Pi-Hole 'invades' my landlord's network

Solution 1:

It sounds like the broadcast domains might not be isolated.

If devices on his segment are actually getting DHCP leases from your router, even if it is 50% of the time, that might explain it.

The only way your Pihole DNS server affects your landlord's subnet is if they are getting it from your DHCP server. Which, if true, means the Pihole is just a symptom of a larger problem. You can also easily test this without turning on the Pihole.

So with your Pihole turned off, see if you can test this. Check the DHCP leases on your DHCP server. Or better yet double check on your landlord's side to see what Lease and DHCP options you get when you connect a new device on his site. ipconfig /all will show you the DHCP server that is providing the lease. Run it a couple of times to see if your DHCP server is serving 50% of the leases. You might have misunderstood the subnets, your router config, or the broadcast zones.