_handlers.py

""" Multicast DNS Service Discovery for Python, v0.14-wmcbrine
    Copyright 2003 Paul Scott-Murphy, 2014 William McBrine

    This module provides a framework for the use of DNS Service Discovery
    using IP multicast.

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library 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
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
    USA
"""

import itertools
import random
from collections import deque
from typing import (
    TYPE_CHECKING,
    Dict,
    Iterable,
    List,
    NamedTuple,
    Optional,
    Set,
    Tuple,
    Union,
    cast,
)

from ._cache import DNSCache, _UniqueRecordsType
from ._dns import DNSAddress, DNSNsec, DNSPointer, DNSQuestion, DNSRecord, DNSRRSet
from ._history import QuestionHistory
from ._logger import log
from ._protocol.incoming import DNSIncoming
from ._protocol.outgoing import DNSOutgoing
from ._services.info import ServiceInfo
from ._services.registry import ServiceRegistry
from ._updates import RecordUpdate, RecordUpdateListener
from ._utils.time import current_time_millis, millis_to_seconds
from .const import (
    _ADDRESS_RECORD_TYPES,
    _CLASS_IN,
    _CLASS_UNIQUE,
    _DNS_OTHER_TTL,
    _DNS_PTR_MIN_TTL,
    _FLAGS_AA,
    _FLAGS_QR_RESPONSE,
    _ONE_SECOND,
    _SERVICE_TYPE_ENUMERATION_NAME,
    _TYPE_A,
    _TYPE_AAAA,
    _TYPE_ANY,
    _TYPE_NSEC,
    _TYPE_PTR,
    _TYPE_SRV,
    _TYPE_TXT,
)

if TYPE_CHECKING:
    from ._core import Zeroconf


_AnswerWithAdditionalsType = Dict[DNSRecord, Set[DNSRecord]]

_MULTICAST_DELAY_RANDOM_INTERVAL = (20, 120)
_RESPOND_IMMEDIATE_TYPES = {_TYPE_NSEC, _TYPE_SRV, *_ADDRESS_RECORD_TYPES}


class QuestionAnswers(NamedTuple):
    ucast: _AnswerWithAdditionalsType
    mcast_now: _AnswerWithAdditionalsType
    mcast_aggregate: _AnswerWithAdditionalsType
    mcast_aggregate_last_second: _AnswerWithAdditionalsType


class AnswerGroup(NamedTuple):
    """A group of answers scheduled to be sent at the same time."""

    send_after: float  # Must be sent after this time
    send_before: float  # Must be sent before this time
    answers: _AnswerWithAdditionalsType


def construct_nsec_record(name: str, types: List[int], now: float) -> DNSNsec:
    """Construct an NSEC record for name and a list of dns types.

    This function should only be used for SRV/A/AAAA records
    which have a TTL of _DNS_OTHER_TTL
    """
    return DNSNsec(name, _TYPE_NSEC, _CLASS_IN | _CLASS_UNIQUE, _DNS_OTHER_TTL, name, types, created=now)


def construct_outgoing_multicast_answers(answers: _AnswerWithAdditionalsType) -> DNSOutgoing:
    """Add answers and additionals to a DNSOutgoing."""
    out = DNSOutgoing(_FLAGS_QR_RESPONSE | _FLAGS_AA, multicast=True)
    _add_answers_additionals(out, answers)
    return out


def construct_outgoing_unicast_answers(
    answers: _AnswerWithAdditionalsType, ucast_source: bool, questions: List[DNSQuestion], id_: int
) -> DNSOutgoing:
    """Add answers and additionals to a DNSOutgoing."""
    out = DNSOutgoing(_FLAGS_QR_RESPONSE | _FLAGS_AA, multicast=False, id_=id_)
    # Adding the questions back when the source is legacy unicast behavior
    if ucast_source:
        for question in questions:
            out.add_question(question)
    _add_answers_additionals(out, answers)
    return out


def _add_answers_additionals(out: DNSOutgoing, answers: _AnswerWithAdditionalsType) -> None:
    # Find additionals and suppress any additionals that are already in answers
    sending: Set[DNSRecord] = set(answers.keys())
    # Answers are sorted to group names together to increase the chance
    # that similar names will end up in the same packet and can reduce the
    # overall size of the outgoing response via name compression
    for answer, additionals in sorted(answers.items(), key=lambda kv: kv[0].name):
        out.add_answer_at_time(answer, 0)
        for additional in additionals:
            if additional not in sending:
                out.add_additional_answer(additional)
                sending.add(additional)


class _QueryResponse:
    """A pair for unicast and multicast DNSOutgoing responses."""

