[NTLUG:Discuss] networking questions

[Steve Baker]

Richard Cobbe wrote:
> 
> Hello, all.  I've got a couple of related networking questions that I'd
> like some input on.  The computer in common between these two scenarios is
> running a fairly stock RH6.2, kernel 2.2.16 (with the redhat patches,
> though I've recompiled it).  My ethernet card is a PCI 3Com 3c905, and I'm
> using the driver that's included in the kernel source: it's apparently
> 3Com's driver, v 1.0.0d (1999).
> 
> 1) A couple of times over the last few months, I've tried to hook two
>    (Linux) computers together---directly---using ethernet 10baseT.  My
>    understanding is that you can do this by hooking the ethernet cards of
>    the two systems together, using a crossover cable.

Yes.  The cable has to be more than some minimum length - I think that's
about 3 feet or something.

>    It doesn't seem to work.  When I try to go from my normal system (stock
>    RH6.2, kernel 2.2.16), it locks up the machine hard enough to require a
>    power cycle.

That's very strange.  A foulup like that shouldn't be capable of locking
the system up.  Getting networking going can be annoyingly fiddly - but
it doesn't tend to foul things up that badly.

My best guess is that your ethernet card is somehow wrongly set up such
that it conflicts with some other hardware - or something like that.

>    Is this, in fact, possible?  If so, what am I missing?  Or do I need a
>    10baseT hub?

I have one of those super-cheap hubs - it's about 4 inches square and cost
less than $30 in Fry's.  I have three machines - and occasionally add a laptop,
so it's worth the expenditure for me.  But a crossover should be all you need
for just two machines.

> 2) Separate issue.  At one point (I don't recall exactly why I got into
>    this situation), I had my system/network set up almost as normal.  The
>    only difference was that the 10baseT crossover cable which normally
>    connects my PC and my DSL modem wasn't plugged into the modem.
> 
>    I tried to ping some host, and the computer immediately hard-booted
>    itself.
> 
>    That shouldn't have happened, yes?  Obviously, if the cable isn't
>    plugged in, I shouldn't get a response, but it shouldn't hardboot the
>    machine, right?

Absolutely not.  I have my laptop set up with the network software all
running - and when I take it away somewhere (disconnected from the
network), it happily boots up with just a couple of complaints - which
are mostly to do with being unable to mount the drives from my NFS server.
It even works OK if I disconnect the Ethernet card as well as the cable.

The network software is pretty resiliant - it has to be.

>    I don't know if it matters, but the cable in question is actually two
>    cables.  There's the two-foot crossover cable which came with the modem,
>    then a straight-through female-female connector, then a ~25-foot patch
>    cable.  The patch cable is plugged into my PC, and the crossover cable
>    is (usually) plugged into the DSL modem.
> 
> Any insights as to what the story is here?

Let me see if I have the facts straight here:

  * The machine works OK when using 10baseT to the modem.
    (Is it running Linux at the time?)

  * The machine locks up hard when that same 10baseT port
    with the same driver is connected up to another machine.

  * The machine even locks up hard when that same 10baseT port
    is not even connected to *anything* ?!

Did the ethernet adaptor come with the modem?  Maybe it's a
special low-cost hack that the DSL people produce to keep
costs down - or to stop you from using someone else's modem
or something evil like that?

This *is* very strange.

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