Rootless mode

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.

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 distributions.

• /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

情報

We recommend that you use the Ubuntu kernel.

Known limitations

• Only the following storage drivers are supported:
  • overlay2 (only if running with kernel 5.11 or later, or Ubuntu-flavored kernel)
  • fuse-overlayfs (only if running with kernel 4.18 or later, and fuse-overlayfs is installed)
  • btrfs (only if running with kernel 4.18 or later, or ~/.local/share/docker is mounted with user_subvol_rm_allowed mount option)
  • vfs
• Cgroup is supported only when running with cgroup v2 and systemd. See Limiting resources.
• Following features are not supported:
  • 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 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.
• NFS mounts as the docker "data-root" is not supported. This limitation is not specific to rootless mode.

Install

メモ

If the system-wide Docker daemon is already running, consider disabling it:

$ sudo systemctl disable --now docker.service docker.socket
$ sudo rm /var/run/docker.sock

Should you choose not to shut down the docker service and socket, you will need to use the --force parameter in the next section. There are no known issues, but until you shutdown and disable you're still running rootful Docker.

$ dockerd-rootless-setuptool.sh install
[INFO] Creating /home/testuser/.config/systemd/user/docker.service
...
[INFO] Installed docker.service successfully.
[INFO] To control docker.service, run: `systemctl --user (start|stop|restart) docker.service`
[INFO] To run docker.service on system startup, run: `sudo loginctl enable-linger testuser`

[INFO] Make sure the following environment variables are set (or add them to ~/.bashrc):

export PATH=/usr/bin:$PATH
export DOCKER_HOST=unix:///run/user/1000/docker.sock

$ sudo apt-get install -y docker-ce-rootless-extras

$ curl -fsSL https://get.docker.com/rootless | sh
...
[INFO] Creating /home/testuser/.config/systemd/user/docker.service
...
[INFO] Installed docker.service successfully.
[INFO] To control docker.service, run: `systemctl --user (start|stop|restart) docker.service`
[INFO] To run docker.service on system startup, run: `sudo loginctl enable-linger testuser`

[INFO] Make sure the following environment variables are set (or add them to ~/.bashrc):

export PATH=/home/testuser/bin:$PATH
export DOCKER_HOST=unix:///run/user/1000/docker.sock

See Troubleshooting if you faced an error.

Uninstall

To remove the systemd service of the Docker daemon, run dockerd-rootless-setuptool.sh uninstall:

$ dockerd-rootless-setuptool.sh uninstall
+ systemctl --user stop docker.service
+ systemctl --user disable docker.service
Removed /home/testuser/.config/systemd/user/default.target.wants/docker.service.
[INFO] Uninstalled docker.service
[INFO] This uninstallation tool does NOT remove Docker binaries and data.
[INFO] To remove data, run: `/usr/bin/rootlesskit rm -rf /home/testuser/.local/share/docker`

Unset environment variables PATH and DOCKER_HOST if you have added them to ~/.bashrc.

To remove the data directory, run rootlesskit rm -rf ~/.local/share/docker.

To remove the binaries, remove docker-ce-rootless-extras package if you installed Docker with package managers. If you installed Docker with https://get.docker.com/rootless ( Install without packages), remove the binary files under ~/bin:

$ cd ~/bin
$ rm -f containerd containerd-shim containerd-shim-runc-v2 ctr docker docker-init docker-proxy dockerd dockerd-rootless-setuptool.sh dockerd-rootless.sh rootlesskit rootlesskit-docker-proxy runc vpnkit

Usage

Daemon

$ systemctl --user start docker

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

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 data dir should not be on NFS.
• The daemon config dir is set to ~/.config/docker by default. This directory is different from ~/.docker that is used by the client.

Client

You need to specify either the socket path or the CLI context 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 CLI context using docker context:

$ 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:25.0-dind-rootless

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 \
  -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 and restart the daemon.

$ sudo setcap cap_net_bind_service=ep $(which rootlesskit)
$ systemctl --user restart docker

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

Limiting resources

Limiting resources with cgroup-related docker run flags such as --cpus, --memory, --pids-limit is supported only when running with cgroup v2 and systemd. See Changing cgroup version to enable cgroup v2.

If docker info shows none as Cgroup Driver, the conditions are not satisfied. When these conditions are not satisfied, rootless mode ignores the cgroup-related docker run flags. See Limiting resources without cgroup for workarounds.

If docker info shows systemd as Cgroup Driver, the conditions are satisfied. However, typically, only memory and pids controllers are delegated to non-root users by default.

$ cat /sys/fs/cgroup/user.slice/user-$(id -u).slice/user@$(id -u).service/cgroup.controllers
memory pids

To allow delegation of all controllers, you need to change the systemd configuration as follows:

# mkdir -p /etc/systemd/system/user@.service.d
# cat > /etc/systemd/system/user@.service.d/delegate.conf << EOF
[Service]
Delegate=cpu cpuset io memory pids
EOF
# systemctl daemon-reload

メモ

Delegating cpuset requires systemd 244 or later.

