LXC container driver

The libvirt LXC driver manages "Linux Containers". At their simplest, containers can just be thought of as a collection of processes, separated from the main host processes via a set of resource namespaces and constrained via control groups resource tunables. The libvirt LXC driver has no dependency on the LXC userspace tools hosted on sourceforge.net. It directly utilizes the relevant kernel features to build the container environment. This allows for sharing of many libvirt technologies across both the QEMU/KVM and LXC drivers. In particular sVirt for mandatory access control, auditing of operations, integration with control groups and many other features.

Control groups Requirements

In order to control the resource usage of processes inside containers, the libvirt LXC driver requires that certain cgroups controllers are mounted on the host OS. The minimum required controllers are 'cpuacct', 'memory' and 'devices', while recommended extra controllers are 'cpu', 'freezer' and 'blkio'. Libvirt will not mount the cgroups filesystem itself, leaving this up to the init system to take care of. Systemd will do the right thing in this respect, while for other init systems the cgconfig init service will be required. For further information, consult the general libvirt cgroups documentation.

Namespace requirements

In order to separate processes inside a container from those in the primary "host" OS environment, the libvirt LXC driver requires that certain kernel namespaces are compiled in. Libvirt currently requires the 'mount', 'ipc', 'pid', and 'uts' namespaces to be available. If separate network interfaces are desired, then the 'net' namespace is required. In the near future, the 'user' namespace will optionally be supported.

NOTE: In the absence of support for the 'user' namespace, processes inside containers cannot be securely isolated from host process without the use of a mandatory access control technology such as SELinux or AppArmor.

Default container setup

Command line arguments

When the container "init" process is started, it will typically not be given any command line arguments (eg the equivalent of the bootloader args visible in /proc/cmdline). If any arguments are desired, then must be explicitly set in the container XML configuration via one or more initarg elements. For example, to run systemd --unit emergency.service would use the following XML

  <os>
    <type arch='x86_64'>exe</type>
    <init>/bin/systemd</init>
    <initarg>--unit</initarg>
    <initarg>emergency.service</initarg>
  </os>

Environment variables

When the container "init" process is started, it will be given several useful environment variables. The following standard environment variables are mandated by systemd container interface to be provided by all container technologies on Linux.

container: The fixed string libvirt-lxc to identify libvirt as the creator
container_uuid: The UUID assigned to the container by libvirt
PATH: The fixed string /bin:/usr/bin
TERM: The fixed string linux

In addition to the standard variables, the following libvirt specific environment variables are also provided

LIBVIRT_LXC_NAME: The name assigned to the container by libvirt
LIBVIRT_LXC_UUID: The UUID assigned to the container by libvirt
LIBVIRT_LXC_CMDLINE: The unparsed command line arguments specified in the container configuration. Use of this is discouraged, in favour of passing arguments directly to the container init process via the initarg config element.

Filesystem mounts

In the absence of any explicit configuration, the container will inherit the host OS filesystem mounts. A number of mount points will be made read only, or re-mounted with new instances to provide container specific data. The following special mounts are setup by libvirt

/dev a new "tmpfs" pre-populated with authorized device nodes
/dev/pts a new private "devpts" instance for console devices
/sys the host "sysfs" instance remounted read-only
/proc a new instance of the "proc" filesystem
/proc/sys the host "/proc/sys" bind-mounted read-only
/sys/fs/selinux the host "selinux" instance remounted read-only
/sys/fs/cgroup/NNNN the host cgroups controllers bind-mounted to only expose the sub-tree associated with the container
/proc/meminfo a FUSE backed file reflecting memory limits of the container

Device nodes

The container init process will be started with CAP_MKNOD capability removed and blocked from re-acquiring it. As such it will not be able to create any device nodes in /dev or anywhere else in its filesystems. Libvirt itself will take care of pre-populating the /dev filesystem with any devices that the container is authorized to use. The current devices that will be made available to all containers are

/dev/zero
/dev/null
/dev/full
/dev/random
/dev/urandom
/dev/stdin symlinked to /proc/self/fd/0
/dev/stdout symlinked to /proc/self/fd/1
/dev/stderr symlinked to /proc/self/fd/2
/dev/fd symlinked to /proc/self/fd
/dev/ptmx symlinked to /dev/pts/ptmx
/dev/console symlinked to /dev/pts/0

In addition, for every console defined in the guest configuration, a symlink will be created from /dev/ttyN symlinked to the corresponding /dev/pts/M pseudo TTY device. The first console will be /dev/tty1, with further consoles numbered incrementally from there.

