What is knocking LAN devices offline?
June 29, 2022 6:38 AM   Subscribe

I'm running a Raspberry Pi on my home LAN which is itself running Home Assistant.

Sometimes the Pi loses connectivity and I have to pull the power plug to restart it. I can't SSH in or access the web GUI.

Sometimes, the Pi goes through phases where rebooting knocks other devices on my LAN offline, taking the internet connection with it!

I'm wondering if it's an IP address conflict, but pinging the Pi's reserved address doesn't return an answering ping unless the Pi is on.

Very strange

Any ideas?!
posted by dance to Computers & Internet (6 answers total)
Is the pi overheating?
posted by fake at 6:39 AM on June 29 [1 favorite]

I've seen routers with built-in DHCP that enforce the policy that static IP assignments have to be outside the range used for DHCP (for example if your DHCP range is to then the router won't let you configure static address assignments unless they're <= .49 or >= .100). Other DHCP servers will let you use addresses from within the pool and in theory should be smart enough not to hand those addresses out to anything but the specified MAC, but not doing so is an easy way to tell at a glance that you shouldn't ever end up with a conflict.

Is everything plugged into one router directly, or do you have one or more network switches in place? This is one of those situations where I'd be tempted to add, remove, or upgrade switch hardware to see if a particular switch is no longer functioning as well as it should.
posted by fedward at 7:38 AM on June 29 [2 favorites]

Shakes Magic 8 Ball....

It's stealing the gateway's IP address. When other devices' ARP cache times out and they re-ARP for the gateway the Pi is also answering so the ARP caches get poisoned and devices can't reach past the router.

You're probably on the right track about IP conflicts getting everything confused. There's also flaky interfaces spewing bogus MAC addresses and overloading the switching fabric's lookup tables.

This would be easier to figure out on a network of 'smart' switches/routers. Also easier wired directly together to a laptop running Wireshark to look at the traffic.

But I'm assuming for a first guess that everything is wireless.

If it's narrowed down to "only happens when the Pi is there", then it's the software doing weird things with the network stack. Or it's the network interface being flakey all by it's self.
posted by zengargoyle at 7:43 AM on June 29 [2 favorites]

I have had cheap home routers that ran out of IP addresses to allocate (more precisely: of log space on its internal disk when it had allocated a certain number of addresses). This was not quite that behaviour, but I can see similar flakiness coming from a grumpy DHCP server not reissuing leases. Check any logs your router has generated.
posted by How much is that froggie in the window at 8:09 AM on June 29 [1 favorite]

Knocking other devices off the LAN sounds a bit like what can happen if a second device on a network tries to become the DHCP server. I recall I managed to do something like that with a Synology once.

Someone reports a similar issue with Home Assistant here. No real solutions there, unfortunately, and DHCP might be a red herring. Another mention of something superficially similar here relating to Pi-hole.
posted by figurant at 9:51 AM on June 29 [1 favorite]

You are not also running PiHole somewhere? It likes to be made DNS on the local network and I had seen older versions steal this away from the router. (Also my Synology has also stolen DNS without careful setup)
posted by rozcakj at 6:08 AM on June 30

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