[TAG] Eth0 debugging

[John Karns]

On Thu, 22 Jun 2006, Bob van der Poel wrote:


> Hi all. I'm having some "odd" connection problems and wondering how to
> track it down.

Well, I have zero experience with satellite equipment, but I have had my
share of connection issues with my ADSL setup over time, so I'll mention
some of the things I do when confronted with such problems.


> The satellite comes to me though a modem which is pretty much the same
> as  cable modem. I just plug the ethernet cable in and it runs. From
> what I've found out, I can not get any info from the modem. Only the ISP
> can do that.
>
> The modem does have some status lights. POWER, CONNECTED, IN and OUT.
> The last 2 indicate computer <> modem traffic. The CONNECTED light is
> solid when there is a connection the satellite.
>
> Now, for the most part all this works just fine. But, at other times I
> seem to lose the connection ... When that happens the CONNECTED light is
> still on, but my computer doesn't appear to have a connection the the
> internet. The IN/OUT lights to blink, so that does tell me that the
> modem/computer link is up?
>
> I blame the modem/satellite. The ISP tells me to reboot the computer
> (too much time debugging windows?). Quite often this does work. But,
> this morning it didn't. But, repowering the modem did the trick.
>
> So, I could call the ISP ... and after a long hold they will tell me
> that it has to be down from them to debug, that it isn't the modem, must
> be me, etc.
>
> What I'm after is some debugging ideas. What can I run on my computer to
> see if there is a problem here.
>
> I figure that we're down to a few possibilities:
>
> 1. The modem is flaky.
>
> 2. The satellite connection is flaky (my friend has the same system and
> is NOT having these problems).

Could be attributable to differences in your respective environments.  I
would assume that a satellite link is somewhat line of sight.  I could be
wrong, but if you're surrounded by a lot of trees for example, where there
is dense foliage between the dish and the satellite, that could affect
signal strength.


> 3. I'm having a software problem. Buffer overflows or something. Which
> means that a reboot would fix it (sometimes it does).

Depends on what kind of connection protocol - if that's the right
terminology - your modem uses.  Perhaps one of the easiest to deal with
is just a plain old ethernet connection, where the port of your modem
provides a routable IP address.

Many ISPs these days have moved to setups using some variant of ppp, such
as PPPoE, which requires an additional daemon running on your connected
host to support it.  On a Linux host using PPPoE, the most popular
software for that is from Roaring Penguin.  Depending on your Linux
distro, that can be tricky to set up.  But since you didn't mention it,
I'll assume that it's not an issue here.


> 4. I'm having a hardware problem with the on board ethernet. Don't know
> if a soft reboot would effect this, but the power cycle today didn't.

Always a possibility, although usually of low probability.  The way to
test for that is with a different host and / or NIC.


> Maybe I should have a router in the chain? Not sure what that would prove.

Principal merits of a router would be in the realm of connection sharing
(NAT, etc) for multiple hosts and firewall setup.

Just a WAG on my part, but I would guess that your modem is rather
similar to ADSL, in that it uses a synchronous connection.  I would
further guess that the CONNECTED light indicates whether or not the modem
is synchronized.  In my experience, that's usually where my connection
problems arise.  Re-initializing the modem usually takes care of that
issue by forcing connection renogociation with the port on the ISP side.

To summarize, my procedure is something like this:

1) From a terminal command prompt: "ifconfig" to see the status of the
host network interfaces.  You should see a response something like:

wlan0     Link encap:Ethernet  HWaddr 00:E0:98:49:85:6D
           inet addr:192.168.1.116  Bcast:192.168.1.255  Mask:255.255.255.0
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:6042 errors:10 dropped:0 overruns:0 frame:10
           TX packets:6281 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:1000
           RX bytes:5052344 (4.8 MiB)  TX bytes:955796 (933.3 KiB)

The wlan0 above is the name that the Linux host is giving to the
interface.  Yours could be different, such as 'eth0'.  One can also ping
the local interface to see if it's talking to the host.  In this case
"ping 192.168.1.116".

2) "netstat -nr" should show the gateway address, indicated with a 'UG'
flag on the same line.  Depending on whether your modem is running bridged
or as a router (depends on the equipment and the ISPs choice of
confiuration), this could be an interface on the modem (functioning as a
router / dhcp server) or an interface that the ISP is providing.  A
successful ping (a response is echoed to your terminal screen, with a time
in seconds indicating the latency of the response) to that IP would
indicate that the problem is outside of your host / modem environment.

It may not be conclusive however, because in some cases, the ISP will
block pings (as mine does).  In that case, you can pick some known
internet address.  Use a number rather than a name though (such as
209.73.186.23 rather than www.yahoo.com), to eliminate DNS issues from
this step.  If you succeed with the ping to the IP address, but not to the
name address, then the problem is not a connection issue, but is in name
resolving - most likely a problem in the ISPs domain.

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.

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

works on most Debian derived distros, as well as SuSE, IIRC.

This procedure is not exhaustive, but should help get you started.

-- 
John Karns