需要在C中构建简单的DNS解析器

unsigned char *query = (unsigned char *)malloc(MAX_BUFFER_SIZE);

int idx = 0;

// 标识符

query[idx++] = (rand() % 256);

query[idx++] = (rand() % 256);

// 标志

query[idx++] = 0x01;

query[idx++] = 0x00;

// 查询记录数

query[idx++] = 0x00;

query[idx++] = 0x01;

// 查询类型：A记录

query[idx++] = 0x00;

query[idx++] = 0x01;

// 类别：Internet

query[idx++] = 0x00;

query[idx++] = 0x01;

// 域名

int len = strlen(domain);

for (int i = 0; i < len; ++i) {

    query[idx++] = domain[i];

}

query[idx++] = 0x00;

*query_size = idx;

return query;

int idx = 0;

// 跳过标识符

idx += 2;

// 检查标志

if ((response[idx++] << 8) | response[idx++] != 0x8180) {

    return -1;

}

// 跳过查询记录数和查询类型

idx += 4;

// 跳过回答记录数

int answer_count = (response[idx++] << 8) | response[idx++];

// 解析回答记录

for (int i = 0; i< answer_count; ++i) {

    // 跳过域名

    while (response[idx++] != 0x00) {}

    // 读取记录类型和类别

    int type = (response[idx++] << 8) | response[idx++];

    int class = (response[idx++] << 8) | response[idx++];

    // 检查类型和类别

    if (type == 0x0001 && class == 0x0001) {

        // 跳过TTL

        idx += 4;

        // 读取IP地址长度

        int ip_len = response[idx++];

        // 读取IP地址

        for (int j = 0; j < ip_len; ++j) {

            sprintf(ip + j * 3, "%d.", response[idx++]);

        }

        ip[ip_len * 3 - 1] = '\0';

        return 0;

    } else {

        // 跳过TTL和IP地址长度

        idx += 5;

        // 跳过IP地址

        int ip_len = response[idx++];

        idx += ip_len;

    }

}

return -1;

if (argc != 2) {

    printf("Usage: %s<domain>\n", argv[0]);

    return 1;

}

const char *domain = argv[1];

int sockfd = socket(AF_INET, SOCK_DGRAM, 0);

if (sockfd < 0) {

    perror("socket");

    return 1;

}

struct sockaddr_in servaddr;

memset(&servaddr, 0, sizeof(servaddr));

servaddr.sin_family = AF_INET;

servaddr.sin_port = htons(DNS_PORT);

if (inet_pton(AF_INET, DNS_SERVER, &servaddr.sin_addr) <= 0) {

    perror("inet_pton");

    return 1;

}

if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {

    perror("connect");

    return 1;

}

int query_size;

unsigned char *query = build_dns_query(domain, &