C语言判断arp协议是免费arp的代码

以下是使用C语言判断ARP协议是免费ARP的代码示例：

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

int is_free_arp(struct ether_arp *arp_packet) {
    if (ntohs(arp_packet->arp_op) != ARPOP_REQUEST) {  // 只处理ARP请求
        return 0;
    }

    if (memcmp(arp_packet->arp_tha, "\x00\x00\x00\x00\x00\x00", 6) != 0) {  // 目标MAC地址不是广播地址
        return 0;
    }

    if (memcmp(arp_packet->arp_tpa, arp_packet->arp_spa, 4) != 0) {  // 目标IP地址不是本地IP地址
        return 0;
    }

    return 1;
}

int main(int argc, char *argv[]) {
    int sockfd;
    struct sockaddr_ll saddr;
    struct ether_arp arp_packet;
    unsigned char buffer[ETH_FRAME_LEN];
    ssize_t n;

    // 创建原始套接字
    sockfd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ARP));
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // 绑定网络接口
    memset(&saddr, 0, sizeof(saddr));
    saddr.sll_family = AF_PACKET;
    saddr.sll_protocol = htons(ETH_P_ARP);
    saddr.sll_ifindex = if_nametoindex("eth0");  // 替换为网络接口名称
    if (bind(sockfd, (struct sockaddr *)&saddr, sizeof(saddr)) < 0) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    // 接收并处理ARP请求
    while (1) {
        n = recv(sockfd, buffer, sizeof(buffer), 0);
        if (n < 0) {
            perror("recv");
            exit(EXIT_FAILURE);
        }

        memcpy(&arp_packet, buffer + sizeof(struct ether_header), sizeof(struct ether_arp));
        if (is_free_arp(&arp_packet)) {
            printf("Received a free ARP request from %s\n", inet_ntoa(*(struct in_addr *)arp_packet.arp_spa));
        }
    }

    close(sockfd);
    return 0;
}

该代码使用了Linux下的原始套接字接收ARP请求，并判断是否为免费ARP请求。其中，`is_free_arp`函数用于判断一个ARP请求是否为免费ARP请求。如果是，则打印源IP地址。需要注意的是，运行该程序需要root权限。