libcap programming

[Neminath]

i am writing a piece of code in c using libcap libraries to separate individual connection packets into a separate file.

i use pcap_dispatch(p, num_packets, got_packet, NULL); function and the handler function is void got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet, pcap_t *p) , but i am getting a warning: passing argument 3 of ‘pcap_dispatch’ from incompatible pointer type.

can somebody help me

[Rubberman]

i am writing a piece of code in c using libcap libraries to separate individual connection packets into a separate file.

i use pcap_dispatch(p, num_packets, got_packet, NULL); function and the handler function is void got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet, pcap_t *p) , but i am getting a warning: passing argument 3 of ‘pcap_dispatch’ from incompatible pointer type.

can somebody help me

I have libcap and libcap-devel packages installed in my system, but I cannot find the headers. Where are they located, do you know?

[Rubberman]

I have libcap and libcap-devel packages installed in my system, but I cannot find the headers. Where are they located, do you know?

I found a linux man page for this on the net, and the signature for pcap_dispatch is:

typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h, const u_char *bytes);
int pcap_dispatch(pcap_t *p, int cnt, pcap_handler callback, u_char *user);

The error you are getting says that the 3rd argument, the pcap_handler callback function pointer, is incorrect. You need to check the signature of that function. From what I see, you have added an additional argument on your handler, pcap_t *p. That's probably why it is giving this error.

[Neminath]

But got_packet is a function which i have definition

i am posting my complete code here

yes without that argument it works fine

but i need that 4th argument in got_packet function for some additional processing

basically i want to dump that packet to a file

#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>


/* default snap length (maximum bytes per packet to capture) */
#define SNAP_LEN 65535

/* ethernet headers are always exactly 14 bytes [1] */
#define SIZE_ETHERNET 14

/* Ethernet addresses are 6 bytes */
#define ETHER_ADDR_LEN 6

/* Ethernet header */
struct sniff_ethernet {
u_char ether_dhost[ETHER_ADDR_LEN]; /* destination host address */
u_char ether_shost[ETHER_ADDR_LEN]; /* source host address */
u_short ether_type; /* IP? ARP? RARP? etc */
};

/* IP header */
struct sniff_ip {
u_char ip_vhl; /* version << 4 | header length >> 2 */
u_char ip_tos; /* type of service */
u_short ip_len; /* total length */
u_short ip_id; /* identification */
u_short ip_off; /* fragment offset field */
#define IP_RF 0x8000 /* reserved fragment flag */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_char ip_ttl; /* time to live */
u_char ip_p; /* protocol */
u_short ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */

};

struct sniff_icmp
{
u_char icmp_type;
u_char icmp_code;
u_short icmp_checksum;
};
#define IP_HL(ip) (((ip)->ip_vhl) & 0x0f)
#define IP_V(ip) (((ip)->ip_vhl) >> 4)
#define IP_Flag(ip) (((ip)->ip_off) & 0xD0)
#define IP_off(ip) (((ip)->ip_off) & 0x1F)
/* TCP header */
typedef u_int tcp_seq;

struct sniff_tcp {
u_short th_sport; /* source port */
u_short th_dport; /* destination port */
tcp_seq th_seq; /* sequence number */
tcp_seq th_ack; /* acknowledgement number */
// tcp_seq th_syn;
// tcp_seq th_fin;
//tcp_seq th_urg;
u_char th_offx2; /* data offset, rsvd */
#define TH_OFF(th) (((th)->th_offx2 & 0xf0) >> 4)
u_char th_flags;
#define TH_FIN 0x01
#define TH_SYN 0x02
#define TH_RST 0x04
#define TH_PUSH 0x08
#define TH_ACK 0x10
#define TH_URG 0x20
#define TH_ECE 0x40
#define TH_CWR 0x80
#define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
u_short th_win; /* window */
u_short th_sum; /* checksum */
u_short th_urp; /* urgent pointer */
};
struct sniff_udp {
u_short udp_sport; /* source port */
u_short udp_dport; /* destination port */
u_short udp_hlen; /* Udp header length*/
u_short udp_chksum; /* Udp Checksum */
};

char ADDRESS_PAIR[100];char PAIR_ADDRESS[100];
char TCP_ACTIVE_ONE[100][100];
char TCP_ACTIVE_TWO[100][100];
char UDP_ACTIVE[100][100];
char SOURCE_IP[20];
char DEST_IP[20];
unsigned short SOURCE_PORT;
unsigned short DEST_PORT;
char buf[5];
char str[32];
int tcp_active_count=0;
int connet_count= 0;
void got_packet( const struct pcap_pkthdr *header, const u_char *packet,pcap_t * p );
char* convert_tostring( unsigned short val);


FILE* outfile;

void got_packet(const struct pcap_pkthdr *header, const u_char *packet,pcap_t * p)
{
const struct sniff_ethernet *ethernet; /* The ethernet header [1] */
const struct sniff_ip *ip; /* The IP header */
const struct sniff_tcp *tcp; /* The TCP header */
// const char *payload; /* Packet payload */
struct sniff_udp *udp; /* The Udp header */
struct sniff_icmp *icmp; /* The ICMP header*/
int size_ip;
int size_tcp;
int size_payload;
int size_udp;

