当存在多个NS记录时解析dns响应

Question

你好，我正在为学校做一个作业，要求你使用java构建自己的DNS CLient，而不使用任何现有的库。我想出了如何解析响应，但在回答部分中有多条记录时出错。

public void processResponse(DatagramPacket packet) throws IOException{
    DataInputStream din = new DataInputStream(new ByteArrayInputStream(packet.getData()));
    din.readShort(); //Transaction ID
    din.readShort(); //Flags

    int questions = din.readShort();
    int answers = din.readShort();
    int authorities = din.readShort();
    int additional = din.readShort();
    int recLen = 0;
    while ((recLen = din.readByte()) > 0) {
        for (int i = 0; i < recLen; i++) {
            din.readByte();
        }
    }
    din.readShort(); // Record type for question
    din.readShort(); // Class for question

    //ANSWERS
    //Just looping for one answer here
    for(int i=0;i<1;i++){
        din.readShort(); // Record name for Answer
        String response = "";
        int typeOfRec = din.readShort();
        din.readShort(); // Class for answers
        int ttl = din.readInt();
        short resLength = din.readShort();
        switch(typeOfRec){
        //A
        case 1:{
            response+="IP\t";
            String ip = "";
            for (int j = 0; j < resLength; j++ ) {
                ip+=("" + String.format("%d", (din.readByte() & 0xFF)) + ".");
            }

            response+=ip+"\t"+ttl;
            System.out.println(response);
            break;
        }
        //NS
        case 2:{
            String ip = "";
            response+="NS\t";
            while ((recLen = din.readByte()) > 0) {
                byte[] record = new byte[recLen];

                for (int j = 0; j < recLen; j++) {
                    record[j] = din.readByte();
                }

                ip+=new String(record, "UTF-8");
                ip+=".";
            }
            response+=ip+"\t"+ttl;
            System.out.println(response);
            break;
        }
        //CNAME
        case 5:{
            response+="CNAME\t";
            break;
        }
        //MX
        case 15:{
            response+="MX\t";
            break;
        }
        default:{
            break;
        }
        }
    }        
}

这是最终处理响应数据包以获得所需数据的函数。但是，当我运行此代码时，响应仅为NS-358。awsdns-44。86399。com不见了

Answer 1

看起来您需要了解名称是如何以DNS wire格式编码的。特别是它们是如何被压缩的。RFC 1035第4.1.4节将是一个良好的起点。