tcp.c

/**
 * inet_cksum from :
 * http://haifux.org/lectures/108/haifux-ping/in_cksum.c
 * 
 * 
 * modified with in_cksum from : 
 * http://stackoverflow.com/questions/14410128/how-to-verify-tcp-checksum
 * 
 **/

/*
 * Copyright (c) 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Mike Muuss.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
#include <sys/types.h>
#include "tcp.h"

unsigned short int inet_cksum(unsigned short int *addr, size_t len, uint16_t init) {
    register int nleft = (int) len;
    register unsigned short int *w = addr;
    unsigned short int answer = 0;
    register int sum = init;

    /*
     *  Our algorithm is simple, using a 32 bit accumulator (sum),
     *  we add sequential 16 bit words to it, and at the end, fold
     *  back all the carry bits from the top 16 bits into the lower
     *  16 bits.
     */
    while (nleft > 1) {
        sum += *w++;
        nleft -= 2;
    }

    /* mop up an odd byte, if necessary */
    if (nleft == 1) {
        *(u_char *) (&answer) = *(u_char *) w;
        sum += answer;
    }

    /*
     * add back carry outs from top 16 bits to low 16 bits
     */
    sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
    sum += (sum >> 16); /* add carry */
    answer = ~sum; /* truncate to 16 bits */
    return (answer);
}

uint16_t tcp_cksum(const void * addr, unsigned len, struct in_addr dest_ip, struct in_addr src_ip) {
    struct tcp_pseudo tcp_pseudo_hdr = {0};
    uint16_t csum;


    tcp_pseudo_hdr.dest_ip = dest_ip;
    tcp_pseudo_hdr.src_ip = src_ip;
    tcp_pseudo_hdr.protocol = IPPROTO_TCP;
    tcp_pseudo_hdr.len = htons(len);

    csum = inet_cksum(&tcp_pseudo_hdr, (unsigned) sizeof (tcp_pseudo_hdr), 0);
    csum = inet_cksum(addr, len, (uint16_t) ~csum);

    return csum;
}