Some LXD containers on a hidden net, others on your lan

Back in November I wrote about Converting eth0 to br0 and getting all your LXC or LXD onto your LAN

It works, but you might not want ALL of your LXD on your LAN.

You’ll still need your LAN interface to be a br0 instead of a device that isn’t a bridge. Go follow the Bridge your interface section of that post to convert your eth0 to br0.

I’ve fully converted to using LXD. I don’t even remember if LXC supports profiles. I think it does, so I think the same idea could be applied to LXC, but I’m only showing this for LXD.

First, copy the default profile:

lxc profile copy default lanbridge

Second, edit the new profile to use br0 instead of lxdbr0:

lxc profile device set lanbridge eth0 parent br0

Third and finally, start instances with that profile:

lxc launch ubuntu-xenial -p lanbridge

In my case, this instance is on my local lan AND on public ipv6 space (thanks Comcast).

heritable-gale    | RUNNING | (eth0) | 2601:400:8000:5ab3:216:3eff:fe73:d242 (eth0)


Cloud-config with LXD

A year ago I wrote

Since then, LXD became the best way to use LXC.

By default, LXD already uses ubuntu-cloudimg images.

The lesser know feature is using cloud-config with LXD. It turns out it is very easy to pass user-data to an LXD instance when you start it, just like you would on any cloud provider.

LXD even has the -e option to make your LXD instance ephemeral. It will be deleted automatically when you stop it.

Just like in that previous blog post, I create a file named one.yaml. The name can be anything. Then i start it:

lxc launch ubuntu:14.04 crisp-Hadley -c user.user-data="$(cat one.yaml)"

That is all there is to it.

Here is an example of config similar to what I used recently to QA a build configuration:

 all: "|tee -a /tmp/cloud.out"
#hostname: {{ hostname }}
 - rm -f /etc/dpkg/dpkg.cfg.d/multiarch
 - source: ppa:yellow/ppa
ssh_import_id: [evarlast] # use -S option
 - make
final_message: "The system is finally up, after $UPTIME seconds"
 - cd /home/ubuntu
 - git clone
 - cd myproject
 - make deps run

Converting eth0 to br0 and getting all your LXC or LXD onto your LAN

Wayne has a great post on the new juju lxd work. I’ve been using it a bit and it is awesome. It is super fast and I can create and destroy environments faster than creating and destroying with juju-local.

One thing which I’ve done which has made all LXC and LXD instances more valuable to me, in my home development environment, is to use a bridge to put them directly on my home LAN.

Normally, LXC creates its own device, lxc-br0, which is managed by the lxc-net service. The service creates the device, brings it up, manages the dnsmasq tied to it (which provides DHCP for the range).

Bridge your interface

Instead of using lxc-br0, I create a br0. I add my eth0 (and in my case other devices) to that br0. Then I configure LXC and LXD to use br0 instead of lxc-br0. I go as far as stopping the lxc-net service, since I’m not using it.

There is one trick if you are going to do that on a remote home system, e.g. I have an old laptop I leave in the basement and I’m really lazy and I don’t want to walk down there and use its console when I screw up its networking. The trick is to make sure eth0 comes up on br0 when its added there.

Before you do anything, make sure bridge-utils is installed. It probably is if you are already using lxc, but if this is a fresh install, you’ll want to apt-get install bridge-utils

Edit your /etc/network/interfaces and disable eth0 by setting it to manual. Add it to br0 by adding a new br0 section and listing eth0 in bridge-ifaces and bridge-ports.

auto br0
iface br0 inet dhcp
    bridge-ifaces eth0
    bridge-ports eth0
    up ifconfig eth0 up

iface eth0 inet manual

Now run sudo ifup br0. At this point something magical happens, the DHCP lease is renewed but this time the IP address is bound to br0. The magical part is that br0 used the eth0 MAC to make the DHCP request and so you get the same IP address in response and even your SSH session stays open. YAY!

wikipedia network bridge

LXC can use any bridge

Now configure LXC to use this bridge.

apt-get install lxc
sed -i 's/ = lxcbr0/ = br0/' /etc/lxc/default.conf

TADA, now any LXC containers you start with lxc-start will use br0 and get an address from your household DHCP server. They will be accessible from any host in your home.

Now what about LXD?

LXD can use any bridge

It turns out, while LXD is a layer on top of LXC, it doesn’t use /etc/lxc/default.conf for its default config, but instead uses its own settings. These are editable with lxc profile edit default. Change the lxcbr0 in your editor and save and exit. You can check that it is correct by using lxc profile show default.

There you have it. LXD instances starting on your local LAN.

Now go read Wayne’s post again and use the Juju LXD provider.

Ubuntu Cloud Image Based Containers with LXC

At a previous employer, we standardized on Ubuntu cloud images on AWS EC2 and in our OpenStack. You can find the images at If you are using Ubuntu on EC2 or another Certified Public Cloud, then its most likely one of these cloud images.

We leveraged cloud-init and extended an already existing simple management system to allow passing user-data to EC2 instances and OpenStack Nova instances. The use of ephemeral instances proved very powerful and influenced our thinking greatly. We came up with great solutions using these very simple techniques.

Even before I left that job, I longed for an easy way to do the same thing for myself. I played a bit with the AWS CLI tool (the newer python boto based tool) and yes, aws ec2 run-instances –user-data works. I always longed to get the same thing on my home server and on my laptop.

