Overlays

Warewulf supplements provisioned node images with an “overlay” system. Overlays are collections of files and Templates that are rendered and built per-node and then applied over the image during the provisioning process.

Overlays are the primary mechanism for adding functionality Warewulf. Much of even core functionality in Warewulf is implemented as distribution overlays, and this flexibility is also available for local, custom overlays. By combining templates with tags, network tags, and resources, the node registry (nodes.conf) can become an expressive metadata store for arbitrary cluster node configuration.

You can list the available overlays with wwctl overlay list, and the files within the overlays with wwctl overlay list --all.

# wwctl overlay list --all fstab
OVERLAY NAME  FILES/DIRS    SITE
------------  ----------    ----
fstab         etc/          false
fstab         etc/fstab.ww  false

Structure

An overlay is a directory that is applied to the root of a cluster node’s runtime file system. The overlay source directory should contain a single rootfs directory which represents the actual root directory for the overlay.

/usr/share/warewulf/overlays/issue
└── rootfs
    └── etc
        └── issue.ww

Adding Overlays to Nodes

A node or profile can configure an overlay in two different ways:

An overlay can be configured to apply only during boot, along with the node image. These overlays are called system overlays.
An overlay can be configured to also apply periodically while the system is running. These overlays are called runtime overlays.

wwctl profile set default \
  --system-overlays="wwinit,,wwclient,fstab,hostname,ssh.host_keys,systemd.netname,NetworkManager" \
  --runime-overlays="hosts,ssh.authorized_keys"

Multiple overlays can be applied to a single node, and overlays from multiple profiles are appended together when applied to a single node.

Building Overlays

Overlays are built (e.g., with wwctl overly build) into compressed overlay images for distribution to cluster nodes. These images typically match these two use cases: system and runtime. As such, each cluster node typically has two overlay images.

# wwctl overlay build
Building system overlay image for n1
Created image for n1 system overlay: /var/lib/warewulf/provision/overlays/n1/__SYSTEM__.img
Compressed image for n1 system overlay: /var/lib/warewulf/provision/overlays/n1/__SYSTEM__.img.gz
Building runtime overlay image for n1
Created image for n1 runtime overlay: /var/lib/warewulf/provision/overlays/n1/__RUNTIME__.img
Compressed image for n1 runtime overlay: /var/lib/warewulf/provision/overlays/n1/__RUNTIME__.img.gz

Overlay images for multiple node are built in parallel. By default, each CPU in the Warewulf server will build overlays independently. The number of workers can be specified with the --workers option.

Warewulf will attempt to build/update overlays as needed (configurable in the warewulf.conf); but not all cases are detected, and manual overlay builds are often necessary.

Creating and Modifying Overlays

You can add a new overlay to Warewulf with wwctl overlay create.

wwctl overlay create issue

A new overlay is just an empty directory. For it to be useful it needs to contain some files.

For example, wwctl overlay import imports files from the Warewulf server into the overlay.

wwctl overlay import --parents issue /etc/issue

This imports /etc/issue from the Warewulf server into the new issue overlay.

Note

The issue overlay already existed as a distribution overlay. Creating one shadows the distribution overlay with a new site overlay, allowing for local modification.

Any modification to a distribution overlay first transparently creates a new site overlay and applies any changes there: distribution overlays should always remain unmodified.

You can also edit a new or existing overlay file in an interactive editor.

wwctl overlay edit issue /etc/issue

Use wwctl overlay show to inspect the content of an overlay file.

wwctl overlay show issue /etc/issue

Overlay files that end with .ww are templates. You can use wwctl overlay show --render=<node> to show how a given template file would be rendered for distribution to a given cluster node.

wwctl overlay delete issue /etc/issue
wwctl overlay import issue /etc/issue /etc/issue.ww
wwctl overlay show issue /etc/issue.ww --render=n1

More information about templates is available in its own section.

The content of the file for the given overlay is displayed with this command. With the --render option a template is rendered as it will be rendered for the given node. The node name is a mandatory argument to the --render flag. Additional information for the file can be suppressed with the --quiet option.

Note

It is not possible to delete files with an overlay.

