xfrout.py.in

#!@PYTHON@

# Copyright (C) 2010-2012  Internet Systems Consortium.
#
# Permission to use, copy, modify, and distribute this software for any
# purpose with or without fee is hereby granted, provided that the above
# copyright notice and this permission notice appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SYSTEMS CONSORTIUM
# DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL
# INTERNET SYSTEMS CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT,
# INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
# FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
# NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
# WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.


import sys; sys.path.append ('@@PYTHONPATH@@')
import isc
import isc.cc
import threading
import struct
import signal
from isc.datasrc import DataSourceClient, ZoneFinder, ZoneJournalReader
from socketserver import *
import os
from isc.config.ccsession import *
from isc.cc import SessionError, SessionTimeout
from isc.statistics import Counters
from isc.notify import notify_out
import isc.util.process
import fcntl
import socket
import select
import errno
from optparse import OptionParser, OptionValueError
from isc.util import socketserver_mixin
import isc.server_common.tsig_keyring

from isc.log_messages.xfrout_messages import *

isc.log.init("b10-xfrout", buffer=True)
logger = isc.log.Logger("xfrout")

# Pending system-wide debug level definitions, the ones we
# use here are hardcoded for now
DBG_PROCESS = logger.DBGLVL_TRACE_BASIC
DBG_COMMANDS = logger.DBGLVL_TRACE_DETAIL

DBG_XFROUT_TRACE = logger.DBGLVL_TRACE_BASIC

try:
    from libutil_io_python import *
    from pydnspp import *
except ImportError as e:
    # C++ loadable module may not be installed; even so the xfrout process
    # must keep running, so we warn about it and move forward.
    logger.error(XFROUT_IMPORT, str(e))

from isc.acl.acl import ACCEPT, REJECT, DROP, LoaderError
from isc.acl.dns import REQUEST_LOADER

isc.util.process.rename()

class XfroutConfigError(Exception):
    """An exception indicating an error in updating xfrout configuration.

    This exception is raised when the xfrout process encouters an error in
    handling configuration updates.  Not all syntax error can be caught
    at the module-CC layer, so xfrout needs to (explicitly or implicitly)
    validate the given configuration data itself.  When it finds an error
    it raises this exception (either directly or by converting an exception
    from other modules) as a unified error in configuration.
    """
    pass

class XfroutSessionError(Exception):
    '''An exception raised for some unexpected events during an xfrout session.
    '''
    pass

def init_paths():
    global SPECFILE_PATH
    global AUTH_SPECFILE_PATH
    global UNIX_SOCKET_FILE
    if "B10_FROM_BUILD" in os.environ:
        SPECFILE_PATH = os.environ["B10_FROM_BUILD"] + "/src/bin/xfrout"
        AUTH_SPECFILE_PATH = os.environ["B10_FROM_BUILD"] + "/src/bin/auth"
        if "B10_FROM_SOURCE_LOCALSTATEDIR" in os.environ:
            UNIX_SOCKET_FILE = os.environ["B10_FROM_SOURCE_LOCALSTATEDIR"] + \
                "/auth_xfrout_conn"
        else:
            UNIX_SOCKET_FILE = os.environ["B10_FROM_BUILD"] + "/auth_xfrout_conn"
    else:
        PREFIX = "@prefix@"
        DATAROOTDIR = "@datarootdir@"
        SPECFILE_PATH = "@datadir@/@PACKAGE@".replace("${datarootdir}", DATAROOTDIR).replace("${prefix}", PREFIX)
        AUTH_SPECFILE_PATH = SPECFILE_PATH
        if "BIND10_XFROUT_SOCKET_FILE" in os.environ:
            UNIX_SOCKET_FILE = os.environ["BIND10_XFROUT_SOCKET_FILE"]
        else:
            UNIX_SOCKET_FILE = "@@LOCALSTATEDIR@@/@PACKAGE_NAME@/auth_xfrout_conn"

init_paths()

SPECFILE_LOCATION = SPECFILE_PATH + "/xfrout.spec"
AUTH_SPECFILE_LOCATION = AUTH_SPECFILE_PATH + os.sep + "auth.spec"
VERBOSE_MODE = False
XFROUT_DNS_HEADER_SIZE = 12     # protocol constant
XFROUT_MAX_MESSAGE_SIZE = 65535 # ditto

# borrowed from xfrin.py @ #1298.  We should eventually unify it.
def format_zone_str(zone_name, zone_class):
    """Helper function to format a zone name and class as a string of
       the form '<name>/<class>'.
       Parameters:
       zone_name (isc.dns.Name) name to format
       zone_class (isc.dns.RRClass) class to format
    """
    return zone_name.to_text(True) + '/' + str(zone_class)

