Run the Docker daemon as a non-root user (Rootless mode)

Estimated reading time: 14 minutes

Rootless mode allows running the Docker daemon and containers as a non-root user to mitigate potential vulnerabilities in the daemon and the container runtime.

Rootless mode does not require root privileges even during the installation of the Docker daemon, as long as the prerequisites are met.

Rootless mode was introduced in Docker Engine v19.03.

Note

Rootless mode is an experimental feature and has some limitations. For details, see Known limitations.

How it works

Rootless mode executes the Docker daemon and containers inside a user namespace. This is very similar to userns-remap mode, except that with userns-remap mode, the daemon itself is running with root privileges, whereas in rootless mode, both the daemon and the container are running without root privileges.

Rootless mode does not use binaries with SETUID bits or file capabilities, except newuidmap and newgidmap, which are needed to allow multiple UIDs/GIDs to be used in the user namespace.

Prerequisites

  • You must install newuidmap and newgidmap on the host. These commands are provided by the uidmap package on most distros.

  • /etc/subuid and /etc/subgid should contain at least 65,536 subordinate UIDs/GIDs for the user. In the following example, the user testuser has 65,536 subordinate UIDs/GIDs (231072-296607).

$ id -u
1001
$ whoami
testuser
$ grep ^$(whoami): /etc/subuid
testuser:231072:65536
$ grep ^$(whoami): /etc/subgid
testuser:231072:65536

Distribution-specific hint

Note: We recommend that you use the Ubuntu kernel.

Ubuntu

  • No preparation is needed.

  • overlay2 storage driver is enabled by default (Ubuntu-specific kernel patch).

  • Known to work on Ubuntu 16.04, 18.04, and 20.04.

Debian GNU/Linux

  • Add kernel.unprivileged_userns_clone=1 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system.

  • To use the overlay2 storage driver (recommended), run sudo modprobe overlay permit_mounts_in_userns=1 (Debian-specific kernel patch, introduced in Debian 10). Add the configuration to /etc/modprobe.d for persistence.

  • Known to work on Debian 9 and 10. overlay2 is only supported since Debian 10 and needs modprobe configuration described above.

Arch Linux

  • Add kernel.unprivileged_userns_clone=1 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system

openSUSE

  • sudo modprobe ip_tables iptable_mangle iptable_nat iptable_filter is required. This might be required on other distros as well depending on the configuration.

  • Known to work on openSUSE 15.

Fedora 31 and later

  • Fedora 31 uses cgroup v2 by default, which is not yet supported by the containerd runtime. Run sudo grubby --update-kernel=ALL --args="systemd.unified_cgroup_hierarchy=0" to use cgroup v1.
  • You might need sudo dnf install -y iptables.

CentOS 8

  • You might need sudo dnf install -y iptables.

CentOS 7

  • Add user.max_user_namespaces=28633 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system.

  • systemctl --user does not work by default. Run the daemon directly without systemd: dockerd-rootless.sh --experimental --storage-driver vfs

  • Known to work on CentOS 7.7. Older releases require additional configuration steps.

  • CentOS 7.6 and older releases require COPR package vbatts/shadow-utils-newxidmap to be installed.

  • CentOS 7.5 and older releases require running sudo grubby --update-kernel=ALL --args="user_namespace.enable=1" and a reboot following this.

Known limitations

  • Only vfs graphdriver is supported. However, on Ubuntu and Debian 10, overlay2 and overlay are also supported.
  • Following features are not supported:
    • Cgroups (including docker top, which depends on the cgroups)
    • AppArmor
    • Checkpoint
    • Overlay network
    • Exposing SCTP ports
  • To use the ping command, see Routing ping packets.
  • To expose privileged TCP/UDP ports (< 1024), see Exposing privileged ports.
  • IPAddress shown in docker inspect and is namespaced inside RootlessKit’s network namespace. This means the IP address is not reachable from the host without nsenter-ing into the network namespace.
  • Host network (docker run --net=host) is also namespaced inside RootlessKit.

Install

The installation script is available at https://get.docker.com/rootless.

$ curl -fsSL https://get.docker.com/rootless | sh

Make sure to run the script as a non-root user. To install Rootless Docker as the root user, see the Manual installation steps.

The script shows environment variables that are required:

$ curl -fsSL https://get.docker.com/rootless | sh
...
# Docker binaries are installed in /home/testuser/bin
# WARN: dockerd is not in your current PATH or pointing to /home/testuser/bin/dockerd
# Make sure the following environment variables are set (or add them to ~/.bashrc):

export PATH=/home/testuser/bin:$PATH
export PATH=$PATH:/sbin
export DOCKER_HOST=unix:///run/user/1001/docker.sock

#
# To control docker service run:
# systemctl --user (start|stop|restart) docker
#

Manual installation

To install the binaries manually without using the installer, extract docker-rootless-extras-<version>.tar.gz along with docker-<version>.tar.gz from https://download.docker.com/linux/static/stable/x86_64/

If you already have the Docker daemon running as the root, you only need to extract docker-rootless-extras-<version>.tar.gz. The archive can be extracted under an arbitrary directory listed in the $PATH. For example, /usr/local/bin, or $HOME/bin.

