capturing user data using pcap in c - libpcap

Question

I wanted to send my own data size of 1024 bytes through a particular interface . Suppose we have two hosts , one is sending and another is receiving.
Receiving host is using pcap mechanism to receive the data from other host.As per my knowledge Pcap receives echo packets from interface.
Here i want my own data to be received. How can i achieve that ??? Im a beginner ,so please help me out how to deal with pcap.
Actually I want to receive all data into a host ,save it and later forward it to my actual destination.
Is it possible using pcap???
client:
import socket
import select
import sys
import time
import json
import os
import pickle
from time import sleep
c=1
while(c):
if os.path.exists('/home/mininet/save.txt'):
s = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)
count=5
while (count):
f1=open("/home/mininet/save.txt","r")
d=f1.read()
f1.close()
if d.strip():
d=int(d)
f=open("/home/mininet/test1.txt", "rb")
l = f.read(1024)
s.sendto(l,("10.0.0.1",9999))
count=count-1
time.sleep(d)
c=0
s.close()
This is the client part and in server the corresponding program to receive these data..
First both client and server was connected with each other . Then that link is broken and placed a host in between them to monitor the traffic.
Each data should reach to that newly created host then that host redirect that data to the server. I wanted to achieve this using pcap.

Answer 1

Here is an example:
/*
* Use pcap_open_live() to open a packet capture device.
* Use pcap_dump() to output the packet capture data in
* binary format to a file for processing later.
*/
#include <unistd.h>
#include <stdio.h>
#include <pcap.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define IFSZ 16
#define FLTRSZ 120
#define MAXHOSTSZ 256
#define PCAP_SAVEFILE "./pcap_savefile"
extern char *inet_ntoa();
int
usage(char *progname)
{
printf("Usage: %s <interface> [<savefile name>]\n", basename(progname));
exit(11);
}
int
main(int argc, char **argv)
{
pcap_t *p; /* packet capture descriptor */
struct pcap_stat ps; /* packet statistics */
pcap_dumper_t *pd; /* pointer to the dump file */
char ifname[IFSZ]; /* interface name (such as "en0") */
char filename[80]; /* name of savefile for dumping packet data */
char errbuf[PCAP_ERRBUF_SIZE]; /* buffer to hold error text */
char lhost[MAXHOSTSZ]; /* local host name */
char fltstr[FLTRSZ]; /* bpf filter string */
char prestr[80]; /* prefix string for errors from pcap_perror */
struct bpf_program prog; /* compiled bpf filter program */
int optimize = 1; /* passed to pcap_compile to do optimization */
int snaplen = 80; /* amount of data per packet */
int promisc = 0; /* do not change mode; if in promiscuous */
/* mode, stay in it, otherwise, do not */
int to_ms = 1000; /* timeout, in milliseconds */
int count = 20; /* number of packets to capture */
u_int32 net = 0; /* network IP address */
u_int32 mask = 0; /* network address mask */
char netstr[INET_ADDRSTRLEN]; /* dotted decimal form of address */
char maskstr[INET_ADDRSTRLEN]; /* dotted decimal form of net mask */
int linktype = 0; /* data link type */
int pcount = 0; /* number of packets actually read */
/*
* For this program, the interface name must be passed to it on the
* command line. The savefile name may be optionally passed in
* as well. If no savefile name is passed in, "./pcap_savefile" is
* used. If there are no arguments, the program has been invoked
* incorrectly.
*/
if (argc < 2)
usage(argv[0]);
if (strlen(argv[1]) > IFSZ) {
fprintf(stderr, "Invalid interface name.\n");
exit(1);
}
strcpy(ifname, argv[1]);
/*
* If there is a second argument (the name of the savefile), save it in
* filename. Otherwise, use the default name.
*/
if (argc >= 3)
strcpy(filename,argv[2]);
else
strcpy(filename, PCAP_SAVEFILE);
/*
* Open the network device for packet capture. This must be called
* before any packets can be captured on the network device.
*/
if (!(p = pcap_open_live(ifname, snaplen, promisc, to_ms, errbuf))) {
fprintf(stderr, "Error opening interface %s: %s\n",
ifname, errbuf);
exit(2);
}
/*
* Look up the network address and subnet mask for the network device
* returned by pcap_lookupdev(). The network mask will be used later
* in the call to pcap_compile().
*/
if (pcap_lookupnet(ifname, &net, &mask, errbuf) < 0) {
