Isolate a service in a private filesystem namespace with RootDirectory/MountAPIVFS/PrivateDevices instead of trusting host paths
domain: systemd · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Choose a base dir containing the service's files, e.g. /srv/app (must hold the binary and its minimal libs, or use a distro/chroot-style tree)
Add RootDirectory=/srv/app to the [Service] section so the service runs with pivot_root() into that directory as its new root
Add MountAPIVFS=yes so /proc, /sys, /dev, /run are mounted into the private namespace with defaults
Add PrivateDevices=yes and PrivateTmp=yes to further restrict what the namespace exposes; consider ProtectSystem=strict for read-only root
If the base dir is a partition or filesystem image, use RootImage=/path/to/image.ext4 or .raw instead of RootDirectory
Re-run systemctl daemon-reload, restart the unit, and verify the service cannot see host files outside the new root (ls / inside the service process)
Combine with BindReadOnlyPaths= to hand the service read-only copies of specific host files it must see
Known gotchas
RootDirectory= only works for system services (root) and, when set, hard-coded host paths in the binary will be relative to the new root - the payload must be self-contained or you get 'No such file'
MountAPIVFS=yes must be set together with RootDirectory= or the new root has no /proc making many binaries and systemd features hang
DynamicUser=yes and RootDirectory= both change the namespace; using PrivateUsers=yes with RootDirectory= adds an extra user-namespace layer and requires unprivileged userns support (kernel.unprivileged_userns_clone)
Per-user instances disable mount option overrides by default for security (polkit may be required to allow specific mount options)
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