Linux Container

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Containers are a lightweight alternative to fully virtualized VMs. Instead of emulating a complete Operating System (OS), containers simply use the OS of the host they run on. This implies that all containers use the same kernel, and that they can access resources from the host directly.

This is great because containers do not waste CPU power nor memory due to kernel emulation. Container run-time costs are close to zero and usually negligible. But there are also some drawbacks you need to consider:

  • You can only run Linux based OS inside containers, i.e. it is not possible to run FreeBSD or MS Windows inside.

  • For security reasons, access to host resources needs to be restricted. This is done with AppArmor, SecComp filters and other kernel features. Be prepared that some syscalls are not allowed inside containers.

Proxmox VE uses LXC as underlying container technology. We consider LXC as low-level library, which provides countless options. It would be too difficult to use those tools directly. Instead, we provide a small wrapper called pct, the "Proxmox Container Toolkit".

The toolkit is tightly coupled with Proxmox VE. That means that it is aware of the cluster setup, and it can use the same network and storage resources as fully virtualized VMs. You can even use the Proxmox VE firewall, or manage containers using the HA framework.

Our primary goal is to offer an environment as one would get from a VM, but without the additional overhead. We call this "System Containers".

Note If you want to run micro-containers (with docker, rkt, …), it is best to run them inside a VM.

Security Considerations

Containers use the same kernel as the host, so there is a big attack surface for malicious users. You should consider this fact if you provide containers to totally untrusted people. In general, fully virtualized VMs provide better isolation.

The good news is that LXC uses many kernel security features like AppArmor, CGroups and PID and user namespaces, which makes containers usage quite secure. We distinguish two types of containers:

Privileged Containers

Security is done by dropping capabilities, using mandatory access control (AppArmor), SecComp filters and namespaces. The LXC team considers this kind of container as unsafe, and they will not consider new container escape exploits to be security issues worthy of a CVE and quick fix. So you should use this kind of containers only inside a trusted environment, or when no untrusted task is running as root in the container.

Unprivileged Containers

This kind of containers use a new kernel feature called user namespaces. The root UID 0 inside the container is mapped to an unprivileged user outside the container. This means that most security issues (container escape, resource abuse, …) in those containers will affect a random unprivileged user, and so would be a generic kernel security bug rather than an LXC issue. The LXC team thinks unprivileged containers are safe by design.


The /etc/pve/lxc/<CTID>.conf file stores container configuration, where <CTID> is the numeric ID of the given container. Like all other files stored inside /etc/pve/, they get automatically replicated to all other cluster nodes.

Note CTIDs < 100 are reserved for internal purposes, and CTIDs need to be unique cluster wide.
Example Container Configuration
ostype: debian
arch: amd64
hostname: www
memory: 512
swap: 512
net0: bridge=vmbr0,hwaddr=66:64:66:64:64:36,ip=dhcp,name=eth0,type=veth
rootfs: local:107/vm-107-disk-1.raw,size=7G

Those configuration files are simple text files, and you can edit them using a normal text editor (vi, nano, …). This is sometimes useful to do small corrections, but keep in mind that you need to restart the container to apply such changes.

For that reason, it is usually better to use the pct command to generate and modify those files, or do the whole thing using the GUI. Our toolkit is smart enough to instantaneously apply most changes to running containers. This feature is called "hot plug", and there is no need to restart the container in that case.

File Format

Container configuration files use a simple colon separated key/value format. Each line has the following format:

# this is a comment
OPTION: value

Blank lines in those files are ignored, and lines starting with a # character are treated as comments and are also ignored.

It is possible to add low-level, LXC style configuration directly, for example:

lxc.init_cmd: /sbin/my_own_init


lxc.init_cmd = /sbin/my_own_init

Those settings are directly passed to the LXC low-level tools.


