swap – What are the explanations for seeing an incomplete ARP?

I add this reply, though it’s (in all probability) in a roundabout way associated to the particular query above, it’s related in some circumstances

If a techniques static ip / dhcp settings are incorrect, this could present up as an oblique facet impact.

In my particular case I had a Linux machine which I moved areas and altered the community settings for.

I forgot to replace them in /and many others/dhcpcd.conf, once I moved the machine again once more.

That is my dhcpcd.conf

interface eth0
        static ip_address=192.168.1.8/24
        static routers=192.168.1.254
        #static routers=192.168.1.1
        static domain_name_servers=127.0.0.1

When it was “damaged”, I had the road static routers=192.168.1.254 commented out, and the road beneath #static routers=192.168.1.1 was uncommented.

This then triggered arp to show an invalid entry (“incomplete”) for the handle 192.168.1.1.

Although I solely moved this technique every week in the past, then moved it again a number of days in the past, I had completely no clue what was improper with it. So far as I might inform, I might join it to my native community, ssh into it high quality, and there have been no points exhibiting up with ip addr. traceroute additionally wasn’t exhibiting any helpful data, it simply urged that every one queries have been going by way of a single hop to “machine host title”. Clearly that made no sense and was improper, however it did not give me any indication as to what the difficulty was – it implied that every one queries have been being resolved (one way or the other) on the identical machine. This machine does DNS by the way in which.

Circuitously associated to the query of OP, however could also be very helpful to others who’ve this difficulty in future, and that is the one means you will have discovered to this point to diagnose it, as not a lot else seems to be improper!

• When utilizing this machine because the DNS server for different machines on the community, these machines have been unable to resolve exterior DNS requests
• I might ssh into the DNS server (that might be anticipated as its IP is ready static, and the swap/router additionally expects to see a machine with this static IP linked on the native ethernet by way of the swap).
• The very first thing that indicated some difficulty was sudo apt replace failed. (Since it is a DNS server, it has no graphical interface and due to this fact though working one thing like an online browser would have indicated this drawback loads sooner, or from the Community desktop tray icon, it does not have any of these issues.)
• ping 8.8.8.8 was additionally failing, nevertheless ssh and pinging native machines was high quality
• I checked /and many others/community/interfaces[.d] nevertheless all information in these dirs have been clean / set to defaults, because the DNS software program manages these
• There have been no errors/warnings within the DNS software program when connecting to the DNS server by way of the online interface provided
• I began checking issues like ip addr (no points) and arp. I already stated traceroute did not point out what the issue is perhaps, however did point out one thing bizarre was happening.
• Trying to find explanation why arp might need an incomplete entry guided me alongside the proper strains
• I discovered this query (https://serverfault.com/questions/765380/when-do-stale-arp-entries-become-failed-when-never-used) and began trying into the way to take away arp entries
• I take away the entry with arp -d 192.168.1.1
• Nevertheless it saved coming again
• I attempted to hook up with 192.168.1.1 on my native community from one other machine, nevertheless I used to be not stunned I could not do that or ping it as a result of there aren’t any machines on my community with this handle
• Normally, this handle could be the handle of your gateway / an ISP router in house purposes
• This jogged my memory that I had set a community adapter within the different location to have the handle 192.168.1.1, and that the pc that was linked to this handle was getting used as a router to attach the networks 192.168.0.X and 192.168.1.X
• The unique location is on community 192.168.1.X, the placement I moved to was on 192.168.0.X
• Since most 192.168.1.X/24 networks use 192.168.1.1 because the default gateway, seeing this IP did not initially arouse any suspicions, and it did not appear “bizarre” for any cause
• Nevertheless I modified the default gateway in dhcpcd.conf as described above and the difficulty was resolved
• Lesson discovered, when making a bizarre computer-to-computer community off website quickly, give the router adapter/NIC a bizarre static IP like 192.168.1.50 – which may assist to remind ones self or point out one thing out of the atypical is ready in a settings file someplace