[ale] Strange Linux networking issue

Derek Atkins derek at ihtfp.com
Tue Apr 20 09:17:21 EDT 2021


On Tue, April 20, 2021 8:52 am, Phil Turmel via Ale wrote:
> I would not expect that layout to work except by chance.  With those
> subnet masks, each box is free to send any packet to 172.16.x.y out
> either interface at any time.  Specifying an interface for ping won't
> prevent the other box from replying out the wrong interface.

The routing table is set up such that they know where to send it.  The (a)
and (d) links have a single-host route on them but no network-route, so
the machines definitely know to reach each other via (b)/(c).  There is
nothing about "chance" here, it's basic IP routing (well, okay, advanced
IP routing).  But certainly nothing out of the ordinary, and stuff I've
done for 25 years.

> To be reliable, (b) and (c) need to be in the same subnet, and (a) and
> (d) need to be in different subnets.   Not necessarily different subnets
> from each other, but definitely different from the b/c subnet.

That's not at all true.  It's all about the routing table and ensuring the
A and B know how to get to each other (via the b/c link).  I've done this
plenty of times before and it's worked just fine.  And indeed, this setup
was working just fine for a week or two before it suddenly stopped

My leading theory right now is what Jim suggested, that the Ethernet
autoconfig is going into a tizzy because the two computers are directly
connected via a patch cable and not a crossover cable, and the r8129
chipsets are getting locked out which is preventing ARP replies.

Granted, if this WERE the case, I'm not sure why ethtool is reporting link
up, and there's nothing in the demsg log about the link going down.

I've suggested to the person I'm helping to put a switch between these
devices, even though it's adding yet another "thing" in there.



> On 4/19/21 1:06 PM, Derek Atkins via Ale wrote:
>> HI.
>> I'm having a strange networking issue between two AWOW devices running
>> Linux that stopped talking to each other.  The devices are configured as
>> below:
>> --(a)[Box 1](b) ----- (c)[Box 2](d)--
>> Where:
>> (a) is
>> (b) is
>> (c) is
>> (d) is
>> The (b)/(c) link is a direct link between the devices, not a crossover
>> cable, and no switch.
>> The two devices were talking fine for a while and then all of a sudden
>> they stopped.  Now that won't talk at all.  When truing to ping .7.100
>> from Box 1, it sends out ARPs but there is no ARP response from Box 2.
>> Similarly, on Box 2 trying to ping .5.1 results in no ARP responses.
>> Both devices show the (b)/(c) link as up.  Running ethtool shows the
>> link
>> is detected.  However arp -an shows an incomplete response (no
>> surprise).
>> The dmesg output doesn't show anything.
>> I am at a loss. I don't understand how or why the two devices would stop
>> talking, and why neither will pick it up again.
>> Any suggestions for where to look or how else to debug this?
>> Thanks,
>> -derek
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       Derek Atkins                 617-623-3745
       derek at ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant

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