Limiting resources without cgroup

Even when cgroup is not available, 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>

Troubleshooting

Unable to install with systemd when systemd is present on the system

$ dockerd-rootless-setuptool.sh install
[INFO] systemd not detected, dockerd-rootless.sh needs to be started manually:
...

rootlesskit cannot detect systemd properly if you switch to your user via sudo su. For users which cannot be logged-in, you must use the machinectl command which is part of the systemd-container package. After installing systemd-container switch to myuser with the following command:

$ sudo machinectl shell myuser@

Where myuser@ is your desired username and @ signifies this machine.

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

メモ

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.

iptables failed: iptables -t nat -N DOCKER: Fatal: can't open lock file /run/xtables.lock: Permission denied

This error may happen with an older version of Docker when SELinux is enabled on the host.

The issue has been fixed in Docker 20.10.8. A known workaround for older version of Docker is to run the following commands to disable SELinux for iptables:

$ sudo dnf install -y policycoreutils-python-utils && sudo semanage permissive -a iptables_t

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: failed to register layer: ApplyLayer exit status 1 stdout: stderr: lchown <FILE>: operation not permitted

This error occurs mostly when ~/.local/share/docker is located on NFS.

A workaround is to specify non-NFS data-root directory in ~/.config/docker/daemon.json as follows:

{"data-root":"/somewhere-out-of-nfs"}

docker run errors

docker: Error response from daemon: OCI runtime create failed: ...: read unix @->/run/systemd/private: read: connection reset by peer: unknown.

This error occurs on cgroup v2 hosts mostly when the dbus daemon is not running for the user.

$ systemctl --user is-active dbus
inactive

$ docker run hello-world
docker: Error response from daemon: OCI runtime create failed: container_linux.go:380: starting container process caused: process_linux.go:385: applying cgroup configuration for process caused: error while starting unit "docker
-931c15729b5a968ce803784d04c7421f791d87e5ca1891f34387bb9f694c488e.scope" with properties [{Name:Description Value:"libcontainer container 931c15729b5a968ce803784d04c7421f791d87e5ca1891f34387bb9f694c488e"} {Name:Slice Value:"use
r.slice"} {Name:PIDs Value:@au [4529]} {Name:Delegate Value:true} {Name:MemoryAccounting Value:true} {Name:CPUAccounting Value:true} {Name:IOAccounting Value:true} {Name:TasksAccounting Value:true} {Name:DefaultDependencies Val
ue:false}]: read unix @->/run/systemd/private: read: connection reset by peer: unknown.

To fix the issue, run sudo apt-get install -y dbus-user-session or sudo dnf install -y dbus-daemon, and then relogin.

If the error still occurs, try running systemctl --user enable --now dbus (without sudo).

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

This is an expected behavior on cgroup v1 mode. To use these flags, the host needs to be configured for enabling cgroup v2. For more information, see Limiting resources.

Networking errors

This section provides troubleshooting tips for networking in rootless mode.

Networking in rootless mode is supported via network and port drivers in RootlessKit. Network performance and characteristics depend on the combination of network and port driver you use. If you're experiencing unexpected behavior or performance related to networking, review the following table which shows the configurations supported by RootlessKit, and how they compare:

For information about troubleshooting specific networking issues, see:

• docker run -p fails with cannot expose privileged port
• Ping doesn't work
• IPAddress shown in docker inspect is unreachable
• --net=host doesn't listen ports on the host network namespace
• Network is slow
• docker run -p does not propagate source IP addresses

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.

Network is slow

Docker with rootless mode uses slirp4netns as the default network stack if slirp4netns v0.4.0 or later is installed. If slirp4netns is not installed, Docker falls back to VPNKit. Installing slirp4netns may improve the network throughput.

For more information about network drivers for RootlessKit, see RootlessKit documentation.

Also, changing MTU value may improve the throughput. The MTU value can be specified by creating ~/.config/systemd/user/docker.service.d/override.conf with the following content:

[Service]
Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=<INTEGER>"

And then restart the daemon:

$ systemctl --user daemon-reload
$ systemctl --user restart docker

docker run -p does not propagate source IP addresses

This is because Docker in rootless mode uses RootlessKit's builtin port driver by default, which doesn't support source IP propagation. To enable source IP propagation, you can:

• Use the slirp4netns RootlessKit port driver
• Use the pasta RootlessKit network driver, with the implicit port driver

The pasta network driver is experimental, but provides improved throughput performance compared to the slirp4netns port driver. The pasta driver requires Docker Engine version 25.0 or later.

To change the RootlessKit networking configuration:

1. Create a file at ~/.config/systemd/user/docker.service.d/override.conf.

2. Add the following contents, depending on which configuration you would like to use:

   • slirp4netns

     [Service]
     Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_NET=slirp4netns"
     Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=slirp4netns"

   • pasta network driver with implicit port driver

     [Service]
     Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_NET=pasta"
     Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=implicit"

3. Restart the daemon:

   $ systemctl --user daemon-reload
   $ systemctl --user restart docker
   

For more information about networking options for RootlessKit, see:

• Network drivers
• Port drivers

Tips for debugging

Entering into dockerd namespaces

The dockerd-rootless.sh script executes dockerd in its own user, mount, and network namespaces.

For debugging, you can enter the namespaces by running nsenter -U --preserve-credentials -n -m -t $(cat $XDG_RUNTIME_DIR/docker.pid).