Hello
I'm David, a PhD student
I'm developing a small sniffer for my project. I'm using libpcap
I built a sniffer for capturing ethernet packets on the cable and it is
working fine.
Now, I would like to use this sniffer for capturing 802.11 WLAN packets.
When I use this sniffer for capturing 802.11 WLAN packets on my adapter, it
looks ok, but when I try to get the MAC and IP addresses, they are wrong.
I supposse that I should eliminate first the WLAN envelopment or something
like tath
I would like to know if you can give a clue or if you have some example
code.
Here you have my code:
#include <pcap.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/if_ether.h>
#include <net/ethernet.h>
#include <netinet/ether.h>
#include <netinet/ip.h>
/* tcpdump header (ether.h) defines ETHER_HDRLEN) */
#ifndef ETHER_HDRLEN
#define ETHER_HDRLEN 14
#endif
/*
* Structure of an internet header, naked of options.
*
* Stolen from tcpdump source (thanks tcpdump people)
*
* We declare ip_len and ip_off to be short, rather than u_short
* pragmatically since otherwise unsigned comparisons can result
* against negative integers quite easily, and fail in subtle ways.
*/
struct my_ip {
u_int8_t ip_vhl; /* header length, version */
#define IP_V(ip) (((ip)->ip_vhl & 0xf0) >> 4)
#define IP_HL(ip) ((ip)->ip_vhl & 0x0f)
u_int8_t ip_tos; /* type of service */
u_int16_t ip_len; /* total length */
u_int16_t ip_id; /* identification */
u_int16_t ip_off; /* fragment offset field */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_int8_t ip_ttl; /* time to live */
u_int8_t ip_p; /* protocol */
u_int16_t ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */
};
main()
{
pcap_if_t *alldevs;
pcap_if_t *d;
pcap_t *p;
pcap_t *adhandle;
int i=0;
int inum;
int res;
int datalink;
char errbuf[PCAP_ERRBUF_SIZE];
char timestr[16];
struct tm *ltime;
struct pcap_pkthdr *header;
struct ether_header *eptr;
const struct my_ip* ip;
u_char *pkt_data;
u_short ether_type;
/* Retrieve the device list */
if (pcap_findalldevs(&alldevs, errbuf) == -1)
{
fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
exit(1);
}
/* Print the list */
for(d=alldevs; d; d=d->next)
{
printf("%d. %s", ++i, d->name);
if (d->description)
printf(" (%s)\n", d->description);
else
printf(" (No description available) \n");
}
if(i==0)
{
printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
return -1;
}
printf("Enter the interface number (1-%d):",i);
scanf("%d", &inum);
if(inum < 1 || inum > i)
{
printf("\nInterface number out of range.\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}
/* Jump to the selected adapter */
for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);
/* Open the adapter */
if ( (adhandle= pcap_open_live(d->name, // name of the device
65536, // portion of the packet to capture.
// 65536 grants that the whole packet will be captured on all the
MACs.
1, // promiscuous mode
1000, // read timeout
errbuf // error buffer
) ) == NULL)
{
fprintf(stderr,"\nUnable to open the adapter. %s is not supported by
WinPcap\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}
printf("\nlistening on %s... \n", d->name);
if(datalink=pcap_datalink(adhandle)<0){
printf("\nDatalink Error: %s\n",errbuf);
}
printf("\nDatalink=%s\n\n", pcap_datalink_val_to_name(datalink));
/* At this point, we don't need any more the device list. Free it */
pcap_freealldevs(alldevs);
/* Retrieve the packets */
while((res = pcap_next_ex( adhandle, &header, &pkt_data)) >= 0){
if(res == 0)
/* Timeout elapsed */
continue;
/* convert the timestamp to readable format */
ltime=localtime(&header->ts.tv_sec);
strftime( timestr, sizeof timestr, "%H:%M:%S", ltime);
printf("%s,%.6d; len:%d; ", timestr, header-> ts.tv_usec,
header->len);
/////////////////////////////////////////// MAC
/* lets start with the ether header... */
eptr = (struct ether_header *) pkt_data;
ether_type = ntohs(eptr->ether_type);
/* check to see if we have an ip packet */
if (ether_type == ETHERTYPE_IP)
{
fprintf(stdout,"Type(IP); ");
}else if (ether_type == ETHERTYPE_ARP)
{
fprintf(stdout,"Type(ARP); ");
}else if (eptr->ether_type == ETHERTYPE_REVARP)
{
fprintf(stdout,"Type(RARP); ");
}else {
fprintf(stdout,"Type(?); ");
}
/* Lets print SOURCE DEST TYPE LENGTH */
fprintf(stdout,"MAC: ");
fprintf(stdout,"%s/"
,ether_ntoa((struct ether_addr*)eptr->ether_shost));
fprintf(stdout,"%s; "
,ether_ntoa((struct ether_addr*)eptr->ether_dhost));
/////////////////////////////////////////// IP
u_int length = header->len;
u_int hlen,off,version;
int len;
/* jump pass the ethernet header */
ip = (struct my_ip*)(pkt_data + sizeof(struct ether_header));
length -= sizeof(struct ether_header);
/* check to see we have a packet of valid length */
if (length < sizeof(struct my_ip))
{
printf("truncated ip %d",length);
return NULL;
}
len = ntohs(ip->ip_len);
hlen = IP_HL(ip); /* header length */
version = IP_V(ip);/* ip version */
/* Check to see if we have the first fragment */
off = ntohs(ip->ip_off);
if((off & 0x1fff) == 0 )/* aka no 1's in first 13 bits */
{/* print SOURCE DESTINATION hlen version len offset */
fprintf(stdout,"IP: ");
fprintf(stdout,"%s/",
inet_ntoa(ip->ip_src));
fprintf(stdout,"%s.\n",
inet_ntoa(ip->ip_dst),
hlen,version,len,off);
} /* check version */
if(version != 4)
{
fprintf(stdout,"Unknown version %d\n",version);
}
/* check header length */
if(hlen < 5 )
{
fprintf(stdout,"bad-hlen %d \n",hlen);
}
/* see if we have as much packet as we should */
if(length < len)
printf("truncated IP - %d bytes missing\n",len - length);
///////////////////////////////////////////
}
if(res == -1){
printf("Error reading the packets: %s\n", pcap_geterr(adhandle));
return -1;
}
return 0;
}
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