以太网的arp数据包结构：

arp结构op操作参数：1为请求，2为应答。

常用的数据结构如下：

1.物理地址结构位于netpacket/packet.h

struct sockaddr_ll
{
    unsigned short int sll_family;
    unsigned short int sll_protocol;
    int sll_ifindex;
    unsigned short int sll_hatype;
    unsigned char sll_pkttype;
    unsigned char sll_halen;
    unsigned char sll_addr[8];
};

sll_ifindex是网络(网卡)接口索引，代表从这个接口收发数据包

2.网络(网卡)接口数据结构位于net/if.h

struct ifreq
{
# define IFHWADDRLEN    6
# define IFNAMSIZ    IF_NAMESIZE
    union
      {
    char ifrn_name[IFNAMSIZ];    /* Interface name, e.g. "en0".  */
      } ifr_ifrn;


    union
      {
    struct sockaddr ifru_addr;
    struct sockaddr ifru_dstaddr;
    struct sockaddr ifru_broadaddr;
    struct sockaddr ifru_netmask;
    struct sockaddr ifru_hwaddr;
    short int ifru_flags;
    int ifru_ivalue;
    int ifru_mtu;
    struct ifmap ifru_map;
    char ifru_slave[IFNAMSIZ];    /* Just fits the size */
    char ifru_newname[IFNAMSIZ];
    __caddr_t ifru_data;
      } ifr_ifru;
};

该结构里面包含2个union，第一个是接口名，如：eth0，wlan0等。可以通过ioctl()函数来获取对应的接口信息，ip地址，mac地址，接口索引等。

3.以太网首部结构位于net/ethernet.h

struct ether_header
{
  u_int8_t  ether_dhost[ETH_ALEN];    /* destination eth addr    */
  u_int8_t  ether_shost[ETH_ALEN];    /* source ether addr    */
  u_int16_t ether_type;                /* packet type ID field    */
} __attribute__ ((__packed__));

ether_type帧类型：常见的有IP，ARP，RARP，都有对应的宏定义。

4.arp包结构位于netinet/if_ether.h

struct    ether_arp {
    struct    arphdr ea_hdr;        /* fixed-size header */
    u_int8_t arp_sha[ETH_ALEN];    /* sender hardware address */
    u_int8_t arp_spa[4];        /* sender protocol address */
    u_int8_t arp_tha[ETH_ALEN];    /* target hardware address */
    u_int8_t arp_tpa[4];        /* target protocol address */
};
#define    arp_hrd    ea_hdr.ar_hrd
#define    arp_pro    ea_hdr.ar_pro
#define    arp_hln    ea_hdr.ar_hln
#define    arp_pln    ea_hdr.ar_pln
#define    arp_op    ea_hdr.ar_op

上面的ether_arp结构还包含一个arp首部，位于net/if_arp.h

struct arphdr
{
    unsigned short int ar_hrd;        /* Format of hardware address.  */
    unsigned short int ar_pro;        /* Format of protocol address.  */
    unsigned char ar_hln;        /* Length of hardware address.  */
    unsigned char ar_pln;        /* Length of protocol address.  */
    unsigned short int ar_op;        /* ARP opcode (command).  */
}

二.arp请求代码                                      

/**
 * @file arp_request.c
 */


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/if_ether.h>
#include <net/ethernet.h>
#include <net/if_arp.h>
#include <net/if.h>
#include <netpacket/packet.h>


/* 以太网帧首部长度 */
#define ETHER_HEADER_LEN sizeof(struct ether_header)
/* 整个arp结构长度 */
#define ETHER_ARP_LEN sizeof(struct ether_arp)
/* 以太网 + 整个arp结构长度 */
#define ETHER_ARP_PACKET_LEN ETHER_HEADER_LEN + ETHER_ARP_LEN
/* IP地址长度 */
#define IP_ADDR_LEN 4
/* 广播地址 */
#define BROADCAST_ADDR {0xff, 0xff, 0xff, 0xff, 0xff, 0xff}


void err_exit(const char *err_msg)
{
    perror(err_msg);
    exit(1);
}


/* 填充arp包 */
struct ether_arp *fill_arp_packet(const unsigned char *src_mac_addr, const char *src_ip, const char *dst_ip)
{
    struct ether_arp *arp_packet;
    struct in_addr src_in_addr, dst_in_addr;
    unsigned char dst_mac_addr[ETH_ALEN] = BROADCAST_ADDR;


    /* IP地址转换 */
    inet_pton(AF_INET, src_ip, &src_in_addr);
    inet_pton(AF_INET, dst_ip, &dst_in_addr);


    /* 整个arp包 */
    arp_packet = (struct ether_arp *)malloc(ETHER_ARP_LEN);
    arp_packet->arp_hrd = htons(ARPHRD_ETHER);
    arp_packet->arp_pro = htons(ETHERTYPE_IP);
    arp_packet->arp_hln = ETH_ALEN;
    arp_packet->arp_pln = IP_ADDR_LEN;
    arp_packet->arp_op = htons(ARPOP_REQUEST);
    memcpy(arp_packet->arp_sha, src_mac_addr, ETH_ALEN);
    memcpy(arp_packet->arp_tha, dst_mac_addr, ETH_ALEN);
    memcpy(arp_packet->arp_spa, &src_in_addr, IP_ADDR_LEN);
    memcpy(arp_packet->arp_tpa, &dst_in_addr, IP_ADDR_LEN);


    return arp_packet;
}


/* arp请求 */
void arp_request(const char *if_name, const char *dst_ip)
{
    struct sockaddr_ll saddr_ll;
    struct ether_header *eth_header;
    struct ether_arp *arp_packet;
    struct ifreq ifr;
    char buf[ETHER_ARP_PACKET_LEN];
    unsigned char src_mac_addr[ETH_ALEN];
    unsigned char dst_mac_addr[ETH_ALEN] = BROADCAST_ADDR;
    char *src_ip;
    int sock_raw_fd, ret_len, i;


    if ((sock_raw_fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ARP))) == -1)
        err_exit("socket()");


    bzero(&saddr_ll, sizeof(struct sockaddr_ll));
    bzero(&ifr, sizeof(struct ifreq));
    /* 网卡接口名 */
    memcpy(ifr.ifr_name, if_name, strlen(if_name));


