[Doh] Dedicated DoH port

[Tomas Krizek <[e-mail]>]

Disclaimer: I don't adocate the use of a dedicated DoH port rather than
using port 443 for most DoH traffic. I'm simply trying to establish
reasonable defaults as a software developer and packager.

Knot Resolver will use 44353 as the default port for DoH. We've
considered using port 443 by default, but it presents many challenges.

If an admin is already running an https service on the machine, the
clash with DoH resolver can be quite problematic. In best case scenario,
the admin runs into an error (not able to bind to port 443 - quite
cryptical for someone trying to run DNS resolver who's not up to date
about DoH development). In a worse case scenario, the DoH service might
actually seem to successfully start and run alongside the unrelated
https service (e.g. when both services use systemd socket activation
with ReusePort=true - basically SO_REUSEPORT under systemd).

Those who know what they're doing will have no issues configuring their
DoH service to run on port 443. However, I think it's reasonable to use
a different, dedicated port as DoH default for packaging, documentation etc.

Since there is currently no IANA assigned DoH port, I've filed the
following user port request with IANA to establish a common default that
could be used among DNS vendors.

Service Name:         [domain-doh]
Desired Port Number:  [44353]
Description:          [DNS query-response protocol over HTTPS]
-- 
Tomas Krizek
PGP: 4A8B A48C 2AED 933B D495  C509 A1FB A5F7 EF8C 4869