When you create a snapshot, pct stores the configuration at snapshot time into a separate snapshot section within the same configuration file. For example, after creating a snapshot called “testsnapshot”, your configuration file will look like this:

Container configuration with snapshot
memory: 512
swap: 512
parent: testsnaphot

memory: 512
swap: 512
snaptime: 1457170803

There are a few snapshot related properties like parent and snaptime. The parent property is used to store the parent/child relationship between snapshots. snaptime is the snapshot creation time stamp (Unix epoch).

Guest Operating System Configuration

We normally try to detect the operating system type inside the container, and then modify some files inside the container to make them work as expected. Here is a short list of things we do at container startup:

set /etc/hostname

to set the container name

modify /etc/hosts

to allow lookup of the local hostname

network setup

pass the complete network setup to the container

configure DNS

pass information about DNS servers

adapt the init system

for example, fix the number of spawned getty processes

set the root password

when creating a new container

rewrite ssh_host_keys

so that each container has unique keys

randomize crontab

so that cron does not start at the same time on all containers

Changes made by Proxmox VE are enclosed by comment markers:

# --- BEGIN PVE ---
# --- END PVE ---

Those markers will be inserted at a reasonable location in the file. If such a section already exists, it will be updated in place and will not be moved.

Modification of a file can be prevented by adding a .pve-ignore. file for it. For instance, if the file /etc/.pve-ignore.hosts exists then the /etc/hosts file will not be touched. This can be a simple empty file creatd via:

# touch /etc/.pve-ignore.hosts

Most modifications are OS dependent, so they differ between different distributions and versions. You can completely disable modifications by manually setting the ostype to unmanaged.

OS type detection is done by testing for certain files inside the container:


inspect /etc/lsb-release (DISTRIB_ID=Ubuntu)


test /etc/debian_version


test /etc/fedora-release

RedHat or CentOS

test /etc/redhat-release


test /etc/arch-release


test /etc/alpine-release


test /etc/gentoo-release

Note Container start fails if the configured ostype differs from the auto detected type.


arch: (amd64 | i386) (default=amd64)

OS architecture type.

cmode: (console | shell | tty) (default=tty)

Console mode. By default, the console command tries to open a connection to one of the available tty devices. By setting cmode to console it tries to attach to /dev/console instead. If you set cmode to shell, it simply invokes a shell inside the container (no login).

console: boolean (default=1)

Attach a console device (/dev/console) to the container.

cpulimit: number (0 - 128) (default=0)

Limit of CPU usage.

Note If the computer has 2 CPUs, it has a total of 2 CPU time. Value 0 indicates no CPU limit.
cpuunits: integer (0 - 500000) (default=1024)

CPU weight for a VM. Argument is used in the kernel fair scheduler. The larger the number is, the more CPU time this VM gets. Number is relative to the weights of all the other running VMs.

Note You can disable fair-scheduler configuration by setting this to 0.
description: string

Container description. Only used on the configuration web interface.

hostname: string

Set a host name for the container.

lock: (backup | migrate | rollback | snapshot)

Lock/unlock the VM.

memory: integer (16 - N) (default=512)

Amount of RAM for the VM in MB.

mp[n]: [volume=]<volume> ,mp=<Path> [,acl=<1|0>] [,backup=<1|0>] [,quota=<1|0>] [,ro=<1|0>] [,size=<DiskSize>]

Use volume as container mount point.


Explicitly enable or disable ACL support.


Whether to include the mountpoint in backups (only used for volume mountpoints).


Path to the mountpoint as seen from inside the container.

Note Must not contain any symlinks for security reasons.

Enable user quotas inside the container (not supported with zfs subvolumes)


Read-only mountpoint


Volume size (read only value).


Volume, device or directory to mount into the container.

nameserver: string

Sets DNS server IP address for a container. Create will automatically use the setting from the host if you neither set searchdomain nor nameserver.

net[n]: name=<string> [,bridge=<bridge>] [,firewall=<1|0>] [,gw=<GatewayIPv4>] [,gw6=<GatewayIPv6>] [,hwaddr=<XX:XX:XX:XX:XX:XX>] [,ip=<IPv4Format/CIDR>] [,ip6=<IPv6Format/CIDR>] [,mtu=<integer>] [,rate=<mbps>] [,tag=<integer>] [,trunks=<vlanid[;vlanid...]>] [,type=<veth>]

Specifies network interfaces for the container.


