Different OSes handle it differently. Windows tries the primary, waits 1 second and then starts trying secondaries.

glibc tries in order. musl (Alpine Linux) tries all in parallel and returns thebl first resolution (If any).

Most systems do not round-robin by default afaik. They just try the listed DNS servers in order (and resolv.conf will only respect the first 3 IIRC).

DNS fallback is often also misunderstood to mean "fall back if the domain is not found", but it really means "fall back if the name server fails to respond". If the domain is not found (i.e. servers returns a valid NXDOMAIN response), most resolvers do not consult any other name servers.

Round-robin, cascading name lookups, and other frequently desired functionality can be obtained through dnsmasq or similar caching/forwarding name servers.

There are also options for libc that affect this behavior that can be set in /etc/resolv.conf like:

    options timeout:2 retries:2 max-inflight:768 max-timeouts:100 single-request-reopen rotate

There are man pages that describe these options.

Can confirm. If I have a secondary DNS specified in my router, other than my pi-hole, it becomes useless. My guess is the router is either measuring response time, and goes with the most efficient, or otherwise round-robining the requests. Either way, requests bypass the pi-hole in such quantities that it became useless.

PS: someone here mentioned that this behavior is OS-dependent - nope, this happens on the router level, and all devices in my apartment suffer.

It depends on how your router's DHCP server is configured. If you configure your router to pass its own IP address out as the DNS server for the local subnet then the router's behavior dictates how DNS works. If your router is passing out an external DNS in the DHCP configuration, then you'll get OS-dependent behavior.

My router uses a DNS resolver internally, and it will spread-cast to multiple DNS servers and use the quickest response it can get. It also caches using the TTL in the DNS response, and so it will serve up cached records transparently.

Then your devices are likely using your router as a DNS resolver, which in turn talks to your pihole and the external one. And thus it depends on your router's OS what it does.

Dnsmasq has different modes if you give it multiple upstream resolvers. By default it only queries the fastest one, which it determines by every now and then sending a query to all the servers and picking the one that replies first. You can tell it to always query all servers, or always query them in the order specified.

If your recursive DNS servers are Unbound DNS, they will keep note of which infra (upstream forwarder) nodes are responding and will blacklist the ones that do not. You can see this with

    unbound-control dump_infra

After the infra ttl expires, they will probe the down nodes. If they respond, then traffic will be distributed among them again.