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
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.