# borrowed from xfrin.py @ #1298.
def format_addrinfo(addrinfo):
    """Helper function to format the addrinfo as a string of the form
       <addr>:<port> (for IPv4) or [<addr>]:port (for IPv6). For unix domain
       sockets, and unknown address families, it returns a basic string
       conversion of the third element of the passed tuple.
       Parameters:
       addrinfo: a 3-tuple consisting of address family, socket type, and,
                 depending on the family, either a 2-tuple with the address
                 and port, or a filename
    """
    try:
        if addrinfo[0] == socket.AF_INET:
            return str(addrinfo[2][0]) + ":" + str(addrinfo[2][1])
        elif addrinfo[0] == socket.AF_INET6:
            return "[" + str(addrinfo[2][0]) + "]:" + str(addrinfo[2][1])
        else:
            return str(addrinfo[2])
    except IndexError:
        raise TypeError("addrinfo argument to format_addrinfo() does not "
                        "appear to be consisting of (family, socktype, (addr, port))")

def get_rrset_len(rrset):
    """Returns the wire length of the given RRset"""
    bytes = bytearray()
    rrset.to_wire(bytes)
    return len(bytes)

def get_soa_serial(soa_rdata):
    '''Extract the serial field of an SOA RDATA and returns it as an Serial object.
    '''
    return Serial(int(soa_rdata.to_text().split()[2]))

def make_blocking(filenum, on):
    """A helper function to change blocking mode of the given socket.

    It sets the mode of blocking I/O for the socket associated with filenum
    (descriptor of the socket) according to parameter 'on': if it's True the
    file will be made blocking; otherwise it will be made non-blocking.

    The given filenum must be a descriptor of a socket (not an ordinary file
    etc), but this function doesn't check that condition.

    filenum(int): file number (descriptor) of the socket to update.
    on(bool): whether enable (True) or disable (False) blocking I/O.

    """
    flags = fcntl.fcntl(filenum, fcntl.F_GETFL)
    if on:                      # make it blocking
        flags &= ~os.O_NONBLOCK
    else:                       # make it non blocking
        flags |= os.O_NONBLOCK
    fcntl.fcntl(filenum, fcntl.F_SETFL, flags)

class XfroutSession():
    def __init__(self, sock_fd, request_data, server, tsig_key_ring, remote,
                 default_acl, zone_config, client_class=DataSourceClient):
        self._sock_fd = sock_fd
        self._request_data = request_data
        self._server = server
        self._tsig_key_ring = tsig_key_ring
        self._tsig_ctx = None
        self._tsig_len = 0
        self._remote = remote
        self._request_type = None
        self._request_typestr = None
        self._acl = default_acl
        self._zone_config = zone_config
        self.ClientClass = client_class # parameterize this for testing
        self._soa = None # will be set in _xfrout_setup or in tests
        self._jnl_reader = None # will be set to a reader for IXFR
        # Creation of self.counters should be done before of
        # invoking self._handle()
        self._counters = Counters(SPECFILE_LOCATION)
        self._handle()

    def create_tsig_ctx(self, tsig_record, tsig_key_ring):
        return TSIGContext(tsig_record.get_name(), tsig_record.get_rdata().get_algorithm(),
                           tsig_key_ring)

    def _handle(self):
        ''' Handle a xfrout query, send xfrout response(s).

        This is separated from the constructor so that we can override
        it from tests.

        '''
        # Check the xfrout quota.  We do both increase/decrease in this
        # method so it's clear we always release it once acquired.
        quota_ok = self._server.increase_transfers_counter()
        ex = None
        try:
            # Before start, make sure the socket uses blocking I/O because
            # responses will be sent in the blocking mode; otherwise it could
            # result in EWOULDBLOCK and disrupt the session.
            make_blocking(self._sock_fd, True)
            self.dns_xfrout_start(self._sock_fd, self._request_data, quota_ok)
        except Exception as e:
            # To avoid resource leak we need catch all possible exceptions
            # We log it later to exclude the case where even logger raises
            # an exception.
            ex = e

        # Release any critical resources
        if quota_ok:
            self._server.decrease_transfers_counter()
        self._close_socket()

        if ex is not None:
            logger.error(XFROUT_HANDLE_QUERY_ERROR, ex)

    def _close_socket(self):
        '''Simply close the socket via the given FD.

        This is a dedicated subroutine of handle() and is sepsarated from it
        for the convenience of tests.

