Lxc-systemd

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Revision as of 15:24, 5 April 2013 by Starfry (Talk | contribs) (updated info re udev disabling)

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Notes: Should be merged to main page after clean up. (Discuss in Talk:Lxc-systemd#)

These are brief notes that describe how to configure a Linux container that runs systemd inside. They ought to be consumed into the main Linux Containers wiki page when that is rewritten.

Without specific configuration, conflicts arise between systemd and lxc in the /dev tree. A new mode called 'autodev' has been added to LXC to help rectify this.

LXC needs to be configured to use its new "autodev" mode which causes it to create a new /dev tree.

 lxc.autodev = 1

This will cause LXC to create its own device tree but this also means that the traditional way of manually creating device nodes in the container rootfs /dev tree will not work because /dev is overmounted by LXC. Should you require (and you probably will) any device nodes that are not created by LXC by default then you will need to use an autodev hook script:

 lxc.hook.autodev = /path/to/script

Where the script is similar to:

 #!/bin/bash
 # LXC Autodev hook.
 cd ${LXC_ROOTFS_MOUNT}/dev
 mknod .....

The other thing you must do is disable services that are not supported inside a container. Chroot into the container rootfs and mask those services:

 ln -s /dev/null /etc/systemd/system/systemd-udevd.service
 ln -s /dev/null /etc/systemd/system/systemd-udevd-control.socket
 ln -s /dev/null /etc/systemd/system/systemd-udevd-kernel.socket
 ln -s /dev/null /etc/systemd/system/proc-sys-fs-binfmt_misc.automount

This disables udev and mounting of /proc/sys/fs/binfmt_misc.

An alternative way to prevent systemd from starting udev is to deny mknod capabilities in the container:

 lxc.cap.drop = mknod

Dropping the mknod capability is described at http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface where it explains that this is necessary to prevent systemd from starting udev. However, masking the systemd services is a better approach because it allows you to retain mknod capability inside a container if you need it.

Additionally you should ensure that you have a pty declaration in your LXC container because the presence of this causes LXC to mount devpts as a new instance (without this the container gets the host's devpts and that is not a good thing - more strange things will happen!):

 lxc.pts = 1024

Note that there is no need to explicitly mount system devices (either via the container config or via its own /etc/fstab) and this should not be done because systemd (or LXC in the case of /dev...) takes care of it:

  • /dev/pts
  • /dev/shm
  • /proc
  • /sys