"""
Misc. DNS related code: query, dynamic update, etc.

Usually, higher level code wants to call the add/update/delete functions.
"""

# time to wait for dns name resolving [s]
RESOLVER_TIMEOUT = 5.0

# time to wait for dns name updating [s]
UPDATE_TIMEOUT = 20.0

# time after we retry to reach a previously unreachable ns [s]
UNAVAILABLE_RETRY = 300.0


from datetime import timedelta

import logging
logger = logging.getLogger(__name__)

import dns.inet
import dns.name
import dns.resolver
import dns.query
import dns.update
import dns.tsig
import dns.tsigkeyring

from django.utils.timezone import now


Timeout = dns.resolver.Timeout


class SameIpError(ValueError):
    """
    raised if an IP address is already present in DNS and and update was
    requested, but is not needed.
    """


class DnsUpdateError(ValueError):
    """
    raised if DNS update return code is not NOERROR
    """


class NameServerNotAvailable(Exception):
    """
    raised if some nameserver was flagged as not available,
    but we tried using it.
    """


def check_ip(ipaddr, keys=('ipv4', 'ipv6')):
    """
    Check if a string is a valid ip address and also
    determine the kind of the address (address family).
    Return first key for v4, second key for v6.

    :param ipaddr: ip address, v4 or v6, str
    :param keys: 2-tuple (v4key, v6key)
    :return: v4key or v6key
    :raises: ValueError if the ip is invalid
    """
    af = dns.inet.af_for_address(ipaddr)
    return keys[af == dns.inet.AF_INET6]


def add(fqdn, ipaddr, ttl=60, origin=None):
    """
    intelligent dns adder - first does a lookup on the master server to find
    the current ip and only sends an 'add' if there is no such entry.
    otherwise send an 'upd' if the if we have a different ip.

    :param fqdn: fully qualified domain name (str)
    :param ipaddr: new ip address
    :param ttl: time to live, default 60s (int)
    :param origin: origin zone (optional, str)
    :raises: SameIpError if new and old IP is the same
    :raises: ValueError if ipaddr is no valid ip address string
    """
    rdtype = check_ip(ipaddr, keys=('A', 'AAAA'))
    try:
        current_ipaddr = query_ns(fqdn, rdtype, origin=origin)
        # check if ip really changed
        ok = ipaddr != current_ipaddr
        action = 'upd'
    except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
        # no dns entry yet, ok
        ok = True
        action = 'add'
    if ok:
        # only send an add/update if the ip really changed as the update
        # causes write I/O on the nameserver and also traffic to the
        # dns slaves (they get a notify if we update the zone).
        update_ns(fqdn, rdtype, ipaddr, action=action, ttl=ttl, origin=origin)
    else:
        raise SameIpError


def delete(fqdn, rdtype=None, origin=None):
    """
    dns deleter

    :param fqdn: fully qualified domain name (str)
    :param rdtype: 'A', 'AAAA' or None (deletes 'A' and 'AAAA')
    :param origin: origin zone (optional, str)
    """
    if rdtype is not None:
        assert rdtype in ['A', 'AAAA', ]
        rdtypes = [rdtype, ]
    else:
        rdtypes = ['A', 'AAAA']
    for rdtype in rdtypes:
        update_ns(fqdn, rdtype, action='del', origin=origin)


def update(fqdn, ipaddr, ttl=60, origin=None):
    """
    intelligent dns updater - first does a lookup on the master server to find
    the current ip and only sends a dynamic update if we have a different ip.

    :param fqdn: fully qualified domain name (str)
    :param ipaddr: new ip address
    :param ttl: time to live, default 60s (int)
    :param origin: origin zone (optional, str)
    :raises: SameIpError if new and old IP is the same
    :raises: ValueError if ipaddr is no valid ip address string
    """
    rdtype = check_ip(ipaddr, keys=('A', 'AAAA'))
    try:
        current_ipaddr = query_ns(fqdn, rdtype, origin=origin)
        # check if ip really changed
        ok = ipaddr != current_ipaddr
    except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
        # no dns entry yet, ok
        ok = True
    if ok:
        # only send an update if the ip really changed as the update
        # causes write I/O on the nameserver and also traffic to the
        # dns slaves (they get a notify if we update the zone).
        update_ns(fqdn, rdtype, ipaddr, action='upd', ttl=ttl, origin=origin)
    else:
        raise SameIpError


def query_ns(qname, rdtype, origin=None):
    """
    query a dns name from our master server

    :param qname: the query name
    :type qname: dns.name.Name object or str
    :param rdtype: the query type
    :type rdtype: int or str
    :param origin: origin zone
    :type origin: str or None
    :return: IP (as str) or "-" if ns is not available
    """
    origin, name = parse_name(qname, origin)
    fqdn = name + origin
    assert fqdn.is_absolute()
    nameserver, origin = get_ns_info(fqdn)[0:2]
    resolver = dns.resolver.Resolver(configure=False)
    # we do not configure it from resolv.conf, but patch in the values we
    # want into the documented attributes:
    resolver.nameservers = [nameserver, ]
    resolver.search = []
    resolver.lifetime = RESOLVER_TIMEOUT
    try:
        answer = resolver.query(fqdn, rdtype)
        ip = str(list(answer)[0])
        logger.debug("query: %s answer: %s" % (fqdn.to_text(), ip))
        return ip
    except (dns.resolver.Timeout, dns.resolver.NoNameservers):  # socket.error also?
        logger.warning("timeout when querying for name '%s' in zone '%s' with rdtype '%s'." % (
                       name, origin, rdtype))
        set_ns_availability(origin, False)
        raise


def parse_name(fqdn, origin=None):
    """
    Parse a fully qualified domain name into a relative name
    and a origin zone. Please note that the origin return value will
    have a trailing dot.

