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So, when the user types in internal.mydomain.com, it gets sent to Cloudflare then they query my public domain server which doesn't have the entry so it falls back to checking the DNS the system has listed. Is Firefox going to cache the IP like the OS does?

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The behaviour depends on a few preferences:

* network.trr.mode can be set to 0 (disabled), 1 (race native vs TRR), 2 (TRR first, OS DNS as fallback), 3 (TRR only), 4 (run native and TRR in parallel but use native results, save TRR timings for telemetry), or 5 (off by choice)

* network.trr.uri configures which DoH endpoint is queried

Firefox does maintain a DNS cache, even if you use the native DNS resolver. You can view the cache at about:networking#dns.

So, I really need to set 5 because 0 will probably later become default consent to use DNS-over-HTTPS.

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