Running without root privileges using Podman

Docker traditionally ran as the root user. Users who wanted to run docker containers needed to be given sudo access and use sudo docker, or be added to the docker group, so they could run docker without typing sudo first. In both cases, they were running docker with root privileges. This is considered a bad security practice, because it effectively grants root host privileges to all docker users. However, namespaces and control groups where not as mature as they are now, and no better alternative was available.

However today docker offers the possibility to run in rootless mode. Podman runs rootless by design.

Podman or Docker?

Podman 4.7 and above includes an extended syntax for --uidmap and --gidmap that makes it straightforward to map additional groups. This feature was contributed by a rocker user, so you are encouraged to try it!

Running a container rootless does not mean that the container does not have any root-like capabilities, it means that the container engine does not run as root. For most rocker-related projects, running rootless is a security advantage.

1 Who am I?

At the host:

Terminal

whoami
# sergio

In the container:

Terminal

podman run --rm docker.io/rocker/rstudio whoami
# root

2 Using apt-get inside a rootless container

It is perfectly possible to run apt-get commands on a rootless container, because it just modifies files inside the container.

At the host:

Terminal

apt-get update
# Reading package lists... Done
# E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)

In the container:

Terminal

podman run --rm docker.io/rocker/rstudio apt-get update
# Get:1 http://security.ubuntu.com/ubuntu jammy-security InRelease [110 kB]
# ...
# Fetched 26.8 MB in 6s (4,750 kB/s)
# Reading package lists...

3 Modifying files

You can bind mount the /etc/ directory (e.g. using -v /etc:/hostetc) but you won’t be able to modify most of its files, since you are not allowed to do that when you are outside the container.

At the host:

Terminal

touch /etc/try-creating-a-file
# touch: cannot touch '/etc/try-creating-a-file': Permission denied

In the container: Rootless means no additional host permissions

Terminal

podman run --rm -v /etc/:/hostetc docker.io/rocker/rstudio \
  touch /hostetc/try-creating-a-file
# touch: cannot touch '/hostetc/try-creating-a-file': Permission denied

However, you can modify the files within the container:

Terminal

podman run --rm docker.io/rocker/rstudio touch /etc/try-creating-a-file

And files from mounted volumes, assuming you have the permissions where they are mounted at the host:

Terminal

podman run \
    --rm \
    --volume "$HOME/workdir:/workdir" \
    docker.io/rocker/rstudio touch /workdir/try-creating-a-file
ls "$HOME/workdir/try-creating-a-file"
rm "$HOME/workdir/try-creating-a-file"

Your user in the host is mapped to the root user in the container.

4 Port binding

You can’t bind your container to host ports lower than 1024, since those are reserved to root (or to be precise reserved to processes with CAP_NET_BIND_SERVICE capability set).

Terminal

podman run --rm -p 80:8787 docker.io/rocker/rstudio
# Error: rootlessport cannot expose privileged port 80, you can add 
# 'net.ipv4.ip_unprivileged_port_start=80' to /etc/sysctl.conf (currently 1024),
# or choose a larger port number (>= 1024):
# listen tcp 0.0.0.0:80: bind: permission denied

However larger port numbers work perfectly fine:

Terminal

podman run --rm -p 8787:8787 docker.io/rocker/rstudio

5 Mounting shared data from an additional group

You may want to mount a directory from a group you belong to, to be able to read and write into it. Let’s say you are ana and you belong to the rfriends group in the host. That group has access to the /shared_data folder, that you would like to access from your container.

Tip

To run R code or an R script using rocker accessing a shared directory, you can skip the instructions below and manage to work on the command line. However, you won’t be able to access that directory if you try to login from RStudio’s web browser. It will only work from process launched from the command line.

Terminal

podman run 
  -ti \
  --rm \
  --group-add keep-groups \
  -v /shared_dir:/shared_dir \
  docker.io/rocker/rstudio R

Find out the group ID (GID) of the rfriends group.
```
Terminal
```
```
getent group rfriends
rfriends:x:2000:ana,sergio
```
The GID is 2000, and both ana and sergio belong to it.
Subordinate that GID to your user. You will need administrative permissions:
```
Terminal
```
```
sudo usermod --add-subgids 2000-2000 ana
```
Update your Podman rootless namespace:
```
Terminal
```
```
podman system migrate
```

You are now able to map the group in the container. How? That depends on your Podman version:

5.1 Podman versions 4.7 and above

To run your container mapping your host GID 2000 to a container GID combine the --group-add keep-groups with the --gidmap option:

Terminal

podman run \
    --rm \
    --group-add keep-groups \
    --gidmap="+g102000:@2000" \
    --volume /shared_dir:/shared_dir \
    docker.io/rocker/rstudio

You will have used --group-add keep-groups so the user in the container still belongs to the subordinated group. The --gidmap argument takes care of appending the mapping of group 2000 from the host to group 102000 in the container. Additioning 100 000 to your GID is an easy way to remember the container GID and avoid collisions with lower container GIDs.

5.2 Podman versions below 4.7

The command will look like:

Terminal

podman run  \
    --rm  \
    --group-add keep-groups \
    --uidmap "0:0:65535"  \
    --gidmap "0:0:1" \
    --gidmap "102000:1:1" \
    --gidmap "1:2:60000" \
    --volume /shared_dir:/shared_dir \
    docker.io/rocker/rstudio

You can notice several differences in the idmapping command:

You must provide a default user id mapping: --uidmap "0:0:65535"
You must provide a full group id mapping:
- The group id mapping should map intermediate GID 0 to container GID 0. --gidmap "0:0:1" This maps your user to root.
- You must find out the intermediate GID mapping for the GID you want to map (using podman unshare cat /proc/self/gid_map).
```
Terminal
```
```
podman unshare cat /proc/self/gid_map
#          0       1000          1
#          1       2000          1
#          2     100000      65536
```
  By looking at the table above, you can find host GID 2000 in the middle column and see it is mapped to intermediate id 1 in the left column.
  
  So your mapping must include intermediate GID 1 to container GID 102000: --gidmap 102000:1:1
- And you must map container IDs from 1 to n, using free intermediate GIDs. Here we map 60000: --gidmap "1:2:60000".

And happy coding!