Permissions

Overlay files are distributed to cluster nodes with the same user, group, and mode that they have on the Warewulf server. Use wwctl overlay chown and wwctl overlay chmod to adjust them as necessary.

wwctl overlay chown issue /etc/issue.ww root root
wwctl overlay chmod issue /etc/issue.ww 0644

Distribution Overlays

Warewulf distinguishes between distribution overlays, which are included with Warewulf, and site overlays, which are created or added locally. A site overlay always takes precedence over a distribution overlay with the same name. Any modification of a distribution overlay with wwctl actually makes changes to an automatically-generated site overlay cloned from the distribution overlay.

Site overlays are often stored at /var/lib/warewulf/overlays/. Distribution overlays are often stored at /usr/share/warewulf/overlays/. But these paths are dependent on compilation, distribution, packaging, and configuration settings.

wwinit

The wwinit overlay performs initial configuration of the Warewulf node. Its wwinit script runs before systemd or other init is called and contains all configurations which are needed to boot.

In particular:

Configure the loopback interface
Configure the BMC based on the node’s configuration
Update PAM configuration to allow missing shadow entries
Relabel the file system for SELinux

Other overlays may place additional scripts in /warewulf/init.d/ to affect node configuration in this pre-boot environment.

wwclient

All configured overlays are provisioned initially along with the node image itself; but wwclient periodically fetches and applies the runtime overlay to allow configuration of some settings without a reboot.

Network interfaces

Warewulf ships with support for many different network interface configuration systems. All of these are applied by default; but the list may be trimmed to the desired system.

ifcfg
NetworkManager
debian.interfaces
wicked

Warewulf also configures both systemd and udev with the intended names of configured network interfaces, typically based on a known MAC address.

systemd.netname
udev.netname

Several of the network configuration overlays support netdev tags to further customize the interface:

``DNS[0-9]*``: one or more DNS servers
``DNSSEARCH``: domain search path
``MASTER``: the master for a bond interface

NetworkManager

``parent_device``: the parent device of a vlan interface
``vlan_id``: the vlan id for a vlan interface
``downdelay``, ``updelay``, ``miimon``, ``mode``, ``xmit_hash_policy``: bond device settings

Basics

The hostname overlay sets the hostname based on the configured Warewulf node name.

The hosts overlay configures /etc/hosts to include all Warewulf nodes.

The issue overlay configures a standard Warewulf status message for display during login.

The resolv overlay configures /etc/resolv.conf based on the value of “DNS” nettags. (In most situations this should be unnecessary, as the network interface configuration should handle this dynamically.)

fstab

The fstab overlay configures /etc/fstab based on the data provided in the “fstab” resource. It also creates entries for file systems defined by Ignition.

nodeprofiles:
  default:
    resources:
      fstab:
        - spec: warewulf:/home
          file: /home
          vfstype: nfs
        - spec: warewulf:/opt
          file: /opt
          vfstype: nfs

ssh

Two SSH overlays configure host keys (one set for all node in the cluster) and authorized_keys for the root account.

ssh.authorized_keys
ssh.host_keys

syncuser

The syncuser overlay updates /etc/passwd and /etc/group to include all users on both the Warewulf server and from the image.

To function properly, wwctl image syncuser (or the --syncuser option during import, exec, shell, or build) must have also been run on the image to synchronize its user and group IDs with those of the server.

If a PasswordlessRoot tag is set to “true”, the overlay will also insert a “passwordless” root entry. This can be particularly useful for accessing a cluster node when its network interface is not properly configured.

ignition

The ignition overlay defines partitions and file systems on local disks.

debug

The debug overlay is not intended to be used in configuration, but is provided as an example. In particular, the provided tstruct.md.ww demonstrates the use of most available template metadata.

wwctl overlay show --render=<nodename> debug tstruct.md.ww

localtime

The localtime overlay configures the timezone of a cluster node to match that of the Warewulf server; alternatively, a different timezone may be specified with a localtime tag.

wwctl profile set default --tagadd="localtime=UTC"

host

Configuration files used for the configuration of the Warewulf host / server are stored in the host overlay. Unlike other overlays, it must have the name host. Existing files on the host are copied to backup files with a wwbackup suffix at the first run. (Subsequent use of the host overlay won’t overwrite existing wwbackup files.)

The following services get configuration files via the host overlay:

ssh keys are created with the scrips ssh_setup.sh and ssh_setup.csh
hosts entries are created by manipulating /etc/hosts with the template hosts.ww
nfs kernel server receives its exports from the template exports.ww
the dhcpd service is configured with dhcpd.conf.ww