/* Validates a packet's reported FCS value */
static int check_fcs(const unsigned char *packet, size_t len)
{
if(!has_rt_header()) return 1;
uint32_t offset = 0, match = 0;
uint32_t fcs = 0, fcs_calc = 0;
struct radio_tap_header *rt_header = NULL;
if(len > 4)
{
/* FCS is not calculated over the radio tap header */
if(len >= sizeof(*rt_header))
{
uint32_t presentflags, flags;
if(!rt_get_presentflags(packet, len, &presentflags, &offset))
goto skip;
if(!(presentflags & (1U << IEEE80211_RADIOTAP_FLAGS)))
goto skip;
offset = rt_get_flag_offset(presentflags, IEEE80211_RADIOTAP_FLAGS, offset);
if(offset < len) {
memcpy(&flags, packet + offset, 4);
flags = end_le32toh(flags);
if(flags & IEEE80211_RADIOTAP_F_BADFCS)
return 0;
if(!(flags & IEEE80211_RADIOTAP_F_FCS))
return 1;
}
skip:
rt_header = (struct radio_tap_header *) packet;
offset = end_le16toh(rt_header->len);
}
/* Get the packet's reported FCS (last 4 bytes of the packet) */
fcs = end_le32toh(*(uint32_t*)(packet + (len-4)));
if(len > offset)
{
/* FCS is the inverse of the CRC32 checksum of the data packet minus the frame's FCS and radio tap header (if any) */
fcs_calc = ~crc32((char *) packet+offset, (len-offset-4));
if(fcs_calc == fcs)
{
match = 1;
}
}
}
return match;
}
/*
* Returns a pointer to the radio tap header. If there is no radio tap header,
* it returns a pointer to a dummy radio tap header.
*/
const u_char *radio_header(const u_char *packet, size_t len)
{
if(has_rt_header())
{
return packet;
}
else
{
return (u_char *) FAKE_RADIO_TAP_HEADER;
}
}
/*
* Returns a pointer to the radio tap header. If there is no radio tap header,
* it returns a pointer to a dummy radio tap header.
*/
unsigned char *radio_header(const unsigned char *packet, size_t len)
{
if(has_rt_header())
{
return (void*)packet;
}
else
{
return FAKE_RADIO_TAP_HEADER;
}
}
/* Extracts the signal strength field (if any) from the packet's radio tap header */
int8_t signal_strength(const u_char *packet, size_t len)
{
int8_t ssi = 0;
int offset = sizeof(struct radio_tap_header);
struct radio_tap_header *header = NULL;
if(has_rt_header() && (len > (sizeof(struct radio_tap_header) + TSFT_SIZE + FLAGS_SIZE + RATE_SIZE + CHANNEL_SIZE + FHSS_FLAG)))
{
header = (struct radio_tap_header *) packet;
if((header->flags & SSI_FLAG) == SSI_FLAG)
{
if((header->flags & TSFT_FLAG) == TSFT_FLAG)
{
offset += TSFT_SIZE;
}
if((header->flags & FLAGS_FLAG) == FLAGS_FLAG)
{
offset += FLAGS_SIZE;
}
if((header->flags & RATE_FLAG) == RATE_FLAG)
{
offset += RATE_SIZE;
}
if((header->flags & CHANNEL_FLAG) == CHANNEL_FLAG)
{
offset += CHANNEL_SIZE;
}
if((header->flags & FHSS_FLAG) == FHSS_FLAG)
{
offset += FHSS_FLAG;
} else {
offset += 12;
}
if(offset < len)
{
ssi = (int8_t) packet[offset];
}
if (ssi > 100) {
ssi = 100 - ssi;
}
}
}
return ssi;
}
/* Extracts the signal strength field (if any) from the packet's radio tap header */
int8_t signal_strength(const unsigned char *packet, size_t len)
{
if(has_rt_header() && (len > (sizeof(struct radio_tap_header))))
{
uint32_t offset, presentflags;
if(!rt_get_presentflags(packet, len, &presentflags, &offset))
return 0;
if(!(presentflags & (1U << IEEE80211_RADIOTAP_DBM_ANTSIGNAL)))
return 0;
offset = rt_get_flag_offset(presentflags, IEEE80211_RADIOTAP_DBM_ANTSIGNAL, offset);
if (offset < len)
return (int8_t) packet[offset];
}
return 0;
}
/* Validates a packet's reported FCS value */
int check_fcs(const u_char *packet, size_t len)
{
int offset = 0, match = 0;
uint32_t fcs = 0, fcs_calc = 0;
struct radio_tap_header *rt_header = NULL;
if(len > 4)
{
/* Get the packet's reported FCS (last 4 bytes of the packet) */
memcpy((uint32_t *) &fcs, (packet + (len-4)), 4);
/* FCS is not calculated over the radio tap header */
if(has_rt_header())
{
rt_header = (struct radio_tap_header *) packet;
#ifdef __APPLE__
unsigned char *body = (unsigned char*) (rt_header+1);
uint32_t present = rt_header->flags;
uint8_t rflags = 0;
int i;
for (i = IEEE80211_RADIOTAP_TSFT; i <= IEEE80211_RADIOTAP_EXT; i++) {
if (!(present & (1 << i))) continue;
switch (i) {
case IEEE80211_RADIOTAP_TSFT:
body += sizeof(uint64_t);
break;
case IEEE80211_RADIOTAP_FLAGS:
rflags = *((uint8_t*)body);
/* fall through */
case IEEE80211_RADIOTAP_RATE:
body += sizeof(uint8_t);
break;
case IEEE80211_RADIOTAP_CHANNEL:
body += sizeof(uint16_t)*2;
break;
case IEEE80211_RADIOTAP_RX_FLAGS:
case IEEE80211_RADIOTAP_FHSS:
body += sizeof(uint16_t);
break;
case IEEE80211_RADIOTAP_DB_ANTSIGNAL:
case IEEE80211_RADIOTAP_DBM_ANTNOISE:
case IEEE80211_RADIOTAP_DBM_ANTSIGNAL:
case IEEE80211_RADIOTAP_DB_ANTNOISE:
case IEEE80211_RADIOTAP_ANTENNA:
body++;
break;
case 18: // IEEE80211_RADIOTAP_XCHANNEL
body += sizeof(uint32_t);
body += sizeof(uint16_t);
body += sizeof(uint8_t);
body += sizeof(uint8_t);
break;
case 19: // IEEE80211_RADIOTAP_MCS
body += 3*sizeof(uint8_t);
break;
default:
i = IEEE80211_RADIOTAP_EXT+1;
break;
}
}
#define IEEE80211_RADIOTAP_F_BADFCS 0x40
if (rflags & IEEE80211_RADIOTAP_F_BADFCS) {
// bad FCS, ignore
return 0;
}
if (!(rflags & IEEE80211_RADIOTAP_F_FCS)) {
// fcs not always present
return 1;
}
#endif
offset += rt_header->len;
}
if(len > offset)
{
/* FCS is the inverse of the CRC32 checksum of the data packet minus the frame's FCS and radio tap header (if any) */
fcs_calc = ~crc32((char *) packet+offset, (len-offset-4));
if(fcs_calc == fcs)
{
match = 1;
}
}
}
return match;
}
