Write short form hostname

[gabriel-samfira]

While setting the hostname as a FQDN is perfectly valid, in most cases the expectation is that /etc/hostname holds the short form hostname and the FQDN is added as a canonical hostname, i.e. as the first column in /etc/hosts after the local IP address.

This will allow clients to use:

:~$ hostname -f
arrakis.local

to get the fqdn and still be able to get the short form hostname by calling:

:~$ hostname
arrakis

[bgilbert]

We arrived at Afterburn's current hostname policy after much debate and debugging, and changing it would likely break existing users. If you'd like to add a command-line option to write only the hostname part, we could consider it.

[gabriel-samfira]

Fair enough! Would a similar command line option like --short-hostname=/etc/hostname be ok? (I am terrible at naming things).

[bgilbert]

Works for me. Note that there isn't a great way to configure the afterburn-hostname.service service at runtime, since it runs in the initrd. (For example, Ignition can't reconfigure the initrd.) So, if you wanted to use this, you'd have to configure a custom unit that runs in the booted system, which probably means you'd be setting the hostname after things have already started using the old one.

[gabriel-samfira]

I was a bit sparse on context 😄 .

The need for this came up recently when we noticed that we can't bring up a k8s cluster on OpenStack using Flatcar as a base OS with kops. Here is the relevant issue with more context: https://github.com/kubernetes/kops/issues/15385

For OpenStack we have a fix merged in coreos-cloudinit, but the goal is to eventually use afterburn to set the hostname. The short hostname option will most likely be (eventually) included in future releases of Flatcar.

[bgilbert]

Ah, okay, cool. Happy to ship a command-line option to be invoked by a distro that integrates Afterburn.

[gabriel-samfira]

I added a new boolean flag --short and placed the --hostname in a group. The --short flag requires the --hostname flag and toggles whether or not to write the short form hostname. I also added a new test.

This is my first interaction with rust, so let me know if the changes are not idiomatic or just plain silly :smile:.

[gabriel-samfira]

Here is an example of the new flag in action.

Without the --short flag:

control-plane-nova-dkfvy2 ~ # ./afterburn --provider openstack-metadata --hostname=/root/test
May 16 14:51:15.352 INFO Fetching http://169.254.169.254/latest/meta-data/hostname: Attempt #1
May 16 14:51:15.542 INFO Fetch successful
May 16 14:51:15.542 INFO wrote hostname control-plane-nova-dkfvy2.novalocal to /root/test
control-plane-nova-dkfvy2 ~ # cat /root/test 
control-plane-nova-dkfvy2.novalocal

With the --short flag:

control-plane-nova-dkfvy2 ~ # ./afterburn --provider openstack-metadata --short --hostname=/root/test
May 16 14:51:24.876 INFO Fetching http://169.254.169.254/latest/meta-data/hostname: Attempt #1
May 16 14:51:24.908 INFO Fetch successful
May 16 14:51:24.908 INFO wrote hostname control-plane-nova-dkfvy2 to /root/test
control-plane-nova-dkfvy2 ~ # cat /root/test 
control-plane-nova-dkfvy2

For systems that don't explicitly set the new flag, things work as they did before. Opting to use the new flag will set the short form hostname.

[bgilbert]

Thanks for the PR! I won't be able to do future review rounds on this, unfortunately, but hopefully someone else on the team can take a look.

Closing/reopening to kick CI.

[gabriel-samfira]

Thanks for the review @bgilbert !

I will make the changes today and push them.