This is a little trick that allows you to override the default base OS in the CoreOS “toolbox“. The toolbox is a neat trick to allow you to debug and troubleshoot issues inside containers on CoreOS without having to do any outside work of setting up a container.
The default toolbox OS defaults to Fedora, which we’re going to change to Ubuntu. There is a custom configuration file that will get read in via the .toolboxrc file, located at /home/core/.toolboxrc by default. To keep things simple we will only be changing the few pieces of the config to get the toolbox to behave how we want. More can be changed but we don’t really need to override anything else.
That’s pretty cool, but what if we want to have this config file be in place for all servers? We don’t want to have to manually write this config file for every server we log in to.
To fix this issue we will add a simple configuration in to the user-data file that gets fed in to the CoreOS cloud-config when the server is created. You can find more information about the CoreOS cloud-configs here.
The bit in the cloud config that needs to change is the following.
-write_files: - path: /home/core/.toolboxrc owner: core content: | TOOLBOX_DOCKER_IMAGE=ubuntu TOOLBOX_DOCKER_TAG=14.04
If you are already using cloud-config then this change should be easy, just add the bit starting with -path to your existing -write_files section. New servers using this config will have the desired toolbox defaults.
This approach gives us an automated, reproducible way to clone our custom toolbox config to every server that uses cloud-config to bootstrap itself. Once the config is in place simply run the “toolbox” command and it should use the custom values to pull the desired Ubuntu image.
Then you can run your Ubuntu commands and debugging tools from within the toolbox. Everything else will be the same, we just use Ubuntu now as our default toolbox OS. Here is the post that gave me the idea to do this originally.