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Thought that many on this list don't get Forbes, so I'd forward a link to
a Stephen Manes column in the latest issue about 3Com's NAT software and
how it changes data at whim and corrects the checksum to spoof the
recipient.<br><br>
Fortunately, the mission critical backup application that discovered the
problem was programmed according to the end-to-end principle: it
didn't depend on the TCP checksum for reliability.<br><br>
While Manes calls it a "programming error", it appears more
likely that it was a design feature - the NAT software scanned the entire
TCP segment for addresses that matched the local address that must be
translated. This kind of solution has been used in some NAT
software I've seen on machines that try to be "automagical"
thinking that the byte sequence 192.168.0.1 is pretty unlikely to appear
in most data packets, so it should be corrected everywhere. 3Com
probably bought the rights to a package like that one (anyone on this
list know the truth?)<br><br>
This is a good argument for adversary-proof checksums (like one-way
signed message digests) I suggested in a recent exchange - clearly there
are devices that behave like adversaries out there in the real world
today, designed by real programmers to change bits in a way that is not
statistically independent of the data.<br><br>
It is also becoming clear that patching the symptoms of a bad design
choice (NAT in this case) is going to be never-ending, and it's time to
obviate the need to perpetuate such kludges. I realize that this
(beginning with IPv6, end-to-end encryption, etc.) is a big job and the
Cisco/3Com/Microsoft axis don't seem to have the guts to do it. But
it is time.<br><br>
<a href="http://www.forbes.com/forbes/2001/0723/118.html" eudora="autourl">http://www.forbes.com/forbes/2001/0723/118.html</a><br><br>
The Four-Byte Shuffle <br>
Stephen Manes, <font color="#0000FF"><u>Forbes Magazine</u></font>,
07.23.01, 12:00 AM ET <br><br>
Digital bits are supposed to be sacrosanct. An error in just one among
billions can create unknowable consequences, from innocuous to
disastrous. I know this firsthand. Four little bytes of data
corruption--just enough to spell a choice expletive--recently wasted many
hours of my life. <br><br>
<...><br>
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- David<br>
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WWW Page:
<a href="http://www.reed.com/dpr.html" eudora="autourl">http://www.reed.com/dpr.html</a><br><br>
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