DNS upstream usage sequence

I did not find the answer in the Wiki, so I wonder if there is any usage sequence if more than one DNS upsream server is used - is it round-robin, or just alphabetical order?

And if I enter only one upstream server, and this one is offline for maintenance, IPfire uses its own recursion - but how often are the upstream servers tested for availability? Is it periodically, e. g. each minute?

Unbound test the servers to determine which is the fastest by sending queries to multiple servers. If a server is down it is blocked for 15min and then tested again.

It try to use the fastest server most of the time.

https://unbound.docs.nlnetlabs.nl/en/latest/reference/history/info-timeout-server-selection.html

unbound will only use recursion if no server is configured. (no auto fallback.)

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