[lug] Linux boxes drop off the net? Router problem?

[rm at mamma.varadinet.de]

On Tue, Feb 06, 2001 at 04:00:31PM -0700, Gary Frerking (TurboPower) wrote:

> 
> Ohhh, it's always possible -- it's obvious that *something* is
> misconfigured. We've checked the Linux boxes (5 of them) over pretty
> carefully though. The 5 boxes were set up independently by three different
> people that all are fairly good with Linux. Is it possible we all made the
> same mistake? I guess so, but the odds seem a bit slim.

Oh, i could tell you ... ;-)

No, i actually don't suspect (is this a verb?) any of the linux
boxes -- in my (humble) experience those are usually the boxes
best understood and maintained. I was more thinking of some mis-
confiugred Windows ActiveDirectory or a DHCP terrorist that doesn't
'know' of the linux boxes and happily gives out their IP-Adresses
to requesting Win-boxes. Do you have DHCP on the net? Does the lease
time have some strange similarity with the up and down of the linix
box?

> >> Is there anyway that you can isolate a ping so that it does NOT hit the
> router? <<
> 
> Nothing immediately comes to mind, but I'll see if I can come up with
> something. I mentioned earlier that we tried the opposite (connected one of
> the Linux boxes directly into the router) and it showed the same symptoms --
> wouldn't that effectively eliminate the switch as being the problem?
> 
> >> I also believe that both the switch and the Cisco router have the ability
> to log into and display current ARP tables. <<
> 
> Probably so -- I'll see if I can get the SysAdmin to do that for me.
> 
> >> Unfortunately this is where my knowledge of routing ends. <<
> 
> I reached my limit some time ago. <g>
> 
> I just installed ntop on one of the machines, I'll let that cook awhile and
> see if it yields any further clues.

If this _is_ an ARP related problem you might want to install an 
ARP watcher like 'arpwatch' or 'karpski' - a graphical arp watcher.
BTW, i consider ARP-watchers an esential tool for any serious net-admin.
I have sooo often seen networks come down because someone brought in
a laptop or home computer and hooked it to the net without setting
the IP settings correctly.

  Ralf