Kernel modules and other settings

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.

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echo nf_conntrack > /etc/modules-load.d/nf.conf

Or, using a Butane Config:

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variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/modules-load.d/nf.conf
      mode: 0644
      contents:
        inline: nf_conntrack

Loading kernel modules with options

The following section demonstrates how to provide module options when loading. After these configs are processed, the dummy module is loaded into the kernel, and five dummy interfaces are added to the network stack.

Further details can be found in the systemd man pages: modules-load.d(5) systemd-modules-load.service(8) modprobe.d(5)

This example Butane Config loads the dummy network interface module with an option specifying the number of interfaces the module should create when loaded (numdummies=5):

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variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/modprobe.d/dummy.conf
      mode: 0644
      contents:
        inline: options dummy numdummies=5
    - path: /etc/modules-load.d/dummy.conf
      mode: 0644
      contents:
        inline: dummy

Tuning sysctl parameters

The Linux kernel offers a plethora of knobs under /proc/sys to control the availability of different features and tune performance parameters. For one-shot changes values can be written directly to the files under /proc/sys but persistent settings must be written to /etc/sysctl.d:

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echo net.netfilter.nf_conntrack_max=131072 > /etc/sysctl.d/nf.conf
sysctl --system

Some parameters, such as the conntrack one above, are only available after the module they control has been loaded. To ensure any modules are loaded in advance use modules-load.d as described above. A complete Butane Config using both would look like:

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variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/modules-load.d/nf.conf
      mode: 0644
      contents:
        inline: |
          nf_conntrack
    - path: /etc/sysctl.d/nf.conf
      mode: 0644
      contents:
        inline: |
          net.netfilter.nf_conntrack_max=131072

Further details can be found in the systemd man pages: sysctl.d(5) systemd-sysctl.service(8)

Adding custom kernel boot options

The Flatcar Container Linux bootloader parses the configuration file /usr/share/oem/grub.cfg, where custom kernel boot options may be set.

The /usr/share/oem/grub.cfg file can be configured with Ignition. Beginning with Flatcar major version 3185 the kernelArguments directive in Ignition v3 allows to add or remove kernel command line parameters and reboot the system directly from the initramfs to apply them as part of the first boot setup. It only works for unconditional set linux_append statements in grub.cfg and any existing linux_console statement is not considered.

Here’s an example for ensuring that flatcar.autologin exists while ensuring that quiet does not exist. First the Butane YAML config and then the transpiled Ignition v3 config:

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variant: flatcar
version: 1.0.0
kernel_arguments:
  should_exist:
    - flatcar.autologin
  should_not_exist:
    - quiet

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{
  "ignition": {
    "version": "3.3.0"
  },
  "kernelArguments": {
    "shouldExist": [
      "flatcar.autologin"
    ],
    "shouldNotExist": [
      "quiet"
    ]
  }
}

Instead of using kernelArguments you can also use the plain file directive in Ignition to write to /usr/share/oem/grub.cfg. However, because Ignition runs after GRUB, the GRUB configuration won’t take effect until the next reboot of the node. This is particularly useful if you are bound to use Ignition V2 (which requires the use of ct instead of butane).

Here’s an example Container Linux Configuration for using the plain file directive (this YAML content has to be transpiled to Ignition JSON with ct):

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storage:
  filesystems:
    - name: "OEM"
      mount:
        device: "/dev/disk/by-label/OEM"
        format: "btrfs"
  files:
    - filesystem: "OEM"
      path: "/grub.cfg"
      mode: 0644
      append: true
      contents:
        inline: |
          set linux_append="$linux_append flatcar.autologin=tty1"

To take effect directly on first boot, the alternative is to create a getty@.service drop-in, here a snippet that will work with ct and butane:

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systemd:
  units:
    - name: getty@.service
      dropins:
        - name: 10-autologin.conf
          contents: |
            [Service]
            ExecStart=
            ExecStart=-/sbin/agetty --noclear %I $TERM

Enable Flatcar Container Linux autologin

To login without a password for the core user on the serial or VGA console on every boot, edit /usr/share/oem/grub.cfg to add a line like this:

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set linux_append="$linux_append flatcar.autologin=tty1"

Without specifying =tty1 any TTY will be used, e.g., the serial console.

To control this setting on provisioning time, use the Ignition v3 kernelArguments directive with shouldExist or shouldNotExist (see the Butane config in the section above).

Enable systemd debug logging

Edit /usr/share/oem/grub.cfg to add the following line, enabling systemd’s most verbose debug-level logging:

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set linux_append="$linux_append systemd.log_level=debug"

Mask a systemd unit

Completely disable the systemd-networkd.service unit by adding this line to /usr/share/oem/grub.cfg:

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set linux_append="$linux_append systemd.mask=systemd-networkd.service"

Adding custom messages to MOTD

When logging in interactively, a brief message (the “Message of the Day (MOTD)”) reports the Flatcar Container Linux release channel, version, and a list of any services or systemd units that have failed. Additional text can be added by dropping text files into /etc/motd.d. The directory may need to be created first, and the drop-in file name must end in .conf. Flatcar Container Linux versions 555.0.0 and greater support customization of the MOTD.

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mkdir -p /etc/motd.d
echo "This machine is dedicated to computing Pi" > /etc/motd.d/pi.conf

Or via a Butane Config:

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variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/motd.d/pi.conf
      mode: 0644
      contents:
        inline: This machine is dedicated to computing Pi

The system boot messages that are printed to the console will be cleared when systemd starts a login prompt. In order to preserve these messages, the getty services will need to have their TTYVTDisallocate setting disabled. This can be achieved with a drop-in for the template unit, getty@.service. Note that the console will still scroll so the login prompt is at the top of the screen, but the boot messages will be available by scrolling.

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mkdir -p '/etc/systemd/system/getty@.service.d'
echo -e '[Service]\nTTYVTDisallocate=no' > '/etc/systemd/system/getty@.service.d/no-disallocate.conf'