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698 lines
24 KiB
XML
698 lines
24 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>LXC container driver</h1>
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<ul id="toc"></ul>
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<p>
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The libvirt LXC driver manages "Linux Containers". At their simplest, containers
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can just be thought of as a collection of processes, separated from the main
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host processes via a set of resource namespaces and constrained via control
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groups resource tunables. The libvirt LXC driver has no dependency on the LXC
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userspace tools hosted on sourceforge.net. It directly utilizes the relevant
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kernel features to build the container environment. This allows for sharing
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of many libvirt technologies across both the QEMU/KVM and LXC drivers. In
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particular sVirt for mandatory access control, auditing of operations,
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integration with control groups and many other features.
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</p>
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<h2><a name="cgroups">Control groups Requirements</a></h2>
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<p>
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In order to control the resource usage of processes inside containers, the
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libvirt LXC driver requires that certain cgroups controllers are mounted on
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the host OS. The minimum required controllers are 'cpuacct', 'memory' and
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'devices', while recommended extra controllers are 'cpu', 'freezer' and
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'blkio'. Libvirt will not mount the cgroups filesystem itself, leaving
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this up to the init system to take care of. Systemd will do the right thing
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in this respect, while for other init systems the <code>cgconfig</code>
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init service will be required. For further information, consult the general
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libvirt <a href="cgroups.html">cgroups documentation</a>.
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</p>
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<h2><a name="namespaces">Namespace requirements</a></h2>
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<p>
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In order to separate processes inside a container from those in the
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primary "host" OS environment, the libvirt LXC driver requires that
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certain kernel namespaces are compiled in. Libvirt currently requires
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the 'mount', 'ipc', 'pid', and 'uts' namespaces to be available. If
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separate network interfaces are desired, then the 'net' namespace is
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required. If the guest configuration declares a
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<a href="formatdomain.html#elementsOSContainer">UID or GID mapping</a>,
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the 'user' namespace will be enabled to apply these. <strong>A suitably
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configured UID/GID mapping is a pre-requisite to making containers
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secure, in the absence of sVirt confinement.</strong>
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</p>
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<h2><a name="init">Default container setup</a></h2>
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<h3><a name="cliargs">Command line arguments</a></h3>
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<p>
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When the container "init" process is started, it will typically
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not be given any command line arguments (eg the equivalent of
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the bootloader args visible in <code>/proc/cmdline</code>). If
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any arguments are desired, then must be explicitly set in the
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container XML configuration via one or more <code>initarg</code>
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elements. For example, to run <code>systemd --unit emergency.service</code>
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would use the following XML
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</p>
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<pre>
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<os>
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<type arch='x86_64'>exe</type>
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<init>/bin/systemd</init>
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<initarg>--unit</initarg>
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<initarg>emergency.service</initarg>
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</os>
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</pre>
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<h3><a name="envvars">Environment variables</a></h3>
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<p>
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When the container "init" process is started, it will be given several useful
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environment variables. The following standard environment variables are mandated
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by <a href="http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">systemd container interface</a>
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to be provided by all container technologies on Linux.
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</p>
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<dl>
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<dt>container</dt>
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<dd>The fixed string <code>libvirt-lxc</code> to identify libvirt as the creator</dd>
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<dt>container_uuid</dt>
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<dd>The UUID assigned to the container by libvirt</dd>
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<dt>PATH</dt>
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<dd>The fixed string <code>/bin:/usr/bin</code></dd>
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<dt>TERM</dt>
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<dd>The fixed string <code>linux</code></dd>
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</dl>
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<p>
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In addition to the standard variables, the following libvirt specific
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environment variables are also provided
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</p>
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<dl>
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<dt>LIBVIRT_LXC_NAME</dt>
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<dd>The name assigned to the container by libvirt</dd>
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<dt>LIBVIRT_LXC_UUID</dt>
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<dd>The UUID assigned to the container by libvirt</dd>
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<dt>LIBVIRT_LXC_CMDLINE</dt>
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<dd>The unparsed command line arguments specified in the container configuration.