fprintf(stderr, "Error looking up network: %s\n", errbuf);
exit(3);
}
/*
* Create the filter and store it in the string called 'fltstr.'
* Here, you want only incoming packets (destined for this host),
* which use port 69 (tftp), and originate from a host on the
* local network.
*/
/* First, get the hostname of the local system */
if (gethostname(lhost,sizeof(lhost)) < 0) {
fprintf(stderr, "Error getting hostname.\n");
exit(4);
}
/*
* Second, get the dotted decimal representation of the network address
* and netmask. These will be used as part of the filter string.
*/
inet_ntop(AF_INET, (char*) &net, netstr, sizeof netstr);
inet_ntop(AF_INET, (char*) &mask, maskstr, sizeof maskstr);
/* Next, put the filter expression into the fltstr string. */
sprintf(fltstr,"dst host %s and src net %s mask %s and udp port 69",
lhost, netstr, maskstr);
/*
* Compile the filter. The filter will be converted from a text
* string to a bpf program that can be used by the Berkely Packet
* Filtering mechanism. The fourth argument, optimize, is set to 1 so
* the resulting bpf program, prog, is compiled for better performance.
*/
if (pcap_compile(p,&prog,fltstr,optimize,mask) < 0) {
/*
* Print out appropriate text, followed by the error message
* generated by the packet capture library.
*/
fprintf(stderr, "Error compiling bpf filter on %s: %s\n",
ifname, pcap_geterr(p));
exit(5);
}
/*
* Load the compiled filter program into the packet capture device.
* This causes the capture of the packets defined by the filter
* program, prog, to begin.
*/
if (pcap_setfilter(p, &prog) < 0) {
/* Copy appropriate error text to prefix string, prestr */
sprintf(prestr, "Error installing bpf filter on interface %s",
ifname);
/*
* Print error to screen. The format will be the prefix string,
* created above, followed by the error message that the packet
* capture library generates.
*/
pcap_perror(p,prestr);
exit(6);
}
/*
* Open dump device for writing packet capture data. In this sample,
* the data will be written to a savefile. The name of the file is
* passed in as the filename string.
*/
if ((pd = pcap_dump_open(p,filename)) == NULL) {
/*
* Print out error message if pcap_dump_open failed. This will
* be the below message followed by the pcap library error text,
* obtained by pcap_geterr().
*/
fprintf(stderr,
"Error opening savefile \"%s\" for writing: %s\n",
filename, pcap_geterr(p));
exit(7);
}
/*
* Call pcap_dispatch() to read and process a maximum of count (20)
* packets. For each captured packet (a packet that matches the filter
* specified to pcap_compile()), pcap_dump() will be called to write
* the packet capture data (in binary format) to the savefile specified
* to pcap_dump_open(). Note that packet in this case may not be a
* complete packet. The amount of data captured per packet is
* determined by the snaplen variable which is passed to
* pcap_open_live().
*/
if ((pcount = pcap_dispatch(p, count, &pcap_dump, (char *)pd)) < 0) {
/*
* Print out appropriate text, followed by the error message
* generated by the packet capture library.
*/
sprintf(prestr,"Error reading packets from interface %s",
ifname);
pcap_perror(p,prestr);
exit(8);
}
printf("Packets received and successfully passed through filter: %d.\n",
pcount);
/*
* Get and print the link layer type for the packet capture device,
* which is the network device selected for packet capture.
*/
if (!(linktype = pcap_datalink(p))) {
fprintf(stderr,
"Error getting link layer type for interface %s",
ifname);
exit(9);
}
printf("The link layer type for packet capture device %s is: %d.\n",
ifname, linktype);
/*
* Get the packet capture statistics associated with this packet
* capture device. The values represent packet statistics from the time
* pcap_open_live() was called up until this call.
*/
if (pcap_stats(p, &ps) != 0) {
fprintf(stderr, "Error getting Packet Capture stats: %s\n",
pcap_geterr(p));
exit(10);
}
/* Print the statistics out */
printf("Packet Capture Statistics:\n");
printf("%d packets received by filter\n", ps.ps_recv);
printf("%d packets dropped by kernel\n", ps.ps_drop);
/*
* Close the savefile opened in pcap_dump_open().
*/
pcap_dump_close(pd);
/*
* Close the packet capture device and free the memory used by the
* packet capture descriptor.
*/
pcap_close(p);
}