Bridge to attach the network device to.


Controls whether this interface’s firewall rules should be used.


Default gateway for IPv4 traffic.


Default gateway for IPv6 traffic.


The interface MAC address. This is dynamically allocated by default, but you can set that statically if needed, for example to always have the same link-local IPv6 address. (


IPv4 address in CIDR format.


IPv6 address in CIDR format.

mtu=integer (64 - N)

Maximum transfer unit of the interface. (


Name of the network device as seen from inside the container. (


Apply rate limiting to the interface

tag=integer (1 - 4094)

VLAN tag for this interface.


VLAN ids to pass through the interface


Network interface type.

onboot: boolean (default=0)

Specifies whether a VM will be started during system bootup.

ostype: (alpine | archlinux | centos | debian | fedora | gentoo | opensuse | ubuntu | unmanaged)

OS type. This is used to setup configuration inside the container, and corresponds to lxc setup scripts in /usr/share/lxc/config/<ostype>.common.conf. Value unmanaged can be used to skip and OS specific setup.

protection: boolean (default=0)

Sets the protection flag of the container. This will prevent the CT or CT’s disk remove/update operation.

rootfs: [volume=]<volume> [,acl=<1|0>] [,quota=<1|0>] [,ro=<1|0>] [,size=<DiskSize>]

Use volume as container root.


Explicitly enable or disable ACL support.


Enable user quotas inside the container (not supported with zfs subvolumes)


Read-only mountpoint


Volume size (read only value).


Volume, device or directory to mount into the container.

searchdomain: string

Sets DNS search domains for a container. Create will automatically use the setting from the host if you neither set searchdomain nor nameserver.

startup: `[[order=]\d+] [,up=\d+] [,down=\d+] `

Startup and shutdown behavior. Order is a non-negative number defining the general startup order. Shutdown in done with reverse ordering. Additionally you can set the up or down delay in seconds, which specifies a delay to wait before the next VM is started or stopped.

swap: integer (0 - N) (default=512)

Amount of SWAP for the VM in MB.

template: boolean (default=0)

Enable/disable Template.

tty: integer (0 - 6) (default=2)

Specify the number of tty available to the container

unprivileged: boolean (default=0)

Makes the container run as unprivileged user. (Should not be modified manually.)

unused[n]: string

Reference to unused volumes. This is used internally, and should not be modified manually.

Container Images

Container images, sometimes also referred to as “templates” or “appliances”, are tar archives which contain everything to run a container. You can think of it as a tidy container backup. Like most modern container toolkits, pct uses those images when you create a new container, for example:

pct create 999 local:vztmpl/debian-8.0-standard_8.0-1_amd64.tar.gz

Proxmox VE itself ships a set of basic templates for most common operating systems, and you can download them using the pveam (short for Proxmox VE Appliance Manager) command line utility. You can also download TurnKey Linux containers using that tool (or the graphical user interface).

Our image repositories contain a list of available images, and there is a cron job run each day to download that list. You can trigger that update manually with:

pveam update

After that you can view the list of available images using:

pveam available

You can restrict this large list by specifying the section you are interested in, for example basic system images:

List available system images
# pveam available --section system
system          archlinux-base_2015-24-29-1_x86_64.tar.gz
system          centos-7-default_20160205_amd64.tar.xz
system          debian-6.0-standard_6.0-7_amd64.tar.gz
system          debian-7.0-standard_7.0-3_amd64.tar.gz
system          debian-8.0-standard_8.0-1_amd64.tar.gz
system          ubuntu-12.04-standard_12.04-1_amd64.tar.gz
system          ubuntu-14.04-standard_14.04-1_amd64.tar.gz
system          ubuntu-15.04-standard_15.04-1_amd64.tar.gz
system          ubuntu-15.10-standard_15.10-1_amd64.tar.gz

