rancherOS/docs/os/configuration/adding-kernel-parameters/index.md

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title: Adding kernel parameters layout: os-default

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Kernel boot parameters

RancherOS parses the Linux kernel boot cmdline to add any keys it understands to its configuration. This allows you to modify what cloud-init sources it will use on boot, to enable rancher.debug logging, or to almost any other configuration setting.

There are two ways to set or modify persistent kernel parameters, in-place (editing the file and reboot) or during installation to disk.

In-place editing

To edit the kernel boot parameters of an already installed RancherOS system, use the new sudo ros config syslinux editing command (uses vi).

To activate this setting, you will need to reboot.

During installation

If you want to set the extra kernel parameters when you are Installing RancherOS to Disk please use the --append parameter.

$ sudo ros install -d /dev/sda --append "rancheros.autologin=tty1"

Graphical boot screen

RancherOS v1.1.0 added a Syslinux boot menu, which allows you to temporarily edit the boot paramters, or to select "Debug logging", "Autologin", both "Debug logging & Autologin" and "Recovery Console".

On desktop systems the Syslinux boot menu can be switched to graphical mode by adding UI vesamenu.c32 to a new line in global.cfg (use sudo ros config syslinux to edit the file).

Useful RancherOS cloud-init or boot settings

Recovery console

rancher.recovery=true will start a single user root bash session as easily in the boot process, with no network, or persitent filesystem mounted. This can be used to fix disk problems, or to debug your system.

Enable/Disable sshd

rancher.ssh.daemon=false (its enabled in the os-config) can be used to start your RancherOS with no sshd daemon. This can be used to further reduce the ports that your system is listening on.

Enable debug logging

rancher.debug=true will log everything to the console for debugging.

Autologin console

rancher.autologin=<tty...> will automatically log in the sepcified console - common values are tty1, ttyS0 and ttyAMA0 - depending on your platform.

Enable/Disable hypervisor service auto-enable

RancherOS v1.1.0 added detetion of Hypervisor, and then will try to download the a service called <hypervisor>-vm-tools. This may cause boot speed issues, and so can be disabled by setting rancher.hypervisor_service=false.