        '''
        os.close(self._sock_fd)

    def _check_request_tsig(self, msg, request_data):
        ''' If request has a tsig record, perform tsig related checks '''
        tsig_record = msg.get_tsig_record()
        if tsig_record is not None:
            self._tsig_len = tsig_record.get_length()
            self._tsig_ctx = self.create_tsig_ctx(tsig_record,
                                                  self._tsig_key_ring)
            tsig_error = self._tsig_ctx.verify(tsig_record, request_data)
            if tsig_error != TSIGError.NOERROR:
                return Rcode.NOTAUTH

        return Rcode.NOERROR

    def _parse_query_message(self, mdata):
        ''' parse query message to [socket,message]'''
        #TODO, need to add parseHeader() in case the message header is invalid
        try:
            msg = Message(Message.PARSE)
            Message.from_wire(msg, mdata)
        except Exception as err: # Exception is too broad
            logger.error(XFROUT_PARSE_QUERY_ERROR, err)
            return Rcode.FORMERR, None

        # TSIG related checks
        rcode = self._check_request_tsig(msg, mdata)
        if rcode != Rcode.NOERROR:
            return rcode, msg

        # Make sure the question is valid.  This should be ensured by
        # the auth server, but since it's far from xfrout itself, we check
        # it by ourselves.  A viloation would be an internal bug, so we
        # raise and stop here rather than returning a FORMERR or SERVFAIL.
        if msg.get_rr_count(Message.SECTION_QUESTION) != 1:
            raise RuntimeError('Invalid number of question for XFR: ' +
                               str(msg.get_rr_count(Message.SECTION_QUESTION)))
        question = msg.get_question()[0]

        # Identify the request type
        self._request_type = question.get_type()
        if self._request_type == RRType.AXFR:
            self._request_typestr = 'AXFR'
        elif self._request_type == RRType.IXFR:
            self._request_typestr = 'IXFR'
        else:
            # Likewise, this should be impossible.
            raise RuntimeError('Unexpected XFR type: ' +
                               str(self._request_type))

        # ACL checks
        zone_name = question.get_name()
        zone_class = question.get_class()
        acl = self._get_transfer_acl(zone_name, zone_class)
        acl_result = acl.execute(
            isc.acl.dns.RequestContext(self._remote[2], msg.get_tsig_record()))
        if acl_result == DROP:
            logger.debug(DBG_XFROUT_TRACE, XFROUT_QUERY_DROPPED,
                         self._request_type, format_addrinfo(self._remote),
                         format_zone_str(zone_name, zone_class))
            return None, None
        elif acl_result == REJECT:
            # count rejected Xfr request by each zone name
            self._counters.inc('zones', zone_class.to_text(),
                               zone_name.to_text(), 'xfrrej')
            logger.debug(DBG_XFROUT_TRACE, XFROUT_QUERY_REJECTED,
                         self._request_type, format_addrinfo(self._remote),
                         format_zone_str(zone_name, zone_class))
            return Rcode.REFUSED, msg

        return rcode, msg

    def _get_transfer_acl(self, zone_name, zone_class):
        '''Return the ACL that should be applied for a given zone.

        The zone is identified by a tuple of name and RR class.
        If a per zone configuration for the zone exists and contains
        transfer_acl, that ACL will be used; otherwise, the default
        ACL will be used.

        '''
        # Internally zone names are managed in lower cased label characters,
        # so we first need to convert the name.
        zone_name_lower = Name(zone_name.to_text(), True)
        config_key = (zone_class.to_text(), zone_name_lower.to_text())
        if config_key in self._zone_config and \
                'transfer_acl' in self._zone_config[config_key]:
            return self._zone_config[config_key]['transfer_acl']
        return self._acl

    def _send_data(self, sock_fd, data):
        size = len(data)
        total_count = 0
        while total_count < size:
            count = os.write(sock_fd, data[total_count:])
            total_count += count


    def _send_message(self, sock_fd, msg, tsig_ctx=None):
        render = MessageRenderer()
        # As defined in RFC5936 section3.4, perform case-preserving name
        # compression for AXFR message.
        render.set_compress_mode(MessageRenderer.CASE_SENSITIVE)
        render.set_length_limit(XFROUT_MAX_MESSAGE_SIZE)

        # XXX Currently, python wrapper doesn't accept 'None' parameter in this case,
        # we should remove the if statement and use a universal interface later.
        if tsig_ctx is not None:
            msg.to_wire(render, tsig_ctx)
        else:
            msg.to_wire(render)

        header_len = struct.pack('H', socket.htons(render.get_length()))
        self._send_data(sock_fd, header_len)
        self._send_data(sock_fd, render.get_data())


    def _reply_query_with_error_rcode(self, msg, sock_fd, rcode_):
        if not msg:
            return # query message is invalid. send nothing back.

        msg.make_response()
        msg.set_rcode(rcode_)
        self._send_message(sock_fd, msg, self._tsig_ctx)

    def _get_zone_soa(self, zone_name):
        '''Retrieve the SOA RR of the given zone.

        It returns a pair of RCODE and the SOA (in the form of RRset).
        On success RCODE is NOERROR and returned SOA is not None;
        on failure RCODE indicates the appropriate code in the context of
        xfr processing, and the returned SOA is None.

