Linux Containers

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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.

Alternatives for using containers are systemd-nspawn, docker or rkt.

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 with additional kernel configuration. This is due to the current Arch linux kernel 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 ps:

[root@unprivileged_container /]# ps -ef | head -n 5
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
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


Required software

Installing lxc and arch-install-scripts 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 (a kernel with CONFIG_USER_NS). All Arch Linux kernels have support for CONFIG_USER_NS. 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 create unprivileged containers:

  • Start your unprivileged containers only as root.
  • Enable the sysctl setting kernel.unprivileged_userns_clone to allow normal users to run unprivileged containers. This can be done for the current session with sysctl kernel.unprivileged_userns_clone=1 and can be made permanent with sysctl.d(5).
  • Enable the cgfs pam module by modifying /etc/pam.d/system-login to additionally contain the following line:
session optional -c freezer,memory,name=systemd,unified

Secondly, modify /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/subuid and /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).

Note: A NAT bridge is a standalone bridge with a private network that is not bridged to the host eth0 or a physical network. It exists as a private subnet in the host.
Tip: This is quite useful when WIFI is the only option. There have been various attempts of creating Bridges on WIFI without much success.

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.

# 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.
# 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,' to have
# container 'mail1' always get ip address

# Uncomment the next line if you want lxcbr0's dnsmasq to resolve the .lxc
# domain.  You can then add "server=/lxc/' (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.
Tip: Make sure the bridges ip-range does not interfere with your local network.

Then we need to modify the LXC container template so our containers use our bridge:

/etc/lxc/default.conf = veth = lxcbr0 = up = 00:16:3e:xx:xx:xx

You also need to install dnsmasq which is a dependency for lxcbr0.

Enable and/or start lxc-net.service to use the bridge:

See Network bridge for more information.

Alternate Network - Bridge on same network as host


Reason: Duplicates Bridge with netctl. (Discuss in Talk:Linux Containers#)

If you would like the containers to be on the same network as your host machine and not utilize NAT and firewall forwarding rules you can do the following.

You will setup a manual bridge using netctl that includes the hosts ethernet adapter. You set your ethernet to not have an ip address and instead assign an ip address to the bridge that it is binded to.

In this scenario you will not be using the lxc-net service stop it if you enabled it.

# systemctl stop lxc-net
# systemctl disable lxc-net
Description="Manual Ethernet setup for bridge"
Interface=eno1     # your interface
Description="Bridge Interface"

Make sure to first shutdown your current ethernet connections prior to bringing these up.

# netctl stop-all
# netctl start ethernet
# netctl start bridge
# netctl enable ethernet
# netctl enable bridge

Now change the default config for your containers.

lxc.idmap = u 0 100000 65536
lxc.idmap = g 0 100000 65536 = veth = br0 = up

Now when you create a new container and start it, it should get an DHCP ip from the same network as the host.

Container creation

Containers are built using lxc-create. With the release of lxc-3.0.0-1, upstream has deprecated locally stored templates.

To build an Arch container, invoke like this:

# lxc-create -n playtime -t download -- --dist archlinux --release current --arch amd64

For other distros, invoke like this and select options from the supported distros displayed in the list:

# lxc-create -n playtime -t download
Tip: Users may optionally install haveged and start haveged.service to 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.
Tip: Users of Btrfs can append -B btrfs to create a Btrfs subvolume for storing containerized rootfs. This comes in handy if cloning containers with the help of lxc-clone command. ZFS users may use -B zfs, correspondingly.
Note: Users wanting the legacy templates can find them in lxc-templatesAUR or alternatively, users can build their own templates with distrobuilderAUR.

Container configuration

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

Note: With the release of lxc-1:2.1.0-1, many of the configuration options have changed. Existing containers need to be updated; users are directed to the table of these changes in the v2.1 release notes.

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 = playtime
lxc.arch = x86_64
lxc.include = /usr/share/lxc/config/common.conf

## network = veth = br0 = up = eth0 = ee:ec:fa:e9:56:7d
# uncomment the next two lines if static IP addresses are needed
# leaving these commented will imply DHCP networking
# = =
Note: The entry is optional and if skipped, a random MAC address will be created automatically. It can be advantageous to define a MAC address for the container to allow the DHCP server to always assign the same IP to the container's NIC (beyond the scope of this article but worth mentioning).

