linuxkit/docs/yaml.md

Yaml configuration

The yaml configuration specifies the components and the build time artifacts. All components are downloaded at build time to create an image. The image is self-contained and immutable, so it can be tested reliably for continuous delivery.

The configuration file is processed in the order kernel, init, onboot, services, files. Each section adds file to the root file system

kernel

This section defines the kernel configuration. The image field specifies the Docker image, which should contain a bzImage (for amd64 architecture, others may vary) and a file called kernel.tar which is a tarball that is unpacked into the root, which should usually contain a kernel modules directory. See kernel/ for source code. cmdline specifies the kernel command line options if required.

init

This section currently just lists images that is used for the init system and are unpacked directly into the root filesystem. This should bring up containerd, start the system and daemon containers, and set up basic filesystem mounts. See pkg/init/ for source code. For ease of modification runc and containerd images, which just contain these programs are added here rather than bundled into the init container.

onboot

These containers are run to completion sequentially, using runc before anything else is started. They can be used to configure one shot settings. For details of the config for each container, see below.

services

These containers are started with containerd and are expected to remain running. Startup order is not guaranteed, so containers should wait on any resources, such as networking, that they need. For details of the config for each container, see below.

trust

This section specifies which build components are to be cryptographically verified with Docker Content Trust prior to pulling. Trust is a central concern in any build system, and LinuxKit's is no exception: Docker Content Trust provides authenticity, integrity, and freshness guarantees for the components it verifies. The LinuxKit maintainers are responsible for signing mobylinux components, though collaborators can sign their own images with Docker Content Trust or Notary.

• image lists which individual images to enforce pulling with Docker Content Trust. The image name may include tag or digest, but the matching also succeeds if the base image name is the same.
• org lists which organizations for which Docker Content Trust is to be enforced across all images (ex: mobylinux is the org for mobylinux/kernel)

Image specification

For each image in the system and daemon sections you can specify the OCI options that are passed to runc, so you can specify what capabilities are needed and so on. Most LinuxKit packages now define defaults in the org.mobyproject.config image label. For more details see the OCI specification.

• name a unique name for the program being executed, used as the containerd id.
• image the Docker image to use for the root filesystem. The default command, path and environment are extracted from this so they need not be filled in.
• capabilities the Linux capabilities required, for example CAP_SYS_ADMIN. If there is a single capability all then all capabilities are added.
• mounts is the full form for specifying a mount, which requires type, source, destination and a list of options. If any fields are omitted, sensible defaults are used if possible, for example if the type is dev it is assumed you want to mount at /dev. The default mounts and their options can be replaced by specifying a mount with new options here at the same mount point.
• binds is a simpler interface to specify bind mounts, accepting a string like /src:/dest:opt1,opt2 similar to the -v option for bind mounts in Docker.
• tmpfs is a simpler interface to mount a tmpfs, like --tmpfs in Docker, taking /dest:opt1,opt2.
• command will override the command and entrypoint in the image with a new list of commands.
• env will override the environment in the image with a new environment list
• cwd will set the working directory, defaults to /.
• net sets the network namespace, either to a path, or if host is specified it will use the host namespace.
• pid sets the pid namespace, either to a path, or if host is specified it will use the host namespace.
• ipc sets the ipc namespace, either to a path, or if host is specified it will use the host namespace.
• uts sets the uts namespace, either to a path, or if host is specified it will use the host namespace.
• readonly sets the root filesystem to read only, and changes the other default filesystems to read only.
• maskedPaths sets paths which should be hidden.
• readonlyPaths sets paths to read only.
• uid sets the user id of the process. Only numbers are accepted.
• gid sets the group id of the process. Only numbers are accepted.
• additionalGids sets additional groups for the process. A list of numbers is accepted.
• noNewPrivileges is true means no additional capabilities can be acquired and suid binaries do not work.
• hostname sets the hostname inside the image.
• oomScoreAdj changes the OOM score.
• disableOOMKiller disables the OOM killer for the service.
• rootfsPropagation sets the rootfs propagation, eg shared, slave or (default) private.
• cgroupsPath sets the path for cgroups.
• sysctl sets a list of sysctl key value pairs that are set inside the container namespace.

Further OCI values will be added, as the list is not yet complete.

Mount Options

When mounting filesystem paths into a container - whether as part of onboot or services - there are several options of which you need to be aware. Using them properly is necessary for your containers to function properly.

For most containers - e.g. nginx or even docker - these options are not needed. Simply doing the following will work fine:

binds:
 - /var:/some/var/path

However, in some circumstances you will need additional options. These options are used primarily if you intend to make changes to mount points from within your container that should be visible from outside the container, e.g., if you intend to mount an external disk from inside the container but have it be visible outside.

In order for new mounts from within a container to be propagated, you must set the following on the container:

1. rootfsPropagation: shared
2. The mount point into the container below which new mounts are to occur must be rshared,rbind. In practice, this is /var (or some subdir of /var), since that is the only true read-write area of the filesystem where you will mount things.

Thus, if you have a regular container that is only reading and writing, go ahead and do:

binds:
 - /var:/some/var/path

On the other hand, if you have a container that will make new mounts that you wish to be visible outside the container, do:

binds:
 - /var:/var:rshared,rbind
rootfsPropagation: shared