[ale] Strange Linux networking issue

Jim Kinney jim.kinney at gmail.com
Tue Apr 20 10:32:27 EDT 2021

Ethtool reports up on electrical connection, not logical. So port sees voltage >0 on pin1,4 but logical state can be fubar.

On April 20, 2021 9:17:21 AM EDT, Derek Atkins via Ale <ale at ale.org> wrote:
>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
>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,
>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)
>> (d) need to be in different subnets.   Not necessarily different
>> 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
>A and B know how to get to each other (via the b/c link).  I've done
>plenty of times before and it's worked just fine.  And indeed, this
>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
>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
>>> 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
>>> cable, and no switch.
>>> The two devices were talking fine for a while and then all of a
>>> they stopped.  Now that won't talk at all.  When truing to ping
>>> from Box 1, it sends out ARPs but there is no ARP response from Box
>>> 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
>>> 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
>Ale mailing list
>Ale at ale.org
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