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Don't do this - wrong fixed ARP

I've just spend hours tracking down a simple issue, there was a static ARP entry in my router, must have been a mistake setting a fixed DHCP lease, probably weeks and weeks back, to be honest I am confused as to when and how, as I've not touched that hardware or IP allocation for ages. I also don't use Static ARP entries at home.

What will happen in this situation (it seems) is that many things will work but underlying services updates etc. probably won't, so http will but ping won't, traceroute may, but then not. I assume it is about TCP/UDP/ICMP and the correctly routed return packet being sent to the wrong hardware address, I can only then assume that the few times it did work is "errors looking like success" (there must be a word for that), or the network items using their own ARP cache.

I know it is unlikely that anyone else will ever do this but to leave some trace in the record, when it "does not work all the time" try spreading tests over different protocols, put another item on the same cable/port/IP address noting changes each time, do of course reboot network items (to clear stale ARP tables) but also assume something is set that you personally "never would".

On the Mikrotik's fixed ARP is indicated by a single character.

It was a good exercise in BeagleBone recovery (using the original SD card) and although a bit slow that was painless and simple. The SD card I used will be labelled "good for recovery" and I'll make better printed notes for the config as I had not kept recent backups and the main edits covered only a few items. If I had made extensive customisations of the bookmarks I would have failed there.

My lessons to take away

  1. Protect one original SD card (to save having to download and write the image)
  2. Print out (PDF even) the config pages if you don't export or back them up
  3. If you have put hours of work into the bookmarks export a few copies
  4. If you create a backup SD card , record the server Port as well as SSH passwords
  5. Don't be me.


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