Strip privileged capabilities from a systemd service using CapabilityBoundingSet and AmbientCapabilities
domain: systemd · 6 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Decide the minimum capability set the binary needs (common: no capabilities for a plain worker, CAP_NET_BIND_SERVICE to bind to a port <1024)
Add CapabilityBoundingSet= to the [Service] section to drop everything: CapabilityBoundingSet= or CapabilityBoundingSet=CAP_NET_BIND_SERVICE CAP_DAC_OVERRIDE
Set AmbientCapabilities= to the same nets you want actually raised at exec time (e.g. AmbientCapabilities=CAP_NET_BIND_SERVICE) - bounding set alone only caps the ceiling
Pair with User=nobody or DynamicUser=yes and NoNewPrivileges=yes; with NoNewPrivileges=yes, ambient capabilities cannot be granted afterward, so set them correctly up front
Run systemctl daemon-reload and restart; verify with grep Cap /proc/<pid>/status (CapBnd, CapPrm, CapAmb lines) or capsh --print inside the service
Keep the bounding set as tight as possible; drop CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_SYS_PTRACE unless strictly required
Known gotchas
CAP_SYS_CHROOT, CAP_SYS_ADMIN, CAP_DAC_OVERRIDE and CAP_NET_ADMIN are commonly left set by default - you must actively clear them via the bounding set for real isolation
Ambient capabilities are reset across execve unless raised in AmbientCapabilities=, so a root-suid helper will not automatically re-gain them
NoNewPrivileges=yes disallows upgrades via execve; combine with AmbientCapabilities= so the intended caps survive the exec
Bounding set is inherited but a misconfigured set can break services that legitimately need capabilities - always test start-after with journalctl -u <name>
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