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).
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.