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Use of this is discouraged, in favour of passing arguments directly to the
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container init process via the <code>initarg</code> config element.</dd>
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</dl>
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<h3><a name="fsmounts">Filesystem mounts</a></h3>
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<p>
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In the absence of any explicit configuration, the container will
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inherit the host OS filesystem mounts. A number of mount points will
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be made read only, or re-mounted with new instances to provide
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container specific data. The following special mounts are setup
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by libvirt
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</p>
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<ul>
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<li><code>/dev</code> a new "tmpfs" pre-populated with authorized device nodes</li>
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<li><code>/dev/pts</code> a new private "devpts" instance for console devices</li>
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<li><code>/sys</code> the host "sysfs" instance remounted read-only</li>
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<li><code>/proc</code> a new instance of the "proc" filesystem</li>
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<li><code>/proc/sys</code> the host "/proc/sys" bind-mounted read-only</li>
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<li><code>/sys/fs/selinux</code> the host "selinux" instance remounted read-only</li>
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<li><code>/sys/fs/cgroup/NNNN</code> the host cgroups controllers bind-mounted to
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only expose the sub-tree associated with the container</li>
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<li><code>/proc/meminfo</code> a FUSE backed file reflecting memory limits of the container</li>
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</ul>
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<h3><a name="devnodes">Device nodes</a></h3>
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<p>
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The container init process will be started with <code>CAP_MKNOD</code>
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capability removed and blocked from re-acquiring it. As such it will
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not be able to create any device nodes in <code>/dev</code> or anywhere
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else in its filesystems. Libvirt itself will take care of pre-populating
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the <code>/dev</code> filesystem with any devices that the container
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is authorized to use. The current devices that will be made available
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to all containers are
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</p>
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<ul>
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<li><code>/dev/zero</code></li>
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<li><code>/dev/null</code></li>
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<li><code>/dev/full</code></li>
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<li><code>/dev/random</code></li>
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<li><code>/dev/urandom</code></li>
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<li><code>/dev/stdin</code> symlinked to <code>/proc/self/fd/0</code></li>
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<li><code>/dev/stdout</code> symlinked to <code>/proc/self/fd/1</code></li>
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<li><code>/dev/stderr</code> symlinked to <code>/proc/self/fd/2</code></li>
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<li><code>/dev/fd</code> symlinked to <code>/proc/self/fd</code></li>
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<li><code>/dev/ptmx</code> symlinked to <code>/dev/pts/ptmx</code></li>
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<li><code>/dev/console</code> symlinked to <code>/dev/pts/0</code></li>
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</ul>
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<p>
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In addition, for every console defined in the guest configuration,
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a symlink will be created from <code>/dev/ttyN</code> symlinked to
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the corresponding <code>/dev/pts/M</code> pseudo TTY device. The
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first console will be <code>/dev/tty1</code>, with further consoles
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numbered incrementally from there.
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</p>
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<p>
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Since /dev/ttyN and /dev/console are linked to the pts devices. The
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tty device of login program is pts device. the pam module securetty
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may prevent root user from logging in container. If you want root
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user to log in container successfully, add the pts device to the file
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/etc/securetty of container.
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</p>
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<p>
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Further block or character devices will be made available to containers
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depending on their configuration.
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</p>
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<h2><a name="security">Security considerations</a></h2>
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<p>
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The libvirt LXC driver is fairly flexible in how it can be configured,
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and as such does not enforce a requirement for strict security
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separation between a container and the host. This allows it to be used
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in scenarios where only resource control capabilities are important,
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and resource sharing is desired. Applications wishing to ensure secure
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isolation between a container and the host must ensure that they are
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writing a suitable configuration.
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</p>
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<h3><a name="securenetworking">Network isolation</a></h3>
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<p>
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If the guest configuration does not list any network interfaces,
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the <code>network</code> namespace will not be activated, and thus
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the container will see all the host's network interfaces. This will
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allow apps in the container to bind to/connect from TCP/UDP addresses
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and ports from the host OS. It also allows applications to access
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UNIX domain sockets associated with the host OS, which are in the
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abstract namespace. If access to UNIX domains sockets in the abstract
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namespace is not wanted, then applications should set the
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<code><privnet/></code> flag in the
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<code><features>....</features></code> element.
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</p>
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<h3><a name="securefs">Filesystem isolation</a></h3>
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<p>
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If the guest configuration does not list any filesystems, then
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the container will be set up with a root filesystem that matches
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the host's root filesystem. As noted earlier, only a few locations
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such as <code>/dev</code>, <code>/proc</code> and <code>/sys</code>
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will be altered. This means that, in the absence of restrictions
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from sVirt, a process running as user/group N:M inside the container
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will be able to access almost exactly the same files as a process
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running as user/group N:M in the host.
