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I have a setup where I have two computers connected to each other (C1 and C2). Both have a static IP and they know each others IP. C2 sends UDP packets to C1 and C1 receives and logs them.

Seems simple but here is the catch: There is about 1 kilometer of coax cable connecting those two devices. The connection between C1 and C2 is through a ethernet over coax converter which also transfers PoE. C2 is at a remote location and the only connection is a COAX wire.

The setup seems to work at times, I have had it running for several hours. But then at another time, without changing the configuration, the connection seems to become unstable. Since it looked like a connection issue I used wireshark to see what is being transmitted over the network.

When the system is running ok I can see all the packages from C2 reaching C1. When there are connection issues I can see that C2 is broadcast the command

Who has ip ip addr C1 - tell ip addr C2

This feels like it has lost it's connection. To verify that it is not a power issue I have checked the uptime of C2 and this is OK. It has been running continuously over the connection problems.

I am trying to figure out what causes this connection problems. The questions I have at this point are:

  1. When does an device emit the 'Who has IP' message?
  2. What tests can I do to check to establish if it is a software or hardware issue?

Final comment: I have placed a router between the devices on the C1 side. The connecting outages till occur but it seems to be less frequent. Is a router able to stabilize connections like these?

  • Host configurations an OSes are off-topic here, so your second question is off-topic here. You could ask about that on Super User. You would need a cable professional to check your cabling – Ron Maupin Mar 14 '18 at 16:10
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Apr 1 '18 at 22:27
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Since C2 is ARPing for C1 (which is presumably answering the ARP request) the probable cause is a communication problem from C1 back to C2 - the ARP response doesn't make it back and transmissions stall. Apparently, C2 hasn't received anything from C1 for a while and the ARP cache entry has timed out.

If the coax link isn't reliable it needs to be fixed. You should test the link by running a few tests across. If the C2 side isn't easily accessible you can set up a remotely controlled machine with an independent (mobile) Internet uplink you can connect in on. If these tests show up everthing is fine the problem is the C2 device, otherwise it's either one of the converters or the cable (which is somewhat unlikely if it's a single cable used bidirectionally).

The router question is impossible to answer without detailed communication analysis.

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  • Thank you for the response. I have found that the arp mapping has to be updated every five minutes or so, that explains why I see this coming up every now and then. Would a manually static arp entry help? What kind of tests could I do? Should I just check the number of packets being transmitted? And the problem could be the C2 device, in what way? It's justing emitting packets. – Steven Mar 15 '18 at 8:15
  • A static ARP entry will only help if ARP is the problem (rather unlikely) - I rather suspect the C1-C2 transmission is unreliable. You can run e.g. iperf to produce and gauge traffic and monitor the switches/ports for anomalies (FCS, runts, ...). If the converter-converter link shows up fine the C2 device is pretty much what's left, possibly it has problems receiving traffic. – Zac67 Mar 15 '18 at 11:51
  • As it turns out it was the coax connection. The bottleneck was the quality of the coax cable. The hardware was unable to transmit over such a long distance with the quality of coax we used. – Steven Apr 3 '18 at 7:26
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Your first question, "When does an device emit the 'Who has IP' message?" is the only on-topic question. What you are seeing is an ARP (Address Resolution Protocol) request. ARP is used to resolve a layer-3, e.g. IPv4, address to a layer-2 ,e.g. MAC, address.

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  • Thanks for the response. Does the router keep track of ARP records as well? For instance if an ARP request is emitted, what reponds: the computer with the MAC address or the router with the table entries? – Steven Mar 15 '18 at 8:21
  • Routers are hosts, too, and they use ARP, just like a PC or printer. – Ron Maupin Mar 15 '18 at 16:39

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