Change the default port of Dns over Quic #1834
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InRFC9250,
4.1.1. Port Selection
By default, a DNS server that supports DoQ MUST listen for and accept
QUIC connections on the dedicated UDP port 853 (Section 8), unless
there is a mutual agreement to use another port.
By default, a DNS client desiring to use DoQ with a particular server
MUST establish a QUIC connection to UDP port 853 on the server,
unless there is a mutual agreement to use another port.
DoQ connections MUST NOT use UDP port 53. This recommendation
against use of port 53 for DoQ is to avoid confusion between DoQ and
the use of DNS over UDP [RFC1035]. The risk of confusion exists even
if two parties agreed on port 53, as other parties without knowledge
of that agreement might still try to use that port.
In the stub to recursive scenario, the use of port 443 as a mutually
agreed alternative port can be operationally beneficial, since port
443 is used by many services using QUIC and HTTP-3 and is thus less
likely to be blocked than other ports. Several mechanisms for stubs
to discover recursives offering encrypted transports, including the
use of custom ports, are the subject of ongoing work.