Dave Horsfall dave at horsfall.org
Sat Sep 14 08:30:42 AEST 2019

On Fri, 13 Sep 2019, Norman Wilson wrote:

> UDP (and TCP) checksums are nearly useless against the sort of 
> corruption Larry described.  Since they are a simple addition with carry 
> wraparound, you can insert or remove any number of word-aligned pairs of 
> zero octets without the checksum changing.

I was thinking of an intermediate router (probably one that you never knew 
about) corrupting the checksum-less UDP packet, recalculating the Ethernet 
checksum, and your kernel happily accepting it; you now have an 
application that fails for some unknown reason.

Never seen it in practice, but I've heard of it happening.

-- Dave

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