        '''
        result, finder = self._datasrc_client.find_zone(zone_name)
        if result != DataSourceClient.SUCCESS:
            return (Rcode.NOTAUTH, None)
        result, soa_rrset, _ = finder.find(zone_name, RRType.SOA)
        if result != ZoneFinder.SUCCESS:
            return (Rcode.SERVFAIL, None)
        # Especially for database-based zones, a working zone may be in
        # a broken state where it has more than one SOA RR.  We proactively
        # check the condition and abort the xfr attempt if we identify it.
        if soa_rrset.get_rdata_count() != 1:
            return (Rcode.SERVFAIL, None)
        return (Rcode.NOERROR, soa_rrset)

    def __axfr_setup(self, zone_name):
        '''Setup a zone iterator for AXFR or AXFR-style IXFR.

        '''
        try:
            # Note that we enable 'separate_rrs'.  In xfr-out we need to
            # preserve as many things as possible (even if it's half broken)
            # stored in the zone.
            self._iterator = self._datasrc_client.get_iterator(zone_name,
                                                               True)
        except isc.datasrc.Error:
            # If the current name server does not have authority for the
            # zone, xfrout can't serve for it, return rcode NOTAUTH.
            # Note: this exception can happen for other reasons.  We should
            # update get_iterator() API so that we can distinguish "no such
            # zone" and other cases (#1373).  For now we consider all these
            # cases as NOTAUTH.
            return Rcode.NOTAUTH

        # If we are an authoritative name server for the zone, but fail
        # to find the zone's SOA record in datasource, xfrout can't
        # provide zone transfer for it.
        self._soa = self._iterator.get_soa()
        if self._soa is None or self._soa.get_rdata_count() != 1:
            return Rcode.SERVFAIL

        return Rcode.NOERROR

    def __ixfr_setup(self, request_msg, zone_name, zone_class):
        '''Setup a zone journal reader for IXFR.

        If the underlying data source does not know the requested range
        of zone differences it automatically falls back to AXFR-style
        IXFR by setting up a zone iterator instead of a journal reader.

        '''
        # Check the authority section.  Look for a SOA record with
        # the same name and class as the question.
        remote_soa = None
        for auth_rrset in request_msg.get_section(Message.SECTION_AUTHORITY):
            # Ignore data whose owner name is not the zone apex, and
            # ignore non-SOA or different class of records.
            if auth_rrset.get_name() != zone_name or \
                    auth_rrset.get_type() != RRType.SOA or \
                    auth_rrset.get_class() != zone_class:
                continue
            if auth_rrset.get_rdata_count() != 1:
                logger.info(XFROUT_IXFR_MULTIPLE_SOA,
                            format_addrinfo(self._remote))
                return Rcode.FORMERR
            remote_soa = auth_rrset
        if remote_soa is None:
            logger.info(XFROUT_IXFR_NO_SOA, format_addrinfo(self._remote))
            return Rcode.FORMERR

        # Retrieve the local SOA
        rcode, self._soa = self._get_zone_soa(zone_name)
        if rcode != Rcode.NOERROR:
            return rcode

        # RFC1995 says "If an IXFR query with the same or newer version
        # number than that of the server is received, it is replied to with
        # a single SOA record of the server's current version, just as
        # in AXFR".  The claim about AXFR is incorrect, but other than that,
        # we do as the RFC says.
        begin_serial = get_soa_serial(remote_soa.get_rdata()[0])
        end_serial = get_soa_serial(self._soa.get_rdata()[0])
        if begin_serial >= end_serial:
            # clear both iterator and jnl_reader to signal we won't do
            # iteration in response generation
            self._iterator = None
            self._jnl_reader = None
            logger.info(XFROUT_IXFR_UPTODATE, format_addrinfo(self._remote),
                        format_zone_str(zone_name, zone_class),
                        begin_serial, end_serial)
            return Rcode.NOERROR

        # Set up the journal reader or fall back to AXFR-style IXFR
        try:
            code, self._jnl_reader = self._datasrc_client.get_journal_reader(
                zone_name, begin_serial.get_value(), end_serial.get_value())
        except isc.datasrc.NotImplemented as ex:
            # The underlying data source doesn't support journaling.
            # Fall back to AXFR-style IXFR.
            logger.info(XFROUT_IXFR_NO_JOURNAL_SUPPORT,
                        format_addrinfo(self._remote),
                        format_zone_str(zone_name, zone_class))
            return self.__axfr_setup(zone_name)
        if code == ZoneJournalReader.NO_SUCH_VERSION:
            logger.info(XFROUT_IXFR_NO_VERSION, format_addrinfo(self._remote),
                        format_zone_str(zone_name, zone_class),
                        begin_serial, end_serial)
            return self.__axfr_setup(zone_name)
        if code == ZoneJournalReader.NO_SUCH_ZONE:
            # this is quite unexpected as we know zone's SOA exists.
            # It might be a bug or the data source is somehow broken,
            # but it can still happen if someone has removed the zone
            # between these two operations.  We treat it as NOTAUTH.
            logger.warn(XFROUT_IXFR_NO_ZONE, format_addrinfo(self._remote),
                        format_zone_str(zone_name, zone_class))
            return Rcode.NOTAUTH

