帮我写个DNS查询CNAME的c++例程

以下是一个使用C++编写的查询DNS CNAME记录的例程：

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>

using namespace std;

int main(int argc, char *argv[]) {
    if (argc != 2) {
        cerr << "Usage: " << argv[0] << " <hostname>" << endl;
        return 1;
    }

    char *hostname = argv[1];
    struct hostent *he;
    struct in_addr **addr_list;

    if ((he = gethostbyname(hostname)) == NULL) {
        cerr << "Failed to resolve hostname" << endl;
        return 1;
    }

    cout << "Hostname: " << hostname << endl;
    cout << "Aliases: ";

    char **aliases = he->h_aliases;
    while (*aliases) {
        cout << *aliases << " ";
        aliases++;
    }
    cout << endl;

    cout << "Addresses: ";
    addr_list = (struct in_addr **) he->h_addr_list;
    for (int i = 0; addr_list[i] != NULL; i++) {
        cout << inet_ntoa(*addr_list[i]) << " ";
    }
    cout << endl;

    if (he->h_addrtype == AF_INET && he->h_length == 4) {
        char cname[1024];
        struct sockaddr_in sa;
        memset(&sa, 0, sizeof(sa));
        sa.sin_family = AF_INET;
        sa.sin_port = htons(53);
        sa.sin_addr = *((struct in_addr *) he->h_addr_list[0]);

        int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
        if (sock < 0) {
            cerr << "Failed to create socket" << endl;
            return 1;
        }

        const char *query = "www";
        int query_len = strlen(query);

        char buf[1024];
        memset(buf, 0, sizeof(buf));

        // 构造DNS查询报文
        buf[1] = 0;
        buf[3] = 1;
        buf[5] = 1;

        char *p = buf + 6;
        memcpy(p, query, query_len);
        p += query_len + 1;

        *p++ = 0;
        *p++ = 1;

        // 发送DNS查询报文
        int sa_len = sizeof(sa);
        int n = sendto(sock, buf, p - buf, 0, (struct sockaddr *) &sa, sa_len);
        if (n < 0) {
            cerr << "Failed to send DNS query" << endl;
            return 1;
        }

        // 接收DNS响应报文
        n = recvfrom(sock, buf, sizeof(buf), 0, (struct sockaddr *) &sa, (socklen_t *) &sa_len);
        if (n < 0) {
            cerr << "Failed to receive DNS response" << endl;
            return 1;
        }

        // 解析DNS响应报文
        uint16_t *p16 = (uint16_t *) buf;
        int id = ntohs(*p16++);
        uint16_t flags = ntohs(*p16++);
        uint16_t qcount = ntohs(*p16++);
        uint16_t acount = ntohs(*p16++);
        uint16_t nscount = ntohs(*p16++);
        uint16_t arcount = ntohs(*p16++);

        for (int i = 0; i < qcount; i++) {
            char *qname = (char *) p16;
            p16 += (strlen(qname) + 5) / 2;
        }

        for (int i = 0; i < acount; i++) {
            char *aname = (char *) p16;
            p16 += (strlen(aname) + 5) / 2;

            uint16_t atype = ntohs(*p16++);
            uint16_t aclass = ntohs(*p16++);
            uint32_t attl = ntohl(*(uint32_t *) p16);
            p16 += 2;

            uint16_t alen = ntohs(*p16++);
            if (atype == 5) { // CNAME
                memcpy(cname, p16, alen);
                cname[alen] = '\0';
            }
            p16 += alen / 2;
        }

        close(sock);

        cout << "CNAME: " << cname << endl;
    }

    return 0;
}

该例程使用了gethostbyname函数获取主机名的IP地址列表，并使用UDP套接字发送DNS查询报文和接收DNS响应报文。查询CNAME记录的过程是在解析DNS响应报文中找到类型为5的资源记录，并解析其数据部分得到CNAME的值。