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</p>
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<p>
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There are multiple options for restricting this. It is possible to
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simply map the existing root filesystem through to the container in
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read-only mode. Alternatively a completely separate root filesystem
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can be configured for the guest. In both cases, further sub-mounts
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can be applied to customize the content that is made visible. Note
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that in the absence of sVirt controls, it is still possible for the
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root user in a container to unmount any sub-mounts applied. The user
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namespace feature can also be used to restrict access to files based
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on the UID/GID mappings.
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</p>
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<p>
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Sharing the host filesystem tree, also allows applications to access
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UNIX domains sockets associated with the host OS, which are in the
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filesystem namespaces. It should be noted that a number of init
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systems including at least <code>systemd</code> and <code>upstart</code>
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have UNIX domain socket which are used to control their operation.
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Thus, if the directory/filesystem holding their UNIX domain socket is
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exposed to the container, it will be possible for a user in the container
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to invoke operations on the init service in the same way it could if
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outside the container. This also applies to other applications in the
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host which use UNIX domain sockets in the filesystem, such as DBus,
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Libvirtd, and many more. If this is not desired, then applications
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should either specify the UID/GID mapping in the configuration to
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enable user namespaces and thus block access to the UNIX domain socket
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based on permissions, or should ensure the relevant directories have
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a bind mount to hide them. This is particularly important for the
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<code>/run</code> or <code>/var/run</code> directories.
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</p>
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<h3><a name="secureusers">User and group isolation</a></h3>
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<p>
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If the guest configuration does not list any ID mapping, then the
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user and group IDs used inside the container will match those used
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outside the container. In addition, the capabilities associated with
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a process in the container will infer the same privileges they would
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for a process in the host. This has obvious implications for security,
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since a root user inside the container will be able to access any
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file owned by root that is visible to the container, and perform more
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or less any privileged kernel operation. In the absence of additional
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protection from sVirt, this means that the root user inside a container
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is effectively as powerful as the root user in the host. There is no
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security isolation of the root user.
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</p>
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<p>
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The ID mapping facility was introduced to allow for stricter control
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over the privileges of users inside the container. It allows apps to
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define rules such as "user ID 0 in the container maps to user ID 1000
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in the host". In addition the privileges associated with capabilities
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are somewhat reduced so that they cannot be used to escape from the
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container environment. A full description of user namespaces is outside
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the scope of this document, however LWN has
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<a href="https://lwn.net/Articles/532593/">a good write-up on the topic</a>.
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From the libvirt point of view, the key thing to remember is that defining
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an ID mapping for users and groups in the container XML configuration
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causes libvirt to activate the user namespace feature.
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</p>
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<h2><a name="activation">Systemd Socket Activation Integration</a></h2>
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<p>
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The libvirt LXC driver provides the ability to pass across pre-opened file
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descriptors when starting LXC guests. This allows for libvirt LXC to support
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systemd's <a href="http://0pointer.de/blog/projects/socket-activated-containers.html">socket
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activation capability</a>, where an incoming client connection
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in the host OS will trigger the startup of a container, which runs another
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copy of systemd which gets passed the server socket, and then activates the
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actual service handler in the container.
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</p>
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<p>
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Let us assume that you already have a LXC guest created, running
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a systemd instance as PID 1 inside the container, which has an
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SSHD service configured. The goal is to automatically activate
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the container when the first SSH connection is made. The first
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step is to create a couple of unit files for the host OS systemd
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instance. The <code>/etc/systemd/system/mycontainer.service</code>
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unit file specifies how systemd will start the libvirt LXC container
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</p>
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<pre>
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[Unit]
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Description=My little container
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[Service]
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ExecStart=/usr/bin/virsh -c lxc:/// start --pass-fds 3 mycontainer
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ExecStop=/usr/bin/virsh -c lxc:/// destroy mycontainer
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Type=oneshot
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RemainAfterExit=yes
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KillMode=none
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</pre>
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<p>
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The <code>--pass-fds 3</code> argument specifies that the file
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descriptor number 3 that <code>virsh</code> inherits from systemd,
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is to be passed into the container. Since <code>virsh</code> will
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exit immediately after starting the container, the <code>RemainAfterExit</code>
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and <code>KillMode</code> settings must be altered from their defaults.
