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：
1.物理地址结构位于netpacket/packet.h
1 struct sockaddr_ll
2 {
3 unsigned short int sll_family;
4 unsigned short int sll_protocol;
5 int sll_ifindex;
6 unsigned short int sll_hatype;
7 unsigned char sll_pkttype;
8 unsigned char sll_halen;
9 unsigned char sll_addr[8];
10 };
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
1 struct ifreq
2 {
3 # define IFHWADDRLEN 6
4 # define IFNAMSIZ IF_NAMESIZE
5 union
6 {
7 char ifrn_name[IFNAMSIZ]; /* Interface name, e.g. "en0". */
8 } ifr_ifrn;
9
10 union
11 {
12 struct sockaddr ifru_addr;
13 struct sockaddr ifru_dstaddr;
14 struct sockaddr ifru_broadaddr;
15 struct sockaddr ifru_netmask;
16 struct sockaddr ifru_hwaddr;
17 short int ifru_flags;
18 int ifru_ivalue;
19 int ifru_mtu;
20 struct ifmap ifru_map;
21 char ifru_slave[IFNAMSIZ]; /* Just fits the size */
22 char ifru_newname[IFNAMSIZ];
23 __caddr_t ifru_data;
24 } ifr_ifru;
25 };
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
1 struct ether_header
2 {
3 u_int8_t ether_dhost[ETH_ALEN]; /* destination eth addr */
4 u_int8_t ether_shost[ETH_ALEN]; /* source ether addr */
5 u_int16_t ether_type; /* packet type ID field */
6 } __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
1 struct ether_arp {
2 struct arphdr ea_hdr; /* fixed-size header */
3 u_int8_t arp_sha[ETH_ALEN]; /* sender hardware address */
4 u_int8_t arp_spa[4]; /* sender protocol address */
5 u_int8_t arp_tha[ETH_ALEN]; /* target hardware address */
6 u_int8_t arp_tpa[4]; /* target protocol address */
7 };
8 #define arp_hrd ea_hdr.ar_hrd
9 #define arp_pro ea_hdr.ar_pro
10 #define arp_hln ea_hdr.ar_hln
11 #define arp_pln ea_hdr.ar_pln
12 #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
1 struct arphdr
2 {
3 unsigned short int ar_hrd; /* Format of hardware address. */
4 unsigned short int ar_pro; /* Format of protocol address. */
5 unsigned char ar_hln; /* Length of hardware address. */
6 unsigned char ar_pln; /* Length of protocol address. */
7 unsigned short int ar_op; /* ARP opcode (command). */
8 }
二.arp请求代码
1 /**
2 * @file arp_request.c
3 */
4
5 #include <stdio.h>
6 #include <stdlib.h>
7 #include <string.h>
8 #include <unistd.h>
9 #include <sys/ioctl.h>
10 #include <sys/socket.h>
11 #include <arpa/inet.h>
12 #include <netinet/in.h>
13 #include <netinet/if_ether.h>
14 #include <net/ethernet.h>
15 #include <net/if_arp.h>
16 #include <net/if.h>
17 #include <netpacket/packet.h>
18
19 /* 以太网帧首部长度 */
20 #define ETHER_HEADER_LEN sizeof(struct ether_header)
21 /* 整个arp结构长度 */
22 #define ETHER_ARP_LEN sizeof(struct ether_arp)
23 /* 以太网 + 整个arp结构长度 */
24 #define ETHER_ARP_PACKET_LEN ETHER_HEADER_LEN + ETHER_ARP_LEN
25 /* IP地址长度 */
26 #define IP_ADDR_LEN 4
27 /* 广播地址 */
28 #define BROADCAST_ADDR {0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
29
30 void err_exit(const char *err_msg)
31 {
32 perror(err_msg);
33 exit(1);
34 }
35
36 /* 填充arp包 */
37 struct ether_arp *fill_arp_packet(const unsigned char *src_mac_addr, const char *src_ip, const char *dst_ip)
38 {
39 struct ether_arp *arp_packet;
40 struct in_addr src_in_addr, dst_in_addr;
41 unsigned char dst_mac_addr[ETH_ALEN] = BROADCAST_ADDR;
42
43 /* IP地址转换 */
44 inet_pton(AF_INET, src_ip, &src_in_addr);
45 inet_pton(AF_INET, dst_ip, &dst_in_addr);
46
47 /* 整个arp包 */
48 arp_packet = (struct ether_arp *)malloc(ETHER_ARP_LEN);
49 arp_packet->arp_hrd = htons(ARPHRD_ETHER);
50 arp_packet->arp_pro = htons(ETHERTYPE_IP);
51 arp_packet->arp_hln = ETH_ALEN;
52 arp_packet->arp_pln = IP_ADDR_LEN;
53 arp_packet->arp_op = htons(ARPOP_REQUEST);
54 memcpy(arp_packet->arp_sha, src_mac_addr, ETH_ALEN);
55 memcpy(arp_packet->arp_tha, dst_mac_addr, ETH_ALEN);
56 memcpy(arp_packet->arp_spa, &src_in_addr, IP_ADDR_LEN);
57 memcpy(arp_packet->arp_tpa, &dst_in_addr, IP_ADDR_LEN);
58
59 return arp_packet;
60 }
61
