Вот есть программа, но...

Сорри, не форматировалась как нужно....


*** CUT HERE ***
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====
*** END CUT ***

/*
* This code is GPL.
*/
#include <linux/netfilter.h>
#include <libipq/libipq.h>
#include <stdio.h>

#define BUFSIZE 2048

//time in which the packet considered duplicate (plus 1 sec max.)
#define DROPTIME 1

unsigned long timetable[0x10000];

unsigned short crc16(unsigned char *buf, int len);

static void die(struct ipq_handle *h,char* msg)
{
   ipq_perror(msg);
   ipq_destroy_handle(h);
   exit(1);
}

int main(int argc, char **argv)
{
   int status;
   unsigned char buf[BUFSIZE];
   struct ipq_handle *h;
   unsigned short crc;
   unsigned long time;

   h = ipq_create_handle(0, PF_INET);
   if (!h)
	die(h,"ipq_create_handle");

   status = ipq_set_mode(h, IPQ_COPY_PACKET, BUFSIZE);
   if (status < 0)
	die(h,"ipq_set_mode");

   do{
	status = ipq_read(h, buf, BUFSIZE, 0);
	if (status < 0)
	     die(h,"ipq_read");

	switch (ipq_message_type(buf)) {
	     case NLMSG_ERROR:
		  fprintf(stderr, "Received error message %d\n",
			  ipq_get_msgerr(buf));
		  break;


	     case IPQM_PACKET: {
		  ipq_packet_msg_t *m = ipq_get_packet(buf);

		  crc = crc16(m->payload,m->data_len);
		  time = timetable[crc];
		  if(0 == m->timestamp_sec) {
		      // allow local packets through
		      status = ipq_set_verdict(h, m->packet_id,
					   NF_ACCEPT, 0, NULL);
		  }
		  else if( time >= m->timestamp_sec-DROPTIME ) {

		      status = ipq_set_verdict(h, m->packet_id,
					       NF_DROP, 0, NULL);
		      printf("drop crc 0x%X time %d timestamp %d\n",crc,time,m->timestamp_sec);
		  }
		  else {
		      timetable[crc] = m->timestamp_sec;
		      status = ipq_set_verdict(h, m->packet_id,
					   NF_ACCEPT, 0, NULL);
		      printf("accept crc 0x%X time %d timestamp %d\n",crc,time,m->timestamp_sec);
		  }
		  if (status < 0)
		       die(h,"ipq_set_verdict");
		  break;
	     }

	     default:
		  fprintf(stderr, "Unknown message type!\n");
		  break;
	}
   } while (1);

   ipq_destroy_handle(h);
   return 0;
}

/* Table of CRCs of all 8-bit messages. */
unsigned long crc_table[256];

/* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0;

/* Make the table for a fast CRC. */
void make_crc_table(void)
{
  unsigned long c;
  int n, k;

  for (n = 0; n < 256; n++) {
    c = (unsigned long) n;
    for (k = 0; k < 8; k++) {
      if (c & 1) {
        c = 0xedb88320L ^ (c >> 1);
      } else {
        c = c >> 1;
      }
    }
    crc_table[n] = c;
  }
  crc_table_computed = 1;
}

/*
   Update a running crc with the bytes buf[0..len-1] and return
 the updated crc. The crc should be initialized to zero. Pre- and
 post-conditioning (one's complement) is performed within this
 function so it shouldn't be done by the caller. Usage example:

   unsigned long crc = 0L;

   while (read_buffer(buffer, length) != EOF) {
     crc = update_crc(crc, buffer, length);
   }
   if (crc != original_crc) error();
*/
unsigned long update_crc(unsigned long crc,
                unsigned char *buf, int len)
{
  unsigned long c = crc ^ 0xffffffffL;
  int n;

  if (!crc_table_computed)
    make_crc_table();
  for (n = 0; n < len; n++) {
    c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
  }
  return c ^ 0xffffffffL;
}

/* Return the CRC of the bytes buf[0..len-1]. */
unsigned short crc16(unsigned char *buf, int len)
{
  return (unsigned short)update_crc(0L, buf, len);
}