    def __init__(self, cache: DNSCache, msgs: List[DNSIncoming]) -> None:
        """Build a query response."""
        self._is_probe = any(msg.is_probe for msg in msgs)
        self._msg = msgs[0]
        self._now = self._msg.now
        self._cache = cache
        self._additionals: _AnswerWithAdditionalsType = {}
        self._ucast: Set[DNSRecord] = set()
        self._mcast_now: Set[DNSRecord] = set()
        self._mcast_aggregate: Set[DNSRecord] = set()
        self._mcast_aggregate_last_second: Set[DNSRecord] = set()

    def add_qu_question_response(self, answers: _AnswerWithAdditionalsType) -> None:
        """Generate a response to a multicast QU query."""
        for record, additionals in answers.items():
            self._additionals[record] = additionals
            if self._is_probe:
                self._ucast.add(record)
            if not self._has_mcast_within_one_quarter_ttl(record):
                self._mcast_now.add(record)
            elif not self._is_probe:
                self._ucast.add(record)

    def add_ucast_question_response(self, answers: _AnswerWithAdditionalsType) -> None:
        """Generate a response to a unicast query."""
        self._additionals.update(answers)
        self._ucast.update(answers.keys())

    def add_mcast_question_response(self, answers: _AnswerWithAdditionalsType) -> None:
        """Generate a response to a multicast query."""
        self._additionals.update(answers)
        for answer in answers:
            if self._is_probe:
                self._mcast_now.add(answer)
                continue

            if self._has_mcast_record_in_last_second(answer):
                self._mcast_aggregate_last_second.add(answer)
            elif len(self._msg.questions) == 1 and self._msg.questions[0].type in _RESPOND_IMMEDIATE_TYPES:
                self._mcast_now.add(answer)
            else:
                self._mcast_aggregate.add(answer)

    def _generate_answers_with_additionals(self, rrset: Set[DNSRecord]) -> _AnswerWithAdditionalsType:
        """Create answers with additionals from an rrset."""
        return {record: self._additionals[record] for record in rrset}

    def answers(
        self,
    ) -> QuestionAnswers:
        """Return answer sets that will be queued."""
        return QuestionAnswers(
            self._generate_answers_with_additionals(self._ucast),
            self._generate_answers_with_additionals(self._mcast_now),
            self._generate_answers_with_additionals(self._mcast_aggregate),
            self._generate_answers_with_additionals(self._mcast_aggregate_last_second),
        )

    def _has_mcast_within_one_quarter_ttl(self, record: DNSRecord) -> bool:
        """Check to see if a record has been mcasted recently.

        https://datatracker.ietf.org/doc/html/rfc6762#section-5.4
        When receiving a question with the unicast-response bit set, a
        responder SHOULD usually respond with a unicast packet directed back
        to the querier.  However, if the responder has not multicast that
        record recently (within one quarter of its TTL), then the responder
        SHOULD instead multicast the response so as to keep all the peer
        caches up to date
        """
        maybe_entry = self._cache.async_get_unique(cast(_UniqueRecordsType, record))
        return bool(maybe_entry and maybe_entry.is_recent(self._now))

    def _has_mcast_record_in_last_second(self, record: DNSRecord) -> bool:
        """Check if an answer was seen in the last second.
        Protect the network against excessive packet flooding
        https://datatracker.ietf.org/doc/html/rfc6762#section-14
        """
        maybe_entry = self._cache.async_get_unique(cast(_UniqueRecordsType, record))
        return bool(maybe_entry and self._now - maybe_entry.created < _ONE_SECOND)


def _get_address_and_nsec_records(service: ServiceInfo, now: float) -> Set[DNSRecord]:
    """Build a set of address records and NSEC records for non-present record types."""
    seen_types: Set[int] = set()
    records: Set[DNSRecord] = set()
    for dns_address in service.dns_addresses(created=now):
        seen_types.add(dns_address.type)
        records.add(dns_address)
    missing_types: Set[int] = _ADDRESS_RECORD_TYPES - seen_types
    if missing_types:
        assert service.server is not None, "Service server must be set for NSEC record."
        records.add(construct_nsec_record(service.server, list(missing_types), now))
    return records


class QueryHandler:
    """Query the ServiceRegistry."""

    def __init__(self, registry: ServiceRegistry, cache: DNSCache, question_history: QuestionHistory) -> None:
        """Init the query handler."""
        self.registry = registry
        self.cache = cache
        self.question_history = question_history

    def _add_service_type_enumeration_query_answers(
        self, answer_set: _AnswerWithAdditionalsType, known_answers: DNSRRSet, now: float
    ) -> None:
        """Provide an answer to a service type enumeration query.

        https://datatracker.ietf.org/doc/html/rfc6763#section-9
        """
        for stype in self.registry.async_get_types():
            dns_pointer = DNSPointer(
                _SERVICE_TYPE_ENUMERATION_NAME, _TYPE_PTR, _CLASS_IN, _DNS_OTHER_TTL, stype, now
            )
            if not known_answers.suppresses(dns_pointer):
                answer_set[dns_pointer] = set()

    def _add_pointer_answers(
        self, lower_name: str, answer_set: _AnswerWithAdditionalsType, known_answers: DNSRRSet, now: float
    ) -> None:
        """Answer PTR/ANY question."""
        for service in self.registry.async_get_infos_type(lower_name):
            # Add recommended additional answers according to
            # https://tools.ietf.org/html/rfc6763#section-12.1.
            dns_pointer = service.dns_pointer(created=now)
            if known_answers.suppresses(dns_pointer):
                continue
            additionals: Set[DNSRecord] = {service.dns_service(created=now), service.dns_text(created=now)}
            additionals |= _get_address_and_nsec_records(service, now)
            answer_set[dns_pointer] = additionals

    def _add_address_answers(
        self,
        lower_name: str,
        answer_set: _AnswerWithAdditionalsType,
        known_answers: DNSRRSet,
        now: float,
        type_: int,
    ) -> None:
        """Answer A/AAAA/ANY question."""
        for service in self.registry.async_get_infos_server(lower_name):
            answers: List[DNSAddress] = []
            additionals: Set[DNSRecord] = set()
            seen_types: Set[int] = set()
            for dns_address in service.dns_addresses(created=now):
                seen_types.add(dns_address.type)
                if dns_address.type != type_:
                    additionals.add(dns_address)
                elif not known_answers.suppresses(dns_address):
                    answers.append(dns_address)
            missing_types: Set[int] = _ADDRESS_RECORD_TYPES - seen_types
            if answers:
                if missing_types:
                    assert service.server is not None, "Service server must be set for NSEC record."
                    additionals.add(construct_nsec_record(service.server, list(missing_types), now))
                for answer in answers:
                    answer_set[answer] = additionals
            elif type_ in missing_types:
                assert service.server is not None, "Service server must be set for NSEC record."
                answer_set[construct_nsec_record(service.server, list(missing_types), now)] = set()

    def _answer_question(
        self,
        question: DNSQuestion,
        known_answers: DNSRRSet,
        now: float,
    ) -> _AnswerWithAdditionalsType:
        answer_set: _AnswerWithAdditionalsType = {}
        question_lower_name = question.name.lower()

        if question.type == _TYPE_PTR and question_lower_name == _SERVICE_TYPE_ENUMERATION_NAME:
            self._add_service_type_enumeration_query_answers(answer_set, known_answers, now)
            return answer_set

        type_ = question.type

        if type_ in (_TYPE_PTR, _TYPE_ANY):
            self._add_pointer_answers(question_lower_name, answer_set, known_answers, now)

        if type_ in (_TYPE_A, _TYPE_AAAA, _TYPE_ANY):
            self._add_address_answers(question_lower_name, answer_set, known_answers, now, type_)

        if type_ in (_TYPE_SRV, _TYPE_TXT, _TYPE_ANY):
            service = self.registry.async_get_info_name(question_lower_name)
            if service is not None:
                if type_ in (_TYPE_SRV, _TYPE_ANY):
                    # Add recommended additional answers according to
                    # https://tools.ietf.org/html/rfc6763#section-12.2.
                    dns_service = service.dns_service(created=now)
                    if not known_answers.suppresses(dns_service):
                        answer_set[dns_service] = _get_address_and_nsec_records(service, now)
                if type_ in (_TYPE_TXT, _TYPE_ANY):
                    dns_text = service.dns_text(created=now)
                    if not known_answers.suppresses(dns_text):
                        answer_set[dns_text] = set()

        return answer_set

    def async_response(  # pylint: disable=unused-argument
        self, msgs: List[DNSIncoming], ucast_source: bool
    ) -> QuestionAnswers:
        """Deal with incoming query packets. Provides a response if possible.

