Question

我编写了一个小工具，它只是在你给它的任何ip和端口号上抛出一个syn数据包 - 当我弹出Wireshark时，发送的数据包显示正常，没有错误或任何东西，它看起来就像你的典型的TCP SYN数据包。

由于某种原因，服务器不发送ACK。那么，有两个问题;一，没有ACK响应。二，我甚至不知道如何通过我的程序接收ack响应（bind（）？）。

我修改了我在网上找到的一个syn flooder中的一些代码，因为对我来说似乎很容易遵循一些脚本小子代码，但我担心它可能会丢失一些东西，可能是服务器的结果拒绝否认。代码已被修改，因此syn请求从合法IP地址发送。以下代码显示了我的ip + tcp标头：

 iph->ihl = 5;
 iph->version = 4;
 iph->tos = 0;
 iph->tot_len = sizeof (struct ip) + sizeof (struct tcphdr);
 iph->id = htonl (54321); //Id of this packet
 iph->frag_off = 0;
 iph->ttl = 255;
 iph->protocol = 6;
 iph->check = 0;  //Set to 0 before calculating checksum
 iph->saddr = 0; //Source ip filled in by kernel
 iph->daddr = sin.sin_addr.s_addr;

 //TCP Header
 tcph->source = htons (9999);
 tcph->dest = htons (atoi(argv[2]));
 tcph->seq = random ();
// tcph->ack_seq = 0;
 tcph->doff = 5;  /* first and only tcp segment */
 tcph->syn = 1;
 tcph->window = htonl (65555); /* maximum allowed window size */
 tcph->check = 0;/* if you set a checksum to zero, your kernel's IP stack
    should fill in the correct checksum during transmission */
 tcph->urg_ptr = 0;
 //Now the IP checksum
 iph->check = csum ((unsigned short *) datagram, iph->tot_len >> 1);

所以，我发送，没有回复。我错过了什么？

Answer 1

该行：

   tcph->window = htonl (65555);

可能会导致问题。因为65555大于0xFFFF。 window字段大小为16位。

所以这可能会更好：

   tcph->window = htons (65535);

注意：htonl已更改为htons。

编辑：

甚至更好（如果有可用的话）：

   tcph->window = htons (TCP_MAXWIN);

问题也可能是缺乏TCP校验和计数。您的代码假定较低层更新TCP校验和字段。由于所有网络接口卡都不支持TCP checksum offload，代码可能无法在每个主机中运行。

Answer 2

你还没有写完整个代码。你可能会遗漏一些区域。

此代码可以帮助您。

#include<stdio.h>
#include<string.h>
#include<sys/socket.h>
#include<stdlib.h>
#include<errno.h>
#include<netinet/tcp.h>
#include<netinet/ip.h>

struct pseudo_header
{
u_int32_t source_address;
u_int32_t dest_address;
u_int8_t placeholder;
u_int8_t protocol;
u_int16_t tcp_length;
};

unsigned short csum(unsigned short *ptr,int nbytes) 
{
register long sum;
unsigned short oddbyte;
register short answer;

sum=0;
while(nbytes>1) {
    sum+=*ptr++;
    nbytes-=2;
}
if(nbytes==1) {
    oddbyte=0;
    *((u_char*)&oddbyte)=*(u_char*)ptr;
    sum+=oddbyte;
}

sum = (sum>>16)+(sum & 0xffff);
sum = sum + (sum>>16);
answer=(short)~sum;

return(answer);
}

int main (void)
{
int s = socket (AF_INET, SOCK_RAW, IPPROTO_TCP);

if(s == -1) {
    perror("Failed to create socket");
    exit(1);
}
char datagram[4096] , source_ip[32] , *data , *pseudogram;
char buffer[4096];

memset (datagram, 0, 4096);

struct iphdr *iph = (struct iphdr *) datagram;
struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct iphdr));

struct sockaddr_in sin;
struct pseudo_header psh;

data = datagram + sizeof(struct iphdr) + sizeof(struct tcphdr);
strcpy(data , "ABCDEFGHIJKLMNOPQRSTUVWXYZ");

strcpy(source_ip , "127.0.0.1");

sin.sin_family = AF_INET;
sin.sin_port = htons(55559);
sin.sin_addr.s_addr = inet_addr ("127.0.0.1");

iph->ihl = 5;
iph->version = 4;
iph->tos = 0;
iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data);
iph->id = htonl (54351);
iph->frag_off = 0;
iph->ttl = 255;
iph->protocol = IPPROTO_TCP;
iph->check = 0;
iph->saddr = inet_addr ( source_ip );
iph->daddr = sin.sin_addr.s_addr;
iph->check = csum ((unsigned short *) datagram, iph->tot_len);

tcph->source = htons (55554);
tcph->dest = sin.sin_port;
tcph->seq = htonl(4362);
tcph->ack_seq = htonl(0);
tcph->doff = 5;
tcph->fin=0;
tcph->syn=1;
tcph->rst=0;
tcph->psh=0;
tcph->ack=0;
tcph->urg=0;
tcph->window = 1;
tcph->check = 0;    
tcph->urg_ptr = 0;

psh.source_address = inet_addr( source_ip );
psh.dest_address = sin.sin_addr.s_addr;
psh.placeholder = 0;
psh.protocol = IPPROTO_TCP;
psh.tcp_length = htons(sizeof(struct tcphdr) + strlen(data) );

int psize = sizeof(struct pseudo_header) + sizeof(struct tcphdr) + strlen(data);
pseudogram = malloc(psize);

memcpy(pseudogram , (char*) &psh , sizeof (struct pseudo_header));
memcpy(pseudogram + sizeof(struct pseudo_header) , tcph , sizeof(struct tcphdr) + strlen(data));

tcph->check = csum( (unsigned short*) pseudogram , psize);

int one = 1;
const int *val = &one;

if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
{
    perror("Error setting IP_HDRINCL");
    exit(0);
}
sendto (s, datagram, iph->tot_len , 0, (struct sockaddr *) &sin, sizeof (sin));
return 0;

}

在syn之后检索ack

2 个答案: