Thursday, December 11, 2014

Snappy Ubuntu Core and cloud-init

Snappy Ubuntu Core was announced this week.  In yesterday's blog post (Snappy Ubuntu Core and uvtool) I showed how you can use uvtool to create and manage snappy instances.

Now that we've got that covered, let’s look deeper into a very cool feature - the ability to customize the instance and automate its startup and configuration.  For example, at instance creation time you can specify a snappy application to be installed.  cloud-init is what allows you to do this, and it is installed inside the Snappy image. cloud-init receives this information from the user in the form of 'user-data'.

One of the formats that can be fed to cloud-init is called ‘cloud-config’.  cloud-config is yaml formatted data that is interpreted and acted on by cloud-init.  For Snappy, we’ve added a couple specific configuration values.  Those are included under the top level 'snappy'.
  • ssh_enabled: determines if 'ssh' service is started or not.  By default ssh is not enabled.
  • packages: A list of snappy packages to install on first boot.  Items in this list are snappy package names.
When running inside snappy, cloud-init still provides many of the features it provides on traditional instances.  Some useful configuration entries:

  • runcmd: A list of commands run after boot has been completed. Commands are run as root. Each entry in the list can be a string or a list.  If the entry is a string, it is interpreted by 'sh'.  If it is a list, it is executed as a command and arguments without shell interpretation.
  • ssh_authorized_keys: This is a list of strings.  Each key present will be put into the default user's ssh authorized keys file.  Note that ssh authorized keys are also accepted via the cloud’s metadata service.
  • write_files: this allows you to write content to the filesystem.  The module is still expected to work, but the user will have to be aware that much of the filesystem is read-only. Specifically, writing to file system locations that are not writable is expected to fail.
Some cloud-init config modules are simply not going to work.  For example, traditional packages will not be installed by 'apt' as the root filesystem is read-only.

Example Cloud Config

Its always easiest to start from a working example.  Below is one that demonstrates the usage of the config options listed above.  Please note that user data intended to be consumed as cloud-config must contain the first line '#cloud-config'.
      ssh_enabled: True
        - xkcd-webserver

     - content: |
        echo "==== Hello Snappy!  It is now $(date -R) ===="
       permissions: '0755'
       path: /writable/greet

     - /writable/greet | tee /run/hello.log

Launching with uvtool

Follow yesterday's blog post to get a functional tool.  Then, save the example config file above to a file, and launch you're instance with it.

$ uvt-kvm create --wait --add-user-data=my-config.yaml snappy1 release=devel

Our user-data instructed cloud-init to do a number of different things. First, it wrote a file via 'write_files' to a writable space on disk, and then executed that file with 'runcmd'. Lets verify that was done:

$ uvt-kvm ssh snappy1 cat /run/hello.log
==== Hello Snappy!  It is now Thu, 11 Dec 2014 18:16:34 +0000 ====

It also instructed cloud-init to install the Snappy 'xkcd-webserver' application.
$ uvt-kvm ssh snappy1 snappy versions
Part            Tag   Installed  Available  Fingerprint     Active 
ubuntu-core     edge  141        -          7f068cb4fa876c  *      
xkcd-webserver  edge  0.3.1      -          3a9152b8bff494  *

There we can see that xkcd-webserver was installed, lets check that it is running:

$ uvt-kvm ip snappy1
$ wget -O - --quiet | grep <title>
<title>XKCD rocks!</title>

Launching on Azure

The same user-data listed above also works on Microsoft Azure.   Follow the instructions for setting up the azure command line tools, and then launch the instance with and provide the '--custom-data' flag.  A full command line might look like:
$ imgid=b39f27a8b8c64d52b05eac6a62ebad85__Ubuntu-core-devel-amd64-20141209-90-en-us-30GB
$ azure vm create snappy-test $imgid ubuntu \
  --location "North Europe" --no-ssh-password \
  --ssh-cert ~/.ssh/azure_pub.pem --ssh \

  --custom-data my-config.yaml

Have fun playing with cloud-init!

Wednesday, December 10, 2014

Snappy Ubuntu Core and uvtool

Earlier this week, Ubuntu announced the Snappy Ubuntu Core . As part of the announcement, a set of qemu based instructions were included for checking out a snappy image on your local system.  In addition to that method, we’ve been working on updates to bring support for the transactional images to uvtool. Have you used uvtool before?  I like it, and tend to use for day to day kvm images as it’s pretty simple. So let’s get to it.