        # Use the reader as the iterator to generate the response.
        self._iterator = self._jnl_reader

        return Rcode.NOERROR

    def _xfrout_setup(self, request_msg, zone_name, zone_class):
        '''Setup a context for xfr responses according to the request type.

        This method identifies the most appropriate data source for the
        request and set up a zone iterator or journal reader depending on
        whether the request is AXFR or IXFR.  If it identifies any protocol
        level error it returns an RCODE other than NOERROR.

        '''

        # Identify the data source for the requested zone and see if it has
        # SOA while initializing objects used for request processing later.
        # We should eventually generalize this so that we can choose the
        # appropriate data source from (possible) multiple candidates.
        # We should eventually take into account the RR class here.
        # For now, we hardcode a particular type (SQLite3-based), and only
        # consider that one.
        datasrc_config = '{ "database_file": "' + \
            self._server.get_db_file() + '"}'
        self._datasrc_client = self.ClientClass('sqlite3', datasrc_config)

        if self._request_type == RRType.AXFR:
            return self.__axfr_setup(zone_name)
        else:
            return self.__ixfr_setup(request_msg, zone_name, zone_class)

    def dns_xfrout_start(self, sock_fd, msg_query, quota_ok=True):
        rcode_, msg = self._parse_query_message(msg_query)
        #TODO. create query message and parse header
        if rcode_ is None: # Dropped by ACL
            return
        elif rcode_ == Rcode.NOTAUTH or rcode_ == Rcode.REFUSED:
            return self._reply_query_with_error_rcode(msg, sock_fd, rcode_)
        elif rcode_ != Rcode.NOERROR:
            return self._reply_query_with_error_rcode(msg, sock_fd,
                                                      Rcode.FORMERR)
        elif not quota_ok:
            logger.warn(XFROUT_QUERY_QUOTA_EXCEEDED, self._request_typestr,
                        format_addrinfo(self._remote),
                        self._server._max_transfers_out)
            return self._reply_query_with_error_rcode(msg, sock_fd,
                                                      Rcode.REFUSED)

        question = msg.get_question()[0]
        zone_name = question.get_name()
        zone_class = question.get_class()
        zone_str = format_zone_str(zone_name, zone_class) # for logging

        try:
            rcode_ = self._xfrout_setup(msg, zone_name, zone_class)
        except Exception as ex:
            logger.error(XFROUT_XFR_TRANSFER_CHECK_ERROR, self._request_typestr,
                         format_addrinfo(self._remote), zone_str, ex)
            rcode_ = Rcode.SERVFAIL
        if rcode_ != Rcode.NOERROR:
            logger.info(XFROUT_XFR_TRANSFER_FAILED, self._request_typestr,
                        format_addrinfo(self._remote), zone_str, rcode_)
            return self._reply_query_with_error_rcode(msg, sock_fd, rcode_)

        try:
            # increment Xfr starts by RRType
            if self._request_type == RRType.AXFR:
                self._counters.inc('axfr_running')
            else:
                self._counters.inc('ixfr_running')
            logger.info(XFROUT_XFR_TRANSFER_STARTED, self._request_typestr,
                        format_addrinfo(self._remote), zone_str)
            self._reply_xfrout_query(msg, sock_fd)
        except Exception as err:
            # count unixsockets send errors
            self._counters.inc('socket', 'unixdomain', 'senderr')
            logger.error(XFROUT_XFR_TRANSFER_ERROR, self._request_typestr,
                    format_addrinfo(self._remote), zone_str, err)
        finally:
            # decrement Xfr starts by RRType
            if self._request_type == RRType.AXFR:
                self._counters.dec('axfr_running')
            else:
                self._counters.dec('ixfr_running')
        # count done Xfr requests by each zone name
        self._counters.inc('zones', zone_class.to_text(),
                           zone_name.to_text(), 'xfrreqdone')
        logger.info(XFROUT_XFR_TRANSFER_DONE, self._request_typestr,
                    format_addrinfo(self._remote), zone_str)

    def _clear_message(self, msg):
        qid = msg.get_qid()
        opcode = msg.get_opcode()
        rcode = msg.get_rcode()

        msg.clear(Message.RENDER)
        msg.set_qid(qid)
        msg.set_opcode(opcode)
        msg.set_rcode(rcode)
        msg.set_header_flag(Message.HEADERFLAG_AA)
        msg.set_header_flag(Message.HEADERFLAG_QR)
        return msg

    def _send_message_with_last_soa(self, msg, sock_fd, rrset_soa,
                                    message_upper_len):
        '''Add the SOA record to the end of message.