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
Note: This will not work without filesystem permission modifications on the host if using unprivileged containers.

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 /var/lib/lxc/playtime/config:

## 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.

Note: This will not work if using unprivileged containers.

OpenVPN considerations

Users wishing to run OpenVPN within the container are direct to either OpenVPN (client) in Linux containers and/or OpenVPN (server) in Linux containers.

Managing containers

Basic usage

To list all installed LXC containers:

# lxc-ls -f

Systemd can be used to start and to stop LXCs via lxc@CONTAINER_NAME.service. Enable lxc@CONTAINER_NAME.service to have it start when the host system boots.

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).

Advanced usage

LXC clones

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.


Reason: The note needs a reference. (Discuss in Talk:Linux Containers#)
Note: overlayfs for unprivileged containers is not supported in the current mainline Arch Linux kernel due to security considerations.

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
Note: If a static IP was defined for the "base" lxc, that will need to manually changed in the config for "snap1" and for "snap2" before starting them. If the process is to be automated, a script using sed can do this automatically although this is beyond the scope of this wiki section.

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

Systemd units and wrapper scripts to manage snapshots for pi-hole and OpenVPN are available to automate the process in lxc-snapshots.

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)), nsexec-bzrAUR 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 journalctl shows:

pam_securetty(login:auth): access denied: tty 'pts/0' is not secure !

Add pts/0 to the list of terminal names in /etc/securetty on the container filesystem, see [1]. You can also opt to delete /etc/securetty on the container to allow always root to login, see [2].

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 /etc/lxc/containername/config. If the virtual interface gets the ip assigned and should be connected to the network correctly.

ip addr show veth0 

You may disable all the relevant static ip formulas and try setting the ip through the booted container-os like you would normaly do.

Example container/config

... = veth = veth0 = up = bridge

And then assign your IP through your preferred method inside the container, see also Network configuration#Network management.

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 --clear-env:

# lxc-attach -n container_name --clear-env

Error: Failed at step KEYRING spawning...

Services in an unprivileged container may fail with the following message

some.service: Failed to change ownership of session keyring: Permission denied
some.service: Failed to set up kernel keyring: Permission denied
some.service: Failed at step KEYRING spawning ....: Permission denied

Create a file /etc/lxc/unpriv.seccomp containing

keyctl errno 38

Then add the following line to the container configuration after lxc.idmap

lxc.seccomp.profile = /etc/lxc/unpriv.seccomp

Errors due to mknod changes in linux 4.18 kernel

Applications may have errors such as the following with nginx:

nginx failes with [emerg]: open("/dev/null") failed (13: Permission denied)

There has been changes in linux 4.18 kernel with how mknod is handled and how it interacts with systemd's Privatedevice feature.

Workaround is to revert to a 4.17 version kernel until fixed upstream

Error: Failed to initialize cgroup driver / PAM unable to open

There is a bug in LXC 3.0.2. If you get these errors:

lxc-start: $NAME: cgroups/cgfsng.c: all_controllers_found: 701 No freezer controller mountpoint found
lxc-start: $NAME: cgroups/cgroup.c: cgroup_init: 44 Failed to initialize cgroup driver                                                                          
lxc-start: $NAME: start.c: lxc_init: 861 Failed to initialize cgroup driver
lxc-start: $NAME: start.c: __lxc_start: 1876 Failed to initialize container "$NAME"
lxc-start: $NAME: tools/lxc_start.c: main: 330 The container failed to start
lxc-start: $NAME: tools/lxc_start.c: main: 336 Additional information can be obtained by setting the --logfile and --logpriority options

associated with these errors in the output of journalctl:

Aug 17 16:06:25 localhost sshd[747]: PAM unable to dlopen(/usr/lib/security/ /usr/lib/security/ undefined symbol: strlcat
Aug 17 16:06:25 localhost sshd[747]: PAM adding faulty module: /usr/lib/security/

It is because of issue 2556, which has been fixed upstream already, but has not been backported to Arch (see FS#59689).

Downgrading to LXC 3.0.1 may solve the problem until the fix is packaged.

See also