[ale] Strange Linux networking issue

[Jim Kinney]

Yep. That's the state of the connection on it's end.

On April 20, 2021 10:36:21 AM EDT, Derek Atkins <derek at ihtfp.com> wrote:
>Gotcha.  That makes sense.  Would it still report 1000Mb Full Duplex if
>it's fubar?
>
>-derek
>
>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:
>>>Hi,
>>>
>>>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
>>>working.
>>>
>>>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.
>>>
>>>Thanks,
>>>
>>>-derek
>>>
>>>> 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 172.16.5.0/16
>>>>> (b) is 172.16.5.1/16
>>>>> (c) is 172.16.7.100/16
>>>>> (d) is 172.16.7.0/16
>>>>>
>>>>> 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
>>>
>>>_______________________________________________
>>>Ale mailing list
>>>Ale at ale.org
>>>https://mail.ale.org/mailman/listinfo/ale
>>>See JOBS, ANNOUNCE and SCHOOLS lists at
>>>http://mail.ale.org/mailman/listinfo
>>
>> --
>> 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

-- 
Computers amplify human error
Super computers are really cool
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