Warning
Create a Time Machine backup before major updates, disk operations, or security changes on macOS.
What you will achieve
Clear stale DNS on Mac after resolver changes or flaky site loads.
1) Flush DNS cache
sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder2) Verify resolver behaviour
scutil --dns | grep 'nameserver\[[0-9]\]'3) Re-test affected domains
Open the previously failing site and check whether stale records are gone.