在Python中读取DNS数据包 [英] reading DNS packets in Python

问题描述

我正在研究Python套接字，并决定是否可以实现一个非常基本的名称服务器（即，用于将域名转换为IP地址的查找表）。因此，到目前为止，我已经将服务器设置为仅转储接收到的数据。

I'm playing around with Python sockets, and decided to see if I could implement a very basic name server (i.e. a lookup table for a domain name to an IP address). So I've set up my server so far to just dump the received data.

#!/usr/bin/python
import socket

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
host = ''
port = 53
size = 512
s.bind((host, port))
while True:
    data, addr = s.recvfrom(size)
    print repr(data)

运行上面的代码并将DNS指向127.0.0.1时，我会得到类似于以下内容的信息：

When I run the above code and point my DNS to 127.0.0.1 I get something akin to the following:

'Y\x04\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x00\x00\x01\x00\x01'
'J\xaa\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x00\x00\x1c\x00\x01'
'Y\x04\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x00\x00\x01\x00\x01'

我假设这与DNS问题数据包结构有关，但我不确定。

I'm assuming that it is something to do with the DNS question packet structure, but I'm not to sure.

A）以上是转义字符吗？特定的文字编码？还是仅仅是字节？

A) Are the above escape characters? A specific text encoding? Or simply just bytes?

B）如何解释数据并使用它？

B) How can I interpret the data and work with it?

编辑：更改套接字以使用原始数据报代替数据报将导致以下结果：

Changing the socket to take raw instead of datagrams results in the following:

'E\x00$\x00\xe4\x96\x00\x00@\x01\x00\x00\x7f\x00\x00\x01\x7f\x00\x00\x01\x03\x03X\xb6\x00\x00\x00\x00E\x00V\x00m\x82\x00\x00\xff\x11\x00\x00\x7f\x00\x00\x01\x7f\x00\x00\x01\xf3\xe1\x005\x00B\x00\x00'

推荐答案

您可以从以下内容开始：

You could start with something like this:

#!/usr/bin/env python

import pprint
import socket
import struct


def decode_labels(message, offset):
    labels = []

    while True:
        length, = struct.unpack_from("!B", message, offset)

        if (length & 0xC0) == 0xC0:
            pointer, = struct.unpack_from("!H", message, offset)
            offset += 2

            return labels + decode_labels(message, pointer & 0x3FFF), offset

        if (length & 0xC0) != 0x00:
            raise StandardError("unknown label encoding")

        offset += 1

        if length == 0:
            return labels, offset

        labels.append(*struct.unpack_from("!%ds" % length, message, offset))
        offset += length


DNS_QUERY_SECTION_FORMAT = struct.Struct("!2H")

def decode_question_section(message, offset, qdcount):
    questions = []

    for _ in range(qdcount):
        qname, offset = decode_labels(message, offset)

        qtype, qclass = DNS_QUERY_SECTION_FORMAT.unpack_from(message, offset)
        offset += DNS_QUERY_SECTION_FORMAT.size

        question = {"domain_name": qname,
                    "query_type": qtype,
                    "query_class": qclass}

        questions.append(question)

    return questions, offset


DNS_QUERY_MESSAGE_HEADER = struct.Struct("!6H")

def decode_dns_message(message):

    id, misc, qdcount, ancount, nscount, arcount = DNS_QUERY_MESSAGE_HEADER.unpack_from(message)

    qr = (misc & 0x8000) != 0
    opcode = (misc & 0x7800) >> 11
    aa = (misc & 0x0400) != 0
    tc = (misc & 0x200) != 0
    rd = (misc & 0x100) != 0
    ra = (misc & 0x80) != 0
    z = (misc & 0x70) >> 4
    rcode = misc & 0xF

    offset = DNS_QUERY_MESSAGE_HEADER.size
    questions, offset = decode_question_section(message, offset, qdcount)

    result = {"id": id,
              "is_response": qr,
              "opcode": opcode,
              "is_authoritative": aa,
              "is_truncated": tc,
              "recursion_desired": rd,
              "recursion_available": ra,
              "reserved": z,
              "response_code": rcode,
              "question_count": qdcount,
              "answer_count": ancount,
              "authority_count": nscount,
              "additional_count": arcount,
              "questions": questions}

    return result


s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
host = ''
port = 53
size = 512
s.bind((host, port))
while True:
    data, addr = s.recvfrom(size)
    pprint.pprint(decode_dns_message(data))

然后为其余记录填写解码功能。

And then fill in the decoding functions for the remaining records.

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