/etc/subuid and /etc/subgid

Rootless Containers implementations mostly expect /etc/subuid to contain at least 65,536 subuids.

In the following example, 65,536 subuids (100000-165535) are allocated for a user named “user1”.

$ cat /etc/subuid
user1:100000:65536

The same applies to subgids defined in /etc/subgid. See also How it works/User Namespaces.

These subuids and subgids are typically automatically configured by the system.

If subuids and subgids are not configured, you need to edit /etc/subuid and /etc/subgid directly with a text editor:

$ sudo vi /etc/subuid

newuidmap and newgidmap needs to be installed on the host. These binaries are typically installed by default.

$ sudo apt-get install -y uidmap

$ sudo dnf install -y shadow-utils