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我正在尝试将 DNS 标头和问题发送到 UDP 套接字。我已经调用了以下命令来建立连接：socket(PF_INET, SOCK_DGRAM, 0)，inet_aton(temp, &servAddr.sin_addr)使用端口 53 和connect(sock, (const sockaddr*) &servAddr, sizeof(servAddr))。

我遇到的问题是编写包含标头和 DNS 查询问题的消息。到目前为止，我的 DNS 标头结构如下所示：

struct DNS_Header {
            uint16_t id;
            struct {
                uint8_t rd: 1;      // recursion desired
                uint8_t tc: 1;      // message was truncated
                uint8_t aa: 1;      // authoritative answer
                uint8_t opcode: 4;  // query type
                // 0 a standard query (QUERY); 1 an inverse query (IQUERY);
                // 2 a server status request (STATUS);
                // 3 - 15 reserved for futre use
                uint8_t qr: 1;      // query(0) or response (1)

                uint8_t rcode: 4;   // response code
                // 0: No error condition; 1:Format error; 2:Server failure;
                // 3: Name Error (authoritative server, name doesn't exist)
                // 4: Not implemented; 5: Refused
                uint8_t z: 3;       // reserved for future use. Must be zero
                uint8_t ra: 1;      // recursion
                } flags;
            uint16_t gdcount;
            uint16_t ancount;
            uint16_t nscount;
            uint16_t arcount;
        };

我创建 DNS 查询标头：

   packetHeader.id = htons(1337);   
    packetHeader.flags.rd = 0;          
    packetHeader.flags.tc = 0;          
    packetHeader.flags.aa = 0;          
    packetHeader.flags.opcode = htons(0);       
    packetHeader.flags.qr = 0;          
    packetHeader.flags.rcode = htons(0);    
    packetHeader.flags.z = htons(0);        
    packetHeader.flags.ra = 0;          
    packetHeader.gdcount = htons(1);        
    packetHeader.ancount = htons(0);        
    packetHeader.nscount = htons(0);        
    packetHeader.arcount = htons(0);

我知道我需要将主机名更改为 DNS 问题名称部分的标签/数据格式。我是这样做的：

static const char* const hex = "0123456789ABCDEF";
    hostname.push_back(' ');
    int len = hostname.length();

    int segCount = 0;
    std::vector<std::string> storage;
    std::string temp = "";

    for (int i  = 0; i < len; i++) {
        if (hostname[i] == '.' || hostname[i] == ' ') {
            storage.push_back(std::to_string(0));
            storage.push_back(std::to_string(segCount));
            storage.push_back(temp);
            // reset
            segCount = 0;
            temp = "";
            continue;
        }
        const unsigned char c = hostname[i];
        std::cout << c << " : " << hex[c >> 4] << " " << hex[c & 15] << std::endl;
        temp.push_back(hex[c >> 4]);
        temp.push_back(hex[c & 15]);
        segCount ++;
        continue;
    }

    storage.push_back(std::to_string(0));
    storage.push_back(std::to_string(0));

    for (int i = 0; i < storage.size(); i++) { qname_labelFormat.append(storage[i]); }

    std::cout << "The hex representation of qname TOTAL: " << qname_labelFormat << std::endl

我使用 DNS_Question 结构将 DNS 查询问题的所有内容放在一起。此结构不包括 DNS 查询问题名称。我将名称存储在变量中：qname_lableFormat. 查看下面的 DNS 查询问题结构：

struct DNS_Question {
            uint16_t qtype;
            uint16_t qclass;
        };

为了填充结构 DNS_Question，我这样做了：

   int queryTypeNum = 0;

    if (queryType == "A")     { queryTypeNum = 1;  }
    if (queryType == "NS")    { queryTypeNum = 2;  }
    if (queryType == "CNAME") { queryTypeNum = 5;  }
    if (queryType == "SOA")   { queryTypeNum = 6;  }
    if (queryType == "WKS")   { queryTypeNum = 11; }
    if (queryType == "PTR")   { queryTypeNum = 12; }
    if (queryType == "MX")    { queryTypeNum = 15; }
    if (queryType == "SRV")   { queryTypeNum = 33; }
    if (queryType == "AAAA")  { queryTypeNum = 28; }
    if (queryType == "")      { queryTypeNum = 1;  }

    packetQuestion.qtype = htons(queryTypeNum);
    std::cout << "QType value set to: " << queryTypeNum << std::endl;
    packetQuestion.qclass = htons(1);

在我完成所有这些之后，我使用套接字，write(int fd, const void *buf, size_t count);我相信这是我的错误发生的地方。没有报告错误消息，但是当我调用read(int fd, void *buf, size_t count);命令时，没有收到任何内容。我的写命令如下。

int size = sizeof(struct DNS_Header);
    size += sizeof(struct DNS_Question);
    char *message [size];
    int totalSize = 0;
    int stringSize = qname_labelFormat.length() + 1;
    char* temp = {0};
    temp = (char*)qname_labelFormat.c_str();

    std::memcpy(message, (struct DNS_Header*)&packetHeader, sizeof(packetHeader));
    totalSize = sizeof(struct DNS_Header);

    std::memcpy(message + totalSize, (char*)temp, sizeof(qname_labelFormat));
    totalSize += sizeof(qname_labelFormat);

    std::memcpy(message + totalSize, (struct DNS_Question*)&packetQuestion, sizeof(packetQuestion));
    totalSize += sizeof(packetQuestion);

    int bytesSent = 0;
    if ((bytesSent = write(sock, message, totalSize)) < 0) {
        logger->printLog ("write Failed");
        std::string error = strerror(errno);
        logger->printLog(error);
    }
    else {
        logger->printLog ("write was successful");
}

如果有人能够提供帮助，我将不胜感激。

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