Listing 1. Protocol Stack-Handling Sniffed Packets
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <linux/in.h>
#include <linux/if_ether.h>
int main(int argc, char **argv) {
int sock, n;
char buffer[2048];
unsigned char *iphead, *ethhead;
if ( (sock=socket(PF_PACKET, SOCK_RAW,
htons(ETH_P_IP)))<0) {
perror("socket");
exit(1);
}
while (1) {
printf("----------\n");
n = recvfrom(sock,buffer,2048,0,NULL,NULL);
printf("%d bytes read\n",n);
/* Check to see if the packet contains at least
* complete Ethernet (14), IP (20) and TCP/UDP
* (8) headers.
*/
if (n<42) {
perror("recvfrom():");
printf("Incomplete packet (errno is %d)\n",
errno);
close(sock);
exit(0);
}
ethhead = buffer;
printf("Source MAC address: "
"%02x:%02x:%02x:%02x:%02x:%02x\n",
ethhead[0],ethhead[1],ethhead[2],
ethhead[3],ethhead[4],ethhead[5]);
printf("Destination MAC address: "
"%02x:%02x:%02x:%02x:%02x:%02x\n",
ethhead[6],ethhead[7],ethhead[8],
ethhead[9],ethhead[10],ethhead[11]);
iphead = buffer+14; /* Skip Ethernet header */
if (*iphead==0x45) { /* Double check for IPv4
* and no options present */
printf("Source host %d.%d.%d.%d\n",
iphead[12],iphead[13],
iphead[14],iphead[15]);
printf("Dest host %d.%d.%d.%d\n",
iphead[16],iphead[17],
iphead[18],iphead[19]);
printf("Source,Dest ports %d,%d\n",
(iphead[20]<<8)+iphead[21],
(iphead[22]<<8)+iphead[23]);
printf("Layer-4 protocol %d\n",iphead[9]);
}
}
}