terminals randomly disconnecting

David Van Assche dvanassche at gmail.com
Mon Mar 10 21:18:28 GMT 2008


   I had a problem recently that was similar to yours where one particular
unit would log out spontaneously at a random time, usually within 30
minutes. Sometimes when the user using that unit would log on they would get
someone else's wallpaper or even home desktop. I couldn't figure out what
was causing it, until I finally saw that one of the computers in a totally
different part of the building had a shoddy job done on the patching of the
ethernet end. The 2 computers were somehow conflicting with each other,
logging themselves out and generally behaving very unstably. I re-grimped
the cable really carefully and the problems subsided. Since then, no such
problems have come back to haunt me.
   The moral of the story is that, especially on gigabit networks, even a
single badly grimped cable (not shielded, badly plugged in or whatever else)
will cause your network to do strange things like what you are describing.
It almost certainly sounds to me like a physical problem, as Charles
suggested. At least for me, every time that kind of behaviour happens, and
it has happened many times, it has been because of a switch, a network
cable, a network card or something else physical.

Good luck,
David

On Mon, Mar 10, 2008 at 9:24 PM, Charles Austin <ceaustin at gmail.com> wrote:

> On Mon, Mar 10, 2008 at 1:59 PM, Krsnendu dasa <krsnendu108 at gmail.com>
> wrote:
> > On 12/01/2008, Krsnendu dasa <krsnendu108 at gmail.com> wrote:
> > > I have noticed a few other problems and narrowed down the problem (I
> > think) to either os problem or motherboard problem. (Don't think it is
> the
> > switch or the network card.) Read below for more details. Thanks for all
> > tips.
> > >
> > > On 07/01/2008, Gavin McCullagh <gmccullagh at gmail.com> wrote:
> > >
> > > > Hi,
> > > >
> > > > On Mon, 07 Jan 2008, Krsnendu dasa wrote:
> > > >
> > > > > I am experiencing an unusual problem. Occasionally (every 15 mins
> or
> > so) all
> > > > > the terminals just disconnect. The terminals need restarting
> before
> > they
> > > > > work again.
> > > >
> > > > What you say all, you mean every thin client connected to the
> server?
> > >
> > >
> > > Yes. Also windows computers that were connecting to the internet and
> using
> > dns through the edubuntu server also had disconnection problems. I
> changed
> > dhcpd.conf to get dns from another server and it seemed to fix the
> windows
> > laptop to internet problem.
> > > I am having intermittent problems with samba also. One minute I can
> > connect to the shares on edubuntu then the next minute I can't then I
> can
> > again. It seems to take a long time for the server to register when new
> > windows computers in the workgroup have connnected. At one point I
> noticed
> > that nmbd wasn't running so I manually restarted it. This is after I had
> > already restarted samba using init.d and webmin.
> > >
> > >
> > > > Every 15 minutes is pretty nasty.  I guess at least it's easy enough
> to
> > > > repeat if you're testing.
> > >
> > >
> > > May not be exactly every 15 minutes, maybe closer to 30mins.
> > >
> > >
> > > > You say the clients need restarting and I gather the server doesn't.
> > Can
> > > > you ping a thin client after this happens (ie without rebooting it)?
>  If
> > > > you press <ctrl><alt><backspace> is that enough to restart the thin
> > client?
> > >
> > >
> > > I haven't tried this as I haven't been at school much. I will try it
> next
> > time I  am at school when it freezes.
> > >
> > >
> > > > > In what I think may be a related issue, some times when someone is
> > logged in
> > > > > their terminal freezes and the panel at the bottom of the screen
> which
> > shows
> > > > > which windows are open just has three tabs that all say untitled
> > window.
> > > >
> > > > Have you had opportunity to watch top for a few minutes and see does
> > some
> > > > process go wild around this point?
> > > I haven't noticed anything. I will look again and see if anything
> stands
> > out.
> > >
> > >
> > > > > Background: I am using 32 Edubuntu 7.04 on Intel Quad Core. 4 Gb
> RAM.
> > I am
> > > > > using directx ldm and smbldap.
> > > >
> > > > Would it be possible for you to try not using direct-x and go back
> to
> > the
> > > > default ssh method?
