c实现dns中继服务器

DNS中继服务器是指将DNS查询请求转发给其他DNS服务器的服务器。下面是一个简单的C语言实现示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define BUF_SIZE 512
#define DNS_PORT 53

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <dns_server>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    char *dns_server = argv[1];

    int sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(DNS_PORT);
    addr.sin_addr.s_addr = INADDR_ANY;

    if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    char buf[BUF_SIZE];
    struct sockaddr_in client_addr;
    socklen_t client_addr_len = sizeof(client_addr);

    while (1) {
        int recv_len = recvfrom(sockfd, buf, BUF_SIZE, 0, (struct sockaddr *)&client_addr, &client_addr_len);
        if (recv_len < 0) {
            perror("recvfrom");
            exit(EXIT_FAILURE);
        }

        printf("Received query from %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

        // 将查询请求转发给DNS服务器
        struct sockaddr_in dns_addr;
        memset(&dns_addr, 0, sizeof(dns_addr));
        dns_addr.sin_family = AF_INET;
        dns_addr.sin_port = htons(DNS_PORT);
        dns_addr.sin_addr.s_addr = inet_addr(dns_server);

        int send_len = sendto(sockfd, buf, recv_len, 0, (struct sockaddr *)&dns_addr, sizeof(dns_addr));
        if (send_len < 0) {
            perror("sendto");
            exit(EXIT_FAILURE);
        }

        printf("Forwarded query to %s\n", dns_server);

        // 接收DNS服务器的响应
        recv_len = recvfrom(sockfd, buf, BUF_SIZE, 0, NULL, NULL);
        if (recv_len < 0) {
            perror("recvfrom");
            exit(EXIT_FAILURE);
        }

        printf("Received response from %s\n", dns_server);

        // 将响应发送给客户端
        send_len = sendto(sockfd, buf, recv_len, 0, (struct sockaddr *)&client_addr, client_addr_len);
        if (send_len < 0) {
            perror("sendto");
            exit(EXIT_FAILURE);
        }

        printf("Sent response to %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
    }

    close(sockfd);
    return 0;
}

在运行程序时，需要传入要转发DNS查询请求的DNS服务器的IP地址或域名，例如：

./dns_relay_server 8.8.8.8

这将启动一个DNS中继服务器，并将DNS查询请求转发给Google Public DNS服务器。