Podman

Podman is an alternative to Docker, providing a similar interface. It supports rootless containers and a shim service for docker-compose.

Installation

Install the podman package. Additionally if you want to build container images look at Buildah.

For container networking, install cni-plugins or netavark since v4.0.

If you want to replace Docker, one can install podman-docker to mimic the docker binary along with man pages.

Unlike Docker, Podman does not require a daemon, but there is one providing an API for services like cockpit via cockpit-podman.

By default, it is only possible to run Podman containers as root. See #Rootless Podman to set up running containers as a non-root user.

Configuration

Configuration files for configuring how containers behave are located at /usr/share/containers/. You must copy necessary files to /etc/containers before editing. To configure the network bridge interface used by Podman, see /etc/cni/net.d/87-podman.conflist.

Rootless Podman

Warning: Rootless Podman relies on the unprivileged user namespace usage (CONFIG_USER_NS_UNPRIVILEGED) which has some serious security implications, see Security#Sandboxing applications for details.

By default, only root is allowed to run containers (or namespaces in kernelspeak). Running rootless Podman improves security as an attacker will not have root privileges over your system, and also allows multiple unprivileged users to run containers on the same machine. See also podman(1) § Rootless mode.

Additional dependencies

The slirp4netns package is installed as a dependency to run Podman in a rootless environment.

If Podman uses the netavark network backend (see containers.conf(5)) then it is required to install aardvark-dns to have name resolution in rootless containers.

Enable native rootless overlays

Previously, it was necessary to use the fuse-overlayfs package for FUSE overlay mounts in a rootless environment. However, modern versions of Podman and Linux kernel support native rootless overlays, which yields better performance. To migrate from fuse-overlayfs, run

$ podman system reset

This command will unfortunately delete all pulled containers. Also make sure that Podman uses overlay driver and that the mount_program parameter is not defined in containers-storage.conf(5). It might also be required to follow instructions from Docker#Enable native overlay diff engine.

To verify that native rootless overlays are enabled, run

$ podman info

It should show graphDriverName: overlay and Native Overlay Diff: "true".

Enable kernel.unprivileged_userns_clone

First, check the value of kernel.unprivileged_userns_clone by running:

$ sysctl kernel.unprivileged_userns_clone

If it is currently set to 0, enable it by setting 1 via sysctl or a kernel parameter.

Note: linux-hardened has kernel.unprivileged_userns_clone set to 0 by default.

Set subuid and subgid

In order for users to run rootless Podman, a subuid(5) and subgid(5) configuration entry must exist for each user that wants to use it. New users created using useradd(8) have these entries by default.

Note:

Users created prior to shadow 4.11.1-3 do not have entries in /etc/subuid and /etc/subgid by default. An entry can be created for them using the usermod(8) command or by manually modifying the files.

The following command enables the username user and group to run Podman containers (or other types of containers in that case). It allocates a given range of UIDs and GIDs to the given user and group.

# usermod --add-subuids 100000-165535 --add-subgids 100000-165535 username

The above range for the user username may already be taken by another user as it defines the default range for the first user on the system. If in doubt, first consult the /etc/subuid and /etc/subgid files to find the already reserved ranges.

Many images require 65536 uids / gids for mapping (notably the base busybox and alpine images). It is recommended that you allocate at least that many uids / gids for each user to maximize compatibility with docker.
If you are using systemd-homed, the minimum UID and GID for containers must be at least 524288 (check the "begin container users" value in the output of userdbctl). [1]

Propagate changes to subuid and subgid

Rootless Podman uses a pause process to keep the unprivileged namespaces alive. This prevents any change to the /etc/subuid and /etc/subgid files from being propagated to the rootless containers while the pause process is running. For these changes to be propagated it is necessary to run:

$ podman system migrate

After this, the user/group specified in the above files is able to start and run Podman containers.

Add SYS_CHROOT capabilities (Optional)

Starting at the 4.4 release, some previously default capabilities were dropped, including SYS_CHROOT (explained in an official blog post). This affects containers which use chroot (like archlinux:base) and thus pacman operations fail within the container (i.e. installing packages which execute post-install scripts). You can identify such issues if when building with podman you get errors like below during build:

...
could not change the root directory (Operation not permitted)
error: command failed to execute correctly
...

To resolve this, edit /etc/containers/containers.conf and add the SYS_CHROOT to the list:

/etc/containers/containers.conf

default_capabilities = [
  "CHOWN",
  "DAC_OVERRIDE",
  "FOWNER",
  "FSETID",
  "KILL",
  "NET_BIND_SERVICE",
  "SETFCAP",
  "SETGID",
  "SETPCAP",
  "SETUID",
  "SYS_CHROOT",
]

You can also do it from the command line temporarily with --cap-add sys_chroot when you execute podman-build(1).

