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92 lines
3.2 KiB
Markdown
92 lines
3.2 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 `/var/config`. For example, sshd config
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files from the metadata are placed under `/var/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 `/var/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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"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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"foo" : {
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"bar" : "foobar",
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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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will generate the following files:
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```
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/var/config/ssh/sshd_config
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/var/config/foo/bar
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/var/config/foo/baz
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```
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Each file can either be:
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- a simple string (as for `foo/bar` above) in which case the file will
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be created with the given contents and read/write (but not execute)
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permissions for user and read permissions for group and everyone else (in octal format `0644`).
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- a map (as for `ssh/sshd_config` and `foo/baz` above) with the
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following mandatory keys:
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- `content`: the contents of the file.
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- `perm`: the permissions to create the file with.
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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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Run `linuxkit run` backends accept a `--data=STRING` option which will
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cause the given string to be passed 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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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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`/var/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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## HyperKit
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HyperKit does not distiguish 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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