在syn后取回ack

Question

我编写了一个小工具，它只是在你给它的任何ip和端口号时抛出一个syn包，当我弹出Wireshark时，发送的包看起来很好，没有错误或任何东西，它看起来就像典型的TCP SYN数据包。

由于某些原因，服务器不会发送ACK。所以，有两个问题：一，没有应答。二，我甚至不知道如何通过我的程序（bind（）？）来接收ack响应。

我修改了我在网上找到的syn flooder中的一些代码，因为它对我来说似乎是一些脚本 - 小孩代码很容易遵循，但是恐怕它可能会丢失一些东西，并且可能是服务器的ACK拒绝。代码已被修改，所以syn请求从合法IP地址发送。下面的代码显示了我的ip + tcp头文件：

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，没有回应。我错过了什么？

Answer 1

线：

tcph->window = htonl (65555); 

可能会引起问题。因为65555大于0xFFFF。 window字段大小是16位。

所以这可能是更好的：

tcph->window = htons (65535); 

注意：htonl改为htons。

编辑：

甚至更​​多更好（如果含量的不同可用）：

tcph->window = htons (TCP_MAXWIN); 

问题可以也缺乏TCP校验计数。 您的代码假定较低层更新TCP校验和字段。 因为所有网络接口卡不支持TCP checksum offload, 该代码可能无法在每个主机中工作。

Answer 2

你还没有写完整的代码。可能有一些地方你会错过一些东西。

此代码将帮助你。

#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; 

}