        This function must be run in the event loop as it is not
        threadsafe.
        """
        known_answers = DNSRRSet(msg.answers for msg in msgs if not msg.is_probe)
        query_res = _QueryResponse(self.cache, msgs)

        for msg in msgs:
            for question in msg.questions:
                if not question.unicast:
                    self.question_history.add_question_at_time(question, msg.now, set(known_answers.lookup))
                answer_set = self._answer_question(question, known_answers, msg.now)
                if not ucast_source and question.unicast:
                    query_res.add_qu_question_response(answer_set)
                    continue
                if ucast_source:
                    query_res.add_ucast_question_response(answer_set)
                # We always multicast as well even if its a unicast
                # source as long as we haven't done it recently (75% of ttl)
                query_res.add_mcast_question_response(answer_set)

        return query_res.answers()


class RecordManager:
    """Process records into the cache and notify listeners."""

    def __init__(self, zeroconf: 'Zeroconf') -> None:
        """Init the record manager."""
        self.zc = zeroconf
        self.cache = zeroconf.cache
        self.listeners: List[RecordUpdateListener] = []

    def async_updates(self, now: float, records: List[RecordUpdate]) -> None:
        """Used to notify listeners of new information that has updated
        a record.

        This method must be called before the cache is updated.

        This method will be run in the event loop.
        """
        for listener in self.listeners:
            listener.async_update_records(self.zc, now, records)

    def async_updates_complete(self, notify: bool) -> None:
        """Used to notify listeners of new information that has updated
        a record.

        This method must be called after the cache is updated.

        This method will be run in the event loop.
        """
        for listener in self.listeners:
            listener.async_update_records_complete()
        if notify:
            self.zc.async_notify_all()

    def async_updates_from_response(self, msg: DNSIncoming) -> None:
        """Deal with incoming response packets.  All answers
        are held in the cache, and listeners are notified.

        This function must be run in the event loop as it is not
        threadsafe.
        """
        updates: List[RecordUpdate] = []
        address_adds: List[DNSRecord] = []
        other_adds: List[DNSRecord] = []
        removes: Set[DNSRecord] = set()
        now = msg.now
        unique_types: Set[Tuple[str, int, int]] = set()

        for record in msg.answers:
            # Protect zeroconf from records that can cause denial of service.
            #
            # We enforce a minimum TTL for PTR records to avoid
            # ServiceBrowsers generating excessive queries refresh queries.
            # Apple uses a 15s minimum TTL, however we do not have the same
            # level of rate limit and safe guards so we use 1/4 of the recommended value.
            if record.ttl and record.type == _TYPE_PTR and record.ttl < _DNS_PTR_MIN_TTL:
                log.debug(
                    "Increasing effective ttl of %s to minimum of %s to protect against excessive refreshes.",
                    record,
                    _DNS_PTR_MIN_TTL,
                )
                record.set_created_ttl(record.created, _DNS_PTR_MIN_TTL)

            if record.unique:  # https://tools.ietf.org/html/rfc6762#section-10.2
                unique_types.add((record.name, record.type, record.class_))

            maybe_entry = self.cache.async_get_unique(cast(_UniqueRecordsType, record))
            if not record.is_expired(now):
                if maybe_entry is not None:
                    maybe_entry.reset_ttl(record)
                else:
                    if record.type in _ADDRESS_RECORD_TYPES:
                        address_adds.append(record)
                    else:
                        other_adds.append(record)
                updates.append(RecordUpdate(record, maybe_entry))
            # This is likely a goodbye since the record is
            # expired and exists in the cache
            elif maybe_entry is not None:
                updates.append(RecordUpdate(record, maybe_entry))
                removes.add(record)

        if unique_types:
            self._async_mark_unique_cached_records_older_than_1s_to_expire(unique_types, msg.answers, now)

        if updates:
            self.async_updates(now, updates)
        # The cache adds must be processed AFTER we trigger
        # the updates since we compare existing data
        # with the new data and updating the cache
        # ahead of update_record will cause listeners
        # to miss changes
        #
        # We must process address adds before non-addresses
        # otherwise a fetch of ServiceInfo may miss an address
        # because it thinks the cache is complete
        #
        # The cache is processed under the context manager to ensure
        # that any ServiceBrowser that is going to call
        # zc.get_service_info will see the cached value
        # but ONLY after all the record updates have been
        # processsed.
        new = False
        if other_adds or address_adds:
            new = self.cache.async_add_records(itertools.chain(address_adds, other_adds))
        # Removes are processed last since
        # ServiceInfo could generate an un-needed query
        # because the data was not yet populated.
        if removes:
            self.cache.async_remove_records(removes)
        if updates:
            self.async_updates_complete(new)

    def _async_mark_unique_cached_records_older_than_1s_to_expire(
        self, unique_types: Set[Tuple[str, int, int]], answers: Iterable[DNSRecord], now: float
    ) -> None:
        # rfc6762#section-10.2 para 2
        # Since unique is set, all old records with that name, rrtype,
        # and rrclass that were received more than one second ago are declared
        # invalid, and marked to expire from the cache in one second.
        answers_rrset = set(answers)
        for name, type_, class_ in unique_types:
            for entry in self.cache.async_all_by_details(name, type_, class_):
                if (now - entry.created > _ONE_SECOND) and entry not in answers_rrset:
                    # Expire in 1s
                    entry.set_created_ttl(now, 1)

    def async_add_listener(
        self, listener: RecordUpdateListener, question: Optional[Union[DNSQuestion, List[DNSQuestion]]]
    ) -> None:
        """Adds a listener for a given question.  The listener will have
        its update_record method called when information is available to
        answer the question(s).