> > >
> > >
> > > I can also try this, but my instinct (and the dns /samba problems)
> seem to
> > point to bigger connection problems than ldm.
> > > I also get disconnected when I log in from home using nx.
> > >
> > >
> > > > > I am using a 100/1000 unmanaged switch which feeds 10/100 switches
> in
> > each
> > > > > class.
> > > > >
> > > > > Things I have already tried.
> > > > > At first I thought it could be the 1000 switch, so I swapped it
> for
> > another
> > > > > one, but the problem remains.
> > > >
> > > > Was it another similar switch?  Could it be a problem between the
> > switches?
> > >
> > >
> > > The second time I used a different brand of switch and a 10/100
> switch, so
> > I don't think the switch is to blame.
> > >
> > >
> > > > > Possible solutions:
> > > > > Could upgrading to Gutsy help?
> > > >
> > > > It's hard to say without knowing what the cause of the problem is.
> > >
> > >
> > > If may be my old windows conditioning, but I feel like a reinstall
> might
> > help... or not.
> > >
> > >
> > > > > Could it be the network card in the server?
> > > >
> > > > Sounds plausible.
> > >
> > >
> > > I tried another network card and the same problems are there.
> > >
> > >
> > > > > Can someone help me debug this? Are there any log files I can
> check to
> > trace
> > > > > the origin of this problem?
> > > >
> > > > Yes:
> > > >         /var/log/syslog
> > > >         /var/log/messages
> > > >         /var/log/kern.log
> > > >         /var/log/daemon.log
> > >
> > >
> > > I had a look there and couldn't find anything that stood out for me. A
> few
> > times I saw something that said "disconnected" or something like that.
> > Perhaps I will copy and paste a section around the time when a
> disconnect
> > happens and post it for any experts to analyse.
> > >
> > >
> > > > > Thanks for any help. This is really frustrating, because I had a
> very
> > stable
> > > > > system with K12LTSP 6, and now with Edubutnu 7.04 I have having
> quite
> > a few
> > > > > problems.
> > > >
> > > > How long have you had this problem?  You've been using 7.04 for a
> while,
> > > > right?  Has it always happened?  Has anything changed to prompt
> this?
> > >
> > >
> > > We installed some time in October. This problem has been only since
> > December. I can't think of anything that changed at that time. The last
> > thing I remember that changed is... A course management software
> web-based
> > application was installed and there were some changes to dns.
> > >
>
>
> Also, keep in mind that seemingly innocent physical changes can impact
> your network.  Was there any maintenance done on the building in the
> December timeframe?  Any new items (non computer) added to the
> electrical system in the building?  Any new patch cables added?  Any
> new network drops?
>
>
> >
> > Back to the drawing board... I have installed Hardy and the problem
> > persists. I will have to investigate the network and system more
> thoroughly
> > and document it robustly.
> >
> > Summary:
> > Initial Problem: Feisty install of Edubuntu - One Network interface:
> Server
> > randomly disconnects from the network about every 20-30 mins. Terminals
> left
> > with 3 unusual untitled windows tabs at the bottom of the screen.
> > ALT-Ctrl-backspace usually allows users to log in again.
> >
> > Tried using another network card, switch etc. Ran Wireshark to look for
> > network problems.
>
>
> Did you set Wireshark up on a separate machine and mirror one of your
> switch ports to it?  This is usually a last resort, but it sounds like
> you are reaching that point.  There is really no way to rule out the
> software changes you made in December without looking at EVERY packet
> during the time it goes haywire.  Wireshark will also clue you in to
> physical interference problems (lots of re-transmitted packets).
>
>
> >
> > Tried changing the system to a new box. Problem persists.
> >
> > SSH mostly seems affected. Even when I can ping the server I can't ssh
> in. A
> > few minutes later I can ssh in. At the same time I can ssh in from one
> > machine, but not from another machine.
> >
> > Sometimes the server can ping a machine, but that machine cannot ping
> the
> > server.
> >
> > I installed Hardy with 1 network card: problem persists.
> >
>
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