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<refentry id="systemd-nsresourced.service" conditional='ENABLE_NSRESOURCED'>

  <refentryinfo>
    <title>systemd-nsresourced.service</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-nsresourced.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-nsresourced.service</refname>
    <refname>systemd-nsresourced</refname>
    <refpurpose>User Namespace Resource Delegation Service</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-nsresourced.service</filename></para>
    <para><filename>/usr/lib/systemd/systemd-nsresourced</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>systemd-nsresourced</command> is a system service that permits transient delegation of a a
    UID/GID range to a user namespace (see <citerefentry
    project='man-pages'><refentrytitle>user_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>)
    allocated by a client, via a Varlink IPC API.</para>

    <para>Unprivileged clients may allocate a user namespace, and then request a UID/GID range to be assigned
    to it via this service. The user namespace may then be used to run containers and other sandboxes, and/or
    apply it to an id-mapped mount.</para>

    <para>Allocations of UIDs/GIDs this way are transient: when a user namespace goes away, its UID/GID range
    is returned to the pool of available ranges. In order to ensure that clients cannot gain persistency in
    their transient UID/GID range a BPF-LSM based policy is enforced that ensures that user namespaces set up
    this way can only write to file systems they allocate themselves or that are explicitly allowlisted via
    <command>systemd-nsresourced</command>.</para>

    <para><command>systemd-nsresourced</command> automatically ensures that any registered UID ranges show up
    in the system's NSS database via the <ulink url="https://systemd.io/USER_GROUP_API">User/Group Record
    Lookup API via Varlink</ulink>.</para>

    <para>Currently, only UID/GID ranges consisting of either exactly 1 or exactly 65536 UIDs/GIDs can be
    registered with this service. Moreover, UIDs and GIDs are always allocated together, and
    symmetrically.</para>

    <para>The service provides API calls to allowlist mounts (referenced via their mount file descriptors as
    per Linux <function>fsmount()</function> API), to pass ownership of a cgroup subtree to the user
    namespace and to delegate a virtual Ethernet device pair to the user namespace. When used in combination
    this is sufficient to implement fully unprivileged container environments, as implemented by
    <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>, fully
    unprivileged <varname>RootImage=</varname> (see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>) or
    fully unprivileged disk image tools such as
    <citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>

    <para>This service provides one <ulink url="https://varlink.org/">Varlink</ulink> service:
    <constant>io.systemd.NamespaceResource</constant> allows registering user namespaces, and assign mounts,
    cgroups and network interfaces to it.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-mountfsd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>user_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>
