Most DNS queries are UDP.

I’d do a modified scream test and change old.domain to something like 1.2.3.4. Then run sudo netstat or ss with -tpn, grepping for 1.2.3.4.

Or something like grep -r old.domain /etc.

Yep, that would work fine for the first line of defense. Eventually, you can expand it to copy, replicate, or drive swap the onprem backups offsite somewhere (e.g., cloud, office, or family member) if you want to protect your data from site loss (e.g., house fire).

The only thing missing is a good backup.

If you are storing anything important – especially Immich and Vaultwarden data – you should have a good offsite protection strategy. And even the HASS config should be backed up with versioning because rebuilding from scratch could be painful once you get deep into it.

I’ll let others chime in on possible good backup options because I use Veeam and Azure, which really isn’t in the spirit of this community, and I’d be interested in good open source options myself.

Also, RAID (mirroring) is NOT a backup.

Lol Microsoft is not even close to a walled garden. This is just them removing the password manager feature that nobody used from their authenticator app.