Linux Containers (LXC) is an operating-system-level virtualization method for running multiple isolated Linux systems (containers) on a single control host (LXC host). It does not provide a virtual machine, but rather provides a virtual environment that has its own CPU, memory, block I/O, network, etc. space and the resource control mechanism. This is provided by namespaces and cgroups features in Linux kernel on LXC host. It is similar to a chroot, but offers much more isolation.
- 1 Privileged containers or unprivileged containers
- 2 Setup
- 2.1 Required software
- 2.2 Host network configuration
- 2.3 Container creation
- 2.4 Container configuration
- 3 Managing containers
- 4 Running Xorg programs
- 5 Troubleshooting
- 6 See also
Privileged containers or unprivileged containers
LXCs can be setup to run in either privileged or unprivileged configurations.
In general, running an unprivileged container is considered safer than running a privileged container since unprivileged containers have an increased degree of isolation by virtue of their design. Key to this is the mapping of the root UID in the container to a non-root UID on the host which makes it more difficult for a hack within the container to lead to consequences on host system. In other words, if an attacker manages to escape the container, he or she should find themselves with no rights on the host.
The Arch packages currently provide out-of-the-box support for privileged containers. Unprivileged containers are only available for the system administrator without additional kernel configuration. This is due to the current Archkernel shipping with user namespaces disabled for normal users. This article contains information for users to run either type of container, but additional setup is required to use unprivileged containers.
An example to illustrate unprivileged containers
To illustrate the power of UID mapping, consider the output below from a running, unprivileged container. Therein, we see the containerized processes owned by the containerized root user in the output of
[root@unprivileged_container /]# ps -ef | head -n 5 UID PID PPID C STIME TTY TIME CMD root 1 0 0 17:49 ? 00:00:00 /sbin/init root 14 1 0 17:49 ? 00:00:00 /usr/lib/systemd/systemd-journald dbus 25 1 0 17:49 ? 00:00:00 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation systemd+ 26 1 0 17:49 ? 00:00:00 /usr/lib/systemd/systemd-networkd
On the host however, those containerized root processes are running as the mapped user (ID>100000) on the host, not as the root user on the host:
[root@host /]# lxc-info -Ssip --name sandbox State: RUNNING PID: 26204 CPU use: 10.51 seconds BlkIO use: 244.00 KiB Memory use: 13.09 MiB KMem use: 7.21 MiB
[root@host /]# ps -ef | grep 26204 | head -n 5 UID PID PPID C STIME TTY TIME CMD 100000 26204 26200 0 12:49 ? 00:00:00 /sbin/init 100000 26256 26204 0 12:49 ? 00:00:00 /usr/lib/systemd/systemd-journald 100081 26282 26204 0 12:49 ? 00:00:00 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation 100000 26284 26204 0 12:49 ? 00:00:00 /usr/lib/systemd/systemd-logind
Installingand will allow the host system to run privileged lxcs.
Enable support to run unprivileged containers (optional)
Users wishing to run unprivileged containers need to complete several additional setup steps.
Firstly, a kernel is required that has support for User Namespaces. However, due to more general security concerns, the default Arch kernel does ship with User Namespaces enabled only for the root user. You have multiple options to use a kernel with
CONFIG_USER_NS and thereby create unprivileged containers:
- Use (v4.14.5 or later) or and start your unprivileged containers only as root
kernel.unprivileged_userns_cloneto allow normal users to run unprivileged containers. This can be done for the current session with
sysctl kernel.unprivileged_userns_clone=1and can be made permanent with
(v4.14.5 or later) or and enable the sysctl setting
- Compile and install your own custom kernel with
CONFIG_USER_NSenabled. See Kernels/Arch Build System for more information on compiling a custom kernel.
/etc/lxc/default.conf to contain the following lines:
lxc.idmap = u 0 100000 65536 lxc.idmap = g 0 100000 65536
Finally, create both
/etc/subgid to contain the mapping to the containerized uid/gid pairs for each user who shall be able to run the containers. The example below is simply for the root user (and systemd system unit):
Host network configuration
LXCs support different virtual network types and devices (see lxc.container.conf(5)). A bridge device on the host is required for most types of virtual networking.
LXC comes with its own NAT Bridge (lxcbr0).
