Can a local DNS cache be configured to use stale records when upstream fails?
Solution 1:
I know similar feature in unbound DNS.
From unbound docs:
serve-expired: <yes or no>
If enabled, unbound attempts to serve old responses from cache
with a TTL of 0 in the response without waiting for the actual
resolution to finish. The actual resolution answer ends up in
the cache later on. Default is "no".