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Date: Thu, 23 Dec 1999 12:56:12 -0600 (CST)
From: Jonathan Lemon <jlemon@monster.americantv.com>
To: FreeBSD-gnats-submit@freebsd.org
Subject: kern/15656: in_cksum computes wrong checksum
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>Number:         15656
>Category:       kern
>Synopsis:       in_cksum computes wrong checksum
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Dec 23 11:00:01 PST 1999
>Closed-Date:
>Last-Modified:
>Originator:     Jonathan Lemon
>Release:        FreeBSD 4.0-CURRENT i386
>Organization:
>Environment:

        4.0-CURRENT, circa Dec 14, 1999

>Description:

        For some data patterns, in_cksum generates a checksum of 0.
        The specification says a checksum of 0 should be transmitted
        as all ones (65535), and a transmitted checksum of 0 indicates
        that the sender did not compute a checksum.

        While 0 == 65535 in 1's complement arithmetic, this is still wrong.
        The error was discovered while doing incremental checksum
        computations on the TCP stream on an intermediate machine.


>How-To-Repeat:

        tcpdump 'tcp[16:2] == 0', (dump TCP packets with a zero checksum)
        and transmit various TCP data streams.

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted:


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