Before you can use such a template, you need to download them into one of your storages. You can simply use storage local for that purpose. For clustered installations, it is preferred to use a shared storage so that all nodes can access those images.

pveam download local debian-8.0-standard_8.0-1_amd64.tar.gz

You are now ready to create containers using that image, and you can list all downloaded images on storage local with:

# pveam list local
local:vztmpl/debian-8.0-standard_8.0-1_amd64.tar.gz  190.20MB

The above command shows you the full Proxmox VE volume identifiers. They include the storage name, and most other Proxmox VE commands can use them. For example you can delete that image later with:

pveam remove local:vztmpl/debian-8.0-standard_8.0-1_amd64.tar.gz

Container Storage

Traditional containers use a very simple storage model, only allowing a single mount point, the root file system. This was further restricted to specific file system types like ext4 and nfs. Additional mounts are often done by user provided scripts. This turned out to be complex and error prone, so we try to avoid that now.

Our new LXC based container model is more flexible regarding storage. First, you can have more than a single mount point. This allows you to choose a suitable storage for each application. For example, you can use a relatively slow (and thus cheap) storage for the container root file system. Then you can use a second mount point to mount a very fast, distributed storage for your database application.

The second big improvement is that you can use any storage type supported by the Proxmox VE storage library. That means that you can store your containers on local lvmthin or zfs, shared iSCSI storage, or even on distributed storage systems like ceph. It also enables us to use advanced storage features like snapshots and clones. vzdump can also use the snapshot feature to provide consistent container backups.

Last but not least, you can also mount local devices directly, or mount local directories using bind mounts. That way you can access local storage inside containers with zero overhead. Such bind mounts also provide an easy way to share data between different containers.

Mount Points

The root mount point is configured with the rootfs property, and you can configure up to 10 additional mount points. The corresponding options are called mp0 to mp9, and they can contain the following setting:

rootfs: [volume=]<volume> [,acl=<1|0>] [,quota=<1|0>] [,ro=<1|0>] [,size=<DiskSize>]

mp[n]: [volume=]<volume> ,mp=<Path> [,acl=<1|0>] [,backup=<1|0>] [,quota=<1|0>] [,ro=<1|0>] [,size=<DiskSize>]

acl: boolean

Explicitly enable or disable ACL support.

backup: boolean

Whether to include the mountpoint in backups (only used for volume mountpoints).

mp: <Path>

Path to the mountpoint as seen from inside the container.

Note Must not contain any symlinks for security reasons.
quota: boolean

Enable user quotas inside the container (not supported with zfs subvolumes)

ro: boolean

Read-only mountpoint

size: <DiskSize>

Volume size (read only value).

volume: <volume>

Volume, device or directory to mount into the container.

Currently there are basically three types of mount points: storage backed mount points, bind mounts and device mounts.

Typical container rootfs configuration
rootfs: thin1:base-100-disk-1,size=8G

Storage Backed Mount Points

Storage backed mount points are managed by the Proxmox VE storage subsystem and come in three different flavors:

  • Image based: these are raw images containing a single ext4 formatted file system.

  • ZFS subvolumes: these are technically bind mounts, but with managed storage, and thus allow resizing and snapshotting.

  • Directories: passing size=0 triggers a special case where instead of a raw image a directory is created.

Bind Mount Points

Bind mounts allow you to access arbitrary directories from your Proxmox VE host inside a container. Some potential use cases are:

  • Accessing your home directory in the guest

  • Accessing an USB device directory in the guest

  • Accessing an NFS mount from the host in the guest

Bind mounts are considered to not be managed by the storage subsystem, so you cannot make snapshots or deal with quotas from inside the container. With unprivileged containers you might run into permission problems caused by the user mapping and cannot use ACLs.

Note The contents of bind mount points are not backed up when using vzdump.
Warning For security reasons, bind mounts should only be established using source directories especially reserved for this purpose, e.g., a directory hierarchy under /mnt/bindmounts. Never bind mount system directories like /, /var or /etc into a container - this poses a great security risk.
Note The bind mount source path must not contain any symlinks.