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</p>
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<p>
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Next, the <code>/etc/systemd/system/mycontainer.socket</code> unit
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file is created to get the host systemd to listen on port 23 for
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TCP connections. When this unit file is activated by the first
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incoming connection, it will cause the <code>mycontainer.service</code>
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unit to be activated with the FD corresponding to the listening TCP
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socket passed in as FD 3.
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</p>
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<pre>
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[Unit]
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Description=The SSH socket of my little container
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[Socket]
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ListenStream=23
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</pre>
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<p>
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Port 23 was picked here so that the container doesn't conflict
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with the host's SSH which is on the normal port 22. That's it
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in terms of host side configuration.
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</p>
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<p>
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Inside the container, the <code>/etc/systemd/system/sshd.socket</code>
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unit file must be created
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</p>
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<pre>
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[Unit]
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Description=SSH Socket for Per-Connection Servers
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[Socket]
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ListenStream=23
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Accept=yes
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</pre>
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<p>
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The <code>ListenStream</code> value listed in this unit file, must
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match the value used in the host file. When systemd in the container
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receives the pre-opened FD from libvirt during container startup, it
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looks at the <code>ListenStream</code> values to figure out which
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FD to give to which service. The actual service to start is defined
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by a correspondingly named <code>/etc/systemd/system/sshd@.service</code>
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</p>
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<pre>
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[Unit]
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Description=SSH Per-Connection Server for %I
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[Service]
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ExecStart=-/usr/sbin/sshd -i
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StandardInput=socket
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</pre>
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<p>
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Finally, make sure this SSH service is set to start on boot of the container,
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by running the following command inside the container:
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</p>
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<pre>
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# mkdir -p /etc/systemd/system/sockets.target.wants/
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# ln -s /etc/systemd/system/sshd.socket /etc/systemd/system/sockets.target.wants/
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</pre>
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<p>
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This example shows how to activate the container based on an incoming
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SSH connection. If the container was also configured to have an httpd
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service, it may be desirable to activate it upon either an httpd or a
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sshd connection attempt. In this case, the <code>mycontainer.socket</code>
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file in the host would simply list multiple socket ports. Inside the
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container a separate <code>xxxxx.socket</code> file would need to be
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created for each service, with a corresponding <code>ListenStream</code>
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value set.
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</p>
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<!--
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<h2>Container configuration</h2>
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<h3>Init process</h3>
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<h3>Console devices</h3>
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<h3>Filesystem devices</h3>
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<h3>Disk devices</h3>
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<h3>Block devices</h3>
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<h3>USB devices</h3>
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<h3>Character devices</h3>
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<h3>Network devices</h3>
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-->
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<h2>Container security</h2>
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<h3>sVirt SELinux</h3>
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<p>
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In the absence of the "user" namespace being used, containers cannot
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be considered secure against exploits of the host OS. The sVirt SELinux
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driver provides a way to secure containers even when the "user" namespace
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is not used. The cost is that writing a policy to allow execution of
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arbitrary OS is not practical. The SELinux sVirt policy is typically
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tailored to work with an simpler application confinement use case,
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as provided by the "libvirt-sandbox" project.
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</p>
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<h3>Auditing</h3>
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<p>
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The LXC driver is integrated with libvirt's auditing subsystem, which
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causes audit messages to be logged whenever there is an operation
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performed against a container which has impact on host resources.
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So for example, start/stop, device hotplug will all log audit messages
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providing details about what action occurred and any resources
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associated with it. There are the following 3 types of audit messages
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</p>
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<ul>
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<li><code>VIRT_MACHINE_ID</code> - details of the SELinux process and
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image security labels assigned to the container.</li>
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<li><code>VIRT_CONTROL</code> - details of an action / operation
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performed against a container. There are the following types of
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operation
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<ul>
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<li><code>op=start</code> - a container has been started. Provides
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the machine name, uuid and PID of the <code>libvirt_lxc</code>
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controller process</li>
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<li><code>op=init</code> - the init PID of the container has been
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started. Provides the machine name, uuid and PID of the
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<code>libvirt_lxc</code> controller process and PID of the
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init process (in the host PID namespace)</li>
|
|
<li><code>op=stop</code> - a container has been stopped. Provides
|
|
the machine name, uuid</li>
|
|
</ul>
|
|
</li>
|
|
<li><code>VIRT_RESOURCE</code> - details of a host resource
|
|
associated with a container action.</li>
|
|
</ul>
|
|
|
|
<h3>Device access</h3>
|
|
|
|
<p>
|
|
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.
