Hostname format check does not allow single labels

[swaeberle]

The hostname format checker currently does not allow hostnames without a domain part, hence a single label like localhost is rejected as invalid hostname.
This is caused by class FQDN from fqdn assuming a minimum of two labels.

AFAIK, single labels are not excluded by the RFC 1123 and class FQDN can be forced to accept also single labels in its constructor (min_labels=1).

If rejecting single labels is not the intended behavior, I will send a PR.

[Julian]

Thanks! The first step here (assuming you're correct, which I have to double check the RFC, but willing to initially take your word for it :) is to add a test upstream to the official test suite (which I also maintain).

That's here -- could you perhaps have a look there at the hostname (and IDN hostname) format tests, see if you follow how to add one, and then once it's merged there we can pull and add the suggested fix to how we call fqdn.

[swaeberle]

See tests PR:

• test single label hostnames json-schema-org/JSON-Schema-Test-Suite#685