ipcsum.c

一个网络流量分析的完整的程序

/*
 * in_cksum --
 *      Checksum routine for Internet Protocol family headers (C Version)
 *
 *      borrowed from the ping program from the Linux NetKit.  
 *      This is a standard C version of the
 *      IP header checksum calculation algorithm.
 */

#include <sys/types.h>

int in_cksum(u_short * addr, int len)
{
    register int nleft = len;
    register u_short *w = addr;
    register int sum = 0;
    u_short answer = 0;

    /*
     * 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);
}