        If it would exceed the maximum allowable size of a message, a new
        message will be created to send out the last SOA.

        We assume a message with a single SOA can always fit the buffer
        with or without TSIG.  In theory this could be wrong if TSIG is
        stupidly large, but in practice this assumption should be reasonable.
        '''
        if message_upper_len + get_rrset_len(rrset_soa) > \
                XFROUT_MAX_MESSAGE_SIZE:
            self._send_message(sock_fd, msg, self._tsig_ctx)
            msg = self._clear_message(msg)

        msg.add_rrset(Message.SECTION_ANSWER, rrset_soa)
        self._send_message(sock_fd, msg, self._tsig_ctx)

    def _reply_xfrout_query(self, msg, sock_fd):
        msg.make_response()
        msg.set_header_flag(Message.HEADERFLAG_AA)
        # Reserved space for the fixed header size, the size of the question
        # section, and TSIG size (when included).  The size of the question
        # section is the sum of the qname length and the size of the
        # fixed-length fields (type and class, 2 bytes each).
        message_upper_len = XFROUT_DNS_HEADER_SIZE + \
            msg.get_question()[0].get_name().get_length() + 4 + \
            self._tsig_len

        # If the iterator is None, we are responding to IXFR with a single
        # SOA RR.
        if self._iterator is None:
            self._send_message_with_last_soa(msg, sock_fd, self._soa,
                                             message_upper_len)
            return

        # Add the beginning SOA
        msg.add_rrset(Message.SECTION_ANSWER, self._soa)
        message_upper_len += get_rrset_len(self._soa)

        # Add the rest of the zone/diff contets
        for rrset in self._iterator:
            # Check if xfrout is shutdown
            if  self._server._shutdown_event.is_set():
                logger.info(XFROUT_STOPPING)
                return

            # For AXFR (or AXFR-style IXFR), in which case _jnl_reader is None,
            # we should skip SOAs from the iterator.
            if self._jnl_reader is None and rrset.get_type() == RRType.SOA:
                continue

            # We calculate the maximum size of the RRset (i.e. the
            # size without compression) and use that to see if we
            # may have reached the limit
            rrset_len = get_rrset_len(rrset)

            if message_upper_len + rrset_len <= XFROUT_MAX_MESSAGE_SIZE:
                msg.add_rrset(Message.SECTION_ANSWER, rrset)
                message_upper_len += rrset_len
                continue

            # RR would not fit.  If there are other RRs in the buffer, send
            # them now and leave this RR to the next message.
            self._send_message(sock_fd, msg, self._tsig_ctx)

            # Create a new message and reserve space for the carried-over
            # RR (and TSIG space in case it's to be TSIG signed)
            msg = self._clear_message(msg)
            message_upper_len = XFROUT_DNS_HEADER_SIZE + rrset_len + \
                self._tsig_len

            # If this RR overflows the buffer all by itself, fail.  In theory
            # some RRs might fit in a TCP message when compressed even if they
            # do not fit when uncompressed, but surely we don't want to send
            # such monstrosities to an unsuspecting slave.
            if message_upper_len > XFROUT_MAX_MESSAGE_SIZE:
                raise XfroutSessionError('RR too large for zone transfer (' +
                                         str(rrset_len) + ' bytes)')

            # Add the RRset to the new message
            msg.add_rrset(Message.SECTION_ANSWER, rrset)

        # Add and send the trailing SOA
        self._send_message_with_last_soa(msg, sock_fd, self._soa,
                                         message_upper_len)

class UnixSockServer(socketserver_mixin.NoPollMixIn,
                     ThreadingUnixStreamServer):
    '''The unix domain socket server which accept xfr query sent from auth server.'''

    def __init__(self, sock_file, handle_class, shutdown_event, config_data,
                 cc):
        self._remove_unused_sock_file(sock_file)
        self._sock_file = sock_file
        socketserver_mixin.NoPollMixIn.__init__(self)
        self._counters = Counters(SPECFILE_LOCATION)
        try:
            ThreadingUnixStreamServer.__init__(self, sock_file, \
                                                   handle_class)
        except:
            self._counters.inc('socket', 'unixdomain', 'openfail')
            raise
        else:
            self._counters.inc('socket', 'unixdomain', 'open')
        self._shutdown_event = shutdown_event
        self._write_sock, self._read_sock = socket.socketpair()
        self._common_init()
        self._cc = cc
        self.update_config_data(config_data)

