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:
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You can only run Linux based OS inside containers, i.e. it is not possible to run FreeBSD or MS Windows inside.
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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.
{pve} 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 {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".
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Note
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If you want to run micro-containers (with docker, rct, …), it is best to run them inside a VM. |
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:
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.
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 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 unique cluster wide. Files are stored inside /etc/pve/, so they get automatically replicated to all other cluster nodes.
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.
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.
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:
memory: 512 swap: 512 parent: testsnaphot ... [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).
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
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to set the container name
- modify /etc/hosts
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to allow lookup of the local hostname
- network setup
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pass the complete network setup to the container
- configure DNS
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pass information about DNS servers
- adapt the init system
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for example, fix the number of spawned getty processes
- set the root password
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when creating a new container
- rewrite ssh_host_keys
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so that each container has unique keys
- randomize crontab
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so that cron does not start at the same time on all containers
The 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
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inspect /etc/lsb-release (DISTRIB_ID=Ubuntu)
- Debian
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test /etc/debian_version
- Fedora
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test /etc/fedora-release
- RedHat or CentOS
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test /etc/redhat-release
- ArchLinux
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test /etc/arch-release
- Alpine
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test /etc/alpine-release
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Note
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Container start fails if the configured ostype differs from the auto detected type. |
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 itself ships a set of basic templates for most common operating systems, and you can download them using the pveam (short for {pve} 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:
# 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 {pve} volume identifiers. They include the storage name, and most other {pve} commands can use them. For examply you can delete that image later with:
pveam remove local:vztmpl/debian-8.0-standard_8.0-1_amd64.tar.gz
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 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 {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. 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.
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 limits.
Create a container based on a Debian template (provided you have already downloaded the template via the webgui)
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
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Simple, and fully integrated into {pve}. Setup looks similar to a normal VM setup.
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Storage (ZFS, LVM, NFS, Ceph, …)
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Network
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Authentification
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Cluster
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Fast: minimal overhead, as fast as bare metal
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High density (perfect for idle workloads)
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REST API
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Direct hardware access
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Integrated into {pve} graphical user interface (GUI)
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cgmanager for cgroup management
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lxcfs to provive containerized /proc file system
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apparmor
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CRIU: for live migration (planned)
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We use latest available kernels (4.2.X)
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Image based deployment (templates)
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Container setup from host (Network, DNS, Storage, …)