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> and <literal>group:</literal> in <filename>/etc/nsswitch.conf</filename>.</para>

    <para>It is recommended to place <literal>systemd</literal> after the <literal>files</literal> or
    <literal>compat</literal> entry of the <filename>/etc/nsswitch.conf</filename> lines so that
    <filename>/etc/passwd</filename> and <filename>/etc/group</filename> based mappings take precedence.</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:         compat <command>systemd</command>
group:          compat [SUCCESS=merge] <command>systemd</command>
shadow:         compat

hosts:          mymachines resolve [!UNAVAIL=return] myhostname files 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>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-mymachines</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-userdbd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-homed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>nsswitch.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>getent</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>