As I briefly covered in the [first post of this series][1], LXD’s focus is system containers. That is, we run a full unmodified Linux distribution inside our containers. LXD for all intent and purposes doesn’t care about the workload running in the container. It just sets up the container namespaces and security policies, then spawns /sbin/init and waits for the container to stop.
Application containers such as those implemented by Docker or Rkt are pretty different in that they are used to distribute applications, will typically run a single main process inside them and be much more ephemeral than a LXD container.
Those two container types aren’t mutually exclusive and we certainly see the value of using Docker containers to distribute applications. That’s why we’ve been working hard over the past year to make it possible to run Docker inside LXD.
This means that with Ubuntu 16.04 and LXD 2.0, you can create containers for your users who will then be able to connect into them just like a normal Ubuntu system and then run Docker to install the services and applications they want.
### Requirements
There are a lot of moving pieces to make all of this working and we got it all included in Ubuntu 16.04:
- A kernel with CGroup namespace support (4.4 Ubuntu or 4.6 mainline)
- LXD 2.0 using LXC 2.0 and LXCFS 2.0
- A custom version of Docker (or one built with all the patches that we submitted)
- A Docker image which behaves when confined by user namespaces, or alternatively make the parent LXD container a privileged container (security.privileged=true)
### Running a basic Docker workload
Enough talking, lets run some Docker containers!
First of all, you need an Ubuntu 16.04 container which you can get with:
The “-p default -p docker” instructs LXD to apply both the “default” and “docker” profiles to the container. The default profile contains the basic network configuration while the docker profile tells LXD to load a few required kernel modules and set up some mounts for the container. The docker profile also enables container nesting.
Now lets make sure the container is up to date and install docker:
```
lxc exec docker -- apt update
lxc exec docker -- apt dist-upgrade -y
lxc exec docker -- apt install docker.io -y
```
And that’s it! You’ve got Docker installed and running in your container.
Now lets start a basic web service made of two Docker containers:
That’s it! It’s really that simple to run Docker containers inside a LXD container.
Now as I mentioned earlier, not all Docker images will behave as well as my example, that’s typically because of the extra confinement that comes with LXD, specifically the user namespace.
Only the overlayfs storage driver of Docker works in this mode. That storage driver may come with its own set of limitation which may further limit how many images will work in this environment.
If your workload doesn’t work properly and you trust the user inside the LXD container, you can try:
```
lxc config set docker security.privileged true
lxc restart docker
```
That will de-activate the user namespace and will run the container in privileged mode.
Note however that in this mode, root inside the container is the same uid as root on the host. There are a number of known ways for users to escape such containers and gain root privileges on the host, so you should only ever do that if you’d trust the user inside your LXD container with root privileges on the host.
### Extra information
The main LXD website is at: <https://linuxcontainers.org/lxd>
Development happens on Github at: <https://github.com/lxc/lxd>
Mailing-list support happens on: <https://lists.linuxcontainers.org>
IRC support happens in: #lxcontainers on irc.freenode.net