Nightly channel

To install a nightly version of the Rootless Docker, run the installation script using CHANNEL="nightly":

$ curl -fsSL https://get.docker.com/rootless | CHANNEL="nightly" sh

The raw binary archives are available at:

  • https://master.dockerproject.org/linux/x86_64/docker-rootless-extras.tgz
  • https://master.dockerproject.org/linux/x86_64/docker.tgz

Usage

Daemon

Use systemctl --user to manage the lifecycle of the daemon:

$ systemctl --user start docker

To launch the daemon on system startup, enable the systemd service and lingering:

$ systemctl --user enable docker
$ sudo loginctl enable-linger $(whoami)

To run the daemon directly without systemd, you need to run dockerd-rootless.sh instead of dockerd:

$ dockerd-rootless.sh --experimental --storage-driver vfs

As Rootless mode is experimental, you need to run dockerd-rootless.sh with --experimental.

You also need --storage-driver vfs unless you are using Ubuntu or Debian 10 kernel. You don’t need to care about these flags if you manage the daemon using systemd, as these flags are automatically added to the systemd unit file.

Remarks about directory paths:

  • The socket path is set to $XDG_RUNTIME_DIR/docker.sock by default. $XDG_RUNTIME_DIR is typically set to /run/user/$UID.
  • The data dir is set to ~/.local/share/docker by default.
  • The exec dir is set to $XDG_RUNTIME_DIR/docker by default.
  • The daemon config dir is set to ~/.config/docker (not ~/.docker, which is used by the client) by default.

Other remarks:

  • The dockerd-rootless.sh script executes dockerd in its own user, mount, and network namespaces. You can enter the namespaces by running nsenter -U --preserve-credentials -n -m -t $(cat $XDG_RUNTIME_DIR/docker.pid).
  • docker info shows rootless in SecurityOptions
  • docker info shows none as Cgroup Driver

Client

You need to specify the socket path explicitly.

To specify the socket path using $DOCKER_HOST:

$ export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/docker.sock
$ docker run -d -p 8080:80 nginx

To specify the socket path using docker context:

$ docker context create rootless --description "for rootless mode" --docker "host=unix://$XDG_RUNTIME_DIR/docker.sock"
rootless
Successfully created context "rootless"
$ docker context use rootless
rootless
Current context is now "rootless"
$ docker run -d -p 8080:80 nginx

Best practices

Rootless Docker in Docker

To run Rootless Docker inside “rootful” Docker, use the docker:<version>-dind-rootless image instead of docker:<version>-dind.

$ docker run -d --name dind-rootless --privileged docker:19.03-dind-rootless --experimental

The docker:<version>-dind-rootless image runs as a non-root user (UID 1000). However, --privileged is required for disabling seccomp, AppArmor, and mount masks.

Expose Docker API socket through TCP

To expose the Docker API socket through TCP, you need to launch dockerd-rootless.sh with DOCKERD_ROOTLESS_ROOTLESSKIT_FLAGS="-p 0.0.0.0:2376:2376/tcp".

$ DOCKERD_ROOTLESS_ROOTLESSKIT_FLAGS="-p 0.0.0.0:2376:2376/tcp" \
  dockerd-rootless.sh --experimental \
  -H tcp://0.0.0.0:2376 \
  --tlsverify --tlscacert=ca.pem --tlscert=cert.pem --tlskey=key.pem

Expose Docker API socket through SSH

To expose the Docker API socket through SSH, you need to make sure $DOCKER_HOST is set on the remote host.

$ ssh -l <REMOTEUSER> <REMOTEHOST> 'echo $DOCKER_HOST'
unix:///run/user/1001/docker.sock
$ docker -H ssh://<REMOTEUSER>@<REMOTEHOST> run ...

Routing ping packets

On some distributions, ping does not work by default.

Add net.ipv4.ping_group_range = 0 2147483647 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system to allow using ping.

Exposing privileged ports

To expose privileged ports (< 1024), set CAP_NET_BIND_SERVICE on rootlesskit binary.

$ sudo setcap cap_net_bind_service=ep $HOME/bin/rootlesskit

Or add net.ipv4.ip_unprivileged_port_start=0 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system.

Limiting resources

In Docker 19.03, rootless mode ignores cgroup-related docker run flags such as --cpus, --memory, --pids-limit`.

However, you can still use the traditional ulimit and cpulimit, though they work in process-granularity rather than in container-granularity, and can be arbitrarily disabled by the container process.

For example:

  • To limit CPU usage to 0.5 cores (similar to docker run --cpus 0.5): docker run <IMAGE> cpulimit --limit=50 --include-children <COMMAND>
  • To limit max VSZ to 64MiB (similar to docker run --memory 64m): docker run <IMAGE> sh -c "ulimit -v 65536; <COMMAND>"

  • To limit max number of processes to 100 per namespaced UID 2000 (similar to docker run --pids-limit=100): docker run --user 2000 --ulimit nproc=100 <IMAGE> <COMMAND>

Changing the network stack

dockerd-rootless.sh uses slirp4netns (if installed) or VPNKit as the network stack by default.

These network stacks run in userspace and might have performance overhead. See RootlessKit documentation for further information.

Optionally, you can use lxc-user-nic instead for the best performance. To use lxc-user-nic, you need to edit /etc/lxc/lxc-usernet and set $DOCKERD_ROOTLESS_ROOTLESSKIT_NET=lxc-user-nic.

Troubleshooting

Errors when starting the Docker daemon

[rootlesskit:parent] error: failed to start the child: fork/exec /proc/self/exe: operation not permitted

This error occurs mostly when the value of /proc/sys/kernel/unprivileged_userns_clone is set to 0:

$ cat /proc/sys/kernel/unprivileged_userns_clone
0

To fix this issue, add kernel.unprivileged_userns_clone=1 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system.

[rootlesskit:parent] error: failed to start the child: fork/exec /proc/self/exe: no space left on device

This error occurs mostly when the value of /proc/sys/user/max_user_namespaces is too small:

$ cat /proc/sys/user/max_user_namespaces
0

To fix this issue, add user.max_user_namespaces=28633 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl --system.

[rootlesskit:parent] error: failed to setup UID/GID map: failed to compute uid/gid map: No subuid ranges found for user 1001 (“testuser”)

This error occurs when /etc/subuid and /etc/subgid are not configured. See Prerequisites.

could not get XDG_RUNTIME_DIR

This error occurs when $XDG_RUNTIME_DIR is not set.

On a non-systemd host, you need to create a directory and then set the path:

$ export XDG_RUNTIME_DIR=$HOME/.docker/xrd
$ rm -rf $XDG_RUNTIME_DIR
$ mkdir -p $XDG_RUNTIME_DIR
$ dockerd-rootless.sh --experimental

Note: You must remove the directory every time you log out.

On a systemd host, log into the host using pam_systemd (see below). The value is automatically set to /run/user/$UID and cleaned up on every logout.

systemctl --user fails with “Failed to connect to bus: No such file or directory”

This error occurs mostly when you switch from the root user to an non-root user with sudo:

# sudo -iu testuser
$ systemctl --user start docker
Failed to connect to bus: No such file or directory

Instead of sudo -iu <USERNAME>, you need to log in using pam_systemd. For example:

  • Log in through the graphic console
  • ssh <USERNAME>@localhost
  • machinectl shell <USERNAME>@

The daemon does not start up automatically

You need sudo loginctl enable-linger $(whoami) to enable the daemon to start up automatically. See Usage.

dockerd fails with “rootless mode is supported only when running in experimental mode”

This error occurs when the daemon is launched without the --experimental flag. See Usage.

docker pull errors

docker: failed to register layer: Error processing tar file(exit status 1): lchown <FILE>: invalid argument

This error occurs when the number of available entries in /etc/subuid or /etc/subgid is not sufficient. The number of entries required vary across images. However, 65,536 entries are sufficient for most images. See Prerequisites.

docker run errors

--cpus, --memory, and --pids-limit are ignored

This is an expected behavior in Docker 19.03. For more information, see Limiting resources.

Error response from daemon: cgroups: cgroup mountpoint does not exist: unknown.

This error occurs mostly when the host is running in cgroup v2. See the section Fedora 31 or later for information on switching the host to use cgroup v1.

Networking errors

docker run -p fails with cannot expose privileged port

docker run -p fails with this error when a privileged port (< 1024) is specified as the host port.

$ docker run -p 80:80 nginx:alpine
docker: Error response from daemon: driver failed programming external connectivity on endpoint focused_swanson (9e2e139a9d8fc92b37c36edfa6214a6e986fa2028c0cc359812f685173fa6df7): Error starting userland proxy: error while calling PortManager.AddPort(): cannot expose privileged port 80, you might need to add "net.ipv4.ip_unprivileged_port_start=0" (currently 1024) to /etc/sysctl.conf, or set CAP_NET_BIND_SERVICE on rootlesskit binary, or choose a larger port number (>= 1024): listen tcp 0.0.0.0:80: bind: permission denied.

When you experience this error, consider using an unprivileged port instead. For example, 8080 instead of 80.

$ docker run -p 8080:80 nginx:alpine

To allow exposing privileged ports, see Exposing privileged ports.

ping doesn’t work

Ping does not work when /proc/sys/net/ipv4/ping_group_range is set to 1 0:

$ cat /proc/sys/net/ipv4/ping_group_range
1       0

For details, see Routing ping packets.

IPAddress shown in docker inspect is unreachable

This is an expected behavior, as the daemon is namespaced inside RootlessKit’s network namespace. Use docker run -p instead.

--net=host doesn’t listen ports on the host network namespace

This is an expected behavior, as the daemon is namespaced inside RootlessKit’s network namespace. Use docker run -p instead.

security, namespaces, rootless