Host Configuration on Flatcar Container Linux

Adding users

Mon, 01 Jan 0001 00:00:00 +0000

You can create user accounts on a Flatcar Container Linux machine manually with useradd or via a Butane Config when the machine is created.

Add Users via Butane Configs

In your Butane Config, you can specify many different parameters for each user. Here’s an example:

variant: flatcar
version: 1.0.0
passwd:
  users:
    - name: core
      ssh_authorized_keys:
        - "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDGdByTgSVHq......."
    - name: elroy
      password_hash: "$6$5s2u6/jR$un0AvWnqilcgaNB3Mkxd5yYv6mTlWfOoCYHZmfi3LDKVltj.E8XNKEcwWm..."
      ssh_authorized_keys:
        - "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDGdByTgSVHq......."
      groups: [ sudo, docker ]

Because usermod does not work to add a user to a predefined system group, you can use systemd-userdb to define membership. Here’s the same example with userdb:

Configuring date and time zone

Mon, 01 Jan 0001 00:00:00 +0000

By default, Flatcar Container Linux machines keep time in the Coordinated Universal Time (UTC) zone and synchronize their clocks with the Network Time Protocol (NTP). This page contains information about customizing those defaults, explains the change in NTP client daemons in recent Flatcar Container Linux versions, and offers advice on best practices for timekeeping in Flatcar Container Linux clusters.

Viewing and changing time and date

The timedatectl(1) command displays and sets the date, time, and time zone.

Using systemd drop-in units

Mon, 01 Jan 0001 00:00:00 +0000

There are two methods of overriding default Flatcar Container Linux settings in unit files: copying the unit file from /usr/lib64/systemd/system to /etc/systemd/system and modifying the chosen settings. Alternatively, one can create a directory named unit.d within /etc/systemd/system and place a drop-in file name.conf there that only changes the specific settings one is interested in. Note that multiple such drop-in files are read if present.

The advantage of the first method is that one easily overrides the complete unit, the default Flatcar Container Linux unit is not parsed at all anymore. It has the disadvantage that improvements to the unit file supplied by Flatcar Container Linux are not automatically incorporated on updates.

Scheduling tasks with systemd timers

Mon, 01 Jan 0001 00:00:00 +0000

Flatcar Container Linux uses systemd timers (cron replacement) to schedule tasks. Here we will show you how you can schedule a periodic job.

Let’s create an alternative for this crontab job:

*/10 * * * * /usr/bin/date >> /tmp/date

Timers work directly with services’ units. So we have to create /etc/systemd/system/date.service first:

[Unit]
Description=Prints date into /tmp/date file

[Service]
Type=oneshot
ExecStart=/usr/bin/sh -c '/usr/bin/date >> /tmp/date'

Then we have to create timer unit with the same name but with *.timer suffix /etc/systemd/system/date.timer:

Using environment variables in systemd units

Mon, 01 Jan 0001 00:00:00 +0000

Environment directive

systemd has an Environment directive which sets environment variables for executed processes. It takes a space-separated list of variable assignments. This option may be specified more than once in which case all listed variables will be set. If the same variable is set twice, the later setting will override the earlier setting. If the empty string is assigned to this option, the list of environment variables is reset, all prior assignments have no effect. Environments directives are used in built-in Flatcar Container Linux systemd units, for example in etcd2 and flannel.

Kernel modules and other settings

Mon, 01 Jan 0001 00:00:00 +0000

Loading kernel modules

Most Linux kernel modules get automatically loaded as-needed but there are a some situations where this doesn’t work. Problems can arise if there is boot-time dependencies are sensitive to exactly when the module gets loaded. Module auto-loading can be broken all-together if the operation requiring the module happens inside of a container. iptables and other netfilter features can easily encounter both of these issues. To force a module to be loaded early during boot simply list them in a file under /etc/modules-load.d. The file name must end in .conf.

Using NVIDIA GPUs on Flatcar

Mon, 01 Jan 0001 00:00:00 +0000

Installation

Flatcar Container Linux offers support for the installation and customization of NVIDIA drivers for Tesla GPUs (both in VMs and on bare metal). Please take note that NVIDIA drivers have been migrated from being solely available on AWS and Azure to being accessible on all platforms with the release of version 3637.0.0. If you are using an older version, please be aware that it is restricted to AWS and Azure only.

Tuning Flatcar Container Linux power management

Mon, 01 Jan 0001 00:00:00 +0000

CPU governor

By default, Flatcar Container Linux uses the “performance” CPU governor meaning that the CPU operates at the maximum frequency regardless of load. This is reasonable for a system that is under constant load or cannot tolerate increased latency. On the other hand, if the system is idle much of the time and latency is not a concern, power savings may be desired.

Several governors are available:

Governor	Description
`performance`	Default. Operate at the maximum frequency
`ondemand`	Dynamically scale frequency at 75% cpu load
`conservative`	Dynamically scale frequency at 95% cpu load
`powersave`	Operate at the minimum frequency
`userspace`	Controlled by a userspace application via the `scaling_setspeed` file

The “conservative” governor can be used instead using the following shell commands: