自己动手学TCP/IP--以太网帧

数据链路层主要的功能

• 把网络层的包封装成帧
• 对帧的校验和确认
• 流量的控制

数据链路层生成以太网帧，以太网帧的主要格式如下。

太网帧的传输大小是由传输媒介决定的，数据帧的大小是64—1518。帧头14个字节，分别是目标MAC地址

（6个字节）+源MAC地址（6个字节）+协议类型（2个字节），帧尾是32位的CRC冗余校验。对于两个字

节协议字段，0×0800主要用于IP协议，还有一些其他的网络协议，比如Novell IPX等。

这里先介绍以太网帧头格式，用libcap抓出来的包最后的CRC冗余校验已经在物理网卡上完成。下面是代码

#include <pcap.h>#include <stdio.h>#include <stdlib.h> #include <string.h>#include <sys/socket.h>#include <arpa/inet.h>#include <net/ethernet.h>#include <netinet/ip_icmp.h>//Provides declarations for icmp header#include <netinet/udp.h>//Provides declarations for udp header#include <netinet/tcp.h>//Provides declarations for tcp header#include <netinet/ip.h>//Provides declarations for ip header#define BUFFER_SIZE 2048#define HIGH4(a) (((a&0xf0))>>4)#define LOW4(a) (a&0x0f)int size ;void process_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *buffer);void print_mac(const u_char *packet_buffer);void mac_to_str(char *szMac, unsigned char mac[6]);int main(int argc,char *argv[]){pcap_if_t *alldevsp , *device;pcap_t *handle; //Handle of the device that shall be sniffedchar errbuf[100] , *devname , devs[100][100];int count = 1 , n;//First get the list of available devicesprintf("Finding available devices ... ");if( pcap_findalldevs( &alldevsp , errbuf) ){printf("Error finding devices : %s" , errbuf);exit(1);}printf("Done");//Print the available devicesprintf("\nAvailable Devices are :\n");for(device = alldevsp ; device != NULL ; device = device->next){printf("%d. %s - %s\n" , count , device->name , device->description);if(device->name != NULL){strcpy(devs[count] , device->name);}count++;}//Ask user which device to sniffprintf("Enter the number of the device you want to sniff : ");scanf("%d" , &n);devname = devs[n];//Open the device for sniffingprintf("Opening device %s for sniffing ... " , devname);handle = pcap_open_live(devname , 65536 , 1 , 0 , errbuf);if (handle == NULL){fprintf(stderr, "Couldn't open device %s : %s\n" , devname , errbuf);exit(1);}printf("Done\n");//Put the device in sniff looppcap_loop(handle , -1 , process_packet , NULL);return 0;}void process_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *buffer){size = header->len;print_mac(buffer);}void print_mac(const u_char *packet_buffer){struct ethhdr *ethernet_header = NULL;//ether headerchar sour_mac[40] = {'\0'};//<linux/if_ether.h>char des_mac[40] = {'\0'};//#define ETH_ALEN   6ethernet_header = (struct ethhdr *)packet_buffer;mac_to_str(sour_mac,ethernet_header->h_source);mac_to_str(des_mac,ethernet_header->h_dest);printf("length of ethernet_head : %d \n",sizeof(struct ethhdr));printf("des_MAC : %s\n",des_mac);printf("sour_MAC : %s\n",sour_mac);printf("Protocal : %x\n",ntohs(ethernet_header->h_proto));}//MAC地址翻译void mac_to_str(char *szMac, unsigned char mac[6]){sprintf(szMac, "%X%X:%X%X:%X%X:%X%X:%X%X:%X%X", HIGH4(mac[0]), LOW4(mac[0]), HIGH4(mac[1]), LOW4(mac[1]), HIGH4(mac[2]), LOW4(mac[2]), HIGH4(mac[3]), LOW4(mac[3]), HIGH4(mac[4]), LOW4(mac[4]), HIGH4(mac[5]), LOW4(mac[5]));}

$ gcc get_mac.c -o get_mac -lpcap