or, it may discard it.
    #
    # Available in DHCPv4 mode only:
    if conf.exists('relay-agents-packets'):
        pkt = '-m ' + conf.return_value('relay-agents-packets')
        relay['options'].append(pkt)

    return relay

def verify(relay):
    # bail out early - looks like removal from running config
    if relay is None:
        return None

    if len(relay['interface']) < 2:
        # We can only issue a warning otherwise old configurations might break
        print('WARNING: At least two interfaces are required for DHCP relay\n' \
              'to work\n')

    if 'lo' in relay['interface']:
        raise ConfigError('DHCP relay does not support the loopback interface.')

    if len(relay['server']) == 0:
        raise ConfigError('No DHCP relay server(s) configured.\n' \
                          'At least one DHCP relay server required.')

    return None

def generate(relay):
    # bail out early - looks like removal from running config
    if relay is None:
        return None

    tmpl = jinja2.Template(config_tmpl)
    config_text = tmpl.render(relay)
    with open(config_file, 'w') as f:
        f.write(config_text)

    return None

def apply(relay):
    if relay is not None:
        os.system('sudo systemctl restart isc-dhcp-relay.service')
    else:
        # DHCP relay support is removed in the commit
        os.system('sudo systemctl stop isc-dhcp-relay.service')
        os.unlink(config_file)

    return None

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