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In the RFC 2782 for SRV record type we can read:

The format of the SRV RR

   Here is the format of the SRV RR, whose DNS type code is 33:

        _Service._Proto.Name TTL Class SRV Priority Weight Port Target

        (There is an example near the end of this document.)

   Service
        The symbolic name of the desired service, as defined in Assigned
        Numbers [STD 2] or locally.  An underscore (_) is prepended to
        the service identifier to avoid collisions with DNS labels that
        occur in nature.
        Some widely used services, notably POP, don't have a single
        universal name.  If Assigned Numbers names the service
        indicated, that name is the only name which is legal for SRV
        lookups.  The Service is case insensitive.

   Proto
        The symbolic name of the desired protocol, with an underscore
        (_) prepended to prevent collisions with DNS labels that occur
        in nature.  _TCP and _UDP are at present the most useful values
        for this field, though any name defined by Assigned Numbers or
        locally may be used (as for Service).  The Proto is case
        insensitive.

   Name
        The domain this RR refers to.  The SRV RR is unique in that the
        name one searches for is not this name; the example near the end
        shows this clearly.

On the other hand in the RFC 6763 for DNS-Based Service Discovery they are using SRV record in such format:

Instance._Service._Proto.Name TTL Class SRV Priority Weight Port Target

where Instance may contain almost all characters:


   It MUST NOT contain ASCII control characters (byte values 0x00-0x1F and
   0x7F) [RFC20] but otherwise is allowed to contain any characters,
   without restriction, including spaces, uppercase, lowercase,
   punctuation -- including dots -- accented characters, non-Roman text,
   and anything else that may be represented using Net-Unicode.

As an example we can get such record from dns-sd.org:

$ dig +noall +answer SRV "Service\032Discovery._http._tcp.dns-sd.org."
Service\032Discovery._http._tcp.dns-sd.org. 60 IN SRV 0 0 80 dns-sd.org.

So my question is: does this SRV record format valid accordingly to RFC 2782? And if it is valid why, for example, cloudflare doesn't allow to add such record?

cloudflare error message

2

To be honest, I have only used (and until now, seen) RFC 2782 formatted SRV records (that look like _Service._Proto.Name TTL Class SRV Priority Weight Port Target) and wouldn't be surprised if GUI's like the one in your example simply aren't programmed to detect, validate and allow RFC 6763 formatted SRV records.

There are many other, arguably legacy and/or obscure, DNS record types and formats that are also not supported by GUI's and vendors like Cloudfare. That is their prerogative.

Apparently Bind DNS server isn't too picky with regards to serving SRV records and neither are resolver libraries in requesting and displaying them, but others may be more strict.
RFC 6763 states in section 13: The following examples were prepared using standard unmodified nslookup and standard unmodified BIND running on GNU/Linux.""

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