python3.5 asyncio dns resolver, based on code from who knows where

dns.py

import os
import socket
import struct
import asyncio
import logging

DNS_SERVER = '8.8.8.8'
DNS_PORT = 53

def encode_header(packet_id):
    return struct.pack('!HBBHHHH', packet_id, 1, 0, 1, 0, 0, 0)

def decode_header(header):
    packet_id, flags1, flags2, qdcount, ancount, nscount, arcount = struct.unpack('!HBBHHHH', header)
    if nscount != 0: # number of authorities
        raise Exception('wrong nscount')
    if arcount != 0:  # number of additionals
        raise Exception('wrong arcount')
    return (packet_id, qdcount, ancount)

def decode_query(data, offset):
    labels = []
    while True:
        part_len = data[offset]
        offset += 1
        if not part_len:
            break
        label = data[offset:offset+part_len]
        offset += part_len
        labels.append(label)
    labels = [label.decode('utf-8') for label in labels]
    host = '.'.join(labels)
    query_type, query_class = struct.unpack('!HH', data[offset:offset+4])
    offset += 4
    if query_class != 1:
        raise Exception('wrong query class')
    return host, query_type, offset

def decode_cname(data):
    data_len = data[0]
    subdomain = data[1:1+data_len]
    subdomain = subdomain.decode('utf-8')
    name = data[1+data_len:]
    if len(name) != 2:
        raise Exception('wrong length for name')
    name = struct.unpack('!H', name)
    return (subdomain, name)

def decode_reply(data, offset):
    name, reply_type, reply_class, ttl, data_len = struct.unpack("!HHHIH", data[offset:offset+12])
    offset += 12
    if reply_class != 1:
        raise Exception('wrong reply class')
    reply = data[offset:offset+data_len]
    offset += data_len
    if reply_type == 1:
        reply = socket.inet_ntop(socket.AF_INET, reply)
    elif reply_type == 5:
        reply = decode_cname(reply)
    reply = (name, reply_type, ttl, reply)
    return reply, offset

def encode_query(host):
    return b''.join( [len(part).to_bytes(1, 'little')+part.encode('utf-8') for part in host.split('.')] + [b'\x00'] + [struct.pack('!HH', 1, 1)])

class UdpDnsClientProtocol(asyncio.Protocol):
    def __init__(self):
        self.fut = asyncio.Future()
        self.transport = asyncio.Future()
        self.packet_id = int.from_bytes(os.urandom(2), 'little')

    def connection_made(self, transport):
        self.transport.set_result(transport)

    async def get_transport(self):
        return await self.transport

    async def resolve(self, host):
        self.fut = asyncio.Future()
        header = encode_header(self.packet_id)
        query = encode_query(host)
        packet = header + query
        t = await self.get_transport()
        t.sendto(packet)
        ip = await asyncio.wait_for(self.fut, 1.0)
        return ip

    def datagram_received(self, data, addr):
        HEADER_LEN = 12
        result = 'done'
        try:
            (packet_id, qdcount, ancount) = decode_header(data[:HEADER_LEN])
            if packet_id != self.packet_id:
                self.error("invalid packet id")
                return

            offset = HEADER_LEN
            for i in range(qdcount):
                host, query_type, offset = decode_query(data, offset)
                logging.debug("QUERY #%s: %r, %s" % (i, host, query_type))
            for i in range(ancount):
                reply, offset = decode_reply(data, offset)
                name, reply_type, ttl, reply = reply
                if reply_type == 1:
                    result = reply
                logging.debug("REPLY #%s: %r" % (i, reply))
        except Exception as exc:
            if not self.fut.cancelled():
                self.fut.set_exception(exc)
                return
        if not self.fut.cancelled() and not self.fut.done():
            self.fut.set_result(result)

    def error_received(self, exc):
        logging.info("ERROR RECEIVED %s" % exc)
        self.fut.set_exception(exc)

async def build_resolver(loop = None):
    loop = loop or asyncio.get_event_loop()
    proto = UdpDnsClientProtocol()
    conn = await loop.create_datagram_endpoint(lambda: proto, remote_addr=(DNS_SERVER, DNS_PORT))
    return proto.resolve

async def lookup(target, resolver = None, loop = None):
    resolver = resolver or await build_resolver(loop)
    return await resolver(target)

if __name__ == "__main__":
    import sys
    loop = asyncio.get_event_loop()
    if '--debug' in sys.argv:
        loop.set_debug(True)
        logging.basicConfig(level=logging.DEBUG)
    print(loop.run_until_complete(lookup(sys.argv[1])))
    loop.close()