        This function is not threadsafe and must be called in the eventloop.
        """
        if not isinstance(listener, RecordUpdateListener):
            log.error(  # type: ignore[unreachable]
                "listeners passed to async_add_listener must inherit from RecordUpdateListener;"
                " In the future this will fail"
            )

        self.listeners.append(listener)

        if question is None:
            return

        questions = [question] if isinstance(question, DNSQuestion) else question
        assert self.zc.loop is not None
        self._async_update_matching_records(listener, questions)

    def _async_update_matching_records(
        self, listener: RecordUpdateListener, questions: List[DNSQuestion]
    ) -> None:
        """Calls back any existing entries in the cache that answer the question.

        This function must be run from the event loop.
        """
        now = current_time_millis()
        records: List[RecordUpdate] = [
            RecordUpdate(record, None)
            for question in questions
            for record in self.cache.async_entries_with_name(question.name)
            if not record.is_expired(now) and question.answered_by(record)
        ]
        if not records:
            return
        listener.async_update_records(self.zc, now, records)
        listener.async_update_records_complete()
        self.zc.async_notify_all()

    def async_remove_listener(self, listener: RecordUpdateListener) -> None:
        """Removes a listener.

        This function is not threadsafe and must be called in the eventloop.
        """
        try:
            self.listeners.remove(listener)
            self.zc.async_notify_all()
        except ValueError as e:
            log.exception('Failed to remove listener: %r', e)


class MulticastOutgoingQueue:
    """An outgoing queue used to aggregate multicast responses."""

    def __init__(self, zeroconf: 'Zeroconf', additional_delay: int, max_aggregation_delay: int) -> None:
        self.zc = zeroconf
        self.queue: deque = deque()
        # Additional delay is used to implement
        # Protect the network against excessive packet flooding
        # https://datatracker.ietf.org/doc/html/rfc6762#section-14
        self.additional_delay = additional_delay
        self.aggregation_delay = max_aggregation_delay

    def async_add(self, now: float, answers: _AnswerWithAdditionalsType) -> None:
        """Add a group of answers with additionals to the outgoing queue."""
        assert self.zc.loop is not None
        random_delay = random.randint(*_MULTICAST_DELAY_RANDOM_INTERVAL) + self.additional_delay
        send_after = now + random_delay
        send_before = now + self.aggregation_delay + self.additional_delay
        if len(self.queue):
            # If we calculate a random delay for the send after time
            # that is less than the last group scheduled to go out,
            # we instead add the answers to the last group as this
            # allows aggregating additonal responses
            last_group = self.queue[-1]
            if send_after <= last_group.send_after:
                last_group.answers.update(answers)
                return
        else:
            self.zc.loop.call_later(millis_to_seconds(random_delay), self.async_ready)
        self.queue.append(AnswerGroup(send_after, send_before, answers))

    def _remove_answers_from_queue(self, answers: _AnswerWithAdditionalsType) -> None:
        """Remove a set of answers from the outgoing queue."""
        for pending in self.queue:
            for record in answers:
                pending.answers.pop(record, None)

    def async_ready(self) -> None:
        """Process anything in the queue that is ready."""
        assert self.zc.loop is not None
        now = current_time_millis()

        if len(self.queue) > 1 and self.queue[0].send_before > now:
            # There is more than one answer in the queue,
            # delay until we have to send it (first answer group reaches send_before)
            self.zc.loop.call_later(millis_to_seconds(self.queue[0].send_before - now), self.async_ready)
            return

        answers: _AnswerWithAdditionalsType = {}
        # Add all groups that can be sent now
        while len(self.queue) and self.queue[0].send_after <= now:
            answers.update(self.queue.popleft().answers)

        if len(self.queue):
            # If there are still groups in the queue that are not ready to send
            # be sure we schedule them to go out later
            self.zc.loop.call_later(millis_to_seconds(self.queue[0].send_after - now), self.async_ready)

        if answers:
            # If we have the same answer scheduled to go out, remove them
            self._remove_answers_from_queue(answers)
            self.zc.async_send(construct_outgoing_multicast_answers(answers))