一.概述
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：
.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 ];
};
sll_ifindex是网络(网卡)接口索引，代表从这个接口收发数据包
.网络(网卡)接口数据结构位于net/if.h
struct ifreq
{
# define IFHWADDRLEN
# 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。
.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。
.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[]; /* sender protocol address */
u_int8_t arp_tha[ETH_ALEN]; /* target hardware address */
u_int8_t arp_tpa[]; /* 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();
}
/* 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;
)
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 */
)
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 */
)
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");
; 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, , (struct sockaddr *)&saddr_ll, sizeof(struct sockaddr_ll));
)
printf("sendto() ok!!!\n");
close(sock_raw_fd);
}
int main(int argc, const char *argv[])
{
)
{
printf(]);
exit();
}
arp_request(argv[], argv[]);
;
}
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();
}
int main(void)
{
struct ether_arp *arp_packet;
char buf[ETHER_ARP_PACKET_LEN];
int sock_raw_fd, ret_len, i;
)
err_exit("socket()");
)
{
bzero(buf, ETHER_ARP_PACKET_LEN);
ret_len = recv(sock_raw_fd, buf, ETHER_ARP_PACKET_LEN, );
)
{
/* 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 */
)
{
printf("==========================arp replay======================\n");
printf("from ip:");
; i < IP_ADDR_LEN; i++)
printf(".%u", arp_packet->arp_spa[i]);
printf("\nfrom mac");
; i < ETH_ALEN; i++)
printf(":%02x", arp_packet->arp_sha[i]);
printf("\n");
}
}
}
close(sock_raw_fd);
;
}
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：
上面可以看到，第一条数据包询问谁是192.168.0.1，然后第二条数据包发送了一个回复，可以看到wireshark里面Opcode:reply(2)。源ip和mac地址跟我们自己的接收程序一样。