containerd/docs/rootless.md

Running containerd as a non-root user

A non-root user can execute containerd by using user_namespaces(7).

For example RootlessKit can be used for setting up a user namespace (along with mount namespace and optionally network namespace). Please refer to RootlessKit documentation for further information.

See also Rootless Docker documentation.

Daemon

$ rootlesskit --net=slirp4netns --copy-up=/etc --copy-up=/run \
  --state-dir=/run/user/1001/rootlesskit-containerd \
  sh -c "rm -f /run/containerd; exec containerd -c config.toml"

• --net=slirp4netns --copy-up=/etc is only required when you want to unshare network namespaces. See RootlessKit documentation for the further information about the network drivers.
• --copy-up=/DIR mounts a writable tmpfs on /DIR with symbolic links to the files under the /DIR on the parent namespace so that the user can add/remove files under /DIR in the mount namespace. --copy-up=/etc and --copy-up=/run are needed on typical setup. Depending on the containerd plugin configuration, you may also need to add more --copy-up options.
• rm -f /run/containerd removes the "copied-up" symbolic link to /run/containerd on the parent namespace (if exists), which cannot be accessed by non-root users. The actual /run/containerd directory on the host is not affected.
• --state-dir is set to a random directory under /tmp if unset. RootlessKit writes the PID to a file named child_pid under this directory.
• You need to provide config.toml with your own path configuration. e.g.

version = 2
root = "/home/penguin/.local/share/containerd"
state = "/run/user/1001/containerd"

[grpc]
  address = "/run/user/1001/containerd/containerd.sock"

Client

A client program such as ctr also needs to be executed inside the daemon namespaces.

$ nsenter -U --preserve-credentials -m -n -t $(cat /run/user/1001/rootlesskit-containerd/child_pid)
$ export CONTAINERD_ADDRESS=/run/user/1001/containerd/containerd.sock
$ export CONTAINERD_SNAPSHOTTER=native
$ ctr images pull docker.io/library/ubuntu:latest
$ ctr run -t --rm --fifo-dir /tmp/foo-fifo --cgroup "" docker.io/library/ubuntu:latest foo

• overlayfs snapshotter does not work inside user namespaces, except on Ubuntu and Debian kernels. However, fuse-overlayfs snapshotter can be used instead if running kernel >= 4.18.
• Enabling cgroup requires cgroup v2 and systemd, e.g. ctr run --cgroup "user.slice:foo:bar" --runc-systemd-cgroup ... . See also runc documentation.