For example, to make the directory /mnt/bindmounts/shared accessible in the container with ID 100 under the path /shared, use a configuration line like mp0: /mnt/bindmounts/shared,mp=/shared in /etc/pve/lxc/100.conf. Alternatively, use pct set 100 -mp0 /mnt/bindmounts/shared,mp=/shared to achieve the same result.

Device Mount Points

Device mount points allow to mount block devices of the host directly into the container. Similar to bind mounts, device mounts are not managed by Proxmox VE’s storage subsystem, but the quota and acl options will be honored.

Note Device mount points should only be used under special circumstances. In most cases a storage backed mount point offers the same performance and a lot more features.
Note The contents of device mount points are not backed up when using vzdump.

FUSE Mounts

Warning Because of existing issues in the Linux kernel’s freezer subsystem the usage of FUSE mounts inside a container is strongly advised against, as containers need to be frozen for suspend or snapshot mode backups.

If FUSE mounts cannot be replaced by other mounting mechanisms or storage technologies, it is possible to establish the FUSE mount on the Proxmox host and use a bind mount point to make it accessible inside the container.

Using Quotas Inside Containers

Quotas allow to set limits inside a container for the amount of disk space that each user can use. This only works on ext4 image based storage types and currently does not work with unprivileged containers.

Activating the quota option causes the following mount options to be used for a mount point: usrjquota=aquota.user,,jqfmt=vfsv0

This allows quotas to be used like you would on any other system. You can initialize the /aquota.user and / files by running

quotacheck -cmug /
quotaon /

and edit the quotas via the edquota command. Refer to the documentation of the distribution running inside the container for details.

Note You need to run the above commands for every mount point by passing the mount point’s path instead of just /.

Using ACLs Inside Containers

The standard Posix Access Control Lists are also available inside containers. ACLs allow you to set more detailed file ownership than the traditional user/ group/others model.

Container Network

You can configure up to 10 network interfaces for a single container. The corresponding options are called net0 to net9, and they can contain the following setting:

net[n]: name=<string> [,bridge=<bridge>] [,firewall=<1|0>] [,gw=<GatewayIPv4>] [,gw6=<GatewayIPv6>] [,hwaddr=<XX:XX:XX:XX:XX:XX>] [,ip=<IPv4Format/CIDR>] [,ip6=<IPv6Format/CIDR>] [,mtu=<integer>] [,rate=<mbps>] [,tag=<integer>] [,trunks=<vlanid[;vlanid...]>] [,type=<veth>]

bridge: <bridge>

Bridge to attach the network device to.

firewall: boolean

Controls whether this interface’s firewall rules should be used.

gw: <GatewayIPv4>

Default gateway for IPv4 traffic.

gw6: <GatewayIPv6>

Default gateway for IPv6 traffic.

hwaddr: <XX:XX:XX:XX:XX:XX>

The interface MAC address. This is dynamically allocated by default, but you can set that statically if needed, for example to always have the same link-local IPv6 address. (

ip: <IPv4Format/CIDR>

IPv4 address in CIDR format.

ip6: <IPv6Format/CIDR>

IPv6 address in CIDR format.

mtu: integer (64 - N)

Maximum transfer unit of the interface. (

name: <string>

Name of the network device as seen from inside the container. (

rate: <mbps>

Apply rate limiting to the interface

tag: integer (1 - 4094)

VLAN tag for this interface.

trunks: <vlanid[;vlanid...]>

VLAN ids to pass through the interface

type: (veth)

Network interface type.

Backup and Restore

Container Backup

It is possible to use the vzdump tool for container backup. Please refer to the vzdump manual page for details.

Restoring Container Backups

Restoring container backups made with vzdump is possible using the pct restore command. By default, pct restore will attempt to restore as much of the backed up container configuration as possible. It is possible to override the backed up configuration by manually setting container options on the command line (see the pct manual page for details).