|
|
</p>
|
|
|
|
<h2><a name="exconfig">Example configurations</a></h2>
|
|
|
|
<h3>Example config version 1</h3>
|
|
<p></p>
|
|
<pre>
|
|
<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>
|
|
</pre>
|
|
|
|
<p>
|
|
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.
|
|
</p>
|
|
|
|
<p>
|
|
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:
|
|
</p>
|
|
<p></p>
|
|
<pre>
|
|
<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>
|
|
</pre>
|
|
|
|
|
|
<h2><a name="usage">Container usage / management</a></h2>
|
|
|
|
<p>
|
|
As with any libvirt virtualization driver, LXC containers can be
|
|
managed via a wide variety of libvirt based tools. At the lowest
|
|
level the <code>virsh</code> command can be used to perform many
|
|
tasks, by passing the <code>-c lxc:///</code> argument. As an
|
|
alternative to repeating the URI with every command, the <code>LIBVIRT_DEFAULT_URI</code>
|
|
environment variable can be set to <code>lxc:///</code>. The
|
|
examples that follow outline some common operations with virsh
|
|
and LXC. For further details about usage of virsh consult its
|
|
manual page.
|
|
</p>
|
|
|
|
<h3><a name="usageSave">Defining (saving) container configuration></a></h3>
|
|
|
|
<p>
|
|
The <code>virsh define</code> command takes an XML configuration
|
|
document and loads it into libvirt, saving the configuration on disk
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// define myguest.xml
|
|
</pre>
|
|
|
|
<h3><a name="usageView">Viewing container configuration</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh dumpxml</code> 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 <code>--inactive</code> flag
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// dumpxml myguest
|
|
</pre>
|
|
|
|
<h3><a name="usageStart">Starting containers</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh start</code> command can be used to start a
|
|
container from a previously defined persistent configuration
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// start myguest
|
|
</pre>
|
|
|
|
<p>
|
|
It is also possible to start so called "transient" containers,
|
|
which do not require a persistent configuration to be saved
|
|
by libvirt, using the <code>virsh create</code> command.
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// create myguest.xml
|
|
</pre>
|
|
|
|
|
|
<h3><a name="usageStop">Stopping containers</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh shutdown</code> 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 <code>/dev/initctl</code> device node.
|
|
If no such device node exists, then it will send SIGTERM
|
|
to PID 1 inside the container.
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// shutdown myguest
|
|
</pre>
|
|
|
|
<p>
|
|
If the container does not respond to the graceful shutdown
|
|
request, it can be forcibly stopped using the <code>virsh destroy</code>
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// destroy myguest
|
|
</pre>
|
|
|
|
|
|
<h3><a name="usageReboot">Rebooting a container</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh reboot</code> 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 <code>/dev/initctl</code> device node.
|
|
If no such device node exists, then it will send SIGHUP
|
|
to PID 1 inside the container.
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// reboot myguest
|
|
</pre>
|
|
|
|
<h3><a name="usageDelete">Undefining (deleting) a container configuration</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh undefine</code> 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.
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// undefine myguest
|
|
</pre>
|
|
|
|
<h3><a name="usageConnect">Connecting to a container console</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh console</code> 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
|
|
<code>--devname</code> argument can be used to choose the
|
|
console to connect to
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// console myguest
|
|
</pre>
|
|
|
|
<h3><a name="usageEnter">Running commands in a container</a></h3>
|
|
|
|
<p>
|
|
The <code>virsh lxc-enter-namespace</code> command can be used
|
|
to enter the namespaces and security context of a container
|
|
and then execute an arbitrary command.
|
|
</p>
|
|
|
|
<pre>
|
|
# virsh -c lxc:/// lxc-enter-namespace myguest -- /bin/ls -al /dev
|
|
</pre>
|
|
|
|
<h3><a name="usageTop">Monitoring container utilization</a></h3>
|
|
|
|
<p>
|
|
The <code>virt-top</code> command can be used to monitor the
|
|
activity and resource utilization of all containers on a
|
|
host
|
|
</p>
|
|
|
|
<pre>
|
|
# virt-top -c lxc:///
|
|
</pre>
|
|
|
|
</body>
|
|
</html>
|