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When attaching a port to an instance, nova will check for DNS support in neutron
and set a 'dns_name' attribute if found. To populate this attribute, nova uses a
sanitised version of the instance name, stored in the instance.hostname
attribute. This sanitisation simply strips out any unicode characters and
replaces underscores and spaces with dashes, before truncating to 63 characters.
It does not currently replace periods and this is the cause of bug 1581977 [1],
where an instance name such as 'ubuntu20.04' will fail to schedule since neutron
identifies '04' as an invalid TLD.

The question now is what to do to resolve this. There are two obvious paths
available to us. The first is to simply catch these invalid hostnames and
replace them with an arbitrary hostname of format 'Server-{serverUUID}'. This is
what we currently do for purely unicode instance names and is what I've proposed
at [2]. The other option is to strip all periods, or rather replace them with
hyphens, when sanitizing the instance name. This is more predictable but breaks
the ability to use the instance name as a FQDN. Such usage is something I'm told
we've never supported, but I'm concerned that there are users out there who are
relying on this all the same and I'd like to get a feel for whether this is the
case first.

So, the question: does anyone currently rely on this inadvertent "feature"?

Cheers,
Stephen

[1] https://launchpad.net/bugs/1581977
[2] https://review.opendev.org/c/openstack/nova/+/764482