systemd/man/nss-systemd.xml

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<refentry id="nss-systemd" conditional='ENABLE_NSS_SYSTEMD'>

  <refentryinfo>
    <title>nss-systemd</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>nss-systemd</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>nss-systemd</refname>
    <refname>libnss_systemd.so.2</refname>
    <refpurpose>UNIX user and group name resolution for user/group lookup via Varlink</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>libnss_systemd.so.2</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>nss-systemd</command> is a plug-in module for the GNU Name Service Switch (NSS)
    functionality of the GNU C Library (<command>glibc</command>), providing UNIX user and group name
    resolution for services implementing the <ulink url="https://systemd.io/USER_GROUP_API">User/Group Record
    Lookup API via Varlink</ulink>, such as the system and service manager
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> (for its
    <varname>DynamicUser=</varname> feature, see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
    details),
    <citerefentry><refentrytitle>systemd-homed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, or <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>

    <para>This module also ensures that the root and nobody users and groups (i.e. the users/groups with the UIDs/GIDs
    0 and 65534) remain resolvable at all times, even if they aren't listed in <filename>/etc/passwd</filename> or
    <filename>/etc/group</filename>, or if these files are missing.</para>

    <para>This module preferably utilizes
    <citerefentry><refentrytitle>systemd-userdbd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    for resolving users and groups, but also works without the service running.</para>

    <para>To activate the NSS module, add <literal>systemd</literal> to the lines starting with
    <literal>passwd:</literal>, <literal>group:</literal>, <literal>shadow:</literal> and
    <literal>gshadow:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>

    <para>It is recommended to place <literal>systemd</literal> after the <literal>files</literal> entry of
    the <filename>/etc/nsswitch.conf</filename> lines so that <filename>/etc/passwd</filename>,
    <filename>/etc/group</filename>, <filename>/etc/shadow</filename> and <filename>/etc/gshadow</filename>
    based mappings take precedence.</para>
  </refsect1>

  <refsect1>
    <title>Static Drop-In JSON User/Group Records</title>

    <para>Besides user/group records acquired via the aforementioned Varlink IPC interfaces and the
    synthesized root and nobody accounts, this module also makes user and group accounts available to the
    system that are defined in static drop-in files in the <filename>/etc/userdb/</filename>,
    <filename>/run/userdb/</filename>, <filename>/run/host/userdb/</filename> and
    <filename>/usr/lib/userdb/</filename> directories.</para>

    <para>This is a simple mechanism to provide static user and group records via JSON drop-in files. Such
    user records should be defined in the format described by the <ulink
    url="https://systemd.io/USER_RECORD">JSON User Records</ulink> specification and be placed in one of the
    aforementioned directories under a file name composed of the user name suffixed with
    <filename>.user</filename>, with a world-readable access mode. A symlink named after the user record's
    UID formatted in decimal and suffixed with <filename>.user</filename> pointing to the primary record file
    should be created as well, in order to allow both lookups by username and by UID. Privileged user record
    data (e.g. hashed UNIX passwords) may optionally be provided as well, in a pair of separate companion
    files with the <filename>.user-privileged</filename> suffix. The data should be stored in a regular file
    named after the user name, suffixed with <filename>.user-privileged</filename>, and a symlink pointing to
    it, named after the used numeric UID formatted in decimal with the same suffix. These companion files
    should not be readable to anyone but root. Example:</para>

    <programlisting>-rw-r--r--. 1 root root  723 May 10 foobar.user
-rw-------. 1 root root  123 May 10 foobar.user-privileged
lrwxrwxrwx. 1 root root   19 May 10 4711.user -> foobar.user
lrwxrwxrwx. 1 root root   19 May 10 4711.user-privileged -> foobar.user-privileged</programlisting>

    <para>Similarly, group records following the format described in <ulink
    url="https://systemd.io/GROUP_RECORD">JSON Group Record</ulink> may be defined, using the file suffixes
    <filename>.group</filename> and <filename>.group-privileged</filename>.</para>

