LXC (Linux Containers) is a virtualization system, making use of the "cgroups" feature. It is conceptually similar to Solaris's Zones and FreeBSD's Jails, so to provide more segregation of a simple chroot without having to incur in the penalties of a full virtualisation solution.
This section is a basic overview of how lxc fits in to the virtualization world, the type of approach it uses, and the benefits and limitations thereof. If you are trying to figure out if lxc is for you, or it's your first time setting up virtualization under Linux, then you should at least skim this section.
Roughly speaking there are two types of virtualization in use today, container-based virtualization and full virtualization.
Container-based Virtualization (lxc)
Container based virtualization is very fast and efficient. It's based on the premise that an OS kernel provides different views of the system to different running processes. This sort of segregation or compartmentalisation (sometimes called "thick sandboxing") can be useful for ensuring guaranteed access to hardware resources such as CPU and IO bandwidth, whilst maintaining security and efficiency.
On the unix family of operating systems, it is said that container based virtualization has its roots in the 1982 release of the chroot tool, a filesystem subsystem specific container-based virtualization tool that was written by Sun Microsystems founder Bill Joy and published as part of 4.2BSD.
Since this early tool, which has become a mainstay of the unix world, a large number of unix developers have worked to mature more powerful container based virtualization solutions. Some examples:
- Solaris Zones
- FreeBSD Jails
- Linux VServer
On Linux, historically the major two techniques have been Linux-VServer (open source / community driven) and OpenVZ (a free spinoff of a commercial product).
However, neither of these will be accepted in to the Linux kernel. Instead Linus has opted for a more flexible, longer-term approach to achieving similar goals, using various new kernel features. lxc is the next-generation container-based virtualization solution that uses these new features.
Conceptually, lxc can be seen as a further development of the existing 'chroot' technique with extra dimensions added. Where 'chroot'-ing only offers isolation at the file system level, lxc offers complete logical isolation from a container to the host and all other containers. In fact, installing a new Gentoo container from scratch is pretty much the same as for any normal Gentoo installation.
Some of the most notably differences are:
- each container will share the kernel with the host (and other containers). No kernel need to be present and/or mounted on the containers /boot directory;
- devices and filesystem will be (more or less) 'inherited' from the host, and need not be configured as would apply for a normal installation;
- if the host is using the OpenRC system for bootstrapping, such configuration items will "automagically" be omitted (i.e. filesystem mounts from fstab).
The last point is important to keep lxc based installation as much as simple and the same as for normal installations (no exceptions).
Full Virtualization (not lxc)
Full virtualization and paravirtualization solutions aim to simulate the underlying hardware. This type of solution, unlike lxc and other container-based solutions, usually allow you to run any operating system. Whilst this may be useful for the purposes of security and server consolidation, it is hugely inefficient compared to container based solutions. The most popular solutions in this area right now are probably VMware, KVM, Xen and VirtualBox.
Limitations of lxc
With lxc, you can efficiently manage resource allocation in real time. In addition, you should be able to run different Linux distributions on the same host kernel in different containers (though there may be teething issues with startup and shutdown 'run control' (rc) scripts, and these may need to be modified slightly to make some guests work. That said, maintainers of tools such as OpenRC are increasingly implementing lxc detection to ensure correct behaviour when their code runs within containers.)
Unlike full virtualization solutions, lxc will not let you run other operating systems (such as proprietary operating systems, or other types of unix).
However, in theory there is no reason why you can't install a full or paravirtualization solution on the same kernel as your lxc host system and run both full/paravirtualised guests in addition to lxc guests at the same time.
Should you elect to do this, there are powerful abstracted virtualization management API under development, such as [libvirt] and [ganeti], that you may wish to check out.
- One kernel
- One operating system
- Many instances
... but can co-exist with other virtualization solutions if required.
MAJOR Temporary Problems with LXC - READ THIS
As documented over here, basically containers are not functional as security containers at present, in that if you have root on a container you have root on the whole box.
- root in a container has all capabilities
- Do not treat root privileges in the container any more lightly than on the host itself.
