Docker inside CT [openvz docker]

2018/10 17 10:10

Docker inside CT

Since OpenVZ kernel 042stab105.4 it is possible to run Docker inside containers. This article describes how.
This page is applicable for OpenVZ 6 (for Virtuozzo 7 see here).


  • Kernel 042stab105.4 or later version
  • Kernel modules tunveth and bridge loaded on host (not required since vzctl 4.9 as it loads it automatically)

Container creation and tuning[编辑]

  • Create CentOS 7 container with enough disk space:
vzctl create $veid --ostemplate centos-7-x86_64 --diskspace 20G
  • Turn on bridge feature to allow docker creating bridged network:
vzctl set $veid --features bridge:on --save
  • Setup Container veth-based network:
vzctl set $veid --netif_add eth0 --save
  • Allow all iptables modules to be used in containers:
vzctl set $veid --netfilter full --save
  • Enable tun device access for container:
vzctl set $veid --devnodes net/tun:rw --save
  • Configure custom cgroups in systemd:
systemd reads /proc/cgroups and mounts all cgroups enabled there, though it doesn’t know there’s a restriction that only freezer,devices and cpuacct,cpu,cpuset can be mounted in container, but not freezer, cpu etc. separately
vzctl mount $veid
echo "JoinControllers=cpu,cpuacct,cpuset freezer,devices" >> /vz/root/$veid/etc/systemd/system.conf
  • Start the container:
vzctl start $veid
  • If you use Debian Wheezy for your CT which does not support systemd, you can run:
mount -t tmpfs tmpfs /sys/fs/cgroup
mkdir /sys/fs/cgroup/freezer,devices
mount -t cgroup cgroup /sys/fs/cgroup/freezer,devices -o freezer,devices
mkdir /sys/fs/cgroup/cpu,cpuacct,cpuset
mount -t cgroup cgroup /sys/fs/cgroup/cpu,cpuacct,cpuset/ -o cpu,cpuacct,cpuset

Prepare Docker in container[编辑]

These steps are to be performed inside the container.

  • Install Docker:
yum -y install docker-io
  • Start docker daemon
dockerd -s vfs

or change line in /etc/sysconfig/docker to:

OPTIONS='--selinux-enabled -s vfs'


service docker start

Example usage[编辑]


Use Docker to start WordPress (official, standard way).

  • Start mysql docker:
docker run --name test-mysql -e MYSQL_ROOT_PASSWORD=123 -d mysql
  • Start wordpress:
docker run --name test-wordpress --link test-mysql:mysql -p 8080:80 -d wordpress
  • Access wordpress server by container IP and port 8080:


  • Only “vfs” Docker graph driver is currently supported
  • Checkpointing and live migration of a container with Docker containers inside is not supported
  • Bridges cannot be created inside Docker containers running inside OpenVZ container
  • Only works with docker versions 1.10 or older. Newer versions will return an error: “Your Linux kernel version 2.6.32-042stab123.2 is not supported for running docker. Please upgrade your kernel to 3.10.0 or newer.” (i.e. switch to Virtuozzo 7 or later)

See also[编辑]


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