c# – 解码DNS标头

2023年1月30日 219次阅读

我正在尝试用c#解码DNS数据包,虽然这并不重要,但我使用的是SharpPcap.

一切都运作良好,但似乎QR和RCODE字段返回错误的值.我将我的结果与Wireshark的结果进行比较.

即使消息是请求,QR也总是1(响应).另一个奇怪的事情是,当消息是查询时返回码为0,当消息是响应时返回码为1.

我用来查找DNS标头的其中一个站点是this个.这里有什么我想念的吗？我用于解码数据包的代码是:(数据是包含有效负载数据的字节数组)

//TransactionID 16 bits (This is correct)
//Not sure why second byte has to be first but that's how wireshark displays it
transactionID = new byte[] { data[1], data[0] }; 

//Flags 16 bits
isResponse = Utilities.getBit(data[2], 0); //always returns 1

//What I'm doing here is converting the boolean returned by getBit into a
//1 or 0 which is used to create a string which will then be converted
//into a UInt16 (If there's an easier way to do this please let me know)
OpCode = Convert.ToUInt16(Convert.ToUInt16(Utilities.getBit(data[2], 1)).ToString() +
         Convert.ToUInt16(Utilities.getBit(data[2], 2)).ToString() +
         Convert.ToUInt16(Utilities.getBit(data[2], 3)).ToString() +
         Convert.ToUInt16(Utilities.getBit(data[2], 4)).ToString(), 2);

//These seem correct (Untested though since wireshark doesn't display them)
authAnswer = Utilities.getBit(data[2], 5);
truncation = Utilities.getBit(data[2], 6);
recursionDesired = Utilities.getBit(data[2], 7);

recursionAvail = Utilities.getBit(data[3], 0);
//skip 3 bits here (reserved and set to 0)...

//ReturnCode returns 0 for queries and 1 for responses, this is pretty weird
returnCode = Convert.ToUInt16(Convert.ToUInt16(Utilities.getBit(data[3], 4)).ToString() +
             Convert.ToUInt16(Utilities.getBit(data[3], 5)).ToString() +
             Convert.ToUInt16(Utilities.getBit(data[3], 6)).ToString() +
             Convert.ToUInt16(Utilities.getBit(data[3], 7)).ToString(), 2);

//Questions count, Answers count, ARR count, Additional ARR: 1 byte each
//These are correct
questionsCount = Convert.ToUInt16(data[4] * 256 + data[5]);
answersCount = Convert.ToUInt16(data[6] * 256 + data[7]);
authorityRRs = Convert.ToUInt16(data[8] * 256 + data[9]);
additionalRRs = Convert.ToUInt16(data[10] * 256 + data[11]);

getBit方法在这里：

public static bool getBit(byte b, int index) {
    //Logical AND with number to find if bit is 0 or 1
    //Shift by index to represent value in binary (2^index)
    bool bit = (b & (1 << index)) != 0;

    return (bit);
}

最佳答案 你尝试过使用
Pcap.Net吗？它完全支持DNS,因此您无需手动解码任何内容.

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