    def server_bind(self):
        """server_bind() overridden for counting unix domain sockets
        bind() failures
        """
        try:
            # call the server_bind() of class
            # ThreadingUnixStreamServer
            return super().server_bind()
        except:
            # count bind failed unixsockets
            self._counters.inc('socket', 'unixdomain', 'bindfail')
            raise

    def get_request(self):
        """get_request() overridden for counting unix domain sockets
        accept() failures and success
        """
        try:
            # call the get_request() of class
            # ThreadingUnixStreamServer
            ret = super().get_request()
            # count successfully accepted unixsockets
            self._counters.inc('socket', 'unixdomain', 'accept')
            return ret
        except:
            # count failed accepted unixsockets
            self._counters.inc('socket', 'unixdomain', 'acceptfail')
            raise

    def _common_init(self):
        '''Initialization shared with the mock server class used for tests'''
        self._lock = threading.Lock()
        self._transfers_counter = 0
        self._zone_config = {}
        self._acl = None # this will be initialized in update_config_data()

    def _receive_query_message(self, sock):
        ''' receive request message from sock'''
        # receive data length
        data_len = sock.recv(2)
        if not data_len:
            return None
        msg_len = struct.unpack('!H', data_len)[0]
        # receive data
        recv_size = 0
        msgdata = b''
        while recv_size < msg_len:
            data = sock.recv(msg_len - recv_size)
            if not data:
                return None
            recv_size += len(data)
            msgdata += data

        return msgdata

    def handle_request(self):
        ''' Enable server handle a request until shutdown or auth is closed.'''
        try:
            request, client_address = self.get_request()
        except socket.error:
            logger.error(XFROUT_FETCH_REQUEST_ERROR)
            return
        self._select_loop(request)

    def _select_loop(self, request_sock):
        '''Main loop for a single session between xfrout and auth.

        This is a dedicated subroutine of handle_request(), but is defined
        as a separate "protected" method for the convenience of tests.
        '''

        # Check self._shutdown_event to ensure the real shutdown comes.
        # Linux could trigger a spurious readable event on the _read_sock
        # due to a bug, so we need perform a double check.
        while not self._shutdown_event.is_set(): # Check if xfrout is shutdown
            try:
                (rlist, wlist, xlist) = select.select([self._read_sock,
                                                       request_sock], [], [])
            except select.error as e:
                if e.args[0] == errno.EINTR:
                    (rlist, wlist, xlist) = ([], [], [])
                    continue
                else:
                    logger.error(XFROUT_SOCKET_SELECT_ERROR, e)
                    break

            # self.server._shutdown_event will be set by now, if it is not a
            # false alarm
            if self._read_sock in rlist:
                continue

            try:
                if not self.process_request(request_sock):
                    break
            except Exception as pre:
                # count unixsockets receive errors
                self._counters.inc('socket', 'unixdomain', 'recverr')
                logger.error(XFROUT_PROCESS_REQUEST_ERROR, pre)
                break

    def _handle_request_noblock(self):
        """Override the function _handle_request_noblock(), it creates a new
        thread to handle requests for each auth"""
        td = threading.Thread(target=self.handle_request)
        td.setDaemon(True)
        td.start()

    def process_request(self, request):
        """Receive socket fd and query message from auth, then
        start a new thread to process the request.

        Return: True if everything is okay; otherwise False, in which case
        the calling thread will terminate.

        """
        sock_fd = recv_fd(request.fileno())
        if sock_fd < 0:
            logger.warn(XFROUT_RECEIVE_FD_FAILED)
            return False

        # receive request msg.  If it fails we simply terminate the thread;
        # it might be possible to recover from this state, but it's more likely
        # that auth and xfrout are in inconsistent states.  So it will make
        # more sense to restart in a new session.
        request_data = self._receive_query_message(request)
        if request_data is None:
            # The specific exception type doesn't matter so we use session
            # error.
            raise XfroutSessionError('Failed to get complete xfr request')

        t = threading.Thread(target=self.finish_request,
                             args=(sock_fd, request_data))
        if self.daemon_threads:
            t.daemon = True
        t.start()
        return True

    def _guess_remote(self, sock_fd):
        """Guess remote address and port of the socket.

        The sock_fd must be a file descriptor of a socket.
        This method retuns a 3-tuple consisting of address family,
        socket type, and a 2-tuple with the address (string) and port (int).