To use LXC's NAT Bridge you need to create its configuration file:
# Leave USE_LXC_BRIDGE as "true" if you want to use lxcbr0 for your # containers. Set to "false" if you'll use virbr0 or another existing # bridge, or mavlan to your host's NIC. USE_LXC_BRIDGE="true" # If you change the LXC_BRIDGE to something other than lxcbr0, then # you will also need to update your /etc/lxc/default.conf as well as the # configuration (/var/lib/lxc/<container>/config) for any containers # already created using the default config to reflect the new bridge # name. # If you have the dnsmasq daemon installed, you'll also have to update # /etc/dnsmasq.d/lxc and restart the system wide dnsmasq daemon. LXC_BRIDGE="lxcbr0" LXC_ADDR="10.0.3.1" LXC_NETMASK="255.255.255.0" LXC_NETWORK="10.0.3.0/24" LXC_DHCP_RANGE="10.0.3.2,10.0.3.254" LXC_DHCP_MAX="253" # Uncomment the next line if you'd like to use a conf-file for the lxcbr0 # dnsmasq. For instance, you can use 'dhcp-host=mail1,10.0.3.100' to have # container 'mail1' always get ip address 10.0.3.100. #LXC_DHCP_CONFILE=/etc/lxc/dnsmasq.conf # Uncomment the next line if you want lxcbr0's dnsmasq to resolve the .lxc # domain. You can then add "server=/lxc/10.0.3.1' (or your actual $LXC_ADDR) # to your system dnsmasq configuration file (normally /etc/dnsmasq.conf, # or /etc/NetworkManager/dnsmasq.d/lxc.conf on systems that use NetworkManager). # Once these changes are made, restart the lxc-net and network-manager services. # 'container1.lxc' will then resolve on your host. #LXC_DOMAIN="lxc"
Then we need to modify the LXC container template so our containers use our bridge:
lxc.net.0.type = veth lxc.net.0.link = lxcbr0 lxc.net.0.flags = up lxc.net.0.hwaddr = 00:16:3e:xx:xx:xx
You also need to install Dnsmasq which is a dependency for lxcbr0.
pacman -S dnsmasq
Then we can start the bridge:
systemctl start lxc-net
If you want the bridge to start at boot-time
systemctl enable lxc-net
For further information including Host bridges users are referred to the Network bridge article.
For privileged containers, simply select a template from
/usr/share/lxc/templates that matches the target distro to containerize. Users wishing to containerize non-Arch distros will need additional packages on the host depending on the target distro:
- Fedora-based: AUR
lxc-create to create the container, which installs the root filesystem of the LXC to
/var/lib/lxc/CONTAINER_NAME/rootfs by default. Example creating an Arch Linux LXC named "playtime":
# lxc-create -n playtime -t /usr/share/lxc/templates/lxc-archlinux
Users wishing to run unprivileged containers should use the -t download directive and select from the images that are displayed. For example:
# lxc-create -n playtime -t download
Alternatively, create a privileged container, and see: #Converting a privileged container to an unprivileged container.
haveged.serviceto avoid a perceived hang during the setup process while waiting for system entropy to be seeded. Without it, the generation of private/GPG keys can add a lengthy wait to the process.
-B btrfsto create a Btrfs subvolume for storing containerized rootfs. This comes in handy if cloning containers with the help of
lxc-clonecommand. ZFS users may use
-B zfs, correspondingly.
The examples below can be used with privileged and unprivileged containers alike. Note that for unprivileged containers, additional lines will be present by default which are not shown in the examples, including the
lxc.idmap = u 0 100000 65536 and the
lxc.idmap = g 0 100000 65536 values optionally defined in the #Enable support to run unprivileged containers (optional) section.
Basic config with networking
System resources to be virtualized/isolated when a process is using the container are defined in
/var/lib/lxc/CONTAINER_NAME/config. By default, the creation process will make a minimum setup without networking support. Below is an example config with networking:
# Template used to create this container: /usr/share/lxc/templates/lxc-archlinux # Parameters passed to the template: # For additional config options, please look at lxc.container.conf(5) ## default values lxc.rootfs.path = /var/lib/lxc/playtime/rootfs lxc.uts.name = playtime lxc.arch = x86_64 lxc.include = /usr/share/lxc/config/archlinux.common.conf ## network lxc.net.0.type = veth lxc.net.0.link = br0 lxc.net.0.flags = up lxc.net.0.name = eth0 lxc.net.0.hwaddr = ee:ec:fa:e9:56:7d # uncomment the next two lines if static IP addresses are needed # leaving these commented will imply DHCP networking # #lxc.net.0.ipv4.address = 192.168.0.3/24 #lxc.net.0.ipv4.gateway = 192.168.0.1
Mounts within the container
For privileged containers, one can select directories on the host to bind mount to the container. This can be advantageous for example if the same architecture is being containerize and one wants to share pacman packages between the host and container. Another example could be shared directories. The syntax is simple:
lxc.mount.entry = /var/cache/pacman/pkg var/cache/pacman/pkg none bind 0 0
Xorg program considerations (optional)
In order to run programs on the host's display, some bind mounts need to be defined so that the containerized programs can access the host's resources. Add the following section to
## for xorg lxc.mount.entry = /dev/dri dev/dri none bind,optional,create=dir lxc.mount.entry = /dev/snd dev/snd none bind,optional,create=dir lxc.mount.entry = /tmp/.X11-unix tmp/.X11-unix none bind,optional,create=dir,ro lxc.mount.entry = /dev/video0 dev/video0 none bind,optional,create=file
If you still get a permission denied error in your LXC guest, then you may need to call
xhost + in your host to allow the guest to connect to the host's display server. Take note of the security concerns of opening up your display server by doing this.
