[NTLUG:Discuss] networking questions

[Richard Cobbe]

Lo, on Saturday, 2 September, 2000, Steve Baker did write:

> 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.

That might have been the problem; I'll check that out.

> 
> >    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.

That's possible, I suppose, but networking, or more specifically, my DSL
link, has generally worked fine for a year.  Plus, the Win95 device manager
indicates that there are no conflicts with the ethernet card.

> >    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?)

Yes.  Both Linux and Win95.  (This in fact my normal setup; the above
situations only came up when I was trying to transfer a lot of software
between two Linux machines.  I removed one end of the ethernet cable from
the DSL modem and plugged it into the second machine.

NORMAL:

             xover
         A --------- DSL modem ---- ISP & the rest of the world.

A is a dual-boot Win95/RH6.2 machine.  Works fine under both OSes.

SITUATION 1:

             xover
         A --------- B               DSL modem --- ISP & rest of the world
                          ^^^^^^^^ 
                              no connection here.

  (Jonathan Brugge's suggestions assume that I'm trying to use one of the
  two machines as a firewall/gateway.  While I'd eventually like to get to
  that point, that's not actually what I'm doing here.  In point of fact,
  this situation first came up when I was trying to install RH 6.2 on
  machine B over an NFS link to A, so I could use B as a firewall/gateway!)

SITUATION 2

             xover
    minbar --------- 

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

Yeah, pretty much.

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

No, it doesn't lock up.  It hard boots!  :-(  After plugging the cable in
and doing an fsck, everything's fine, but really!

> 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?

No, it's a standard 3Com 3c905B-TXNM, which I bought separately over the
internet.

> This *is* very strange.

Yup.  Pretty obnoxious, too.

Richard