Storage

The configuration for how and where container images and instances are stored takes place in /etc/containers/storage.conf.

Note: When using #Rootless Podman overrides to the storage settings can be added to $XDG_CONFIG_HOME/containers/storage.conf on a per-user basis.

Set the driver according to the filesystem in use for the storage location (see containers-storage.conf(5) § STORAGE_TABLE).

Foreign architectures

Podman is able to run images built for different CPU architecture than the host using the Wikipedia:binfmt_misc system.

To enable it, install qemu-user-static and qemu-user-static-binfmt.

systemd comes with the systemd-binfmt.service service which should enable new rules.

Verify that binfmt rules have been added:

$ ls /proc/sys/fs/binfmt_misc

DOSWin        qemu-cris        qemu-ppc      qemu-sh4eb        status
qemu-aarch64  qemu-m68k        qemu-ppc64    qemu-sparc        
qemu-alpha    qemu-microblaze  qemu-riscv64  qemu-sparc32plus  
qemu-arm      qemu-mips        qemu-s390x    qemu-sparc64      
qemu-armeb    qemu-mipsel      qemu-sh4      register

Podman should now be able to run foreign architecture images. Most commands use the foreign architecture when --arch option is passed.

Example:

# podman run --arch arm64 'docker.io/alpine:latest' arch

aarch64

Docker Compose

Podman 3.0.0 introduces docker-compose support. This requires enabling a Podman socket which pretends to be docker; start the podman.service unit. For rootless containers, this requires you to start the podman.service user unit instead and set the DOCKER_HOST variable:

$ export DOCKER_HOST="unix://$XDG_RUNTIME_DIR/podman/podman.sock"

To get hostname resolution between containers running install podman-dnsname.

Note: If you have enabled buildkit in docker, the integration will not work. You need to disable buildkit: $ export DOCKER_BUILDKIT=0

NVIDIA GPUs

NVIDIA Container Toolkit provides container runtime for NVIDIA GPUs. Install the nvidia-container-toolkit^AUR package.

Test the setup:

$ podman run --rm nvidia/cuda:12.0.0-runtime-ubuntu20.04 nvidia-smi

To be able to run rootless containers with podman, the no-cgroups setting must be set to true in /etc/nvidia-container-runtime/config.toml.

Images

Note: You may omit the registry prefix from the images, as Podman will automatically search for the image in all registries defined in /etc/containers/registries.conf at unqualified-search-registries in the defined order. The following images will always contain the prefix, to allow for configurations without docker.io in the configuration.

Arch Linux

The following command pulls the Arch Linux x86_64 image from Docker Hub.

# podman pull docker.io/archlinux

See the Docker Hub page for a full list of available tags, including versions with and without build tools.

Alpine Linux

Alpine Linux is a popular choice for small container images, especially for software compiled as static binaries. The following command pulls the latest Alpine Linux image from Docker Hub:

# podman pull docker.io/alpine

Alpine Linux uses the musl libc implementation instead of the glibc libc implementation used by most Linux distributions. Because Arch Linux uses glibc, there are a number of functional differences between an Arch Linux host and an Alpine Linux container that can impact the performance and correctness of software. A list of these differences is documented in https://wiki.musl-libc.org/functional-differences-from-glibc.html.

Note that dynamically linked software built on Arch Linux (or any other system using glibc) may have bugs and performance problems when run on Alpine Linux (or any other system using a different libc). See [2], [3] and [4] for examples.

CentOS

The following command pulls the latest CentOS image from Docker Hub:

# podman pull docker.io/centos

See the Docker Hub page for a full list of available tags for each CentOS release.

Debian

The following command pulls the latest Debian image from Docker Hub:

# podman pull docker.io/debian

See the Docker Hub page for a full list of available tags, including both standard and slim versions for each Debian release.

Troubleshooting

Add pause to process

WARN[0000] Failed to add pause process to systemd sandbox cgroup: Process org.freedesktop.systemd1 exited with status 1

Can be solved using: https://github.com/containers/crun/issues/704

# echo +cpu +cpuset +io +memory +pids > /sys/fs/cgroup/cgroup.subtree_control

Container DNS will not be enabled

WARN[0000]  binary not found, container DNS will not be enabled

When using netavark as the podman network backend, you need to install aardvark-dns.

Containers terminate on shell logout

After logging out from machine, Podman containers are stopped for some users. To prevent that, enable lingering for users running containers.

You can also create user systemd unit as described in podman-auto-update(1) § EXAMPLES.

Failed to move rootless netns

$ docker-compose up

ERRO[0000] failed to move the rootless netns slirp4netns process to the systemd user.slice: Process org.freedesktop.systemd1 exited with status 1

Can be solved by starting/enabling podman.service.

Error building pause image after Podman upgrade 3.x to 4.0

Error: building local pause image: finding pause binary: exec: "catatonit": executable file not found in $PATH

Install the catatonit package to fix the error.

