How I Solved my Ethernet Woes

["John J. Trammell" <trammell@nitz.fnal.gov>]

Hello all:

I've had some minor difficulties diagnosing the problem I had on my
small home network, so now that it's working I thought I'd write it up
to (perhaps) save some poor soul the trouble I had.

I'm trying to network two 486'es, call them, say, 'pico' and 'sol'.
Both are running Linux 5.1 kernels, with 10Base-T NE2000 clones.

The problem occurs with either a hub or crossover-cable configuration;
the mechanical part of the connections are good (both 'OK' LEDs on the
NICs are lit) but pinging 'sol' from 'pico' fails.  A little investi-
gation yields that 'pico' is sending out ARP requests to 'sol', and
indeed 'sol' is responding with its MAC address, but somehow 'pico'
isn't caching the ARP data like it should -- an 'arp -a' shows that
'pico' knows it should have 'sol's MAC address but doesn't.  Hmm!

It turns out the problem was the configuration of eth0 on 'pico' --
I'd torqued the IRQ (the PnP setting wasn't the same as the insmod
setting), yet somehow had gotten the NIC to respond.  I'm thinking
the behavior I got was the NIC equivalent of when modem IRQ's are set
poorly, that is, marginally correct behavior (e.g. sending out the ARP
request packet) but some sort of messed-up timing behavior (e.g. fail-
ing to catch and save the ARP reply).

What helped make this hard to diagnose was that the signal-to-noise
ratio in comp.os.linux.* is so pathetic.  In searching DejaNews I saw
four or five people posting similar questions, but most of the responses
blamed DNS, and sent me off on a wild goose chase.  Bastards!

Anyhow, I doubt this will help anyone immediately, but I'm hoping this
will eventually find its way into AltaVista or some such, and help
some newbie.

Thanks,
John