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.

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

Or, using a Container Linux Config:

storage:
  files:
    - path: /etc/modules-load.d/nf.conf
      filesystem: root
      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 Container Linux Config loads the dummy network interface module with an option specifying the number of interfaces the module should create when loaded (numdummies=5):

storage:
  files:
    - path: /etc/modprobe.d/dummy.conf
      filesystem: root
      mode: 0644
      contents:
        inline: options dummy numdummies=5
    - path: /etc/modules-load.d/dummy.conf
      filesystem: root
      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:

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 Container Linux Config using both would look like:

storage:
  files:
    - path: /etc/modules-load.d/nf.conf
      filesystem: root
      mode: 0644
      contents:
        inline: |
                    nf_conntrack
    - path: /etc/sysctl.d/nf.conf
      filesystem: root
      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. Note that Ignition runs after GRUB. Therefore, the GRUB configuration won’t take effect until the next reboot of the node.

Here’s an example configuration:

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"

Enable Flatcar Container Linux autologin

To login without a password on every boot, edit /usr/share/oem/grub.cfg to add the line:

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

Enable systemd debug logging

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

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:

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.

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

Or via a Container Linux Config:

storage:
  files:
    - path: /etc/motd.d/pi.conf
      filesystem: root
      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, [email protected]. 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.

mkdir -p '/etc/systemd/system/[email protected]'
echo -e '[Service]\nTTYVTDisallocate=no' > '/etc/systemd/system/[email protected]/no-disallocate.conf'