In addition you might need to add the following line
lxc.mount.entry = tmpfs tmp tmpfs defaults
before the bind mount lines.
To list all installed LXC containers:
# lxc-ls -f
Users can also start/stop LXCs without systemd. Start a container:
# lxc-start -n CONTAINER_NAME
Stop a container:
# lxc-stop -n CONTAINER_NAME
To login into a container:
# lxc-console -n CONTAINER_NAME
If when login you get pts/0 and lxc/tty1 use:
# lxc-console -n CONTAINER_NAME -t 0
Once logged, treat the container like any other linux system, set the root password, create users, install packages, etc.
To attach to a container:
# lxc-attach -n CONTAINER_NAME --clear-env
It works nearly the same as lxc-console, but you are automatically accessing root prompt inside the container, bypassing login. Without the
--clear-env flag, the host will pass its own environment variables into the container (including
$PATH, so some commands will not work when the containers are based on another distribution).
Users with a need to run multiple containers can simplify administrative overhead (user management, system updates, etc.) by using snapshots. The strategy is to setup and keep up-to-date a single base container, then, as needed, clone (snapshot) it. The power in this strategy is that the disk space and system overhead are truly minimized since the snapshots use an overlayfs mount to only write out to disk, only the differences in data. The base system is read-only but changes to it in the snapshots are allowed via the overlayfs.
For example, setup a container as outlined above. We will call it "base" for the purposes of this guide. Now create 2 snapshots of "base" which we will call "snap1" and "snap2" with these commands:
# lxc-copy -n base -N snap1 -B overlayfs -s # lxc-copy -n base -N snap2 -B overlayfs -s
The snapshots can be started/stopped like any other container. Users can optionally destroy the snapshots and all new data therein with the following command. Note that the underlying "base" lxc is untouched:
# lxc-destroy -n snap1 -f
Converting a privileged container to an unprivileged container
Once the system has been configured to use unprivileged containers (see, #Enable support to run unprivileged containers (optional)), AUR contains a utility called
uidmapshift which is able to convert an existing privileged container to an unprivileged container to avoid a total rebuild of the image.
- It is recommended to backup the existing image before using this utility!
- This utility will not shift UIDs and GIDs in ACL, you will need to shift them on your own.
Invoke the utility to convert over like so:
# uidmapshift -b /var/lib/lxc/foo 0 100000 65536
Additional options are available simply by calling
uidmapshift without any arguments.
Running Xorg programs
Either attach to or SSH into the target container and prefix the call to the program with the DISPLAY ID of the host's X session. For most simple setups, the display is always 0.
An example of running Firefox from the container in the host's display:
$ DISPLAY=:0 firefox
Alternatively, to avoid directly attaching to or connecting to the container, the following can be used on the host to automate the process:
# lxc-attach -n playtime --clear-env -- sudo -u YOURUSER env DISPLAY=:0 firefox
Root login fails
If you get the following error when you try to login using lxc-console:
login: root Login incorrect
And the container's
pam_securetty(login:auth): access denied: tty 'pts/0' is not secure !
Alternatively, create a new user in lxc-attach and use it for logging in to the system, then switch to root.
# lxc-attach -n playtime [root@playtime]# useradd -m -Gwheel newuser [root@playtime]# passwd newuser [root@playtime]# passwd root [root@playtime]# exit # lxc-console -n playtime [newuser@playtime]$ su
No network-connection with veth in container config
If you cannot access your LAN or WAN with a networking interface configured as veth and setup through
If the virtual interface gets the ip assigned and should be connected to the network correctly.
ip addr show veth0 inet 192.168.1.111/24
You may disable all the relevant static ip formulas and try setting the ip through the booted container-os like you would normaly do.
... lxc.net.0.type = veth lxc.net.0.name = veth0 lxc.net.0.flags = up lxc.net.0.link =
Error: unknown command
The error may happen when you type a basic command (ls, cat, etc.) on an attached container that have different Linux distribution from the host system (e.g. Debian container in Arch Linux host system). When you attach, use the argument
# lxc-attach -n container_name --clear-env