Why pcap_set_rfmon fails each time? - c

Question

I'm working on a low level sniffer based on libpcap. Everything works perfectly but the assert with pcap_set_rfmon() fails each time. I don't think that I made any mistake to get this result. I post a little snippet of the main function. If you have some ideas about a solution, it would be helpful. I tried with various NICs and dongles but on none of them, the monitor mode can be set.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pcap.h>
#define JUMBO_FRAMES_MTU 9000
#define BIGGER_THAN_ALL_MTUS (64*1024)
#define ERR(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
int main(int argc, char **argv)
{
char errbuf[PCAP_ERRBUF_SIZE];
pcap_t *handle;
if (argc < 2){
ERR("usage : ./prog itf-name\n");
}
/* exiting if strlen of itf > 14 */
if (strlen(argv[1]) > IFNAMSIZ){
ERR("Interface name too long");
}
strncpy(opt_args->device, argv[1], strlen(argv[1]) + 1);
handle = pcap_create(opt_args->device, errbuf);
if (handle == NULL){
(void)fprintf(stderr, "FATAL ERROR : couldn't create sock handle : %s\n", errbuf);
goto fatal_error;
}
/* setting snaplen to 1500 */
assert(pcap_set_snaplen(handle, ETHERNET_MTU) == 0);
assert(pcap_setnonblock(handle, -1, errbuf) != -1);
/* if we can't put the device in monitor mode, so we display a warning
but keep doing the capture */
/* I have a warning here */
if (pcap_can_set_rfmon(handle) != 1){
(void)fprintf(stderr, "WARNING : device can't be set up in monitor mode : %s\n",
pcap_geterr(handle));
} else{
assert(pcap_set_rfmon(handle, 1) == 0);
}
/* we need now to launch the session capture */
if (pcap_activate(handle) < 0){
(void)fprintf(stderr, "FATAL ERROR : couldn't activate PCAP sock : %s\n",
pcap_geterr(handle));
goto fatal_error;
}
/* pcap loop ... */
pcap_close(handle);
return 0;
fatal_error;
pcap_close(handle);
return 1;
}