[TAG] Eth0 debugging

[John Karns]

On Fri, 23 Jun 2006, Bob van der Poel wrote:

> Well ... the friend was running his system direct to a XP box, bu has since 
> had a linux box installed which he is using a router. He's distributing his 
> connection to some neighbours.

The thought later occured to me that, your friend willing, you might
consider swapping modems with him for a few days to compare performance.


> Can we call a sat link at 22500 miles "line of sight"?

Maybe not the best choice of words, and I rather doubt it myself - just
another point to consider, along with my disclaimer that I have no
experience with a satellite link :).  Probably better said that a
satellite link would be to some degree vulnerable to changes in
atmospheric conditions.


> Yes. I think that is the case here. The IP is dynamic, but I don't think it 
> changes very often. Hmmm, could this be a DHCP issue?

Certainly not out of the realm of possibility.


> Question: WHen pinging the local host like this does the chain leave the 
> local box?

No it doesn't.  The whole point with that is to confirm that your NIC,
as part of the link, is functional at that particular time.  The general
approach I used was to start the diagnostic process at the closest point
of origin to the host, and worked outward.  The link to the outside
world obviously includes things other than your modem.

Although you say "I blame the modem/satellite", a comprehensive
troubleshooting process should, IMO, include looking at other components
involved as well.  Although rarely, I _have_ experienced situations when
the link problem was due to the NIC, and a reload of the NIC driver
resolved the issue.


> I'm not sure what this proves other than the fact the IP address 
> is valid and correct? Or does it have to go the modem first? Which would be 
> an indication that the modem is "active".

No, just that the card is responding to the host.


>> 3) Re-init the modem.  Even with a steady-on sync lite, the connection
>> status could be flaky, such as might be caoused by a short interruption
>> in the power to the modem.

Step 3) is important, particularly if your power line has noise spikes,
brown-outs, or occasional interruptions (fractions of a second of AC
cut-outs can leave your modem in an undetermined state, without it
being apparent from looking at the status lights.  I'm just giving
broad suggsetions here of potential problem issues.  Obviously I have
no direct knowledge of the particulars of your AC power conditions.


>> 4) Depending on your distro (Slakware being an exception, as it uses a
>> BSD style init rather than SysV - could be different now though), you can
>> try re-initializing the networking subsystem on your host:
>> 
>> /etc/init.d/networking restart
>
> Mandrake. Yup, tried that without success.

That often does it for me, especially after resuming from a suspend, if
my link dies.


> Question: would pulling the ethernet cable in/out prove or do anything?

Not for any reason that I can cite, except for the possibility of a
flaky cable - not to be dismissed out of hand.  Swapping the cable
would be the tack to take there.  Dis/re-connecting just stirs it up a
bit, but would be quite hit or miss.

-- 
John Karns