For details on upgrading from 3.x to 4.0, see the official blog article^{[dead link 2023-05-06 ⓘ]}.

Error on commit in rootless mode

Error committing the finished image: error adding layer with blob "sha256:02823fca9b5444c196f1f406aa235213254af9909fca270f462e32793e2260d8": Error processing tar file(exit status 1) permitted operation

Check that the storage driver is overlay in the storage configuration.

Error when creating a container with bridge network in rootless mode

If you are using AppArmor you might end up with problems when creating container using a bridge network with the dnsname plugin enabled:

$ podman network create foo

/home/user/.config/cni/net.d/foo.conflist

$ podman run --rm -it --network=foo docker.io/library/alpine:latest ip addr

Error: command rootless-cni-infra [alloc 89398a9315256cb1938075c377275d29c2b6ebdd75a96b5c26051a89541eb928 foo festive_hofstadter    ] in container 1f4344bbd1087c892a18bacc35f4fdafbb61106c146952426488bc940a751efe failed with status 1, stdout="", stderr="exit status 3\n"

This can be solved by adding the following lines to /etc/apparmor.d/local/usr.sbin.dnsmasq:

owner /run/user/[0-9]*/containers/cni/dnsname/*/dnsmasq.conf r,
owner /run/user/[0-9]*/containers/cni/dnsname/*/addnhosts r,
owner /run/user/[0-9]*/containers/cni/dnsname/*/pidfile rw,

And then reloading the AppArmor profile:

# apparmor_parser -R /etc/apparmor.d/usr.sbin.dnsmasq
# apparmor_parser /etc/apparmor.d/usr.sbin.dnsmasq

No image found

By default, the registry list is not populated as the files in the package come from upstream. This means that by default, trying to pull any image without specifying the registry will result in an error similar to the following:

Error: short-name "archlinux" did not resolve to an alias and no unqualified-search registries are defined in "/etc/containers/registries.conf"

A starting configuration could be the following:

/etc/containers/registries.conf.d/00-unqualified-search-registries.conf

unqualified-search-registries = ["docker.io"]

/etc/containers/registries.conf.d/01-registries.conf

[[registry]]
location = "docker.io"

This is equivalent to the default docker configuration.

A less convenient alternative, but having a higher compatibility with systems without configured shortnames, use the full registry path in the Containerfile or Dockerfile.

Containerfile

FROM docker.io/archlinux/archlinux

Permission denied: OCI permission denied

$ podman exec openvas_openvas_1 bash

Error: crun: writing file `/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/user.slice/libpod-b3e8048a9b91e43c214b4d850ac7132155a684d6502e12e22ceb6f73848d117a.scope/container/cgroup.procs`: Permission denied: OCI permission denied

Can be solved: BBS#253966

$ env DBUS_SESSION_BUS_ADDRESS= podman ...
$ env DBUS_SESSION_BUS_ADDRESS= podman-compose ...

Pushing images to Docker Hub: access denied/authentication required

When using podman push to push container images to Docker Hub, the following errors could occur: Requested access to the resource is denied or Authentication required. The following hints can help to fix potential issues:

Tag the local image:

# podman tag <localImage> docker.io/<dockerHubUsername>/<dockerHubRepository>:<Tag>

Push the tagged image:

# podman push docker.io/<dockerHubUsername>/<dockerHubRepository>:<Tag> docker://docker.io/<dockerHubUsername>/<dockerHubRepository>:<Tag>

Login to docker.io, the Docker Hub repository and Docker Hub Registry server:

# podman login -u <DockerHubUsername> -p <DockerHubPassword> registry-1.docker.io
# podman login -u <DockerHubUsername> -p <DockerHubPassword> docker.io/<dockerHubUsername>/<dockerHubRepository>
# podman login -u <DockerHubUsername> -p <DockerHubPassword> docker.io

Logout from all registries before the login, e.g.,
```
# podman logout --all
```
Add <dockerHubUsername> as collaborator in the Docker Hub Collaborators tab of the repository

WARN[0000] "/" is not a shared mount, this could cause issues or missing mounts with rootless containers

Buildah/Podman running as rootless expects the bind mount to be shared, check if it is set to private:

$ findmnt -o PROPAGATION /

PROPAGATION
private

In this case see mount(8) § Shared_subtree_operations and set temporarily the mount as shared with:

# mount --make-shared /

To set it permanently edit /etc/fstab and add the shared option to the desired mount and reboot. It will result in a entry like:

/etc/fstab

# <device>                                <dir> <type> <options> <dump> <fsck>
UUID=0a3407de-014b-458b-b5c1-848e92a327a3 /     ext4   defaults,shared   0      1

Containers with restart policy do not start automatically

Start/enable podman-restart.service.