Use SVCB (RFC9460) instead of SRV (RFC2782)

[mdavids]

UPDATE: I noticed that the patched Chrony uses getdns, which is able to fetch A/AAAA records for the SRV targets, so perhaps that eliminated the need for this suggestion.

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I suggest considering the use of SVCB records rather than SRV records.

The main reason is the availability of the ipv4hint= and ipv6hint= parameters.

Here's a possible use case: imagine a theoretical scenario in which a Time User only has IPv4 connectivity, while all Time Sources except one only have IPv6 connectivity. A DNS query for the SRV record of srv.experimental.ntspooltest.org would return all available servers, after which the client would have to try them one by one only to discover that most of them are unusable.

With SVCB records, the ipv4hint= and ipv6hint= parameters in the DNS response would immediately indicate which of the returned servers are actually reachable.

If this were considered, I would also suggest using underscored node names, for example: _ntspool.example.org..

Possible example record:

_ntspool.example.com. 3600 IN SVCB 1 nts1.example.net. (
    alpn="ntske"
    port="4460"
    ipv4hint=192.0.2.1
    ipv6hint=2001:db8::1
)