minirighi_udp_checksum.c

// Rawsock library, licensed under GPLv2
// Version 0.3.4
#include "minirighi_udp_checksum.h"

uint16_t minirighi_udp_checksum(const void *buff, size_t len, in_addr_t src_addr, in_addr_t dest_addr) {
    const uint16_t *buf=buff;
    uint16_t *ip_src=(void *)&src_addr, *ip_dst=(void *)&dest_addr;
    uint32_t sum;
    size_t length=len;

    sum = 0;
    while (len > 1) {
        sum += *buf++;
        if (sum & 0x80000000)
            sum = (sum & 0xFFFF) + (sum >> 16);
        len -= 2;
    }

    if ( len & 1 )
        sum += *((uint8_t *)buf);

    sum += *(ip_src++);
    sum += *ip_src;
    sum += *(ip_dst++);
    sum += *ip_dst;

    sum += htons(IPPROTO_UDP);
    sum += htons(length);

    while (sum >> 16)
        sum = (sum & 0xFFFF) + (sum >> 16);

    return ( (uint16_t)(~sum)  );
}