62 /* arp请求 */
63 void arp_request(const char *if_name, const char *dst_ip)
64 {
65 struct sockaddr_ll saddr_ll;
66 struct ether_header *eth_header;
67 struct ether_arp *arp_packet;
68 struct ifreq ifr;
69 char buf[ETHER_ARP_PACKET_LEN];
70 unsigned char src_mac_addr[ETH_ALEN];
71 unsigned char dst_mac_addr[ETH_ALEN] = BROADCAST_ADDR;
72 char *src_ip;
73 int sock_raw_fd, ret_len, i;
74
75 if ((sock_raw_fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ARP))) == -1)
76 err_exit("socket()");
77
78 bzero(&saddr_ll, sizeof(struct sockaddr_ll));
79 bzero(&ifr, sizeof(struct ifreq));
80 /* 网卡接口名 */
81 memcpy(ifr.ifr_name, if_name, strlen(if_name));
82
83 /* 获取网卡接口索引 */
84 if (ioctl(sock_raw_fd, SIOCGIFINDEX, &ifr) == -1)
85 err_exit("ioctl() get ifindex");
86 saddr_ll.sll_ifindex = ifr.ifr_ifindex;
87 saddr_ll.sll_family = PF_PACKET;
88
89 /* 获取网卡接口IP */
90 if (ioctl(sock_raw_fd, SIOCGIFADDR, &ifr) == -1)
91 err_exit("ioctl() get ip");
92 src_ip = inet_ntoa(((struct sockaddr_in *)&(ifr.ifr_addr))->sin_addr);
93 printf("local ip:%s\n", src_ip);
94
95 /* 获取网卡接口MAC地址 */
96 if (ioctl(sock_raw_fd, SIOCGIFHWADDR, &ifr))
97 err_exit("ioctl() get mac");
98 memcpy(src_mac_addr, ifr.ifr_hwaddr.sa_data, ETH_ALEN);
99 printf("local mac");
100 for (i = 0; i < ETH_ALEN; i++)
101 printf(":%02x", src_mac_addr[i]);
102 printf("\n");
103
104 bzero(buf, ETHER_ARP_PACKET_LEN);
105 /* 填充以太首部 */
106 eth_header = (struct ether_header *)buf;
107 memcpy(eth_header->ether_shost, src_mac_addr, ETH_ALEN);
108 memcpy(eth_header->ether_dhost, dst_mac_addr, ETH_ALEN);
109 eth_header->ether_type = htons(ETHERTYPE_ARP);
110 /* arp包 */
111 arp_packet = fill_arp_packet(src_mac_addr, src_ip, dst_ip);
112 memcpy(buf + ETHER_HEADER_LEN, arp_packet, ETHER_ARP_LEN);
113
114 /* 发送请求 */
115 ret_len = sendto(sock_raw_fd, buf, ETHER_ARP_PACKET_LEN, 0, (struct sockaddr *)&saddr_ll, sizeof(struct sockaddr_ll));
116 if ( ret_len > 0)
117 printf("sendto() ok!!!\n");
118
119 close(sock_raw_fd);
120 }
121
122 int main(int argc, const char *argv[])
123 {
124 if (argc != 3)
125 {
126 printf("usage:%s device_name dst_ip\n", argv[0]);
127 exit(1);
128 }
129
130 arp_request(argv[1], argv[2]);
131
132 return 0;
133 }
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
1 /**
2 * @file arp_recv.c
3 */
4
5 #include <stdio.h>
6 #include <stdlib.h>
7 #include <string.h>
8 #include <unistd.h>
9 #include <sys/socket.h>
10 #include <arpa/inet.h>
11 #include <netinet/in.h>
12 #include <netinet/if_ether.h>
13 #include <net/if_arp.h>
14 #include <net/ethernet.h>
15
16 /* 以太网帧首部长度 */
17 #define ETHER_HEADER_LEN sizeof(struct ether_header)
18 /* 整个arp结构长度 */
19 #define ETHER_ARP_LEN sizeof(struct ether_arp)
20 /* 以太网 + 整个arp结构长度 */
21 #define ETHER_ARP_PACKET_LEN ETHER_HEADER_LEN + ETHER_ARP_LEN
22 /* IP地址长度 */
23 #define IP_ADDR_LEN 4
24
25 void err_exit(const char *err_msg)
26 {
27 perror(err_msg);
28 exit(1);
29 }
30
31 int main(void)
32 {
33 struct ether_arp *arp_packet;
34 char buf[ETHER_ARP_PACKET_LEN];
35 int sock_raw_fd, ret_len, i;
36
37 if ((sock_raw_fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ARP))) == -1)
38 err_exit("socket()");
39
40 while (1)
41 {
42 bzero(buf, ETHER_ARP_PACKET_LEN);
43 ret_len = recv(sock_raw_fd, buf, ETHER_ARP_PACKET_LEN, 0);
44 if (ret_len > 0)
45 {
46 /* 剥去以太头部 */
47 arp_packet = (struct ether_arp *)(buf + ETHER_HEADER_LEN);
48 /* arp操作码为2代表arp应答 */
49 if (ntohs(arp_packet->arp_op) == 2)
50 {
51 printf("==========================arp replay======================\n");
52 printf("from ip:");
53 for (i = 0; i < IP_ADDR_LEN; i++)
54 printf(".%u", arp_packet->arp_spa[i]);
55 printf("\nfrom mac");
56 for (i = 0; i < ETH_ALEN; i++)
57 printf(":%02x", arp_packet->arp_sha[i]);
58 printf("\n");
59 }
60 }
61 }
62
63 close(sock_raw_fd);
64 return 0;
65 }
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。