[FR] support dohpath key in SVCB records

[crolfe]

This a request to implement/support the Service Binding Mapping for DNS Servers RFC, which defines a new SvcParamKey (i.e. dohpath) for use in SVCB records. This would be an extension of the current work to add SVCB support to Unbound.

Current behavior
Once this PR lands, Unbound will support a list of standard SvcParamKeys in a SVCB / HTTPS record. If an answer contains an unknown SvcParamKey (e.g. dohpath) it will replaced by key65535, as that is what the RFC says to use for "Invalid key".

;ANSWER
resolver.arpa. 300 IN SVCB 1 . alpn="h2"  key65535="/dns-query?{dns}"

Describe the desired feature
Unbound will recognize dohpath as a valid SvcParamKey in SVCB records. When a client queries for a SVCB record that contains a dohpath, this should be present in the answer:

;ANSWER
resolver.arpa. 300 IN SVCB 1 . alpn="h2" dohpath="/dns-query?{dns}"

Potential use-case

• Unbound instances will be able to support Discovery of Desginated Resolvers, which defines a mechanism for DNS clients to discover encrypted DNS services (e.g. DoH, DoT, DoQ) provided by their dns53 resolver.

Current Blockers

• The SVCB PR is still a draft. I am not sure what the timelines are for getting this ready for review / merged
• An IANA number needs to be allocated for the new dohpath SvcParamKey, which to my knowledge, has not happened yet

[okanaganrusty]

It looks like dohpath will be accepted as parameter key 7.

https://www.ietf.org/archive/id/draft-ietf-add-svcb-dns-00.html#name-dohpath

[jp-coding]

Adressed by #739?

[gthess]

This is addressed by #739 which is now merged; closing.