pct.adoc: improve configuration format documenation

[pve-docs.git] / pct.adoc

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NAME
----

pct - Tool to manage Linux Containers (LXC) on Proxmox VE


SYNOPSYS
--------

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DESCRIPTION
-----------
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Proxmox Container Toolkit
=========================
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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 Free BSD or MS Windows inside.

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

{pve} uses https://linuxcontainers.org/[LXC] as underlying container
technology. We consider LXC as low-level library, which provides
countless options. It would be to difficult to use those tools
directly. Instead, we provide a small wrapper called `pct`, the
"Proxmox Container Toolkit".

The toolkit it tightly coupled with {pve}. 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 {pve}
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, rct, ...), 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 VM provides 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 a LXC issue. LXC people think
unprivileged containers are safe by design.


Configuration
-------------

The '/etc/pve/lxc/<CTID>.conf' files stores container configuration,
where '<CTID>' is the numeric ID of the given container. Note that
CTIDs < 100 are reserved for internal purposes, and CTIDs need to be
cluster wide unique. Files are stored inside '/etc/pve/', so they get
automatically replicated to all other cluster nodes.

.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

or

 lxc.init_cmd = /sbin/my_own_init

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

Snapshots
~~~~~~~~~

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
...

[testsnaphot]
memory: 512
swap: 512
snaptime: 1457170803
...
----

There are a view snapshot related properties like 'parent' and
'snaptime'. They '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:: allow to lookup the local hostname

network setup:: pass the complete network setup to the container

configure DNS:: pass information about DNS servers

adopt the init system:: for example, fix the number os 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 same time on all containers

Above task depends on the OS type, so the implementation is different
for each OS type. You can also disable any modifications by manually
setting the 'ostype' to 'unmanaged'.

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

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

Debian:: test /etc/debian_version

Fedora:: test /etc/fedora-release

RedHat or CentOS:: test /etc/redhat-release

ArchLinux:: test /etc/arch-release

Alpine:: test /etc/alpine-release

NOTE: Container start fails is configured 'ostype' differs from auto
detected type.

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 turend
out to be complex and error prone, so we trie 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 {pve} 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'. And it enables us
to use advanced storage features like snapshots and clones. 'vzdump'
can also use the snapshots 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 provides an easy way to share data between different containers.


Managing Containers with 'pct'
------------------------------

'pct' is the tool to manage Linux Containers on {pve}. 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.

CLI Usage Examples
------------------

Create a container based on a Debian template (provided you downloaded
the template via the webgui before)

 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=192.168.15.147/24,gw=192.168.15.1

Reduce the memory of the container to 512MB

 pct set -memory 512 100

Files
------

'/etc/pve/lxc/<CTID>.conf'::

Configuration file for the container '<CTID>'.


Container Advantages
--------------------

- Simple, and fully integrated into {pve}. Setup looks similar to a normal
  VM setup. 

  * Storage (ZFS, LVM, NFS, Ceph, ...)

  * Network

  * Authentification

  * Cluster

- Fast: minimal overhead, as fast as bare metal

- High density (perfect for idle workloads)

- REST API

- Direct hardware access


Technology Overview
-------------------

- Integrated into {pve} graphical user interface (GUI)

- LXC (https://linuxcontainers.org/)

- cgmanager for cgroup management

- lxcfs to provive containerized /proc file system

- apparmor

- CRIU: for live migration (planned)

- We use latest available kernels (4.2.X)

- image based deployment (templates)

- Container setup from host (Network, DNS, Storage, ...)


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