    /* 获取网卡接口索引 */
    if (ioctl(sock_raw_fd, SIOCGIFINDEX, &ifr) == -1)
        err_exit("ioctl() get ifindex");
    saddr_ll.sll_ifindex = ifr.ifr_ifindex;
    saddr_ll.sll_family = PF_PACKET;


    /* 获取网卡接口IP */
    if (ioctl(sock_raw_fd, SIOCGIFADDR, &ifr) == -1)
        err_exit("ioctl() get ip");
    src_ip = inet_ntoa(((struct sockaddr_in *)&(ifr.ifr_addr))->sin_addr);
    printf("local ip:%s\n", src_ip);


    /* 获取网卡接口MAC地址 */
    if (ioctl(sock_raw_fd, SIOCGIFHWADDR, &ifr))
        err_exit("ioctl() get mac");
    memcpy(src_mac_addr, ifr.ifr_hwaddr.sa_data, ETH_ALEN);
    printf("local mac");
    for (i = 0; i < ETH_ALEN; i++)
        printf(":%02x", src_mac_addr[i]);
    printf("\n");


    bzero(buf, ETHER_ARP_PACKET_LEN);
    /* 填充以太首部 */
    eth_header = (struct ether_header *)buf;
    memcpy(eth_header->ether_shost, src_mac_addr, ETH_ALEN);
    memcpy(eth_header->ether_dhost, dst_mac_addr, ETH_ALEN);
    eth_header->ether_type = htons(ETHERTYPE_ARP);
    /* arp包 */
    arp_packet = fill_arp_packet(src_mac_addr, src_ip, dst_ip);
    memcpy(buf + ETHER_HEADER_LEN, arp_packet, ETHER_ARP_LEN);


    /* 发送请求 */
    ret_len = sendto(sock_raw_fd, buf, ETHER_ARP_PACKET_LEN, 0, (struct sockaddr *)&saddr_ll, sizeof(struct sockaddr_ll));
    if ( ret_len > 0)
        printf("sendto() ok!!!\n");


    close(sock_raw_fd);
}


int main(int argc, const char *argv[])
{
    if (argc != 3)
    {
        printf("usage:%s device_name dst_ip\n", argv[0]);
        exit(1);
    }


    arp_request(argv[1], argv[2]);
    
    return 0;
}

流程：命令行接收网卡接口名和要请求的目标IP地址，传入arp_request()函数。用PF_PACKET选项创建ARP类型的原始套接字。用ioctl()函数通过网卡接口名来获取该接口对应的mac地址，ip地址，接口索引。接口索引填充到物理地址sockaddr_ll里面。然后填充以太首部，源地址对应刚刚的网卡接口mac地址，目标地址填广播地址(第28行定义的宏)。以太首部帧类型是ETHERTYPE_ARP，代表arp类型。接着填充arp数据包结构，同样要填充源/目标的ip地址和mac地址，arp包的操作选项填写ARPOP_REQUEST，代表请求操作。填充完成后发送到刚刚的物理地址sockaddr_ll。

三.接收arp数据包  

/**
 * @file arp_recv.c
 */


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


/* 以太网帧首部长度 */
#define ETHER_HEADER_LEN sizeof(struct ether_header)
/* 整个arp结构长度 */
#define ETHER_ARP_LEN sizeof(struct ether_arp)
/* 以太网 + 整个arp结构长度 */
#define ETHER_ARP_PACKET_LEN ETHER_HEADER_LEN + ETHER_ARP_LEN
/* IP地址长度 */
#define IP_ADDR_LEN 4


void err_exit(const char *err_msg)
{
    perror(err_msg);
    exit(1);
}


int main(void)
{
    struct ether_arp *arp_packet;
    char buf[ETHER_ARP_PACKET_LEN];
    int sock_raw_fd, ret_len, i;


    if ((sock_raw_fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ARP))) == -1)
        err_exit("socket()");


    while (1)
    {
        bzero(buf, ETHER_ARP_PACKET_LEN);
        ret_len = recv(sock_raw_fd, buf, ETHER_ARP_PACKET_LEN, 0);
        if (ret_len > 0)
        {
            /* 剥去以太头部 */
            arp_packet = (struct ether_arp *)(buf + ETHER_HEADER_LEN);
            /* arp操作码为2代表arp应答 */
            if (ntohs(arp_packet->arp_op) == 2)
            {
                printf("==========================arp replay======================\n");
                printf("from ip:");
                for (i = 0; i < IP_ADDR_LEN; i++)
                    printf(".%u", arp_packet->arp_spa[i]);
                printf("\nfrom mac");
                for (i = 0; i < ETH_ALEN; i++)
                    printf(":%02x", arp_packet->arp_sha[i]);
                printf("\n");
            }
        }
    }


    close(sock_raw_fd);
    return 0;
}

流程：创建ARP类型的原始套接字。直接调用接收函数，会收到网卡接收的arp数据包，判断收到的arp包操作是arp应答，操作码是2。然后剥去以太首部，取出源mac地址和ip地址！！！

四.实验                                                  

为了更直观，我们打开wireshark一起观察，我这里是wlan环境，监听wlan0。原始套接字要以root身份运行，先运行arp_recv，然后运行arp_request发送arp请求：

wireshark结果：