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docs: clarify subuid/subgid mapping in rootless vs userns-remap #24845

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docs: clarify subuid/subgid mapping in rootless vs userns-remap #24845

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docs: clarify subuid/subgid mapping in rootless vs userns-remap
lacatoire Apr 21, 2026
2d4ef46
docs: move UID/GID mapping details to a subpage
lacatoire Apr 21, 2026
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15 changes: 15 additions & 0 deletions content/manuals/engine/security/rootless/_index.md
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Expand Up @@ -20,6 +20,21 @@ 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.

The two modes also differ in how they map container UIDs and GIDs to the host:

- In `userns-remap` mode, container UID `0` is mapped to the first subordinate
UID listed in `/etc/subuid` for the remap user, and container UID `n` is
mapped to `subuid + n`.
- In rootless mode, container UID `0` is mapped to the host UID of the user
running rootless Docker (the result of `id -u`); container UID `n` (for
`n >= 1`) is mapped to `subuid + (n - 1)`.

GIDs follow the same rules using `/etc/subgid`.

This difference matters when setting file permissions on bind-mounted
directories: in rootless mode, files owned by your host user appear as owned
by `root` inside the container.

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@AkihiroSuda AkihiroSuda Apr 21, 2026

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The changes should be a subpage to avoid inflating the amount of the contents in a single page

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.
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