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sendack.c

/*
   sendack - RST trigger
   ---------------------

   This is a trivial code to send a stray ACK packet to a remote
   host. The main purpose of this is to gather new RST ("connection dropped") 
   signatures quickly, but you can also use it for silent active 
   fingerprinting when you can't or don't want to use -A mode.

   THIS PROGRAM IS NOT SUITABLE FOR GATHERING "Connection refused" SIGNATURES.

   Run p0f in the background in -R mode, then run sendack, observe
   results, if any. This code uses a distinct WSS of 12345. If you see
   it in the signature returned by p0f, you need to wildcard the value
   (p0f does the first step for you), as it appears to be dependent on the 
   original packet and may vary on the other party's stack.

   Linux code, may not work on systems that use different mechanism to
   access raw sockets.

   Copyright (C) 2003-2006 by Michal Zalewski <lcamtuf@coredump.cx>

 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/udp.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <errno.h>

#include "../types.h"

#define fatal(x) do { perror(x); exit(1); } while (0)

static _u8 synpacket[] = {

  /* IP HEADER */

  /* IHL    */ 0x45,
  /* ToS    */ 0x00,
  /* totlen */ 0x00, 0x28,
  /* ID     */ 0x00, 0x00,
  /* offset */ 0x00, 0x00,
  /* TTL    */ 0xFF,
  /* proto  */ 0x06,
  /* cksum */  0x00, 0x00,
  /* saddr */  0, 0, 0, 0,   /* src: [12] */
  /* daddr */  0, 0, 0, 0,   /* dst: [16] */

  /* TCP HEADER - [20] */

  /* sport */  0xCA, 0xFE,
  /* dport */  0, 0,         /* dp: [22] */
  /* SEQ   */  0x0D, 0xEF, 0xAC, 0xED,
  /* ACK   */  0xDE, 0xAD, 0xBE, 0xEF,
  /* doff  */  0x50,
  /* flags */  0x10,         /* just ACK */
  /* wss   */  0x30, 0x39,   /* 12345 */
  /* cksum */  0x00, 0x00,
  /* urg   */  0x00, 0x00

};


_u16 simple_tcp_cksum(void) {
  _u32 sum = 26 /* tcp, len 20 */;
  _u8  i;
  _u8* p = synpacket + 20;

  for (i=0;i<10;i++) {
    sum += (*p << 8) + *(p+1);
    p+=2;
  }

  p = synpacket + 12;
  
  for (i=0;i<4;i++) {
    sum += (*p << 8) + *(p+1);
    p+=2;
  }

  return ~(sum + (sum >> 16));

}



int main(int argc, char** argv) {

  static struct sockaddr_in sain;
  _s32 sad,dad;
  _s32 sock, one = 1;
  _u16 p,ck;

  if (argc - 4 || (sad=inet_addr(argv[1])) == INADDR_NONE || 
     (dad=inet_addr(argv[2])) == INADDR_NONE || !(p=atoi(argv[3]))) {
    fprintf(stderr,"Usage: %s src_ip dst_ip port\n",argv[0]);
    exit(1);
  }
 
  sock=socket(AF_INET,SOCK_RAW,IPPROTO_RAW);
  
  if (sock<0) fatal("socket");
  
  if (setsockopt(sock,IPPROTO_IP,IP_HDRINCL,(char *)&one,sizeof(one)))
    fatal("setsockopt");
    
  sain.sin_family = AF_INET;
  memcpy(&sain.sin_addr.s_addr,&dad,4);
  memcpy(synpacket+12,&sad,4);
  memcpy(synpacket+16,&dad,4);

  p=htons(p);
  memcpy(synpacket+22,&p,2);

  ck=simple_tcp_cksum();
  ck=htons(ck);
  memcpy(synpacket+36,&ck,2);

  if (sendto(sock,synpacket,sizeof(synpacket), 0,(struct sockaddr *)&sain,
    sizeof(struct sockaddr)) < 0) perror("sendto");
  else 
    printf("Stray ACK sent to %s to port %d.\n",argv[2],ntohs(p));

  return 0;
  
}


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