Re: Wireless TCP aborts

On Sun, Mar 7, 2010 at 1:08 PM, Chuck Swiger <cswiger_at_mac.com> wrote:

> Hi, Tim--
>
> On Mar 7, 2010, at 2:16 PM, Tim Kientzle wrote:
> [ ... ]
> >  Corrupted MAC on input.
> >  Disconnecting: Packet corrupt
> >  rsync: connection unexpectedly closed ... [receiver]
> >
> > and then the rsync session is dead.
> >
> > It seems odd that a single corrupt packet would terminate the TCP
> connection.
>
> It's not TCP itself which is closing the connection; if a garbage packet as
> far as TCP or IP layer checksums was seen, it would be dropped and normal
> resend mechanisms would compensate.  However, SSH adds it's own layer of
> data integrity checking called HMAC, which uses block-based hashes like MD5
> or SHA, and is much stronger than the 32-bit CRCs used at TCP/IP layers.
>
> See:
>
>   /usr/src/crypto/openssh/packet.c
>   http://en.wikipedia.org/wiki/HMAC
>
> The implication is that the data is being corrupted before it gets munged
> into network packets; failing memory is a likely cause, but bugs in the
> network stack, the NIC driver, or OpenSSL are also lower-order
> possibilities.
>
> Regards,
> --
> -Chuck
>
>
I have seen this same symptom many times with PCEngines ALIX boards but on
the vr interfaces.  It seems to come and go as I update kernels and worse
yet, it seems to vary by board -  have four boards all purchased at the same
time a couple of years ago and some seem more prone to it than others.  All
have worked and failed at one time or another.

I had, at one point, thought it to be related to power problems, because a
high-power mini-PCI WiFi card seemed to exacerbate the problem, but removing
the card made no difference, and the problem cleared up with an update and
recompile of my source tree.

It's a very disconcerting problem because FTP'ed files were being silently
corrupted, so it's not just an SSH problem either.

dave c