        """
        # This uses a trick. If the socket is IPv4 in reality and we pretend
        # it to be IPv6, it returns IPv4 address anyway. This doesn't seem
        # to care about the SOCK_STREAM parameter at all (which it really is,
        # except for testing)
        if socket.has_ipv6:
            sock_domain = socket.AF_INET6
        else:
            # To make it work even on hosts without IPv6 support
            # (Any idea how to simulate this in test?)
            sock_domain = socket.AF_INET

        sock = socket.fromfd(sock_fd, sock_domain, socket.SOCK_STREAM)
        peer = sock.getpeername()
        sock.close()

        # Identify the correct socket family.  Due to the above "trick",
        # we cannot simply use sock.family.
        family = socket.AF_INET6
        try:
            socket.inet_pton(socket.AF_INET6, peer[0])
        except socket.error:
            family = socket.AF_INET

        return (family, socket.SOCK_STREAM, peer)

    def finish_request(self, sock_fd, request_data):
        '''Finish one request by instantiating RequestHandlerClass.

        This is an entry point of a separate thread spawned in
        UnixSockServer.process_request().

        This method creates a XfroutSession object.
        '''
        self._lock.acquire()
        acl = self._acl
        zone_config = self._zone_config
        self._lock.release()
        self.RequestHandlerClass(sock_fd, request_data, self,
                                 isc.server_common.tsig_keyring.get_keyring(),
                                 self._guess_remote(sock_fd), acl, zone_config)

    def _remove_unused_sock_file(self, sock_file):
        '''Try to remove the socket file. If the file is being used
        by one running xfrout process, exit from python.
        If it's not a socket file or nobody is listening
        , it will be removed. If it can't be removed, exit from python. '''
        if self._sock_file_in_use(sock_file):
            logger.error(XFROUT_UNIX_SOCKET_FILE_IN_USE, sock_file)
            sys.exit(0)
        else:
            if not os.path.exists(sock_file):
                return

            try:
                os.unlink(sock_file)
            except OSError as err:
                logger.error(XFROUT_REMOVE_OLD_UNIX_SOCKET_FILE_ERROR, sock_file, str(err))
                sys.exit(0)

    def _sock_file_in_use(self, sock_file):
        '''Check whether the socket file 'sock_file' exists and
        is being used by one running xfrout process. If it is,
        return True, or else return False. '''
        try:
            sock = socket.socket(socket.AF_UNIX)
            sock.connect(sock_file)
        except socket.error as err:
            sock.close()
            return False
        else:
            sock.close()
            return True

    def shutdown(self):
        self._write_sock.send(b"shutdown") #terminate the xfrout session thread
        super().shutdown() # call the shutdown() of class socketserver_mixin.NoPollMixIn
        # count closed unixsockets
        self._counters.inc('socket', 'unixdomain', 'close')
        try:
            os.unlink(self._sock_file)
        except Exception as e:
            logger.error(XFROUT_REMOVE_UNIX_SOCKET_FILE_ERROR, self._sock_file, str(e))

    def update_config_data(self, new_config):
        '''Apply the new config setting of xfrout module.

        '''
        self._lock.acquire()
        try:
            logger.info(XFROUT_NEW_CONFIG)
            new_acl = self._acl
            if 'transfer_acl' in new_config:
                try:
                    new_acl = REQUEST_LOADER.load(new_config['transfer_acl'])
                except LoaderError as e:
                    raise XfroutConfigError('Failed to parse transfer_acl: ' +
                                            str(e))

            new_zone_config = self._zone_config
            zconfig_data = new_config.get('zone_config')
            if zconfig_data is not None:
                new_zone_config = self.__create_zone_config(zconfig_data)

            self._acl = new_acl
            self._zone_config = new_zone_config
            self._max_transfers_out = new_config.get('transfers_out')
        except Exception as e:
            self._lock.release()
            raise e
        self._lock.release()
        logger.info(XFROUT_NEW_CONFIG_DONE)

    def __create_zone_config(self, zone_config_list):
        new_config = {}
        for zconf in zone_config_list:
            # convert the class, origin (name) pair.  First build pydnspp
            # object to reject invalid input.
            zclass_str = zconf.get('class')
            if zclass_str is None:
                #zclass_str = 'IN' # temporary
                zclass_str = self._cc.get_default_value('zone_config/class')
            zclass = RRClass(zclass_str)
            zorigin = Name(zconf['origin'], True)
            config_key = (zclass.to_text(), zorigin.to_text())

            # reject duplicate config
            if config_key in new_config:
                raise XfroutConfigError('Duplicate zone_config for ' +
                                        str(zorigin) + '/' + str(zclass))

            # create a new config entry, build any given (and known) config
            new_config[config_key] = {}
            if 'transfer_acl' in zconf:
                try:
                    new_config[config_key]['transfer_acl'] = \
                        REQUEST_LOADER.load(zconf['transfer_acl'])
                except LoaderError as e:
                    raise XfroutConfigError('Failed to parse transfer_acl ' +
                                            'for ' + zorigin.to_text() + '/' +
                                            zclass_str + ': ' + str(e))
        return new_config

    def get_db_file(self):