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Operator Notes

  1. HTTP Backoff and Retry
  2. AWS and IAM roles
  3. HTTP headers
  4. Filesystem-Reuse Semantics
  5. Path Traversal and Following Symlinks
  6. SELinux
  7. Partition Reuse Semantics
    1. Partition Matching
    2. Partition number 0
    3. Partition start 0
    4. Unspecified partition start
    5. Partition size 0
    6. Unspecified partition size
  8. Config Merging
    1. Child configs take precedence when specified
    2. Most lists are deduplicated
    3. Files, Directories, and Links are deduplicated across each other
    4. Configs are merged in a depth first traversal
    5. HTTP headers merging
  9. LUKS
    1. Clevis Based Devices

HTTP Backoff and Retry

When Ignition is fetching a resource over http(s), if the resource is unavailable Ignition will continually retry to fetch the resource with an exponential backoff between requests.

For a given retry attempt, Ignition will wait 10 seconds for the server to send the response headers for the request. If response headers are not received in this time, or an HTTP 5XX error code is received, the request is cancelled, Ignition waits for the backoff, and a new request is made.

Any HTTP response code less than 500 results in the request being completed, and either the resource will be fetched or Ignition will fail.

Ignition will initially wait 100 milliseconds between failed attempts, and the amount of time to wait doubles for each failed attempt until it reaches 5 seconds.

AWS and IAM roles

Ignition has support for fetching files over the S3 protocol. When Ignition is running in Amazon EC2, it supports using the IAM role given to the EC2 instance to fetch protected assets from S3. If IAM credentials are not successfully fetched, Ignition will attempt to fetch the file with no credentials.

HTTP headers

When fetching data from an HTTP URL for config references, CA references and file contents, additional headers can be attached to the request using the httpHeaders attribute. This allows downloading data from servers that require authentication or some additional parameters from your request.

Headers can be attached only when source has http or https scheme.

If multiple values are to be set for the same header, they must be separated by a comma. Example: {"name": "Accept", "value": "text/html, application/json"}.

If the remote HTTP server returns a redirect status code (3xx), then additional headers are not included in the redirected request.

If a specified header is one that Ignition sets by default, such as Accept or User-Agent, the specified value overrides Ignition’s default.

Filesystem-Reuse Semantics

When a machine first boots, it’s possible that an earlier installation or other process has already provisioned the disks. The Ignition config can specify the intended filesystem for a given device, and there are three possibilities when Ignition runs:

  • There is no preexisting filesystem.
  • There is a preexisting filesystem of the correct type, label, or UUID (e.g. the Ignition config says /dev/sda should be ext4, and it is ext4).
  • There is a preexisting filesystem of an incorrect type, label, or UUID (e.g. the Ignition config says /dev/sda should be ext4, and it is btrfs).

In the first case, when there is no preexisting filesystem, Ignition will always create the desired filesystem.

In the second two cases, where there is a preexisting filesystem, Ignition’s behavior is controlled by the wipeFilesystem flag in the filesystem section.

If wipeFilesystem is set to true, Ignition will always wipe any preexisting filesystem and create the desired filesystem (or skip creation if the format is set to none). Note this will result in any data on the old filesystem being lost.

If wipeFilesystem is set to false, Ignition will then attempt to reuse the existing filesystem. If the filesystem is of the correct type, has a matching label, and has a matching UUID, then Ignition will reuse the filesystem. If the label or UUID is not set in the Ignition config, they don’t need to match for Ignition to reuse the filesystem. Any preexisting data will be left on the device and will be available to the installation. If the preexisting filesystem is not of the correct type, then Ignition will fail, and the machine will fail to boot. Similarly, if the format is set to none, then any preexisting filesystem will cause Ignition to fail.

When resolving paths, Ignition follows symlinks on all but the last element of a path. This ensures existing symlinks on a filesystem can be overwritten while still following symlinks as expected. When writing files, links, or directories, Ignition does not allow following symlinks outside the specified filesystem. When writing files, links, or directories on the root filesystem, Ignition follows symlinks as if it were executing in that root; a symlink to /etc is followed to /etc on the root filesystem. When writing files, links, or directories to any other filesystem, Ignition fails if it tries to follow a symlink outside that filesystem.