    <para>The primary user/group record files (i.e. those with the <filename>.user</filename> and
    <filename>.group</filename> suffixes) should not contain the <literal>privileged</literal> section as
    described in the specifications. The privileged user/group record files (i.e. those with the
    <filename>.user-privileged</filename> and <filename>.group-privileged</filename> suffixes) should
    contain this section, exclusively.</para>

    <para>In addition to the two types of user record files and the two types of group record files there's a
    fifth type of file that may be placed in the searched directories: files that indicate membership of
    users in groups. Specifically, for every pair of user/group where the user shall be a member of a group a
    file named
    <literal><replaceable>username</replaceable>:<replaceable>groupname</replaceable>.membership</literal>
    should be created, i.e. the textual UNIX user name, followed by a colon, followed by the textual UNIX
    group name, suffixed by <literal>.membership</literal>. The contents of these files are currently not
    read, and the files should be created empty. The mere existence of these files is enough to effect a
    user/group membership. If a program provides user and/or group record files in the searched directories,
    it should always also create such files, both for primary and auxiliary group memberships.</para>

    <para>Note that static user/group records generally do not override conflicting records in
    <filename>/etc/passwd</filename> or <filename>/etc/group</filename> or other account databases. In fact,
    before dropping in these files a reasonable level of care should be taken to avoid user/group name and
    UID/GID conflicts.</para>
  </refsect1>

  <refsect1>
    <title>Configuration in <filename>/etc/nsswitch.conf</filename></title>

    <para>Here is an example <filename>/etc/nsswitch.conf</filename> file that enables
    <command>nss-systemd</command> correctly:</para>

    <!-- synchronize with other nss-* man pages and factory/etc/nsswitch.conf -->
    <programlisting>passwd:         files <command>systemd</command>
group:          files <command>[SUCCESS=merge] systemd</command>
shadow:         files <command>systemd</command>
gshadow:        files <command>systemd</command>

hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis</programlisting>

  </refsect1>

  <refsect1>
    <title>Example: Mappings provided by <filename>systemd-machined.service</filename></title>

    <para>The container <literal>rawhide</literal> is spawned using
    <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>:
    </para>

    <programlisting># systemd-nspawn -M rawhide --boot --network-veth --private-users=pick
Spawning container rawhide on /var/lib/machines/rawhide.
Selected user namespace base 20119552 and range 65536.
...

$ machinectl --max-addresses=3
MACHINE CLASS     SERVICE        OS     VERSION ADDRESSES
rawhide container systemd-nspawn fedora 30      169.254.40.164 fe80::94aa:3aff:fe7b:d4b9

$ getent passwd vu-rawhide-0 vu-rawhide-81
vu-rawhide-0:*:20119552:65534:vu-rawhide-0:/:/usr/sbin/nologin
vu-rawhide-81:*:20119633:65534:vu-rawhide-81:/:/usr/sbin/nologin

$ getent group vg-rawhide-0 vg-rawhide-81
vg-rawhide-0:*:20119552:
vg-rawhide-81:*:20119633:

$ ps -o user:15,pid,tty,command -e|grep '^vu-rawhide'
vu-rawhide-0      692 ?        /usr/lib/systemd/systemd
vu-rawhide-0      731 ?        /usr/lib/systemd/systemd-journald
vu-rawhide-192    734 ?        /usr/lib/systemd/systemd-networkd
vu-rawhide-193    738 ?        /usr/lib/systemd/systemd-resolved
vu-rawhide-0      742 ?        /usr/lib/systemd/systemd-logind
vu-rawhide-81     744 ?        /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
vu-rawhide-0      746 ?        /usr/sbin/sshd -D ...
vu-rawhide-0      752 ?        /usr/lib/systemd/systemd --user
vu-rawhide-0      753 ?        (sd-pam)
vu-rawhide-0     1628 ?        login -- zbyszek
vu-rawhide-1000  1630 ?        /usr/lib/systemd/systemd --user
vu-rawhide-1000  1631 ?        (sd-pam)
vu-rawhide-1000  1637 pts/8    -zsh
</programlisting>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-userdbd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-homed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>