- legacy UID/GID comparisons in many parts of the kernel code are dumb and will not respect containers
- Do not mount parts of external filesystems within a container, except ro (read only).
- Do not re-use UIDs/GIDs between the container and the host
- shutdown and halt will run over the host system.
- Restrict/Replace them in the container
Containers are still useful for isolating applications, including their networking interfaces, and applying resource limits and accounting to those applications. As the above issues are resolved, they will also become functional security containers.
If you are designing a virtualisation solution for the long term and want a timeframe, then with appropriate disclaimers, judging from various comments and experience, an extremely rough timeframe might be 'circa end of 2012'. But no guarantees.
See also CAP_SYS_ADMIN: the new root.
lxc uses two new / lesser known kernel features known as 'control groups' and 'POSIX file capabilities'. It also includes 'template scripts' to setup different guest environments.
Control Groups are a multi-hierarchy, multi-subsystem resource management / control framework for the Linux kernel.
In simpler language, what this means is that unlike the old chroot tool which was limited to the file subsystem, control groups let you define a 'group' encompassing one or more processes (eg: sshd, Apache) and then specify a variety of resource control and accounting options for that control group against multiple subsystems, such as:
- filesystem access
- general device access
- memory resources
- network device resources
- CPU bandwidth
- block device IO bandwidth
- various other aspects of a control group's view of the system
The user-space access to these new kernel features is a kernel-provided filesystem, known as 'cgroup'. It is typically mounted at /cgroup and provides files similar to /proc and /sys representing the running environment and various kernel configuration options.
POSIX File Capabilities
POSIX file capabilities are a way to allocate privileges to a process that allow for more specific security controls than the traditional 'root' vs. 'user' privilege separation on unix family operating systems.
To get an lxc-capable host system working you will need the following steps.
Kernel with the appropriate LXC options enabled
If you are unfamiliar with recompiling kernels, see the copious documentation available on that subject in addition to the notes below.
Kernel options required
The app-emulation/lxc ebuild will check for the most important options for the kernel that are required to set up a LXC host. This is, though, not a fatal check which means you have to make sure to have the options correctly enabled manually. The package also comes with an upstream-provided lxc-checkconfig script that should report on the proper options.
Freezer support allows you to 'freeze' and 'thaw' a running guest, something like 'suspend' under VMware products. It appears to be under heavy development as of October 2010 (LXC list) but is apparently mostly functional. Please add additional notes on this page if you explore further.
CONFIG_CGROUP_FREEZER / "Freeze/thaw support" ('General Setup -> Control Group support -> Freezer cgroup subsystem')
Scheduling allows you to specify how much hardware access (CPU bandwidth, block device bandwidth, etc.) control groups have.
CONFIG_CGROUP_SCHED / "Cgroup sched" ('General Setup -> Control Group support -> Group CPU scheduler') FAIR_GROUP_SCHED / "Group scheduling for SCHED_OTHER" ('General Setup -> Control Group support -> Group CPU scheduler -> Group scheduling for SCHED_OTHER') CONFIG_BLK_CGROUP / "Block IO controller" ('General Setup -> Control Group support -> Block IO controller') CONFIG_CFQ_GROUP_IOSCHED / "CFQ Group Scheduling support" ('Enable the block layer -> IO Schedulers -> CFQ I/O scheduler -> CFQ Group Scheduling support')
Resource Counters (Memory/Swap Accounting)
Resource counters are an 'accounting' feature - they allow you to measure resource utilisation in your guest. They are also an apparent prerequisite for limiting memory and swap utilisation.
CONFIG_RESOURCE_COUNTERS / "Resource counters" ('General Setup -> Control Group support -> Resource counters')
For memory resources...
CONFIG_CGROUP_MEM_RES_CTLR / "Cgroup memory controller" ('General Setup -> Control Group support -> Resource counters -> Memory Resource Controller for Control Groups')
If you want to also count swap utilisation, also select...
