Re: [arch-projects] [netctl] netctl, cloud-init, and systemd

Hi again, thank you both for your input! See comments inline: On 6/17/19 10:20 PM, Jouke Witteveen via arch-projects wrote:

On Mon, Jun 17, 2019 at 9:45 PM Erich Eckner via arch-projects wrote:

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In case you are not familiar with cloud-init, the idea is that you can build a single OS image that runs cloud-init on boot, and cloud-init will take care of such things as network configuration, so that the same image will work regardless of the network setup you choose for the cloud instance.

Does cloud-init run before or after systemd? In other words: is it a systemd unit of some kind or is it rather an init daemon itself which chain-loads systemd?

Cloud-init comes with multiple systemd units and as such is is run by systemd multiple times at different stages during the boot process. The cloud-init wiki page has a rough overview: https://wiki.archlinux.org/index.php/Cloud-init#Systemd_integration

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The current cloud-init implementation for Arch uses netctl [3]. The implementation is correct in such a way that it does indeed render the right netctl profile(s) and enables them. However there is a problem: they are not being started. AFAICT this is because cloud-init does this while the systemd boot is already in process, and changing the dependency graph (by adding new units) does not have any effect until the next run (everything works right on second boot). Note that I even tried having cloud-init run `systemd daemon-reload` after enabling the units, but it didn't help either.

Did you try cloud-init to issue "systemctl start $unitname.service" additionally to "systemctl enable $unitname.service"? This seems to me to be the right way.

It might be worth taking another look at that, but let me quickly lay out why I didn't try this yet: when cloud-init runs for the first time, it goes through a bunch of plugins called "data sources", which will probe different aspects of the environment to determine the cloud provider it is running in, use that knowledge to retrieve vendor-specific configuration details, and use that to write e.g. network config, hostname, etc. The tricky part is that for example the EC2 data source uses a magic IP to retrieve this config (see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.ht...), and other data sources might do similar things. Hence, I was worried that prematurely re-configuring the network might interfere with such actions (the unit running this has "Before=network-pre.target"). However, if cloud-init first fetches all data and then configures things it might not be a problem. I'll take a closer look at what is happening there and maybe try to get a statement from the cloud-init folks.

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The reason I am posting this here is that this seems to be an issue due to the particular way netctl use systemd units. Since you don't know the names or the number of profiles (units) that will be generated during image creation, you cannot enable them at that time. But doing so during first boot does not seem to work.

I would rather say it's due to the way, cloud-init uses systemd units: it enables them, but that's only relevant for successive boots, so it should rather enable and start them (systemd should still honor the dependencies of the units and postpone the start to the point where all of the dependencies are loaded, too).

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Just for comparison, if one were to use e.g. systemd-networkd instead, you would just enable the systemd-networkd unit during image creation, cloud-init could generate the appropriate config for any number of devices, and when the unit starts it will do the right thing. Likewise on other distros, e.g. Debian with /etc/network/interfaces or such.

Now, from my point of view, there could be several approaches to solve

this:

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1. systemd supports updates of the dep graph during boot 2. support such a use case in netctl 3. change cloud-init to use systemd-networkd for Arch

Let me quickly elaborate:

1. is intentionally not phrased as something to be done. It might already be a thing, I just couldn't figure out how to do it. If someone knows more about this, I would love to hear about it. If this works, it would be the easiest solution. However, if it doesn't, I don't have my hopes up high for this being added to systemd anytime soon.

This would mean, if I "systemctl enable $some.service", it will be started right away, too - probably not, what systemd devs want (at least it's not, what systemd currently does).

`systemctl enable --now <SERVICE>` starts a service in addition to enabling it.

Might be an option, see above.

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2. is the main reason I am writing this. Things that came to mind were another special unit (netctl-all?), or even just a well-defined interface to write devices into the state file, so that the plain netctl unit would work. I would be very interested to hear how such a thing sounds to you, the developers?

There is currently netctl-auto@.service, but this requires to know the interfaces in advance. Maybe the netctl devs can consider adding another unit which is interface agnostic? "netctl-auto.service" maybe? (I'm not familiar with netctl's interna - maybe this is not possible at all)

Indeed, there are two more options to achieve what I think you want. 1. Use "netctl-ifplugd@<INTERFACE>", see also: netctl.special(7). This requires ifplugd to be installed and takes all profiles for an interface into consideration, so you don't need to know the name of the profile in advance. Of course, you do need to know the name of the interface.

Same problem, the interface names are also unknown, but could this maybe be combined with the "Using any interface" method? https://wiki.archlinux.org/index.php/Netctl#Using_any_interface I'll take a closer look at that as well.

2. Use "netctl(.service)", see also: netctl.special(7), and write the profile name to "/var/lib/netctl/netctl.state". This only works if cloud-init runs before systemd, or at least finishes before the netctl service is started.

That's pretty much what I was referring to with "the plain netctl unit", except of course it's a service :/ This was a likely candidate for me, I just noticed that one can override the location of the file via environment variable, but I suppose considering that in code would be easy. I just thought it would be even cleaner if there were a netctl command for that (like `netctl store PROFILE` to force-store a profile?). Is this something you would consider? :)

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3. Is of course an option, but would require quite a bit of work in cloud-init. That work, if done right, might however at some point benefit other distros, should they be using systemd-networkd as well. The main reason I am also bringing this up that I was wondering if there are possibly any plans to abandon netctl anyways at some point in favor of distro-agnostic solutions (be it systemd-networkd or any other).

netctl is stable and I intend to keep maintaining it. It should work without adaptations on any linux distribution that uses systemd.

Ok, that's great to know, I'd prefer sticking to that then! Thanks a lot again, this gives me a bit more stuff to work with, I'll keep you posted, and hopefully we have a fully supported Arch in cloud-init soon :) Cheers, Conrad