Finally, I figured out how to do this with LXC. Its simple yes, but I finally learned how to do what I want.

tl;dr example:

lxc-create -n crisp-Hadley -t ubuntu-cloud -- -r trusty -S ~/.ssh/ -u one.yaml

cat > one.yaml
 all: "|tee -a /tmp/cloud.out"

 - rm -f /etc/dpkg/dpkg.cfg.d/multiarch
 - for i in 1 2 3 4 5 ; do curl -s | apt-key add - && break ; sleep 2 ; done
 - source: deb stable main
 - source: ppa:evarlast/experimental
 - mongodb
final_message: "The system is finally up, after $UPTIME seconds"
 - service myapp start


Most LXC tutorials that I’ve seen walk the user through using the download template. The download template is not bad for new users, but I want something more powerful. It turns out there are a number of templates available by default in /usr/share/lxc/templates and you can even create your own.

The template I am interested in is the ubuntu-cloud template. These lxc templates are not so much templates at all as they are scripts. Some of them use other scripts called hooks defined in /usr/share/lxc/hooks. The ubuntu-cloud template, defined in /usr/share/lxc/templates/lxc-ubuntu-cloud.


The help for templates is a little hidden and lxc is a little stupid at letting you view the help. You COULD run lxc-create, use the — to pass options to the template and use -h. That has the unfortunate side effect of creating the container anyway. You’d have to lxc-destroy it even though you only used -h. Instead, it is easier to invoke the template directly and get help.

$ /usr/share/lxc/templates/lxc-ubuntu-cloud -h
LXC Container configuration for Ubuntu Cloud images.

Generic Options
[ -r | --release <release> ]: Release name of container, defaults to host
[ --rootfs <path> ]: Path in which rootfs will be placed
[ -a | --arch ]: Architecture of container, defaults to host architecture
[ -T | --tarball ]: Location of tarball
[ -d | --debug ]: Run with 'set -x' to debug errors
[ -s | --stream]: Use specified stream rather than 'tryreleased'

Additionally, clone hooks can be passed through (ie, --userdata). For those,
 /usr/share/lxc/hooks/ubuntu-cloud-prep --help

Here, we see that if we don’t specify the -r option, it defaults to match the host. I’m running vivid on my host, but I’d really like to stick with trusty inside of containers. The -a is interesting, and I can only guess that it only works where compatible. -a i386 would let me use the i386 cloud image on an amd64 host. I can’t think of any other use where a mixing architecture would work in a container.

But there is nothing here about cloud-init

cloud-init via cloud-prep

The last line of help says clone hooks can be passed through. This is useful and IMO the most important item. Run the help for ubuntu-cloud-prep exactly as suggested.

$ /usr/share/lxc/hooks/ubuntu-cloud-prep --help
Usage: ubuntu-cloud-prep [options] root-dir

  root-dir is the root directory to operate on

  [ -C | --cloud  ]:       do not configure a datasource.  incompatible with
                           options marked '[ds]'
  [ -i | --instance-id]:   instance-id for cloud-init, defaults to random [ds]
  [ -L | --nolocales ]:    Do not copy host's locales into container
  [ -S | --auth-key ]:     ssh public key file for datasource [ds]
  [ -u | --userdata ]:     user-data file for cloud-init [ds]

Options for –userdata and –auth-key. Are those what I think they are? It turns out, yes, they work exactly like choosing a public key and user-data when starting an EC2 or Nova instance.

Putting all this together you can create cloud-config yaml files and specify and ssh key, and starting an LXC is just like starting a public cloud instance.

For example, want a postgresql server running?

$ cat > psql.yaml
 all: "|tee -a /tmp/cloud.out"
 - postgresql
 - echo "listen_addresses = '*'" >>/etc/postgresql/9.3/main/postgresql.conf
 - sudo -u postgres createuser -D -R -S myuser
 - sudo -u postgres createdb -E utf8 -O myuser mydb
 - echo host mydb myuser trust >> /etc/postgresql/9.3/main/pg_hba.conf
 - service postgresql restart 
$ lxc-create -n mypostgresql -t ubuntu-cloud -- -r trusty -S ~/.ssh/ -u psql.yaml
$ lxc-start -n mypostgresql
$ lxc-info -n mypostgresql
Name:           mypostgresql
State:          RUNNING
PID:            6899
CPU use:        3.67 seconds
BlkIO use:      168.00 KiB
Memory use:     23.21 MiB
KMem use:       0 bytes
Link:           veth452FOE
 TX bytes:      3.84 KiB
 RX bytes:      20.36 KiB
 Total bytes:   24.20 KiB
$ psql -h -U myuser -d mydb
psql (9.4.2, server 9.3.7)
SSL connection (protocol: TLSv1.2, cipher: DHE-RSA-AES256-GCM-SHA384, bits: 256, compression: off)
Type "help" for help.

mydb=> \q

One of my favorite things about using the cloud-image like this is that unlike the download images, openssh server is running and listening by default. The user ubuntu has the public key which you provided in its authorized_keys file. Everything is ready to go.

11 lines of config, 373 bytes is not much at all for a running postgresql server.

When I don’t want to use juju, this is my go to option.