Ubuntu Core in LXD containers

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What’s Ubuntu Core?

Ubuntu Core is a version of Ubuntu that’s fully transactional and entirely based on snap packages.

Most of the system is read-only. All installed applications come from snap packages and all updates are done using transactions. Meaning that should anything go wrong at any point during a package or system update, the system will be able to revert to the previous state and report the failure.

The current release of Ubuntu Core is called series 16 and was released in November 2016.

Note that on Ubuntu Core systems, only snap packages using confinement can be installed (no “classic” snaps) and that a good number of snaps will not fully work in this environment or will require some manual intervention (creating user and groups, …). Ubuntu Core gets improved on a weekly basis as new releases of snapd and the “core” snap are put out.


As far as LXD is concerned, Ubuntu Core is just another Linux distribution. That being said, snapd does require unprivileged FUSE mounts and AppArmor namespacing and stacking, so you will need the following:

  • An up to date Ubuntu system using the official Ubuntu kernel
  • An up to date version of LXD

Creating an Ubuntu Core container

The Ubuntu Core images are currently published on the community image server.
You can launch a new container with:

stgraber@dakara:~$ lxc launch images:ubuntu-core/16 ubuntu-core
Creating ubuntu-core
Starting ubuntu-core

The container will take a few seconds to start, first executing a first stage loader that determines what read-only image to use and setup the writable layers. You don’t want to interrupt the container in that stage and “lxc exec” will likely just fail as pretty much nothing is available at that point.

Seconds later, “lxc list” will show the container IP address, indicating that it’s booted into Ubuntu Core:

stgraber@dakara:~$ lxc list
|     NAME    |  STATE  |          IPV4        |                      IPV6                    |    TYPE    | SNAPSHOTS |
| ubuntu-core | RUNNING | (eth0) | 2001:470:b368:b2b5:216:3eff:fee1:296f (eth0) | PERSISTENT | 0         |

You can then interact with that container the same way you would any other:

stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap list
Name       Version     Rev  Developer  Notes
core       16.04.1     394  canonical  -
pc         16.04-0.8   9    canonical  -
pc-kernel  4.4.0-45-4  37   canonical  -

Updating the container

If you’ve been tracking the development of Ubuntu Core, you’ll know that those versions above are pretty old. That’s because the disk images that are used as the source for the Ubuntu Core LXD images are only refreshed every few months. Ubuntu Core systems will automatically update once a day and then automatically reboot to boot onto the new version (and revert if this fails).

If you want to immediately force an update, you can do it with:

stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap refresh
pc-kernel (stable) 4.4.0-53-1 from 'canonical' upgraded
core (stable) 16.04.1 from 'canonical' upgraded
root@ubuntu-core:~# snap version
snap 2.17
snapd 2.17
series 16

And then reboot the system and check the snapd version again:

root@ubuntu-core:~# reboot

stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap version
snap 2.21
snapd 2.21
series 16

You can get an history of all snapd interactions with

stgraber@dakara:~$ lxc exec ubuntu-core snap changes
ID  Status  Spawn                 Ready                 Summary
1   Done    2017-01-31T05:14:38Z  2017-01-31T05:14:44Z  Initialize system state
2   Done    2017-01-31T05:14:40Z  2017-01-31T05:14:45Z  Initialize device
3   Done    2017-01-31T05:21:30Z  2017-01-31T05:22:45Z  Refresh all snaps in the system

Installing some snaps

Let’s start with the simplest snaps of all, the good old Hello World:

stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap install hello-world
hello-world 6.3 from 'canonical' installed
root@ubuntu-core:~# hello-world
Hello World!

And then move on to something a bit more useful:

stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap install nextcloud
nextcloud 11.0.1snap2 from 'nextcloud' installed

Then hit your container over HTTP and you’ll get to your newly deployed Nextcloud instance.

If you feel like testing the latest LXD straight from git, you can do so with:

stgraber@dakara:~$ lxc config set ubuntu-core security.nesting true
stgraber@dakara:~$ lxc exec ubuntu-core bash
root@ubuntu-core:~# snap install lxd --edge
lxd (edge) git-c6006fb from 'canonical' installed
root@ubuntu-core:~# lxd init
Name of the storage backend to use (dir or zfs) [default=dir]: 

We detected that you are running inside an unprivileged container.
This means that unless you manually configured your host otherwise,
you will not have enough uid and gid to allocate to your containers.

LXD can re-use your container's own allocation to avoid the problem.
Doing so makes your nested containers slightly less safe as they could
in theory attack their parent container and gain more privileges than
they otherwise would.

Would you like to have your containers share their parent's allocation (yes/no) [default=yes]? 
Would you like LXD to be available over the network (yes/no) [default=no]? 
Would you like stale cached images to be updated automatically (yes/no) [default=yes]? 
Would you like to create a new network bridge (yes/no) [default=yes]? 
What should the new bridge be called [default=lxdbr0]? 
What IPv4 address should be used (CIDR subnet notation, “auto” or “none”) [default=auto]? 
What IPv6 address should be used (CIDR subnet notation, “auto” or “none”) [default=auto]? 
LXD has been successfully configured.

And because container inception never gets old, lets run Ubuntu Core 16 inside Ubuntu Core 16:

root@ubuntu-core:~# lxc launch images:ubuntu-core/16 nested-core
Creating nested-core
Starting nested-core 
root@ubuntu-core:~# lxc list
|    NAME     |  STATE  |         IPV4        |                       IPV6                    |    TYPE    | SNAPSHOTS |
| nested-core | RUNNING | (eth0) | fd42:2861:5aad:3842:216:3eff:feaf:e6bd (eth0) | PERSISTENT | 0         |


If you ever wanted to try Ubuntu Core, this is a great way to do it. It’s also a great tool for snap authors to make sure their snap is fully self-contained and will work in all environments.

