Capturing beacon frames in 802.11 - wifi

Question

How can I capture 802.11 beacon frames on my linux machine using a C program.Also how can I send a frame response using a C program?

Answer 1

Try using libpcap. See this answer on SO for a basic example.

Answer 2

below program only work :-
1. if your device support link layer type as DLT_IEEE802_11_RADIO
2. if you ahev libpacp installed in your machine.
3. compile it with libcap library( ex:- cc prog.c -lpcap
4. during run provide interface on command line ( ex:- a.out wlan0)
#include <pcap.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
void packet_view(unsigned char *args,const struct pcap_pkthdr *h,const unsigned char *p);
#define SNAP_LEN 3000
void packet_view(
unsigned char *args,
const struct pcap_pkthdr *h,
const unsigned char *p
){
int len;
len = 0;
printf("PACKET\n");
while(len < h->len) {
printf("%02x ", *(p++));
if(!(++len % 16))
printf("\n");
}
printf("\n");
return ;
}
int main(int argc, char **argv)
{
char *dev = NULL; /* capture device name */
char errbuf[PCAP_ERRBUF_SIZE]; /* error buffer */
pcap_t *handle; /* packet capture handle */
char filter_exp[] = "wlan type mgt subtype beacon"; /* filter expression [3] */
// char filter_exp[] = "ip"; /* filter expression [3] */
struct bpf_program fp; /* compiled filter program (expression) */
bpf_u_int32 mask; /* subnet mask */
bpf_u_int32 net; /* ip */
int num_packets = 10; /* number of packets to capture */
/* check for capture device name on command-line */
if (argc == 2) {
dev = argv[1];
}
else if (argc > 2) {
fprintf(stderr, "error: unrecognized command-line options\n\n");
exit(EXIT_FAILURE);
}
else {
/* find a capture device if not specified on command-line */
dev = pcap_lookupdev(errbuf);
if (dev == NULL) {
fprintf(stderr, "Couldn't find default device: %s\n",
errbuf);
exit(EXIT_FAILURE);
}
}
/* get network number and mask associated with capture device */
if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
fprintf(stderr, "Couldn't get netmask for device %s: %s\n",
dev, errbuf);
net = 0;
mask = 0;
}
/* print capture info */
printf("Device: %s\n", dev);
printf("Number of packets: %d\n", num_packets);
printf("Filter expression: %s\n", filter_exp);
/* open capture device */
handle = pcap_open_live(dev, SNAP_LEN, 1, -1, errbuf);
if (handle == NULL) {
fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
exit(EXIT_FAILURE);
}
/* for Ethernet device change the type to DLT_EN10MB */
/* your wlan device need to supprot this link layer type otherwise failure */
if (pcap_datalink(handle) != DLT_IEEE802_11_RADIO) {
fprintf(stderr, "%s is not an Wlan packet\n", dev);
exit(EXIT_FAILURE);
}
/* compile the filter expression */
if (pcap_compile(handle, &fp, filter_exp, 1,PCAP_NETMASK_UNKNOWN) == -1) {
fprintf(stderr, "Couldn't parse filter %s: %s\n",
filter_exp, pcap_geterr(handle));
exit(EXIT_FAILURE);
}
/* apply the compiled filter */
if (pcap_setfilter(handle, &fp) == -1) {
fprintf(stderr, "Couldn't install filter %s: %s\n %s: %s\n",
filter_exp, pcap_geterr(handle));
exit(EXIT_FAILURE);
}
/* now we can set our callback function */
pcap_loop(handle, num_packets, packet_view, NULL);
/* cleanup */
pcap_freecode(&fp);
pcap_close(handle);
printf("\nCapture complete.\n");
return 0;
}