#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <pcap.h>
#include <net/ethernet.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>

/*
 * To compile :
 *
 * gcc -o extract extract.c -lpcap
 *
 */

u_int baseseq;
int output;

void handle_packet(u_char *args, const struct pcap_pkthdr* header,
		const u_char* packet) 
{
struct ether_header *ethernet_frame;
struct ip *ip_packet;
struct tcphdr *tcp_packet;
u_int ip_header_len, ip_len, tcp_header_len, len;
off_t position;

	ethernet_frame = (struct ether_header *)packet;
	if(ntohs(ethernet_frame->ether_type) == ETHERTYPE_IP)
	{
		packet += sizeof(struct ether_header);
		ip_packet = (struct ip*)(packet);
		ip_header_len = ip_packet->ip_hl * 4;
		ip_len = ntohs(ip_packet->ip_len);

		if (ip_packet->ip_p == IPPROTO_TCP) {
			packet += ip_header_len;
			tcp_packet=(struct tcphdr *)(packet);
			tcp_header_len = tcp_packet->doff * 4;

			packet += tcp_header_len;


			if(baseseq==0) {
				baseseq=ntohl(tcp_packet->seq);
			}
			len = ip_len - ip_header_len - tcp_header_len;
			if(len>0) {
				position=(ntohl(tcp_packet->seq)-baseseq)-1;
				lseek(output,position,SEEK_SET);
				write(output,packet,len);
			}
		}
	}

}

void die(char *msg, int code) {
	fprintf(stderr,"%s\n",msg); exit(code); 
}

#define BUFSIZE 1024

int main(int argc, char **argv)
{
char errbuf[PCAP_ERRBUF_SIZE];
pcap_t *descr;
struct bpf_program filter;
bpf_u_int32 netp;
u_char* args = NULL;
int input,nbread;
char filename[16],buf[BUFSIZE+1];

	baseseq=0;

	if( ( descr = pcap_open_offline("-",errbuf) ) == NULL) {
      fprintf(stderr,"Error in pcap_open_offline :\n%s\n",errbuf);
      exit(1);
    }

	if(pcap_compile(descr,&filter,argv[1],0,netp) == -1) {
    	die("Error in pcap_compile",1);
	}

	if(pcap_setfilter(descr,&filter) == -1) {
		die("Error in set_filtern",1);
	}

	strcpy(filename,"/tmp/edaXXXXXX");
	mktemp(filename);
	if( (output=creat(filename,0600)) == -1) {
		die("Failed to create file",1);
	}

	pcap_loop(descr,-1,handle_packet,args);

	close(output);
	
	if( (input=open(filename,O_RDONLY)) == -1) {
		die("Failed to open file",1);
	}
	while((nbread=read(input,buf,BUFSIZE))>0)
	{
		write(1,buf,nbread);
	}
	
	unlink(filename);

	return(0);
}