Почему в DNSServer () dnslib есть и преобразователь, и обработчик?

Question

Я пытаюсь понять процесс разрешения в dnslib.В частности, я использую пример proxy.py для реализации локального прокси-сервера DNS, который будет отправлять запрос на определенные серверы на основе запроса.

(копия proxy.py):

# -*- coding: utf-8 -*-

from __future__ import print_function

import binascii,socket,struct

from dnslib import DNSRecord,RCODE
from dnslib.server import DNSServer,DNSHandler,BaseResolver,DNSLogger

class ProxyResolver(BaseResolver):
    """
        Proxy resolver - passes all requests to upstream DNS server and
        returns response

        Note that the request/response will be each be decoded/re-encoded
        twice:

        a) Request packet received by DNSHandler and parsed into DNSRecord
        b) DNSRecord passed to ProxyResolver, serialised back into packet
           and sent to upstream DNS server
        c) Upstream DNS server returns response packet which is parsed into
           DNSRecord
        d) ProxyResolver returns DNSRecord to DNSHandler which re-serialises
           this into packet and returns to client

        In practice this is actually fairly useful for testing but for a
        'real' transparent proxy option the DNSHandler logic needs to be
        modified (see PassthroughDNSHandler)

    """

    def __init__(self,address,port,timeout=0):
        self.address = address
        self.port = port
        self.timeout = timeout

    def resolve(self,request,handler):
        try:
            if handler.protocol == 'udp':
                proxy_r = request.send(self.address,self.port,
                                timeout=self.timeout)
            else:
                proxy_r = request.send(self.address,self.port,
                                tcp=True,timeout=self.timeout)
            reply = DNSRecord.parse(proxy_r)
        except socket.timeout:
            reply = request.reply()
            reply.header.rcode = getattr(RCODE,'NXDOMAIN')

        return reply

class PassthroughDNSHandler(DNSHandler):
    """
        Modify DNSHandler logic (get_reply method) to send directly to
        upstream DNS server rather then decoding/encoding packet and
        passing to Resolver (The request/response packets are still
        parsed and logged but this is not inline)
    """
    def get_reply(self,data):
        host,port = self.server.resolver.address,self.server.resolver.port

        request = DNSRecord.parse(data)
        self.server.logger.log_request(self,request)

        if self.protocol == 'tcp':
            data = struct.pack("!H",len(data)) + data
            response = send_tcp(data,host,port)
            response = response[2:]
        else:
            response = send_udp(data,host,port)

        reply = DNSRecord.parse(response)
        self.server.logger.log_reply(self,reply)

        return response

def send_tcp(data,host,port):
    """
        Helper function to send/receive DNS TCP request
        (in/out packets will have prepended TCP length header)
    """
    sock = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
    sock.connect((host,port))
    sock.sendall(data)
    response = sock.recv(8192)
    length = struct.unpack("!H",bytes(response[:2]))[0]
    while len(response) - 2 < length:
        response += sock.recv(8192)
    sock.close()
    return response

def send_udp(data,host,port):
    """
        Helper function to send/receive DNS UDP request
    """
    sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)
    sock.sendto(data,(host,port))
    response,server = sock.recvfrom(8192)
    sock.close()
    return response

if __name__ == '__main__':

    import argparse,sys,time

    p = argparse.ArgumentParser(description="DNS Proxy")
    p.add_argument("--port","-p",type=int,default=53,
                    metavar="<port>",
                    help="Local proxy port (default:53)")
    p.add_argument("--address","-a",default="",
                    metavar="<address>",
                    help="Local proxy listen address (default:all)")
    p.add_argument("--upstream","-u",default="8.8.8.8:53",
            metavar="<dns server:port>",
                    help="Upstream DNS server:port (default:8.8.8.8:53)")
    p.add_argument("--tcp",action='store_true',default=False,
                    help="TCP proxy (default: UDP only)")
    p.add_argument("--timeout","-o",type=float,default=5,
                    metavar="<timeout>",
                    help="Upstream timeout (default: 5s)")
    p.add_argument("--passthrough",action='store_true',default=False,
                    help="Dont decode/re-encode request/response (default: off)")
    p.add_argument("--log",default="request,reply,truncated,error",
                    help="Log hooks to enable (default: +request,+reply,+truncated,+error,-recv,-send,-data)")
    p.add_argument("--log-prefix",action='store_true',default=False,
                    help="Log prefix (timestamp/handler/resolver) (default: False)")
    args = p.parse_args()

    args.dns,_,args.dns_port = args.upstream.partition(':')
    args.dns_port = int(args.dns_port or 53)

    print("Starting Proxy Resolver (%s:%d -> %s:%d) [%s]" % (
                        args.address or "*",args.port,
                        args.dns,args.dns_port,
                        "UDP/TCP" if args.tcp else "UDP"))

    resolver = ProxyResolver(args.dns,args.dns_port,args.timeout)
    handler = PassthroughDNSHandler if args.passthrough else DNSHandler
    logger = DNSLogger(args.log,args.log_prefix)
    udp_server = DNSServer(resolver,
                           port=args.port,
                           address=args.address,
                           logger=logger,
                           handler=handler)
    udp_server.start_thread()

    if args.tcp:
        tcp_server = DNSServer(resolver,
                               port=args.port,
                               address=args.address,
                               tcp=True,
                               logger=logger,
                               handler=handler)
        tcp_server.start_thread()

    while udp_server.isAlive():
        time.sleep(1)

Я успешно ввел бизнес-логику своих взаимодействий в методе get_reply PassthroughDNSHandler:

def get_reply(self, data):
    host, port = self.server.resolver.address, self.server.resolver.port

    request = DNSRecord.parse(data)
    query = str(request.questions[0].qname)
    if query.endswith('.example.info.'):
        server = "192.168.10.1"
    elif any(query.endswith(x) for x in ["example.net.", "example.com."]):
        server = "10.24.131.10"
    else:
        server = "1.1.1.1"

    log.debug(f"{query} redirected to {server}")
    response = send_udp(data, server, port)
    reply = DNSRecord.parse(response)

Это работает, как и ожидалось, запрашиваются нужные DNS-серверы в зависимости отпо запросу.

Часть, которую я не понимаю, это участие ProxyResolver в инициализации сервера.

resolver = ProxyResolver(args.dns, args.dns_port, args.timeout)
    udp_server = DNSServer(resolver, port=53, address="127.0.0.1", handler=PassthroughDNSHandler)

Что такое resolver нужен для?

Насколько я понимаю, пакет, полученный на 127.0.0.1:53, передается через handler на PassthroughDNSHandler и фактически обрабатывается в get_reply().

Затем он далее отправляется на соответствующий вышестоящий сервер через send_udp(), и ответ пересылается обратно запрашивающему клиенту.

В какой момент resolver попадает вкартинка и какова ее роль?

Я поставил точку останова в методе resolve() для ProxyResolver, и он никогда не срабатывает.