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. 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:

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

    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):

    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 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:

    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:

    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
    

    Prevent login prompts from clearing the console

    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'
    

    Or via a Container Linux Config:

    systemd:
      units:
        - name: [email protected]
          dropins:
            - name: no-disallocate.conf
              contents: |
                [Service]
                TTYVTDisallocate=no            
    

    When the TTYVTDisallocate setting is disabled, the console scrollback is not cleared on logout, not even by the clear command in the default .bash_logout file. Scrollback must be cleared explicitly, e.g. by running echo -en '\033[3J' > /dev/console as the root user.