Sniffing an ethernet interface on linux - c++

Question

I am trying to capture only packets from a specific interface and instead I am getting packets from all of the interfaces. what am I doing wrong?
bool Snooper::raw_init (const char *device)
{
uid_t privid = geteuid();
int retval;
bool retVal = false;
do {
if ((retval = setuid(0)) != 0) {
perror("seteuid error");
break;
}
cap_t caps = cap_get_proc();
cap_value_t cap_list[2];
cap_list[0] = CAP_NET_RAW;
cap_list[1] = CAP_SETUID;
if ((retval = cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_list, CAP_SET)) == -1) {
perror("cap_set_flag error");
break;
}
if ((retval = cap_set_proc(caps)) == -1) {
perror("cap_set_proc error");
break;
}
struct ifreq ifr;
memset(&ifr, 0, sizeof (struct ifreq));
/* Open A Raw Socket */
if ((m_sockfd = socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL))) < 1) {
perror("Snooper::raw_init:socket Error");
break;
}
/* Set the device to use */
strncpy(ifr.ifr_name, device, strlen(device) + 1);
/* Get the current flags that the device might have */
if (ioctl(m_sockfd, SIOCGIFFLAGS, &ifr) == -1) {
perror("Error: Could not retrieve the flags from the device.\n");
break;
}
printf("The interface is ::: %s\n", device);
perror("Retrieved flags from interface successfully");
/* Set the old flags plus the IFF_PROMISC flag */
ifr.ifr_flags |= IFF_PROMISC;
if (ioctl(m_sockfd, SIOCSIFFLAGS, &ifr) == -1) {
perror("Error: Could not set flag IFF_PROMISC");
break;
}
printf("Setting interface ::: %s ::: to promisc\n", device);
/* Configure the device */
if (ioctl(m_sockfd, SIOCGIFINDEX, &ifr) < 0) {
perror("Error: Error getting the device index.\n");
break;
}
retVal = true;
} while(false);
if ((retval = seteuid(privid)) != 0) {
perror("seteuid error");
}
return retVal;
}
I first validate that I can suid to root since IFF_PROMISC requires it. Then create the socket for UDP traffic, preform the IOCtl for the device, and finally IOCtl for PROMISC.
Now that I have a socket ready I loop on a recv, however I get packets from the other interfaces as well.

Answer 1

To capture packets from a specific interface, you have to bind your socket to that interface using bind function. You can check out this answer for an example.

Answer 2

A small pcap Program that might be able to help You
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<pcap/pcap.h>
#include<netinet/if_ether.h>
#include<netinet/ip.h>
#include<netinet/tcp.h>
void process_packet(u_char *args, const struct pcap_pkthdr *header,
const u_char *buffer)
{
// printf("In Process_Packet\n");
struct ethhdr *eth = (struct ethhdr *)(buffer);
printf("%.2x: %.2x: %.2x: %.2x: %.2x: %.2x: %.2x:\n ", eth->h_dest[0], eth->h_dest[1], eth->h_dest[2], eth->h_dest[3], eth->h_dest[4], eth->h_dest[5], eth->h_dest[6]);
printf("%x \n", htons(eth->h_proto));
if(htons(eth->h_proto)== ETHERTYPE_ARP)
{
printf("ARP PACKET\n");
}
struct iphdr *iph = (struct iphdr*)(buffer + sizeof(struct ethhdr));
int ipheader = iph-> ihl *4;
printf("Source IP Address :%s\n ", inet_ntoa(iph->saddr));
printf("Destination IP Address :%s\n ", inet_ntoa(iph->daddr));
}
int main()
{
pcap_if_t *alldevspec,*devices;
pcap_addr_t *a;
pcap_t *handle;
const char filter_exp[]="IP";
bpf_u_int32 netp;
char errbuf[PCAP_ERRBUF_SIZE];
struct bpf_program fp;
int ret=0, count =1, n=0;
char devs[100][100],*devicename;
ret = pcap_findalldevs(&alldevspec,errbuf);
if(ret < 0)
{
printf("Error in finding the devices\n");
return -1;
}
for(devices = alldevspec; devices!= NULL; devices = devices->next)
{
printf("%d %s-%s \n",count, devices->name,devices->description);
for(a=devices->addresses;a;a=a->next)
{
printf("family %d \n", (a->addr)->sa_family);
if(devices->name != NULL)
{
strcpy(devs[count], devices->name);
}
switch((a->addr)->sa_family)
{
case AF_INET:
printf("%s \n",inet_ntoa(((struct sockaddr_in*)a->addr)->sin_addr.s_addr));
break;
case AF_INET6:
break;
}
}
++count;
}
printf("Enter the device u want to select\n");
scanf("%d",&n);
devicename = devs[n];
handle = pcap_open_live(devicename,65536,1,-1,errbuf);
if(handle == NULL)
{
printf("Error in opening the device\n");
return -1;
}
pcap_compile(handle,&fp, filter_exp,-1,netp);
pcap_setfilter(handle, &fp);
pcap_loop(handle,-1,process_packet,NULL);
return 0;
}