<질문>raw socket을 활용한 3way handshake

// 체크섬 루틴이 추가된 raw_socket.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <linux/ip.h>
#include <linux/tcp.h>
// 체크섬을 구하는 함수 선언/정의.
unsigned short in_cksum(u_short *addr, int len)
{
        int         sum=0;
        int         nleft=len;
        u_short     *w=addr;
        u_short     answer=0;
 
        while (nleft > 1){
        sum += *w++;
        nleft -= 2;
        }
 
        if (nleft == 1){
        *(u_char *)(&answer) = *(u_char *)w ;
        sum += answer;
        }
 
        sum = (sum >> 16) + (sum & 0xffff);
        sum += (sum >> 16);
        answer = ~sum;
        return(answer);
}
// 가상 헤더 구조체 선언
struct pseudohdr {
        u_int32_t   saddr;
        u_int32_t   daddr;
        u_int8_t    useless;
        u_int8_t    protocol;
        u_int16_t   tcplength;
};
int main()
{
        unsigned char packet[40];
        int raw_socket;
        int on = 1 ;
        struct iphdr *iphdr;
        struct tcphdr *tcphdr;
        // 주소 정보를 담을 변수. 매번 IP를 적기 귀찮으니 변수로 사용합니다.
        struct in_addr source_address, dest_address;
        struct sockaddr_in address;
        // 가상 헤더의 구조체 변수.
        struct pseudohdr *pseudo_header;
        source_address.s_addr = inet_addr( "115.145.179.46" );
        dest_address.s_addr = inet_addr( "222.122.134.138" );
 
        raw_socket = socket( AF_INET, SOCK_RAW, IPPROTO_RAW );
        setsockopt( raw_socket, IPPROTO_IP, IP_HDRINCL, (char *)&on, sizeof(on));
 
        iphdr = (struct iphdr *)packet;
        memset( (char *)iphdr, 0, 20 );
tcphdr = (struct tcphdr *)(packet + 20 );
        memset( (char *)tcphdr, 0, 20 );
 
        tcphdr->source = htons( 60861 );
        tcphdr->dest = htons( 80 );
        tcphdr->seq = htonl( 0 );
        tcphdr->ack_seq = htonl( 0 );
        tcphdr->doff = 5;
        tcphdr->syn = 1;
        tcphdr->window = htons( 5840 );
 
        // 가상 헤더 생성.
        pseudo_header = (struct pseudohdr *)((char*)tcphdr-sizeof(struct pseudohdr));
        pseudo_header->saddr = source_address.s_addr;
        pseudo_header->daddr = dest_address.s_addr;
        pseudo_header->protocol = IPPROTO_TCP;
        pseudo_header->tcplength = htons( sizeof(struct tcphdr) );
 
        // TCP 체크섬 계산.
        tcphdr->check = in_cksum( (u_short *)pseudo_header,
        sizeof(struct pseudohdr) + sizeof(struct tcphdr) );
 
        iphdr->version = 4;
        iphdr->ihl = 5;
        iphdr->protocol = IPPROTO_TCP;
        iphdr->tot_len = 40;
        iphdr->id = htons( 65139 );
        iphdr->ttl = 64;
        iphdr->saddr = source_address.s_addr;
        iphdr->daddr = dest_address.s_addr;
        // IP 체크섬 계산.
        iphdr->check = in_cksum( (u_short *)iphdr, sizeof(struct iphdr));
        address.sin_family = AF_INET;
        address.sin_port = htons( 80 );
        address.sin_addr.s_addr = dest_address.s_addr;
        sendto( raw_socket, &packet, sizeof(packet), 0x0, (struct sockaddr *)&address, sizeof(address));
 
}