Get Stacker

Stacker is a single static binary tool with most of the dependencies built into the binary. Stacker, however, depends on specific kernel capabilities and system tools to provide various features.

wget https://github.com/project-stacker/stacker/releases/latest/download/stacker
chmod +x ./stacker
sudo cp ./stacker /usr/bin/stacker
stacker --version
stacker check

Kernel dependencies¶

Stacker requires overlayfs backend, and that works with any kernel >= 4.14. However, for unprivileged use, the overlayfs backend requires a reasonably new kernel change available on all kernels >= 5.8.

Some distributions may have ported these patches into older versions of their kernels. For example, Ubuntu 20.04 and 22.04 kernels already have these patches.

Stacker has checks to ensure that it can run with all these environments requirements, and will fail fast if it can't do something it should be able to do.

stacker check && echo "stacker is ready to use!"

Overlay filesystem¶

An underlying overlayfs cannot back stacker since the stacker needs to create whiteout files, and the kernel (rightfully) forbids manual creation of whiteout files on overlay filesystems. No additional userspace dependencies are required to use the overlayfs backend.

Unprivileged setup¶

Running stacker as an unprivileged user requires stacker to run inside a user namespace owned by the user that executed the command, and stacker will try to map 65k user and group ids to meet the POSIX standard. So, to run stacker, the user's /etc/sub{u,g}id should be configured with enough uids to map things correctly. This configuration can be done automatically via stacker unpriv-setup.

sudo stacker unpriv-setup
cat /etc/subgid
cat /etc/subuid

Squashfs support¶

In order to generate squashfs images, stacker invokes the mksquashfs binary. This binary needs to be installed and present in $PATH.

sudo apt-get install -y squashfs-tools