Setting up a local Snappy Ubuntu Core environment with uvtool

As I’ve already mentioned Ubuntu has a very simple set of tools for creating virtual machines using cloud images, called 'uvtool'.  Uvtool offers a easy way to bring up images on your system in a kvm environment. Before we use uvtool to get snappy on your local environment, you’ll need install the special version that has snappy supported added to it:

$ sudo apt-add-repository ppa:snappy-dev/tools
$ sudo apt-get update
$ sudo apt-get install uvtool
$ newgrp libvirtd

You only need to do 'newgrp libvirtd' during the initial setup, and only if you were not already in the libvirtd group which you can check by running the 'groups' command. A reboot or logout would have the same effect.

uvtool uses ssh key authorization so that you can connect to your instances without being prompted for a password. If you do not have a ssh key in '~/.ssh/', you can create one now with:

$ ssh-keygen

We’re ready to roll.  Let’s download the images:

$ uvt-simplestreams-libvirt sync --snappy flavor=core release=devel

This will download a pre-made cloud image of the latest Snappy Core image from It will download about 110M, so be prepared to wait a little bit.

Now let’s start up an instance called 'snappy-test':

$ uvt-kvm create --wait snappy-test flavor=core

This will do the magic of setting up a libvirt domain, starting it and waiting for it to boot (via the --wait flag).  Time to ssh into it:

$ uvt-kvm ssh snappy-test

You now have a Snappy image which you’re sshd into.

If you want to manually ssh, or test that your snappy install of xkcd-webserver worked, you can get the IP address of the system with:

$ uvt-kvm ip snappy-test

When you're done playing, just destroy the instance with:
$ uvt-kvm destroy snappy-test

Have fun!

Thursday, August 14, 2014

mount-image-callback: easily modify the content of a disk image file

Lots of times when dealing with virtualization or cloud you find yourself working with "images".  Operating on these images can be difficult.  As an example, often times you may have a disk image, and need to change a single file inside it.

There is a tool called 'mount-image-callback' in cloud-utils that takes care of mounting and unmounting a disk image.  It allows you to focus on exactly what you need to do. It supports mounting partitioned or unpartitioned images in any format that qemu can read (thanks to qemu-nbd).

Heres how you can use it interactively:

  $ mount-image-callback disk1.img -- chroot _MOUNTPOINT_
  % echo "I'm chrooted inside the image here"
  % echo "" >> /etc/hosts
  % exit 0

or non-interactively:

  mount-image-callback disk1.img -- \
   sh -c 'rm -Rf $MOUNTPOINT/var/cache/apt'

or one of my typical use cases, to add a package to an image.

  mount-image-callback --system-mounts --resolv-conf --
    chroot _MOUNTPOINT_ apt-get install --assume-yes pastebinit

Above, mount-image-callback handled setting up the loopback or qemu-nbd devices required to mount the image and then mounted it to a temporary directory.  It then runs the command you provide, unmounts the image, and exits with the return code of the provided command.

If the command you provide has the literal argument '_MOUNTPOINT_' then it will substitute the path to the mount.  It also makes that path available in the environment variable MOUNTPOINT.  Adding '--system-mounts' and '--resolv-conf' address the common need to mount proc, dev or sys, and to modify and replace /etc/resolv.conf in the filesystem so that networking will work in a chroot.

mount-image-callback supports mounting either an unpartitioned image (ie, dd if=/dev/sda1 of=my.img) or the first partition of a partitioned image (dd if=/dev/sda of=my.img).  Two improvements I'd like to make are to allow the user to tell it which partition to mount (rather than expecting the first) and also to do so automatically by finding an /etc/fstab and mounting other relevant mounts as well.

Why not libguestfs?

libguestfs is a great tool for doing this.  It operates essentially by launching a qemu (or kvm) guest, and attaching disk images to the guest and then letting the guest's linux kernel and qemu to do the heavy lifting.  Doing this provides security benefits, as mounting untrusted filesystems could cause kernel crash.  However, it also has performance costs and limitations, and also doesn't provide "direct" access as you'd get via just mounting a filesystem.

Much of my work is done inside a cloud instance, and done by automation.  As a result, the security benefits of using a layer of virtualization to access disk images are less important.  Also, I'm likely operating on an official Ubuntu cloud image or other vendor provided image where trust is assumed.

In short, mounting an image and changing files or chrooting is acceptable in many cases and offers more "direct" path to doing so.