Ubuntu Core is a great fit for environments where you want to ensure that your system is always up to date and is entirely reproducible. This does come with a number of constraints that may or may not work for you.

And lastly, a word of warning. Those images are considered as good enough for testing, but aren’t officially supported at this point. We are working towards getting fully supported Ubuntu Core LXD images on the official Ubuntu image server in the near future.

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
Try LXD online: https://linuxcontainers.org/lxd/try-it

About Stéphane Graber

Project leader of Linux Containers, Linux hacker, Ubuntu core developer, conference organizer and speaker.
This entry was posted in Canonical voices, LXD, Planet Ubuntu and tagged . Bookmark the permalink.

10 Responses to Ubuntu Core in LXD containers

  1. Bob says:

    This is great!
    one issue I get on refresh:

    root@core1:~# snap refresh
    error: cannot perform the following tasks:
    – Download snap “core” (1222) from channel “stable” (sha3-384 mismatch downloading core: got 40d096c525d9325d43da759ed7c59660f5b0d06fb4f023dceb2c783c55ba68aa51e5fbeedb8c8d7cdc17e34ecda462a1 but expected abb6ba696c70a69cc9546795f95a093f49adb3ca90c85a5335b5edcc9770fb86a7610e13735f12b81ab971ba1d8d9ac7)

    1. Do you have some kind of proxy server on your network?

      I’ve seen that error before on my network and it was usually caused by the snappy CDN having returned a bad answer and my proxy server caching it. If you don’t have some kind of caching server, I’d expect a retry to eventually get the content downloaded properly from the CDN.

  2. Bob says:

    Seems to be an issue with lxc, installing/refreshing snaps directly on the host work every time.
    But both ubuntu core and ubuntu server 16.04 running in lxc containers are having issues installing and refreshing.
    Errors are either connection reset by peer or hash mismatch. Smaller snaps have a greater chance of success, core universally fails.

    1. Chris Chou says:

      Any update to this update issue? Unable to refresh snap from ubuntu-core in lxd container.

      1. Chris Chou says:

        Just tried it on the demo instance at “https://linuxcontainers.org/lxd/try-it/” and got the same error seen on my local machine:

        root@tryit-chief:~# lxc exec ubuntu-core bash
        root@ubuntu-core:~# snap refresh
        error: cannot refresh []: Post https://search.apps.ubuntu.com/api/v1/snaps/metadata: net/http: request canceled while waiting for connection (Client.Timeout exceeded whil
        e awaiting headers)
        root@ubuntu-core:~# snap –version
        snap 2.23.1
        snapd 2.23.1
        series 16
        kernel 4.4.0-77-generic

  3. * says:

    ARM images seems missing…

    1. Yes, that’s intentional. The way the ARM images work, with regard to bootloader handling is completely different from the x86 ones so we’d need a completely different boot code and conversion tool for those.

  4. Nils says:

    Container does not seem to start for me on a up to date 16.04.2 server.
    cat /var/log/lxd/ubuntu-core/lxc.log
    lxc 20170309192503.621 WARN lxc_start – start.c:signal_handler:322 – Invalid pid for SIGCHLD. Received pid 9718, expected pid 9752.
    lxc 20170309192504.229 WARN lxc_commands – commands.c:lxc_cmd_rsp_recv:172 – Command get_cgroup failed to receive response: Connection reset by peer.
    What am I missing?

  5. * says:

    Something gets messed up on “images:” store server today:

    root@tryit-social:~# lxc launch images:ubuntu-core/16 ubuntu-core
    Creating ubuntu-core
    error: Failed container creation:
    https://images.linuxcontainers.org: Provided partial image fingerprint matches more than one image

    root@tryit-social:~# lxc image show images:ubuntu-core/16
    error: The requested image couldn’t be found.

    root@tryit-social:~# lxc image list images:ubuntu-core/16 | grep “de1512d589a7”
    | plamo/6.x/i386 (1 more) | de1512d589a7 | yes | Plamo 6.x i386 (20170709_21:36) | i686 | 279.14MB | Jul 9, 2017 at 12:00am (UTC) |
    | ubuntu-core/16 (3 more) | de1512d589a7 | yes | Ubuntu-Core 16 amd64 (20170704_19:01) | x86_64 | 203.31MB | Jul 4, 2017 at 12:00am (UTC) |
    | ubuntu-core/16/i386 (1 more) | de1512d589a7 | yes | Ubuntu-Core 16 i386 (20170704_19:01) | i686 | 197.57MB | Jul 4, 2017 at 12:00am (UTC) |
    | | de1512d589a7 | yes | Ubuntu-Core 16 amd64 (20170620_19:01) | x86_64 | 203.31MB | Jun 20, 2017 at 12:00am (UTC) |
    | | de1512d589a7 | yes | Ubuntu-Core 16 amd64 (20170627_19:01) | x86_64 | 203.31MB | Jun 27, 2017 at 12:00am (UTC) |
    | | de1512d589a7 | yes | Ubuntu-Core 16 i386 (20170620_19:01) | i686 | 197.57MB | Jun 20, 2017 at 12:00am (UTC) |
    | | de1512d589a7 | yes | Ubuntu-Core 16 i386 (20170627_19:01) | i686 | 197.57MB | Jun 27, 2017 at 12:00am (UTC) |

  6. Kyle Strickland says:

    The ubuntu-core/16 image doesn’t appear to be available (nor /18 nor /20) on the image server. Is this still being maintained, and is there another location we can use to download this?

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