1. 什么是TCP三次握手 四次挥手?

客户端IP: 192.168.0.106
服务端IP: 1.1.1.1

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
void main()
{
	int socketfd;
        struct sockaddr_in srvaddr;
	socketfd = socket(AF_INET,SOCK_STREAM,0);

	memset(&srvaddr,0,sizeof(srvaddr));
	srvaddr.sin_family = AF_INET;
	inet_pton(AF_INET,"1.1.1.1",&srvaddr.sin_addr);
	srvaddr.sin_port = htons(80);

	connect(socketfd,(struct sockaddr*)&srvaddr,sizeof(srvaddr));

	close(socketfd);
	return;
}

2. 如何使用虚假源IP地址?

客户端IP: 1.2.3.4
服务端IP: 1.1.1.1

#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 (PF_INET, SOCK_RAW, IPPROTO_TCP);
     
    char datagram[4096] , source_ip[32] , *data , *pseudogram;
    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 , "hackbiji.top");
     
    strcpy(source_ip , "1.2.3.4");
    sin.sin_family = AF_INET;
    sin.sin_port = htons(80);
    sin.sin_addr.s_addr = inet_addr ("1.1.1.1");
     
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data);
    iph->id = htonl (12345);
    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 (10010);
    tcph->dest = htons (80);
    tcph->seq = 0;
    tcph->ack_seq = 0;
    tcph->doff = 5;
    tcph->fin=0;
    tcph->syn=1;
    tcph->rst=0;
    tcph->psh=0;
    tcph->ack=0;
    tcph->urg=0;
    tcph->window = htons (666);
    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;
    setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one));
    
    sendto (s, datagram, iph->tot_len ,  0, (struct sockaddr *) &sin, sizeof (sin));
    printf ("Packet Send.Length:%d\tValue:%s\n",iph->tot_len,data);
    return 0;
}