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# 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 the namespaces and cgroups features in the Linux kernel on the LXC host. It is similar to a chroot, but offers much more isolation.

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

## 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 within the container to a non-root UID on the host, which makes it more difficult for a hack inside the container to lead to consequences on the host system. In other words, if an attacker manages to escape the container, he or she should find themselves with limited or no rights on the host.

The Arch linux, linux-lts and linux-zen kernel packages currently provide out-of-the-box support for unprivileged containers. Similarly, with the linux-hardened package, unprivileged containers are only available for the system administrator; with additional kernel configuration changes required, as user namespaces are disabled by default for normal users there. This article contains information for users to run either type of container, but additional steps may be required in order 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
    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 actually shown to be running as the mapped user (ID>100000), rather than the host's actual root user:

    [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

## Setup

### Required software

Installing lxc and arch-install-scripts will allow the host system to run privileged lxcs.

#### Enable support to run unprivileged containers (optional)

Enable the control groups PAM module by modifying `/etc/pam.d/system-login` to additionally contain the following line:

    session    optional   pam_cgfs.so -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):

    /etc/subuid

    root:100000:65536

    /etc/subgid

    root:100000:65536

Users wishing to run unprivileged containers on linux-hardened or their custom kernel 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 linux-hardened kernel does ship with User Namespaces enabled only for the root user. There are two options to create unprivileged containers there:

- Start the 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).

### 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 which is illustrated in this section.

There are several main setups to consider:

1. A host bridge
2. A NAT bridge

The host bridge requires the host's network manager to manage a shared bridge interface. The host and any lxc will be assigned an IP address in the same network (for example 192.168.1.x). This might be more simplistic in cases where the goal is to containerize some network-exposed service like a webserver, or VPN server. The user can think of the lxc as just another PC on the physical LAN, and forward the needed ports in the router accordingly. The added simplicity can also be thought of as an added threat vector, again, if WAN traffic is being forwarded to the lxc, having it running on a separate range presents a smaller threat surface.

The NAT bridge does not require the host's network manager to manage the bridge. lxc ships with `lxc-net` which creates a NAT bridge called `lxcbr0`. The NAT bridge is a standalone bridge with a private network that is not bridged to the host's ethernet device or to a physical network. It exists as a private subnet in the host.

#### Using a host bridge

See Network bridge.

#### Using a NAT bridge

Install dnsmasq which is a dependency for `lxc-net` and before starting the bridge, first create a configuration file for it:

    /etc/default/lxc-net

    # 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"

Tip: Make sure the bridge's IP range does not interfere with the local network.

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

    /etc/lxc/default.conf

    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

Optionally create a configuration file to manually define the IP address of any containers:

    /etc/lxc/dnsmasq.conf

    dhcp-host=playtime,10.0.3.100

Now start and enable `lxc-net.service` to create the bridge interface.

##### Firewall considerations

Since the lxc is running on the 10.0.3.x subnet, access to services such as ssh, httpd, etc. will need to be actively forwarded to the lxc. In principal, the firewall on the host needs to forward traffic incoming traffic on the expected port on the container.

###### Example iptables rule

The goal of this rule is to allow ssh traffic to the lxc:

    # iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 2221 -j DNAT --to-destination 10.0.3.100:22

This rule forwards tcp traffic originating on port 2221 to the IP address of the lxc on port 22.

Note: Make sure to allow traffic on 2221/tcp on the host and to allow 22/tcp traffic on the lxc.

To ssh into the container from another PC on the LAN, one needs to ssh on port 2221 to the host. The host will then forward that traffic to the container.

    $ ssh -p 2221 host.lan

###### Example ufw rule

If using ufw, append the following at the bottom of `/etc/ufw/before.rules` to make this persistent:

    /etc/ufw/before.rules

    *nat
    :PREROUTING ACCEPT [0:0]
    -A PREROUTING -i eth0 -p tcp --dport 2221 -j DNAT --to-destination 10.0.3.101:22
    COMMIT

##### Running containers as non-root user

To create and start containers as a non-root user, extra configuration must be applied.

Create the usernet file under `/etc/lxc/lxc-usernet`. According to the `lxc-usernet` man page, the entry per line is:

    user type bridge number

Configure the file with the user needing to create containers. The bridge will be the same as defined in `/etc/default/lxc-net`.

A copy of the `/etc/lxc/default.conf` is needed in the non-root user's home directory, e.g. `~/.config/lxc/default.conf` (create the directory if needed).

Running containers as a non-root user requires `+x` permissions on `~/.local/share/`. Make that change with chmod before starting a container.

### 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 supplied by `lxc-net.service`:

    /var/lib/lxc/playtime/config

    # 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)

    # Distribution configuration
    lxc.include = /usr/share/lxc/config/common.conf
    lxc.arch = x86_64

    # Container specific configuration
    lxc.rootfs.path = dir:/var/lib/lxc/playtime/rootfs
    lxc.uts.name = playtime

    # Network configuration
    lxc.net.0.type = veth
    lxc.net.0.link = lxcbr0
    lxc.net.0.flags = up
    lxc.net.0.hwaddr = ee:ec:fa:e9:56:7d

#### 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 still experiencing a permission denied error in the LXC guest, call `xhost +` in the host to allow the guest to connect to the host's display server. Take note of the security concerns of opening up the display server by doing this. In addition, add the following line before the above bind mount lines.

    lxc.mount.entry = tmpfs tmp tmpfs defaults

Note: This will not work if using unprivileged containers.

#### OpenVPN considerations

Users wishing to run OpenVPN within the container are requested to 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

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

That works nearly the same as lxc-console, but it causes starts with a 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.

This article or section needs expansion.

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-service-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.

Warning:

- It is recommended to backup the existing image before using this utility!
- This utility will not shift UIDs and GIDs in ACL, users will need to shift them manually!

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

## Troubleshooting

### Root login fails

If presented with following error upon trying 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 !

Delete `/etc/securetty`[1] and `/usr/share/factory/etc/securetty` on the container file system. Optionally add them to NoExtract in `/etc/pacman.conf` to prevent them from getting reinstalled. See FS#45903 for details.

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
    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.

Example `container/config`

    ...
    lxc.net.0.type = veth
    lxc.net.0.name = veth0
    lxc.net.0.flags = up
    lxc.net.0.link = bridge
    ...

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

### Error: unknown command

The error may happen when a basic command (ls, cat, etc.) on an attached container is typed hen a different Linux distribution is containerized relative to the host system (e.g. Debian container in Arch Linux host system). Upon attaching, 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

    /etc/lxc/unpriv.seccomp

    2
    blacklist
    [all]
    keyctl errno 38

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

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

## See also

- LXC 1.0 Blog Post Series
- LXD 2.0: Blog post series
- LXC@developerWorks
- LXC articles on l3net

Categories:

- Virtualization
- Sandboxing

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