checksum

checksum.c

#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>

/*
IP Checksum

Benefits: fast, easy to compute and check which is motivated by earliest
software implementation.

Drawbacks: poor error detection
1. Only guarantees detecting a single bit error;
2. Can detect other errors, but actual guarantees are both weak and complex.
*/

typedef struct
{
    int srcIp;
    int dstIp;
    unsigned short udp_len;
    char rsv;
    char protocol;
    unsigned short src_port;
    unsigned short dst_port;
    unsigned short len;
    unsigned short check_sum;
    char data[2];
} UDPHDR;

char arr[100] = {0xc0, 0xa8, 0xd1, 0x80, 0xc0, 0xa8, 0xd1, 0x01,
                 0x00, 0x0a, 0x00, 0x11, 0x13, 0x88, 0x13, 0x88,
                 0x00, 0x0a, 0x00, 0x00, 0x61, 0x66};

unsigned short check_sum(unsigned short *a, int len);

int main()
{
    unsigned short b = 0;
    UDPHDR udphdr    = {0};

    udphdr.srcIp     = inet_addr("192.168.209.128");
    udphdr.dstIp     = inet_addr("192.168.209.1");
    udphdr.udp_len   = htons(10);
    udphdr.protocol  = 0x11;
    udphdr.rsv       = 0;
    udphdr.src_port  = htons(5000);
    udphdr.dst_port  = htons(5000);
    udphdr.len       = htons(10);
    udphdr.check_sum = 0;
    udphdr.data[0]   = 0x61;
    udphdr.data[1]   = 0x66;

    b = check_sum((unsigned short *)&udphdr, 22);
    printf("[test ...] b = Ox%04x\n", b & 0xffff);

    b = check_sum((unsigned short *)arr, 22);
    printf("[test arr] b = Ox%04x\n", b & 0xffff);

    return 0;
}

unsigned short check_sum(unsigned short *a, int len)
{
    unsigned int sum = 0;

    while (len > 1)
    {
        sum += *a++;
        len -= 2;
    }

    if (len)
    {
        sum += *(unsigned char *)a;
    }

    while (sum >> 16)
    {
        sum = (sum >> 16) + (sum & 0xffff);
    }

    return (unsigned short)(~sum);
}