CONFIG_CGROUP_MEM_RES_CTLR_SWAP / "Memory Resource Controller Swap Extension(EXPERIMENTAL)" ('General Setup -> Control Group support -> Resource counters -> Memory Resource Controller for Control Groups -> Memory Resource Controller Swap Extension')
This allows you to measure the CPU utilisation of your control groups.
CONFIG_CGROUP_CPUACCT / "Cgroup cpu account" ('General Setup -> Control Group support -> Simple CPU accounting cgroup subsystem')
Ethernet bridging, veth, macvlan and vlan (802.1q) support are optional, but you probably want at least one of these:
CONFIG_BRIDGE / "802.1d Ethernet Bridging" ('Networking support -> Networking options -> 802.1d Ethernet Bridging') CONFIG_VETH / "Veth pair device" CONFIG_MACVLAN / "Macvlan" CONFIG_VLAN_8021Q / "Vlan"
Further details about LXC networking options are available on Flameeyes's Weblog
lxc userspace utilities
Due to LXC's still unstable nature, Gentoo provides ebuilds only for the most recent version available, therefore make sure to update your Portage tree before proceeding:
Mounted cgroup filesystem
The 'cgroup' filesystem provides user-space access to the required kernel control group features, and is required by the lxc userspace utilities. Up to kernel 3.1, the filesystem's mountpoint wasn't well defined; nowadays it's defined to be mounted (split) within /sys/fs/cgroup. Recent OpenRC versions already mount it during boot, and the app-emulation/lxc ebuild already depends on a new enough version.
You can check it using:
There are three main options for giving network access to containers:
- Network bridges (the simplest solution in most cases);
- isolated network;
Guest configuration for network bridges
Your guest network configuration resides in the guest's lxc.conf file. Documentation for this file is accessible with:
If you have used a template script to create your guest, this will typically reside in the parent directory of the guest's root filesystem. However, using /etc/lxc/ to store guest configurations is also common.
Your guest configuration should include the following network-related lines:
If, like me, you are using dhcp inside the container to get an IP address, then run it once as shown. LXC will generate a random MAC address for the interface. To keep your DHCP server from getting confused, you will want to use that MAC address all the time. So find out what it is, and then uncomment the 'lxc.network.hwaddr' line and specify it there.
A number of 'template scripts' are distributed with the lxc package. These scripts assist with generating various guest environments.
Template scripts live in /usr/share/lxc/templates but should be executed via the lxc-create tool as follows:
The rootfs of linux container is stored in /etc/lxc/guestname/
Configuration files (the -f configuration-file option) are usually used to specify the network settings for the initial guest configuration. For example:
lxc.network.type=veth lxc.network.link=br0 lxc.network.flags=up
Automatic setup: lxc-gentoo
The lxc-gentoo tool can download, extract and configure a gentoo guest for you. It fixes a lot of little issues that you may otherwise find tedious and are not yet outlined in the manual guest configuration section, below.
You can download it here: lxc-gentoo page
Additional developers, bug fixes, comments, etc. are welcome.
Manual Guest Configuration
LXC allows a configuration file for each guest container, specifying name, IP address, etc. yu can use lxc-gentoo script to create a gentoo guest:
After creating the guest you can manage it as usual.
If you want to share distfiles from your host, you can set the PORTAGE_RO_DISTDIRS variable to a space-separated list of directories to search. Portage will create a symlink in DISTDIR to the first matching file found in PORTAGE_RO_DISTDIRS if the file does not already exist in DISTDIR.
In the latest lxc-gentoo script (comment=269ea1735503cd932421d7c63d729f849279690d), the fstab is hardcoded. Therefore, if you want to mount the shared distfiles, you should add lxc.mount option to utsname.conf file. You can get more information from flameeyes's blog.
- Fixme: this template script cannot be executed in gentoo linux directly, because it contains "apt-get" command when downloading Alt linux guest.
