linux原始套接字(1)-arp请求与接收

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。