Container Management

Since the inception of Warewulf over 20 years ago, Warewulf has used the model of the “Virtual Node File System” (VNFS) as a template image for the compute nodes. This is similar to a golden master image, except that the node file system exists within a directory on the Warewulf control node (e.g. a chroot()).

In hindsight, we’ve been using containers all along, but the buzzword just didn’t exist. Over the last 5-6 years, the enterprise has created a lot of tooling and standards around defining, building, distributing, securing, and managing containers, so Warewulf v4 now integrates directly within the container ecosystem to facilitate the process of VNFS image management.

If you are not currently leveraging the container ecosystem in any other way, you can still build your own chroot directories and use Warewulf as before.

It is important to understand that Warewulf is not running a container runtime on cluster nodes. While it is absolutely possible to run containers on cluster nodes, Warewulf is provisioning the container image to the bare metal and booting it. This container will be used as the base operating system and, by default, it will run entirely in memory. This means that when you reboot the node, the node retains no information about Warewulf or how it booted.

Container Tools

There are different container managment tools available. Docker is probably the most recognizable one in the enterprise. Podman is another one that is gaining traction on the RHEL platforms. In HPC, Apptainer is the most utilized container management tool. You can use any of these to create and manage the containers to be later imported into Warewulf.

Importing Containers

Warewulf supports importing an image from any OCI compliant registry. This means you can import from a public registry or from a private registry.

Here is an example of importing from Docker Hub.

# wwctl container import docker:// rocky-8
Getting image source signatures
Copying blob d7f16ed6f451 done
Copying config da2ca70704 done
Writing manifest to image destination
Storing signatures
[LOG]       info unpack layer: sha256:d7f16ed6f45129c7f4adb3773412def4ba2bf9902de42e86e77379a65d90a984
Updating the container's /etc/resolv.conf
Building container: rocky-8


Most containers in Docker Hub are not “bootable”, in that, they have a limited version of Systemd to make them lighter weight for container purposes. For this reason, don’t expect any base Docker container (e.g. docker://rockylinux or docker://debian) to boot properly. They will not, as they will get stuck into a single user mode. The containers in are not limited and thus they boot as you would expect.

Private Registry

It is possible to use a private registry that is password protected or does not have the requirement for TLS. In order to do so, you have two choices for handling the credentials.

  • Set environmental variables

  • Use docker login or podman login which will store the credentials locally

Please note, there is no requirement to install and use docker or podman on your control node just for importing images into Warewulf.

Here are the environmental variables that can be used.


Here is an example:

# export WAREWULF_OCI_USERNAME=privateuser
# export WAREWULF_OCI_PASSWORD=super-secret-password-or-token
# wwctl import docker://

The above is just an example. Consideration should be done before doing it this way if you are in a security sensitive environment or shared environments. You would not want these showing up in bash history or logs.

Local Files

It is also possible to import a container from a local file or directory. For example, Podman can save a .tar archive of an OCI image. This archive can be directly imported into Warewulf, no registry required.

# podman save alpine:latest >alpine.tar
# wwctl container import alpine.tar alpine

Chroot directories and Apptainer sandbox images can also be imported directly.

$ apptainer build --sandbox ./rockylinux-8/ docker://
$ sudo wwctl container import ./rockylinux-8/ rockylinux-8

HTTP proxies

You can set HTTP_PROXY, HTTPS_PROXY, and NO_PROXY (or their lower-case versions) to use a proxy during wwctl container import.

export HTTPS_PROXY=squid.localdomain
wwctl conatiner import docker://

See ProxyFromEnvironment For more information.


OCI and ORAS registries typically use HTTPS, so you probably need to set HTTPS_PROXY or https_proxy rather than the HTTP variants.


At import time Warewulf checks if the names of the users on the host match the users and UIDs/GIDs in the imported container. If there is mismatch, the import command will print out a warning. By setting the --syncuser flag you advise Warewulf to try to syncronize the users from the host to the container, which means that /etc/passwd and /etc/group of the imported container are updated and all the files belonging to these UIDs and GIDs will also be updated.

A check if the users of the host and container matches can be triggered with the syncuser command.

# wwctl container syncuser container-name

With the --write flag it will update the container to match the user database of the host as described above.

wwctl container syncuser --write container-name

Listing All Imported Containers

Once the container has been imported, you can list them all with the following command:

# wwctl container list
CONTAINER NAME                      BUILT  NODES
rocky-8                             true   0

Once a container has been imported and showing up in this list you can configure it to boot compute nodes.

