[olug] Linux networking weirdness

Obi-Wan obiwan at jedi.com
Mon Oct 26 19:53:59 CDT 2015

Except that the firewall was able to talk to my laptop over the external NIC just fine.

Ben "Obi-Wan" Hollingsworth, www.Jedi.com
Sent from my Ting Samsung Galaxy Note® II

<div>-------- Original message --------</div><div>From: Dan Linder <dan at linder.org> </div><div>Date:2015/10/26  4:41 PM  (GMT-06:00) </div><div>To: Omaha Linux User Group <olug at olug.org> </div><div>Subject: Re: [olug] Linux networking weirdness </div><div>
</div>Could it be something as simple as a dead/dying NIC in the firewall?

Where you've replaced cables and the laptop was able to communicate (albeit
with a new MAC), that's what jumps to my mind.


On Mon, Oct 26, 2015 at 12:10 PM, Obi-Wan <obiwan at jedi.com> wrote:

> Thanks, I'll inspect those ARP requests.
> I did put the firewall's IP on my laptop when I hooked it up in place of
> the firewall.  I didn't snoop for ARP traffic then, but since I was able to
> send traffic through that gateway, I have to assume that I got an ARP
> response.
> Hard coding a MAC address might get things working for now, but that
> sounds like it's just asking for trouble down the road when my ISP changes
> hardware without telling me and I've forgotten about that hardcoded entry.
> It's worth a shot for debugging purposes, though.
> On 10/26/2015 11:58 AM, Matthew G. Marsh wrote:
>> If you connect your laptop again & do a TCPdump do you see the ARP
>> answers from the ISP gateway?
>> Then if you put your Servers MAC address on your laptop do you still see
>> the ARP answers?
>> If so maybe look and see what is different between the ARP packets. You
>> can also try hard coding the ISP MAC into your ARP table on the firewall to
>> force the sending. Just use the MAC address given to your laptop.
>> That is what I saw when reading through.
>> HTH
>> mgm
>> On Mon, 26 Oct 2015, Obi-Wan wrote:
>> Hey folks,
>>> My home Internet stopped working suddenly last Friday night, and I'm at
>>> a loss to explain what I'm seeing.  It was an instantaneous failure, not a
>>> slow degradation, and nobody was doing anything on my firewall at the
>>> time.  The kids were just web browsing on their tablets, which is how we
>>> first saw the problem.  If any of you have any suggestions after reading
>>> this entire treatise, I'd love to hear them.  Here's what I think I know:
>>> Normal setup:  Internet comes wirelessly via a Future Tech radio dish on
>>> my roof.  An ethernet cable (with POE) connects the radio to my firewall,
>>> which is a dedicated Linux server.  Only the POE power injector sits
>>> between the two.  The firewall has a static public IP address on a /25
>>> network that sends traffic to a gateway at my ISP's site.  The firewall
>>> runs IPtables and handles NATting / DNS / DHCP for my home LAN.
>>> Problem symptoms:
>>> My LAN (both wired & WiFi) can still reach the firewall from the inside
>>> just fine.  The firewall can no longer reach the ISP's gateway IP or hence
>>> the Internet at large.  TCPdump on the firewall's external NIC shows
>>> repeated unanswered ARP requests for the gateway from my firewall.  I tried
>>> turning off IPtables entirely, but that had no effect on my firewall's
>>> ability to see the outside world.  The firewall's external NIC still shows
>>> link lights and traffic flashing.  I've tried replacing all the short
>>> cables, and the visible portion of the long cable running from my roof to
>>> my basement shows no visible damage.  I've tried powering down &
>>> un/re-plugging all the related equipment, but to no effect.
>>> The ISP can connect to the rooftop radio from the outside, so that link
>>> to my house seems to be good.
>>> If I disconnect my firewall from the radio and plug my linux laptop
>>> directly into the radio (configuring it to have the firewall's static IP),
>>> then my laptop can get out to the Internet just fine. That seems to
>>> indicate that the POE injector, the long cable, and the gateway
>>> configuration are fine. Physical distances forced me to use a different
>>> cable to connect my laptop to the POE injector than I use to connect the
>>> firewall to the POE injector.
>>> If I connect my laptop directly to the external NIC on my firewall using
>>> a crossover cable (configuring my laptop to be a different IP on the
>>> external /25 subnet), then the laptop & the firewall can communicate with
>>> each other just fine.  That seems to indicate that the firewall is working
>>> just fine.
>>> If I connect the rooftop radio directly into my LAN switch (bypassing
>>> the linux firewall) and let the radio handle NAT / DHCP on a non-routable
>>> subnet that it provides, then the rest of my LAN can get to the Internet at
>>> large, but at an unusably slow speed (240 Kbps download).  That's how I
>>> left things at the moment.  I didn't have to change any config on the radio
>>> to make this happen, so apparently it's able to do this and serve my normal
>>> static IP simultaneously.
>>> If both the radio and the firewall both test fine, and the cable between
>>> them has already been replaced, why isn't this working? What else should I
>>> be looking at?
>>> I tried calling Future Tech's phone support on Saturday, but I could
>>> hear the guy's eyes glazing over when I described my normal setup with a
>>> linux firewall.  He wasn't able to offer any suggestions.
>>> As I type this, it occurs to me that it's *possible* (though highly
>>> unlikely) that *both* the cables I tried using to connect the POE injector
>>> to the firewall could be bad.  I'll have to verify that when I get home
>>> tonight.  In the mean time, I'm at a complete loss.
>> --------------------------------------------------
>> Matthew G. Marsh
>> Special Email Addr for OLUG ;-}
>> Phone: (402) 932-7250
>> Email: olug4mgm at paktronix.com
>> WWW:  http://www.paksecured.org
>> --------------------------------------------------
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> --
> *Ben "Obi-Wan" Hollingsworth* obiwan at jedi.com <mailto:obiwan at jedi.com>
> www.Jedi.com <http://www.jedi.com>
> The stuff of earth competes for the allegiance I owe only to the
> Giver of all good things, so if I stand, let me stand on the
> promise that You will pull me through. /-- Rich Mullins/
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