SELinux

Ignition fully supports distributions which have SELinux enabled. It requires that the distribution ships the setfiles utility. The kernel must be at least v5.5 or alternatively have this patch backported.

Partition Reuse Semantics

The wipePartitionEntry and shouldExist flags control what Ignition will do when it encounters an existing partition. wipePartitionEntry specifies whether Ignition is permitted to delete partition entries in the partition table. shouldExist specifies whether a partition with that number should exist or not (it is invalid to specify a partition should not exist and specify its attributes, such as size or label).

The following table shows the possible combinations of whether or not a partition with the specified number is present, shouldExist, and wipePartitionEntry, and the action Ignition will take:

Partition presentshouldExistwipePartitionEntryAction Ignition takes
falsefalsefalseDo nothing
falsefalsetrueDo nothing
falsetruefalseCreate specified partition
falsetruetrueCreate specified partition
truefalsefalseFail
truefalsetrueDelete existing partition
truetruefalseVerify existing partition matches the specified one, otherwise resize it if resize field is true and partition matches in all respects except size, otherwise fail
truetruetrueCheck if existing partition matches the specified one, delete existing partition and create specified partition if it does not match

Partition Matching

A partition matches if all of the specified attributes (label, start, size, uuid, and typeGuid) are the same. Specifying uuid or typeGuid as an empty string is the same as not specifying them. When 0 is specified for start or size, Ignition checks if the existing partition’s start / size match what they would be if all of the partitions specified were to be deleted (if allowed by wipePartitionEntry), then recreated if shouldExist is true.

Partition number 0

Specifying number as 0 will use the next available partition number. Partition number 0 is disallowed on disks with partitions that specify shouldExist as false. If number is not specified it will be treated as 0.

Partition start 0

Specifying start as 0 will use the starting sector of the largest available block. This is not necessarily the first available block large enough.

Unspecified partition start

If start is not specified and a partition with the same number exists, Ignition will use the start of the existing partition, unless wipePartitionEntry is set. If start is not specified and there is no existing partition, or wipePartitionEntry is set, Ignition will use the starting sector of the largest block, as if start were set to 0.

Partition size 0

Specifying size as 0 means the partition should span to the end of the largest available block. If the starting sector is not within the largest available block, Ignition will fail.

Unspecified partition size

If size is not specified and a partition with the same number exists, it will use the value of the existing partition, unless wipePartitionEntry is set. If size is not specified and there is no existing partition, or wipePartitionEntry is set, size act as if it were set to 0 and use the size of the largest block.

Config Merging

Ignition supports fetching and merging multiple configs. This replaces the append functionality of the Ignition 2.x.0 specification. There are several rules that determine how configs get merged. When a child config is merged with a parent, generally the child config’s values override the parent config’s values.

Child configs take precedence when specified

If a parent and child object are being merged, the fields in the child object take precedence over the fields in the parent config. If a field in the child object is not specified, the field from the parent is used instead.

Most lists are deduplicated

All lists of objects have a field that uniquely identifies that object. If a child config contains an entry that matches an entry already specified in the parent config, those entries are merged. A few sections of the config are exempt from this behavior. See the configuration specification for a complete listing. Generally the only lists that are simply appended are those that specify arguments to commands like mkfs or mdadm.

Since files, directories, and links all describe filesystem entries can conflict, these lists are deduplicated across each other. This means a file in a child config can replace a link in the parent, or a directory in a child config can replace a file in the parent.

Configs are merged in a depth first traversal

A child config can specify children of its own. Those children are merged into their parent config before that config is merged into its own parent. If a config specifies multiple children, those children are merged in the order they appear.

HTTP headers merging

If names of the parent and child headers match, the result will be to replace the value of the parent header with that of the child.

If a child header has no value, the parent header with the same name will be removed.

LUKS

Ignition has support for creating both purely key-file based LUKS2 devices as well as Tang/TPM2 backed (via clevis) devices.

If a key-file is not specified one will be generated for the device. Key-files will be stored at /etc/luks/<deviceName> (this path can be overriden via build flags).

Ignition generates entries in /etc/crypttab for each device and expects that the operating system has hooks to be able to unlock the device (e.x.: systemd-cryptsetup-generator).

Clevis Based Devices

When creating clevis based devices to utilize Tang or TPM2 Ignition will use an SSS Pin and will create the relevant configuration JSON from the provided attributes.