FQDN validator regex is preventing a valid dns name to be set, specifically, for SRV record
For example
set service dns forwarding authoritative-domain _tcp.db.mongors1.example.com records srv _mongodb entry 0 hostname 'mongors1.example.com'
This will fail.
In vyos-1x/src/validatos/fqdn
The validator doesn't accept underscore _
In https://www.rfc-editor.org/rfc/rfc2181#section-11
any binary string whatever can be used as the label of any resource record. Similarly, any binary string can serve as the value of any record that includes a domain name as some or all of its value (SOA, NS, MX, PTR, CNAME, and any others that may be added). Implementations of the DNS protocols must not place any restrictions on the labels that can be used. In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs. A DNS server may be configurable to issue warnings when loading, or even to refuse to load, a primary zone containing labels that might be considered questionable, however this should not happen by default.
It seems that there was no limitation on the character that can be used, even binary are acceptable.
But it might be better if we use some form of validation that matches with the real-world use case.