Note pvesm extractconfig can be used to view the backed up configuration contained in a vzdump archive.

There are two basic restore modes, only differing by their handling of mount points:

“Simple” Restore Mode

If neither the rootfs parameter nor any of the optional mpX parameters are explicitly set, the mount point configuration from the backed up configuration file is restored using the following steps:

  1. Extract mount points and their options from backup

  2. Create volumes for storage backed mount points (on storage provided with the storage parameter, or default local storage if unset)

  3. Extract files from backup archive

  4. Add bind and device mount points to restored configuration (limited to root user)

Note Since bind and device mount points are never backed up, no files are restored in the last step, but only the configuration options. The assumption is that such mount points are either backed up with another mechanism (e.g., NFS space that is bind mounted into many containers), or not intended to be backed up at all.

This simple mode is also used by the container restore operations in the web interface.

“Advanced” Restore Mode

By setting the rootfs parameter (and optionally, any combination of mpX parameters), the pct restore command is automatically switched into an advanced mode. This advanced mode completely ignores the rootfs and mpX configuration options contained in the backup archive, and instead only uses the options explicitly provided as parameters.

This mode allows flexible configuration of mount point settings at restore time, for example:

  • Set target storages, volume sizes and other options for each mount point individually

  • Redistribute backed up files according to new mount point scheme

  • Restore to device and/or bind mount points (limited to root user)

Managing Containers with pct

pct is the tool to manage Linux Containers on Proxmox VE. You can create and destroy containers, and control execution (start, stop, migrate, …). You can use pct to set parameters in the associated config file, like network configuration or memory limits.

CLI Usage Examples

Create a container based on a Debian template (provided you have already downloaded the template via the web interface)

pct create 100 /var/lib/vz/template/cache/debian-8.0-standard_8.0-1_amd64.tar.gz

Start container 100

pct start 100

Start a login session via getty

pct console 100

Enter the LXC namespace and run a shell as root user

pct enter 100

Display the configuration

pct config 100

Add a network interface called eth0, bridged to the host bridge vmbr0, set the address and gateway, while it’s running

pct set 100 -net0 name=eth0,bridge=vmbr0,ip=,gw=

Reduce the memory of the container to 512MB

pct set 100 -memory 512

Obtaining Debugging Logs

In case pct start is unable to start a specific container, it might be helpful to collect debugging output by running lxc-start (replace ID with the container’s ID):

lxc-start -n ID -F -l DEBUG -o /tmp/lxc-ID.log

This command will attempt to start the container in foreground mode, to stop the container run pct shutdown ID or pct stop ID in a second terminal.

The collected debug log is written to /tmp/lxc-ID.log.

Note If you have changed the container’s configuration since the last start attempt with pct start, you need to run pct start at least once to also update the configuration used by lxc-start.



Configuration file for the container <CTID>.

Container Advantages

  • Simple, and fully integrated into Proxmox VE. Setup looks similar to a normal VM setup.

    • Storage (ZFS, LVM, NFS, Ceph, …)

    • Network

    • Authentication

    • Cluster

  • Fast: minimal overhead, as fast as bare metal

  • High density (perfect for idle workloads)


  • Direct hardware access

Technology Overview

  • Integrated into Proxmox VE graphical user interface (GUI)

  • LXC (

  • lxcfs to provide containerized /proc file system

  • AppArmor

  • CRIU: for live migration (planned)

  • We use latest available kernels (4.4.X)

  • Image based deployment (templates)

  • Container setup from host (network, DNS, storage, …)


LXC Containers use an AppArmor profile to provide ressource isolation in the container. This works by blocking system calls like 'mount' who are denied being executed in the container. You can trace the AppArmor activity with:

dmesg | grep apparmor 

If you want to disable AppArmor for a container, you can add the stanza

lxc.aa_profile = unconfined 

at the end of the configuration file ( located in /etc/pve/lxc/CTID.conf ) Note that this is not a recommended setup for production.

Migrate container from OpenVZ to Linux container

Follow this howto: