Had to play IT support at my parent's place again.
They have a single shitty Vodafone router in the cellar and I sprinkled around two more old WiFi access points that I had laying around to cover the house with WiFi. No need for huge performance, it just needs to cover the basic needs and we have plenty of free RF spectrum here on the countryside.
Anyway, somehow my dad managed to reset one of these access points and revert it to factory defaults because it sat on his desk and he shoved it around which seems to have pressed the non-recessed reset switch on the back.
And because every single plastic router assumes that it's the only device on the network it enabled DHCP. Funnily enough the default settings on this thing and the current settings of the main router used the same private subnet.
Now we had two DCHP servers distributing leases for the same subnet with the same default gateway IP and local DNS resolver. That kind of worked. Most of the time, but if it did it did way too well.
The main router won the race for the ARP replies most of the time, so packets destined for the internet went through the right gateway. Only very occasionally the other router won and "poisoned' the ARP table of a single machine which lost internet access.
I only noticed that one router reverted to default settings, because it had the wrong WiFi SSID and no enabled encryption. Great default settings.
Time to gift them two small Mikrotik access points and throw out this 15 year old Chinese plastic garbage I grabbed from the bottom of the box with all the cables you keep because you might need them at some point in time.
edit: Now that I think of it... did the real router really win the race for the ARP replies? Which reply is going to be saved in the ARP table? The first to arrive? The last because it's treated as a more recent table entry which overwrites the first like in a gratuitous ARP case? I haven't actually checked and just fixed the problem.
