[ale] Strange Linux networking issue

Derek Atkins derek at ihtfp.com
Tue Apr 20 10:36:21 EDT 2021

Gotcha.  That makes sense.  Would it still report 1000Mb Full Duplex if
it's fubar?


On Tue, April 20, 2021 10:32 am, Jim Kinney wrote:
> 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
>>See JOBS, ANNOUNCE and SCHOOLS lists at
> --
> Computers amplify human error
> Super computers are really cool

       Derek Atkins                 617-623-3745
       derek at ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant

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