#include <stdio.h>
#include <Windows.h>

#pragma comment (lib, "ws2_32.lib" )

unsigned char data[] = "\x45\x00\x00\x30\x00\x00\x40\x00\x3c\x06\x00\x00\xa5\xf6\x0c\xd7\xa5\xf6\x43\xd3";

char *src_ip = "165.246.12.215";
char *dest_ip = "165.246.67.211";

struct ip_header
{
        unsigned char ip_header_len:4;
        unsigned char ip_version:4;
        unsigned char ip_tos;
        unsigned short ip_total_length;
        unsigned short ip_id;
        unsigned char ip_frag_offset:5;
        unsigned char ip_more_fragment:1;
        unsigned char ip_dont_fragment:1;
        unsigned char ip_reserved_zero:1;
        unsigned char ip_frag_offset1;
        unsigned char ip_ttl;
        unsigned char ip_protocol;
        unsigned short ip_checksum;
        struct in_addr ip_srcaddr;
        struct in_addr ip_destaddr;
};

u_short ip_sum_calc( u_short len_ip_header, u_short * buff )
{
    u_short word16;
    u_int sum = 0;
    u_short i;
    // make 16 bit words out of every two adjacent 8 bit words in the packet
    // and add them up
    for( i = 0; i < len_ip_header; i = i+2 )
    {
        word16 = ( ( buff[i]<<8) & 0xFF00 )+( buff[i+1] & 0xFF );
        sum = sum + (u_int) word16;
    }
    // take only 16 bits out of the 32 bit sum and add up the carries
    while( sum >> 16 )
        sum = ( sum & 0xFFFF ) + ( sum >> 16 );
    // one's complement the result
    sum = ~sum;

    return ((u_short) sum);
}

void main()
{
    unsigned short chksum;
    u_short ipdata[20]; //checksum을 계산할기위한 버퍼
    ip_header *myih;
    char *ptr;

    myih = (ip_header*)malloc(sizeof(myih));
    ptr = (char*)myih;
    printf("\n");
    // IP 패킷에 대한 정보들을 저장한다.
    myih->ip_header_len=0x5;
    myih->ip_version = 0x4;
    myih->ip_tos = 0x0;
    myih->ip_total_length = ntohs(0x30);
    myih->ip_id = htons(0x00);
    myih->ip_frag_offset = 0;
    myih->ip_more_fragment = 0;
    myih->ip_dont_fragment = 1;
    myih->ip_reserved_zero =0;
    myih->ip_frag_offset1 =0;
    myih->ip_ttl = 0x3c;
    myih->ip_protocol = 0x06;
    myih->ip_checksum = 0;
    myih->ip_srcaddr.S_un.S_addr = inet_addr(src_ip);
    myih->ip_destaddr.S_un.S_addr = inet_addr(dest_ip);
    //모든 정보를 세팅하고 체크섬을 구한다.
    int i;
    // 1byte 단위 값을 2바이트 배열에 저장함
    for (i = 0; i < 20; i ++) {
        ipdata[i] = *(unsigned char*)ptr++;
    }
    myih->ip_checksum = ip_sum_calc(20, ipdata);
    printf("checksum = %x\n",myih->ip_checksum);
}