/* define ethernet header */
ethernet = (struct sniff_ethernet*)(packet);
//printf("\nethernet header starts at %d",ethernet);
/* define/compute ip header offset */
ip = (struct sniff_ip*)(packet + SIZE_ETHERNET);
//printf("\nip header starts at %d",ip);
// printf("\ni m here %d",IP_HL(ip));
size_ip = IP_HL(ip)*4;
/*if (size_ip < 20) {
printf(" * Invalid IP header length: %u bytes\n", size_ip);

}*/
/* print source and destination IP addresses */
//printf("source address %s \n", inet_ntoa(ip->ip_src));
//printf("dest address %s \n", inet_ntoa(ip->ip_dst));
//printf("dest IP starts at %d",ip->ip_src);
strcpy(SOURCE_IP , inet_ntoa(ip->ip_src));
strcpy(DEST_IP ,inet_ntoa(ip->ip_dst));
//printf("\n%d\n",ip->ip_p);
switch(ip->ip_p)
{
case 6: //printf("i m here %d",size_ip);
tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip);
// printf("\n %d\n",(packet ));
// printf("\n %d\n",(packet + SIZE_ETHERNET + size_ip));
size_tcp = TH_OFF(tcp)*4;
if (size_tcp < 20)
{
printf(" * Invalid TCP header length: %u bytes\n", size_tcp);
return;
}
SOURCE_PORT = ntohs(tcp->th_sport);
// printf("source port is %d\n",ntohs(tcp->th_sport));
DEST_PORT = ntohs(tcp->th_dport);
// printf("dest port is %d\n",ntohs(tcp->th_dport));
ADDRESS_PAIR[0] ='\0';PAIR_ADDRESS[0]='\0';
// printf("the sequence no is %u\n",tcp->th_seq);
strcpy(ADDRESS_PAIR,SOURCE_IP);
strcat(ADDRESS_PAIR,DEST_IP);
strcpy(PAIR_ADDRESS,DEST_IP);
strcat(PAIR_ADDRESS,SOURCE_IP);
convert_tostring(SOURCE_PORT);
strcat(ADDRESS_PAIR,str);
convert_tostring(DEST_PORT);
strcat(ADDRESS_PAIR,str);
strcat(PAIR_ADDRESS,str);
convert_tostring(SOURCE_PORT);
strcat(PAIR_ADDRESS,str);
printf("the address line for this packet is %s\n",ADDRESS_PAIR);
printf("the address line for this packet is %s\n",PAIR_ADDRESS);
if(((tcp->th_flags)&TH_SYN) == 2 )
{
int i;
for(i=0;i<tcp_active_count;i++)
{
if(strcmp(TCP_ACTIVE_ONE[i],ADDRESS_PAIR)==0 )
{
break;
}

}
// pcap_dumper_t * pc = pcap_dump_open(p, "outfile.tcpdump");
//printf("file opened for writing");

strcpy(TCP_ACTIVE_ONE[tcp_active_count],ADDRESS_PAIR);
strcpy(TCP_ACTIVE_TWO[tcp_active_count++],PAIR_ADDRESS);
//printf("\n%s",TCP_ACTIVE_ONE[tcp_active_count-1]);
//printf("\n%s",TCP_ACTIVE_TWO[tcp_active_count-1]);
printf("it is a SYN packet");

}
connet_count++;
/* int i;
for(i=0;i<tcp_active_count;i++)
{
if(strcmp(TCP_ACTIVE_ONE[i],ADDRESS_PAIR)==0 )
{

}
}*/

if(((tcp->th_flags)&TH_FIN) == 1 |((tcp->th_flags)&TH_RST) == 4 )
{
int i=0;
for(i=0;i<tcp_active_count;i++)
{
if(strcmp(TCP_ACTIVE_ONE[i],ADDRESS_PAIR)==0 | strcmp(TCP_ACTIVE_TWO[i],ADDRESS_PAIR)==0 )
{
printf("connection ending here");
printf("total number of packets in this connection %d", connet_count);
exit(0);
}
}
}

break;
/*case IPPROTO_UDP: udp = (struct sniff_udp*)(packet + SIZE_ETHERNET + size_ip);
SOURCE_PORT = udp->th_sport;
DEST_PORT = udp->th_dport;
break; */
default: break;
}

}
char* convert_tostring( unsigned short val)
{
char revertedstr[32];
int length=0;

while (val > 0)
{
int a = val % 10;
revertedstr[length++] = a | '0';
val /= 10;
}

length--;
int rev = 0;
while (length >= 0)
{
str[rev++] = revertedstr[length--];
}

str[rev] = '\0';

return str;
}

int main(int argc, char **argv)
{ long int num_packets=0;
char filename[] = "/home/neminath/Desktop/1.tcpdump";
FILE *file ;
file= fopen(filename,"r");
char *ebuf;
pcap_t * p = pcap_fopen_offline(file, ebuf);
num_packets = pcap_dispatch(p, num_packets, got_packet, NULL);
return 0;
}

[Neminath]

on my ubuntu box its here

/usr/include/pcap.h

[Rubberman]

The code is correct, but the last argument, "pcap_t* p", is unused as far as I can tell from reading the code. Remove it. FWIW, your compiler, if warnings are turned up, will complain about the unused argument; however, it first gave the error about the incorrect callback signature.

[Neminath]

yeah u r right there.

[Rubberman]

I determined that I didn't have the developer version of the library installed, which is why my system didn't have /usr/include/pcap.h - a situation which I quickly rectified, and verified that the current version on CentOS/RHEL has defined the callback signature as I indicated previously. You simply need to remove that last argument from the function.