https://support.mozilla.org/en-US/kb/canary-domain-use-application-dnsnet

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        [*] Firefox
        [Version 113]
        [Windows 8      ]

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Canary domain - use-application-dns.net

To signal that their local DNS resolver implements special features
that make the network unsuitable for DNS-over-HTTPS (DoH), network
administrators may configure their networks to modify DNS requests
for the following special-purpose domain, called a canary domain:
use-application-dns.net.

Note: The canary domain only applies to users who have DoH enabled as
the default option. It does not apply for users who have made the
choice to turn on DoH by themselves.

Firefox will attempt to resolve this domain using the DNS server(s)
configured in the operating system of the device, and examine the
result. The result will be considered negative if:

  * A response code other than NOERROR is returned, such as NXDOMAIN
    (non-existent domain) or SERVFAIL.
  * A NOERROR response code is returned, but contains neither A nor
    AAAA records.

The result will be considered positive if the query completes with
NOERROR and contains A or AAAA records (or both).

A negative result will be a signal to disable application DNS, (i.e.,
DoH).

The use of this domain is specified by Mozilla, as a limited-time
measure until a method for signaling the presence of DNS-based
content filtering is defined and adopted by an Internet standards
body.

Note: Some existing DNS filtering providers implement similar domains
for users to verify that filtering is working. This canary domain
differs by being intended to be checked by software such as Firefox,
rather than checked explicitly by the user, and by working across
filtering providers.


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These fine people helped write this article:

AliceWyman, Mozinet, Joni, Angela Lazar
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