Making Changes To Containers

Warewulf has a minimal container runtime built into it. This means you can run commands inside of any of the containers and make changes to them as follows:

# wwctl container exec rocky-8 /bin/sh
[rocky-8] Warewulf> cat /etc/rocky-release
Rocky Linux release 8.4 (Green Obsidian)
[rocky-8] Warewulf> exit
Rebuilding container...
[INFO]     Skipping (VNFS is current)

You can also --bind directories from your host into the container when using the exec command. This works as follows:

# wwctl container exec --bind /tmp:/mnt rocky-8 /bin/sh
[rocky-8] Warewulf>


As with any mount command, both the source and the target must exist. This is why the example uses the /mnt/ directory location, as it is almost always present and empty in every Linux distribution (as prescribed by the LSB file hierarchy standard).

When the command completes, if anything within the container changed, the container will be rebuilt into a bootable static object automatically.

If the files /etc/passwd or /etc/group were updated, there will be an additional check to confirm if the users are in sync as described in Syncuser section.

Excluding Files from a Container

Warewulf can exclude files from a source container to prevent them from being delivered to the compute node. This is typically used to reduce the size of the image when some files are unnecessary.

Patterns for excluded files are read from the file /etc/warewulf/excludes in the container image itself. For example, the default Rocky Linux images exclude these paths:


/etc/warewulf/excludes supports the patterns implemented by filepath.Match.

Preparing a container for build

Warewulf executes the script /etc/warewulf/ after a wwctl container shell or wwctl container exec and prior to (re)building the final node image for delivery. This is typically used to remove cache or log files that may have been generated by the executed command or interactive session.

For example, the default Rocky Linux images runs dnf clean all to remove any package repository caches that may have been generated.

Creating Containers From Scratch

You can also create containers from scratch and import those containers into Warewulf as previous versions of Warewulf did.

Building A Container From Your Host

RPM based distributions, as well as Debian variants can all bootstrap mini chroot() directories which can then be used to bootstrap your node’s container.

For example, on an RPM based Linux distribution with YUM or DNF, you can do something like the following:

# yum install --installroot /tmp/newroot basesystem bash \
    chkconfig coreutils e2fsprogs ethtool filesystem findutils \
    gawk grep initscripts iproute iputils net-tools nfs-utils pam \
    psmisc rsync sed setup shadow-utils rsyslog tzdata util-linux \
    words zlib tar less gzip which util-linux openssh-clients \
    openssh-server dhclient pciutils vim-minimal shadow-utils \
    strace cronie crontabs cpio wget rocky-release ipmitool yum \

You can do something similar with Debian-based distributions:

# apt-get install debootstrap
# debootstrap stable /tmp/newroot

Once you have created and modified your new chroot(), you can import it into Warewulf with the following command:

# wwctl container import /tmp/newroot containername

Building A Container Using Apptainer

Apptainer, a container platform for HPC and performance intensive applications, can also be used to create node containers for Warewulf. There are several Apptainer container recipes in the containers/Apptainer/ directory and can be found on GitHub at

You can use these as starting points and adding any additional steps you want in the %post section of the recipe file. Once you’ve done that, installing Apptainer, building a container sandbox and importing into Warewulf can be done with the following steps:

# yum install epel-release
# yum install Apptainer
# Apptainer build --sandbox /tmp/newroot /path/to/Apptainer/recipe.def
# wwctl container import /tmp/newroot containername

Building A Container Using Podman

You can also build a container using podman via a Dockerfile. For this step the container must be exported to a tar archive, which then can be imported to Warewulf. The following steps will create an openSUSE Leap container and import it to Warewulf:

# podman build -f containers/Docker/openSUSE/Containerfile --tag leap-ww
# podman save localhost/leap-ww:latest  -o ~/leap-ww.tar
# wwctl container import file://root/leap-ww.tar leap-ww

Container Size Considerations

Base compute node container images start quite small (a few hundred megabytes), but can grow quickly as packages and other files are added to them. Even these larger images are typically not an issue in modern environments; but some architectural limits exist that can impede the use of images larger than a few gigabytes. Workarounds exist for these issues in most circumstances:

  • Systems booting in legacy / BIOS mode, being a 32-bit environment, cannot boot an image that requires more than 4GB to decompress. This means that the compressed image and the decompressed image together must be < 4GB. This is typically reported by the system as “No space left on device (”

    The best work-around for this limitation is to switch to UEFI. UEFI is 64-bit and should support booting significantly larger images, though sometimes system-specific implementation details have led to artificial limitations on image size.

  • The Linux kernel itself can only decompress an image up to 4GB due to the use of 32-bit integers in critical sections of the kernel initrd decompression code.

    The best work-around for this limitation is to use an iPXE with support for imgextract. This allows iPXE to decompress the image rather than the kernel.

  • Some BIOS / firmware retain a “memory hole” feature for legacy devices, e.g., reserving a 1MB block of memory at the 15MB-16MB address range. this feature can interfere with booting stateless node images.

    If you are still getting “Not enough memory” or “No space left on device” errors, try disabling any “memory hole” features or updating your system BIOS or firmware.

Duplicating a container

It is possible to duplicate an installed image by using :


This kind of duplication can be useful if you are looking for canary tests.