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118 lines
4.2 KiB
Markdown
118 lines
4.2 KiB
Markdown
# Metadata and Userdata handling
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Most providers offer two general mechanisms to provide compute instances
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with information about the instance that cannot be discovered by any other
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means. There are usually two types of information, namely _metadata_ and
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_user-data_. Metadata is usually set by the provider (e.g. geographical
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region of the datacentre, name given to the instance, external IP address,
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tags and other similar information), while userdata is fully custom,
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hence the name, and it is the information that user may supply to their
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instances before launch (it is immutable in most providers).
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The [metadata package](../pkg/metadata/) handles both metadata and
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userdata for a number of providers (see below). It abstracts over
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the provider differences by exposing both metadata and userdata in
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a directory hierarchy under `/run/config`. For example, sshd config
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files from the metadata are placed under `/run/config/ssh`.
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Userdata is assumed to be a single string and the contents will be
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stored under `/run/config/userdata`. If userdata is a JSON file, the
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contents will be further processed, where different keys cause
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directories to be created and the directories are populated with files.
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For example, the following userdata file:
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```JSON
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{
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"ssh": {
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"entries": {
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"sshd_config": {
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"perm": "0600",
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"content": "PermitRootLogin yes\nPasswordAuthentication no"
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}
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}
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},
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"foo": {
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"entries": {
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"bar": {
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"content": "foobar"
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},
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"baz": {
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"perm": "0600",
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"content": "bar"
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}
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}
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}
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}
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```
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will generate the following files:
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```
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/run/config/ssh/sshd_config
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/run/config/foo/bar
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/run/config/foo/baz
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```
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The JSON file consists of a map from `name` to an entry object. Each entry object has the following fields:
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- `content`: if present then the entry is a file. The value is a string containing the desired contents of the file.
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- `entries`: if present then the entry is a directory. The value is a map from `name` to entry objects.
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- `perm`: the permissions to create the file with.
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The `content` and `entries` fields are mutually exclusive, it is an error to include both,
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one or the other _must_ be present.
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The file or directory's name in each case is the same as the key which referred to that entry.
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This hierarchy can then be used by individual containers, who can bind
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mount the config sub-directory into their namespace where it is
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needed.
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# Metadata image creation
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`linuxkit run` backends accept two options to pass metadata to the VM in a platform specific
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manner to be picked up by the `pkg/metadata` component:
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* `-data=STRING` will cause the given `STRING` to be passed to the VM
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* `-data-file=PATH` will cause the contents of the file at `PATH` to be passed to the VM
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Alternatively `linuxkit metadata create meta.iso STRING` will produce
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a correctly formatted ISO image which can be passed to a VM as a CDROM
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device for consumption by the `pkg/metadata` component.
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# Providers
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Below is a list of supported providers and notes on what is supported. We will add more over time.
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## GCP
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GCP metadata is reached via a well known URL
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(`http://metadata.google.internal/`) and currently
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we extract the hostname and populate the
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`/run/config/ssh/authorized_keys` from metadata. In the future we'll
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add more complete SSH support.
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GCP userdata is extracted from `/computeMetadata/v1/instance/attributes/userdata`
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and made available in `/run/config/userdata`.
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## AWS
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AWS metadata is reached via the following URL
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(`http://169.254.169.254/latest/meta-data/`) and currently we extract the
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hostname and populate the `/run/config/ssh/authorized_keys` from metadata.
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AWS userdata is extracted from `http://169.254.169.254/latest/user-data` and
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and made available in `/run/config/userdata`.
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## Hetzner
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Hetzner metadata is reached via the following URL
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(`http://169.254.169.254/latest/meta-data/`) and currently we extract the
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hostname and populate the `/run/config/ssh/authorized_keys` from metadata.
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Hetzner userdata is extracted from `http://169.254.169.254/latest/user-data` and
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and made available in `/run/config/userdata`.
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## HyperKit
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HyperKit does not distinguish metadata and userdata, it's simply
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refered to as data, which is passed to the VM as a disk image
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in ISO9660 format.
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