[RndTbl] sshd: Corrupted MAC on input.

Trevor Cordes trevor at tecnopolis.ca
Fri Jul 31 04:10:51 CDT 2020

On 2020-07-30 Gilbert E. Detillieux wrote:
> > You could also try using nc from /dev/zero from the server to the
> > client into a file, then use a script (or something) to check if the
> > file is all zeros.  
> A script?  Just using "od" would tell me that.  :)

Ya but not if your test nc/file is several GB or TB! :-)  You need
something that will tell you if there are non-zero bytes in the final
file.  Well, maybe od can do that if you output in lines and can then
use grep to match non-all-zero lines.  I'm pretty sure I could make a
solution faster in a perl -e one-liner than with od and grep! :-)

Hmm... or maybe cmp -l file /dev/zero .....

> TCP checksums aren't perfect, and with very large transfers, there is
> a statistically significant probability of errors getting through, if
> the underlying layers aren't doing their job.  (Normally, Ethernet
> frame errors are more likely to weed out the bad packets than TCP
> checksums, but I remember in the days of PPP over dial-up, that TCP
> checksums were often inadequate.  If we've got problems with
> something in the Ethernet data path letting through bad packets, sshd
> could be seeing errors that TCP misses.)

Oh, they should be "perfect enough" so you don't get what you're seeing
on a regular basis.  Maybe someone can whip off the spec and we can do
the math.  I think they are 32-bit checksums in TCP?  Yes, between the
lower layer checksums and TCP my gut says errors should be rare.  Maybe
the math will spell differently though on really junky connections...
However short of wireless, no one should really have that junky a
connection anymore.

> So, looking for things I can test remotely, at the moment...

Computer manufacturers should start including a camera and light
*inside* every case pointing down to the mobo so one can inspect the
caps at will remotely! :-)

Either that or spend the extra 5c per cap and not use the no-name
Chinese caps in the first place!

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