new bug in iproute.c

[karn@thumper.bellcore.com (Phil R. Karn)]

I inadvertently introduced a bug into iproute.c a few weeks ago when I
added a performance enhancement. Instead of recalculating the IP
header checksum after decrementing the TTL field, I simply added 0x100
to the checksum. Unfortunately, this doesn't account for the
end-around carry that occurs when the high byte of the checksum is
incremented from FF to 00 hex.

The symptom is that a long burst of packets is generated with incorrect
IP checksums; the residual as displayed by the trace command is 1.
As the IP ID counter increments, the problem will seem to go away
for a while only to return later.

The fix is straightforward. In iproute.c, function ip_route(),
change the line

        ip.checksum += 0x100;

to

        if(((ip.checksum += 0x100) & 0xff00) == 0)
                ip.checksum++;  /* End-around carry */

I've tested this code by pinging a nearby machine over 60,000 times
without fail, so I think I've got it right now...

Phil