Fixme: It seems the pacman-4.0.1 cannot work correctly in gentoo linux
Fixme: The archlinux template does not create a working container (app-emulation/lxc-0.8.0_rc2-r1), giving an error on not being able to find /sbin/init (the file /usr/lib/systemd/systemd does not exists). Chrooting into the linux container (the rootfs directory) and issuing:
solves this issue. Also, you need CONFIG_DEVTMPFS activated in the kernel configuration if you configure the container as stated in the archlinux wiki
lxc contains a minimal template script for busybox. Busybox is basically a base system oriented towards embedded use, where many base utilities exist in an optimized form within one stripped binary to save on memory. Busybox is installed as part of the base gentoo system, so the script works right away. Example:
You will need to install dev-util/debootstrap package:
You can then use the lxc supplied debian template script to download all required files, generate a configuration file and a root filesystem for your guest.
lxc-fedora template assists with setting up Fedora guests. Note that in order to use lxc-fedora, you must:
lxc-opensuse template assists with setting up opensuse linux guest. Fixme: lack of zypper command line package manager tool in gentoo portage.
lxc contains a minimal template script for sshd guests. You can create the sshd guest through:
lxc contains a minimal template script for Ubuntu guests (see ubuntu.com). Note that in order to use lxc-ubuntu, you must:
Usage is as follows...
Or, in versions < app-emulation/lxc-0.8.0_rc2-r1
This will create the folder ubuntu-guest. Inside the folder, there will be a file called config. It takes a very long time to create a ubuntu guest, please be patient. To start the container, issue the following command:
You should use the username and password of the existing system user used when creating the container. To set root password, enter the directory ubuntu-guest and you will see the directory rootfs. Issue:
Set the password with the command:
Using the Guest Container
To start and stop the guest, simply run:
Please be aware, that when you have daemonized the booting process (-d), you will not get any output on screen. This might happen when you conveniently use an alias wich daemonizes by default, and forgot about it. You may get puzzled later by this if there is problem while booting a new the container that has not been configured properly (e.g. network).
Use from gentoo init system
Gentoo's ebuild (without the vanilla USE flag enabled), provides an init script to manage containers and start them at boot time. To make use of the init script you just have to create a symlink in the /etc/init.d/ folder:
Of course, the use of such scripts is primarily intended for booting and stopping the system. To make a guest to the rc chain, run:
To enter a (already started) guest directly from the host machine, see the lxc-console section below.
Accessing the guest
Using lxc-console provides console access to the guest. To use type:
If you get a message saying lxc-console: console denied by guestname, then you need to add to your container config:
lxc.tty = 1
To exit the console, use:
Note that unless you log out inside the guest, you will remain logged on, so the next time you run lxc-console, you will return to the same session.
Usage of lxc-console should be restricted to root. It should be primarily a tool for system administrators (root) to enter a (newly) container after it is first created, e.g. when the network connection is not properly configured yet. Using multiple instances of lxc-console on distinct guests works fine, but starting a second instance for a guest that is already governed by another lxc-console session, leads to redirection of keyboard input and terminal output. Best is to avoid use of lxc-console at all. (Perhaps lxc developers should enhance the tool in such way that is only possible for singleton use per guest. ;-)
Accessing the container with sshd
A common technique to allow users direct access into a system container is to run a separate sshd inside the container. Users then connect to that sshd directly. In this way, you can treat the container just like you treat a full virtual machine where you grant external access. If you give the container a routable address, then users can reach it without using ssh tunneling.
If you set up the container with a virtual ethernet interface connected to a bridge on the host, then it can have its own ethernet address on the LAN, and you should be able to connect directly to it without logically involving the host (the host will transparently relay all traffic destined for the container, without the need for any special considerations). You should be able to simply 'ssh <container_ip>'.
Some of the lxc tools apparently assume that /etc/lxc/<guestname>/ exists. However, you should keep the guests' root filesystems out of /etc since it's not a path that's supposed to store large volumes of binary data.
The templates of lxc will use the following locations::
- /etc/lxc/<guestname>/config = guest configuration file
- /etc/lxc/<guestname>/fstab = optional guest fstab file
- /var/lxc/<guestname>/rootfs = root filesystem image
- /var/log/lxc/<guestname> = lxc-start logfile