Further block or character devices will be made available to containers depending on their configuration.

Systemd Socket Activation Integration

The libvirt LXC driver provides the ability to pass across pre-opened file descriptors when starting LXC guests. This allows for libvirt LXC to support systemd's socket activation capability, where an incoming client connection in the host OS will trigger the startup of a container, which runs another copy of systemd which gets passed the server socket, and then activates the actual service handler in the container.

Let us assume that you already have a LXC guest created, running a systemd instance as PID 1 inside the container, which has an SSHD service configured. The goal is to automatically activate the container when the first SSH connection is made. The first step is to create a couple of unit files for the host OS systemd instance. The /etc/systemd/system/mycontainer.service unit file specifies how systemd will start the libvirt LXC container

[Unit]
Description=My little container

[Service]
ExecStart=/usr/bin/virsh -c lxc:/// start --pass-fds 3 mycontainer
ExecStop=/usr/bin/virsh -c lxc:/// destroy mycontainer
Type=oneshot
RemainAfterExit=yes
KillMode=none

The --pass-fds 3 argument specifies that the file descriptor number 3 that virsh inherits from systemd, is to be passed into the container. Since virsh will exit immediately after starting the container, the RemainAfterExit and KillMode settings must be altered from their defaults.

Next, the /etc/systemd/system/mycontainer.socket unit file is created to get the host systemd to listen on port 23 for TCP connections. When this unit file is activated by the first incoming connection, it will cause the mycontainer.service unit to be activated with the FD corresponding to the listening TCP socket passed in as FD 3.

[Unit]
Description=The SSH socket of my little container

[Socket]
ListenStream=23

Port 23 was picked here so that the container doesn't conflict with the host's SSH which is on the normal port 22. That's it in terms of host side configuration.

Inside the container, the /etc/systemd/system/sshd.socket unit file must be created

[Unit]
Description=SSH Socket for Per-Connection Servers

[Socket]
ListenStream=23
Accept=yes

The ListenStream value listed in this unit file, must match the value used in the host file. When systemd in the container receives the pre-opened FD from libvirt during container startup, it looks at the ListenStream values to figure out which FD to give to which service. The actual service to start is defined by a correspondingly named /etc/systemd/system/sshd@.service

[Unit]
Description=SSH Per-Connection Server for %I

[Service]
ExecStart=-/usr/sbin/sshd -i
StandardInput=socket

Finally, make sure this SSH service is set to start on boot of the container, by running the following command inside the container:

# mkdir -p /etc/systemd/system/sockets.target.wants/
# ln -s /etc/systemd/system/sshd.socket /etc/systemd/system/sockets.target.wants/

This example shows how to activate the container based on an incoming SSH connection. If the container was also configured to have an httpd service, it may be desirable to activate it upon either an httpd or a sshd connection attempt. In this case, the mycontainer.socket file in the host would simply list multiple socket ports. Inside the container a separate xxxxx.socket file would need to be created for each service, with a corresponding ListenStream value set.

Container security

sVirt SELinux

In the absence of the "user" namespace being used, containers cannot be considered secure against exploits of the host OS. The sVirt SELinux driver provides a way to secure containers even when the "user" namespace is not used. The cost is that writing a policy to allow execution of arbitrary OS is not practical. The SELinux sVirt policy is typically tailored to work with an simpler application confinement use case, as provided by the "libvirt-sandbox" project.

Auditing

The LXC driver is integrated with libvirt's auditing subsystem, which causes audit messages to be logged whenever there is an operation performed against a container which has impact on host resources. So for example, start/stop, device hotplug will all log audit messages providing details about what action occurred and any resources associated with it. There are the following 3 types of audit messages

VIRT_MACHINE_ID - details of the SELinux process and image security labels assigned to the container.
VIRT_CONTROL - details of an action / operation performed against a container. There are the following types of operation
- op=start - a container has been started. Provides the machine name, uuid and PID of the libvirt_lxc controller process
- op=init - the init PID of the container has been started. Provides the machine name, uuid and PID of the libvirt_lxc controller process and PID of the init process (in the host PID namespace)
- op=stop - a container has been stopped. Provides the machine name, uuid
VIRT_RESOURCE - details of a host resource associated with a container action.

