: ${USER:=$LOGNAME}
: ${USER:=unknown}
cat > /tmp/$USER.c <<EOM
#include <stdio.h>
#include <sys/types.h>

unsigned short in_cksum(addr, len)
    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);
}

main () {
	char packet[] = "\377\377\377\377abcdefg";

	printf("%d\n", in_cksum(packet, sizeof(packet)));

	/* simulate a fairly common form of xmit error */
	* (int *)packet = 0x0000;
	printf("%d\n", in_cksum(packet, sizeof(packet)));
}
EOM
gcc -g /tmp/$USER.c -o /tmp/$USER
/tmp/$USER
#
# output:
# 53870
# 53870
#
# I believe the IP and TCP checksum are the same algorithm, so will likely
# fail in the same way with this corruption.  Ethernet checksums are
# irrelevant if this corruption occurs within the router or host.
# I should probably read RFC 1071, 1141 and 1624, but am too lazy.