    :param fqdn: fully qualified domain name (str)
    :param origin: origin zone (optional, str)
    :return: origin, relative name (both dns.name.Name)
    """
    fqdn = dns.name.from_text(fqdn)
    if origin is None:
        origin = dns.resolver.zone_for_name(fqdn)
        rel_name = fqdn.relativize(origin)
    else:
        origin = dns.name.from_text(origin)
        rel_name = fqdn - origin
    return origin, rel_name


def get_ns_info(fqdn, origin=None):
    """
    Get the master nameserver for the <origin> zone, the key needed
    to update the zone and the key algorithm used.

    :param fqdn: the fully qualified hostname we are dealing with (str)
    :param origin: zone we are dealing with, must be with trailing dot (default:autodetect) (str)
    :return: master nameserver, origin, domain, update keyname, update key, update algo
    :raises: NameServerNotAvailable if ns was flagged unavailable in the db
    """
    fqdn_str = str(fqdn)
    origin, name = parse_name(fqdn_str, origin)
    origin_str = str(origin)
    from .models import Domain
    try:
        # first we check if we have an entry for the fqdn
        # single-host update secret use case
        # XXX we need 2 DB accesses for the usual case just to support this rare case
        domain = fqdn_str.rstrip('.')
        d = Domain.objects.get(domain=domain)
        keyname = fqdn_str
    except Domain.DoesNotExist:
        # now check the base zone, the usual case
        # zone update secret use case
        domain = origin_str.rstrip('.')
        d = Domain.objects.get(domain=domain)
        keyname = origin_str
    if not d.available:
        if d.last_update + timedelta(seconds=UNAVAILABLE_RETRY) > now():
            # if there are troubles with a nameserver, we set available=False
            # and stop trying working with that nameserver for a while
            raise NameServerNotAvailable("nameserver for domain %s at IP %s was flagged unavailable" % (
                                         domain, d.nameserver_ip, ))
        else:
            # retry timeout is over, set it available again
            set_ns_availability(domain, True)
    algorithm = getattr(dns.tsig, d.nameserver_update_algorithm)
    return d.nameserver_ip, origin, domain, name, keyname, d.nameserver_update_key, algorithm


def update_ns(fqdn, rdtype='A', ipaddr=None, origin=None, action='upd', ttl=60,
              raise_badsig=False):
    """
    update our master server

    :param fqdn: the fully qualified domain name to update (str)
    :param rdtype: the record type (default: 'A') (str)
    :param ipaddr: ip address (v4 or v6), if needed (str)
    :param origin: the origin zone to update (default; autodetect) (str)
    :param action: 'add', 'del' or 'upd'
    :param ttl: time to live for the added/updated resource, default 60s (int)
    :return: dns response
    """
    assert action in ['add', 'del', 'upd', ]
    nameserver, origin, domain, name, keyname, key, algo = get_ns_info(fqdn, origin)
    upd = dns.update.Update(origin,
                            keyring=dns.tsigkeyring.from_text({keyname: key}),
                            keyalgorithm=algo)
    if action == 'add':
        assert ipaddr is not None
        upd.add(name, ttl, rdtype, ipaddr)
    elif action == 'del':
        upd.delete(name, rdtype)
    elif action == 'upd':
        assert ipaddr is not None
        upd.replace(name, ttl, rdtype, ipaddr)
    logger.debug("performing %s for name %s and origin %s with rdtype %s and ipaddr %s" % (
                 action, name, origin, rdtype, ipaddr))
    try:
        response = dns.query.tcp(upd, nameserver, timeout=UPDATE_TIMEOUT)
        rcode = response.rcode()
        if rcode != dns.rcode.NOERROR:
            rcode_text = dns.rcode.to_text(rcode)
            logger.warning("DNS error [%s] performing %s for name %s and origin %s with rdtype %s and ipaddr %s" % (
                           rcode_text, action, name, origin, rdtype, ipaddr))
            raise DnsUpdateError(rcode_text)
        return response
    except dns.exception.Timeout:
        logger.warning("timeout when performing %s for name %s and origin %s with rdtype %s and ipaddr %s" % (
                       action, name, origin, rdtype, ipaddr))
        set_ns_availability(domain, False)
        raise
    except dns.tsig.PeerBadSignature:
        logger.error("PeerBadSignature - shared secret mismatch? zone: %s" % (origin, ))
        if raise_badsig:
            raise
        else:
            set_ns_availability(domain, False)


def set_ns_availability(domain, available):
    """
    Set availability of the master nameserver for <domain>.

    As each Timeout takes quite a while, we want to avoid it.

    :param domain: domain object or string, may end with "."
    :param available: True/False for availability of ns
    """
    from .models import Domain
    domain = str(domain).rstrip('.')
    d = Domain.objects.get(domain=domain)
    d.available = available
    d.save()
    if available:
        logger.info("set zone '%s' to available" % domain)
    else:
        logger.warning("set zone '%s' to unavailable" % domain)