Chapter 7. Appendix: Managing users, groups, SSH keys, and secrets in image mode for RHEL

In package-oriented systems, you can use the derived build to inject users and credentials by using the following command:

RUN useradd someuser

You can find issues in the default shadow-utils implementation of useradd: Users and groups IDs are allocated dynamically, and this can cause drift.

For systems configured with persistent /home /var/home, any changes to /var made in the container image after initial installation will not be applied on subsequent updates.

For example, if you inject /var/home/someuser/.ssh/authorized_keys into a container build, existing systems do not get the updated authorized_keys file.

Use the systemd DynamicUser=yes option where possible for system users.

This is significantly better than the pattern of allocating users or groups at package install time, because it avoids potential UID or GID drift.

Use systemd-sysusers, for example, in your derived build. For more information, see the systemd -sysusers documentation.

COPY mycustom-user.conf /usr/lib/sysusers.d

The sysusers tool makes changes to the traditional /etc/passwd file as necessary during boot time. If /etc is persistent, this can avoid UID or GID drift. It means that the UID or GID allocation depends on how a specific machine was upgraded over time.

With nss-altfiles, you can remove the systemd JSON user records. It splits system users into /usr/lib/passwd and /usr/lib/group, aligning with the way the OSTree project handles the 3 way merge for /etc as it relates to /etc/passwd. Currently, if the /etc/passwd file is modified in any way on the local system, then subsequent changes to /etc/passwd in the container image are not applied.

Base images built by rpm-ostree have nns-altfiles enabled by default.

Also, base images have a system users pre-allocated and managed by the NSS file to avoid UID or GID drift.

In a derived container build, you can also append users to /usr/lib/passwd, for example. Use sysusers.d or DynamicUser=yes.

The filesystem layout depends on the base image.

By default, the user data is stored in both /etc, /etc/passwd, /etc/shadow and groups, and /home, depending on the base image. However, the generic base images have to both be machine-local persistent state. In this model /home is a symlink to /var/home/user.

For base images where /etc and /var are configured to persist by default, you can inject users by using installers such as Anaconda or Kickstart.

Typically, generic installers are designed for one time bootstrap. Then, the configuration becomes a mutable machine-local state that you can change in Day 2 operations, by using some other mechanism.

You can use the Anaconda installer to set the initial password. However, changing this initial password requires a different in-system tool, such as passwd.

These flows work equivalently in a bootc-compatible system, to support users directly installing generic base images, without requiring changes to the different in-system tool.

Many operating system deployments minimize persistent, mutable, and executable state. This can damage user home directories.

The /home directory can be set as tmpfs, to ensure that user data is cleared across reboots. This approach works especially well when combined with a transient /etc directory.

To set up the user’s home directory to, for example, inject SSH authorized_keys or other files, use the systemd tmpfiles.d snippets:

f~ /home/user/.ssh/authorized_keys 600 user user - <base64 encoded data>

SSH is embedded in the image as: /usr/lib/tmpfiles.d/<username-keys.conf. Another example is a service embedded in the image that can fetch keys from the network and write them. This is the pattern used by cloud-init.