#!/usr/bin/env python3
#
# dyndns.py
#
# a simple ssh based dyndns updater for bind zone files
#
# Copyright (C) 2013 Christian Pointner <equinox@spreadspace.org>
#
# This file is part of dyndns.py.
#
# dyndns.py is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 2 of the License, or
# any later version.
#
# dyndns.py 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 dyndns.py. If not, see <http://www.gnu.org/licenses/>.
#

import subprocess
import fcntl
import shutil
import hashlib
import tempfile
import sys
import os

if len(sys.argv) != 2:
    print("ERROR: the base-domain-name must be the sole parameter")
    sys.exit(1)


domain = sys.argv[1]
base_d = '/var/lib/dyndns'
rndc = '/usr/sbin/rndc'

digest_fn = os.path.join(base_d, 'digest.'+domain)
header_fn = os.path.join(base_d, 'header.'+domain)
records_fn = os.path.join(base_d, 'records.'+domain)
tmpl_fn = os.path.join(base_d, 'tmpl.'+domain)
serial_fn = os.path.join(base_d, 'serial.'+domain)
db_fn = os.path.join(base_d, 'db.'+domain)

digest_fd = open(digest_fn, 'a+')
fcntl.flock(digest_fd, fcntl.LOCK_EX)
digest_fd.seek(0)

h = hashlib.blake2b()
with open(tmpl_fn, 'wb+') as tmpl_fd:
    with open(header_fn, 'rb') as header_fd:
        shutil.copyfileobj(header_fd, tmpl_fd)
    try:
        with open(records_fn, 'rb') as records_fd:
            shutil.copyfileobj(records_fd, tmpl_fd)
    except FileNotFoundError:
        pass

    tmpl_fd.seek(0)
    while True:
        data = tmpl_fd.read(64*1024)
        if not data:
            break
        h.update(data)

old_digest = digest_fd.readline().strip()
new_digest = h.hexdigest()
if old_digest == new_digest:
    print("OK: already up to date.")
    sys.exit(0)

serial = 0
with open(serial_fn, 'a+') as serial_fd:
    serial_fd.seek(0)
    serial = int(serial_fd.readline()) + 1
    serial_fd.truncate(0)
    serial_fd.write(str(serial)+'\n')

with tempfile.NamedTemporaryFile(dir=base_d, delete=False, mode='w') as out_fd:
    with open(tmpl_fn, 'r') as tmpl_fd:
        while True:
            line = tmpl_fd.readline()
            if not line:
                break
            line = line.replace('__SERIAL__', str(serial))
            out_fd.write(line)
    os.chmod(out_fd.name, 0o644)
    os.rename(out_fd.name, db_fn)


cmd = [rndc, 'reload', domain]
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
rndc_output = ''
for line in p.stdout.readlines():
    rndc_output = rndc_output + line.decode('utf-8')

r = p.wait()
if r != 0:
    print("ERROR: rndc returned %d\n\n%s" % (r, rndc_output))
    sys.exit(1)

digest_fd.truncate(0)
digest_fd.write(new_digest + '\n')

print("OK: " + rndc_output)