Device access

All containers are launched with the CAP_MKNOD capability cleared and removed from the bounding set. Libvirt will ensure that the /dev filesystem is pre-populated with all devices that a container is allowed to use. In addition, the cgroup "device" controller is configured to block read/write/mknod from all devices except those that a container is authorized to use.

Example configurations

Example config version 1

<domain type='lxc'>
  <name>vm1</name>
  <memory>500000</memory>
  <os>
    <type>exe</type>
    <init>/bin/sh</init>
  </os>
  <vcpu>1</vcpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/libvirt_lxc</emulator>
    <interface type='network'>
      <source network='default'/>
    </interface>
    <console type='pty' />
  </devices>
</domain>

In the <emulator> element, be sure you specify the correct path to libvirt_lxc, if it does not live in /usr/libexec on your system.

The next example assumes there is a private root filesystem (perhaps hand-crafted using busybox, or installed from media, debootstrap, whatever) under /opt/vm-1-root:

<domain type='lxc'>
  <name>vm1</name>
  <memory>32768</memory>
  <os>
    <type>exe</type>
    <init>/init</init>
  </os>
  <vcpu>1</vcpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/libvirt_lxc</emulator>
    <filesystem type='mount'>
      <source dir='/opt/vm-1-root'/>
      <target dir='/'/>
    </filesystem>
    <interface type='network'>
      <source network='default'/>
    </interface>
    <console type='pty' />
  </devices>
</domain>

Container usage / management

As with any libvirt virtualization driver, LXC containers can be managed via a wide variety of libvirt based tools. At the lowest level the virsh command can be used to perform many tasks, by passing the -c lxc:/// argument. As an alternative to repeating the URI with every command, the LIBVIRT_DEFAULT_URI environment variable can be set to lxc:///. The examples that follow outline some common operations with virsh and LXC. For further details about usage of virsh consult its manual page.

Defining (saving) container configuration>

The virsh define command takes an XML configuration document and loads it into libvirt, saving the configuration on disk

# virsh -c lxc:/// define myguest.xml

Viewing container configuration

The virsh dumpxml command can be used to view the current XML configuration of a container. By default the XML output reflects the current state of the container. If the container is running, it is possible to explicitly request the persistent configuration, instead of the current live configuration using the --inactive flag

# virsh -c lxc:/// dumpxml myguest

Starting containers

The virsh start command can be used to start a container from a previously defined persistent configuration

# virsh -c lxc:/// start myguest

It is also possible to start so called "transient" containers, which do not require a persistent configuration to be saved by libvirt, using the virsh create command.

# virsh -c lxc:/// create myguest.xml

Stopping containers

The virsh shutdown command can be used to request a graceful shutdown of the container. By default this command will first attempt to send a message to the init process via the /dev/initctl device node. If no such device node exists, then it will send SIGTERM to PID 1 inside the container.

# virsh -c lxc:/// shutdown myguest

If the container does not respond to the graceful shutdown request, it can be forceably stopped using the virsh destroy

# virsh -c lxc:/// destroy myguest

Rebooting a container

The virsh reboot command can be used to request a graceful shutdown of the container. By default this command will first attempt to send a message to the init process via the /dev/initctl device node. If no such device node exists, then it will send SIGHUP to PID 1 inside the container.

# virsh -c lxc:/// reboot myguest

Undefining (deleting) a container configuration

The virsh undefine command can be used to delete the persistent configuration of a container. If the guest is currently running, this will turn it into a "transient" guest.

# virsh -c lxc:/// undefine myguest

Connecting to a container console

The virsh console command can be used to connect to the text console associated with a container. If the container has been configured with multiple console devices, then the --devname argument can be used to choose the console to connect to

# virsh -c lxc:/// console myguest

Running commands in a container

The virsh lxc-enter-namespace command can be used to enter the namespaces and security context of a container and then execute an arbitrary command.

# virsh -c lxc:/// lxc-enter-namespace myguest -- /bin/ls -al /dev

Monitoring container utilization

The virt-top command can be used to monitor the activity and resource utilization of all containers on a host

# virt-top -c lxc:///