systemd/man/systemd-journald.service.xml

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<refentry id="systemd-journald.service">

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

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

  <refnamediv>
    <refname>systemd-journald.service</refname>
    <refname>systemd-journald.socket</refname>
    <refname>systemd-journald-dev-log.socket</refname>
    <refname>systemd-journald-audit.socket</refname>
    <refname>systemd-journald@.service</refname>
    <refname>systemd-journald@.socket</refname>
    <refname>systemd-journald-varlink@.socket</refname>
    <refname>systemd-journald</refname>
    <refpurpose>Journal service</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-journald.service</filename></para>
    <para><filename>systemd-journald.socket</filename></para>
    <para><filename>systemd-journald-dev-log.socket</filename></para>
    <para><filename>systemd-journald-audit.socket</filename></para>
    <para><filename>systemd-journald@.service</filename></para>
    <para><filename>systemd-journald@.socket</filename></para>
    <para><filename>systemd-journald-varlink@.socket</filename></para>
    <para><filename>/usr/lib/systemd/systemd-journald</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-journald</filename> is a system service
    that collects and stores logging data. It creates and maintains
    structured, indexed journals based on logging information that is
    received from a variety of sources:</para>

    <itemizedlist>
      <listitem><para>Kernel log messages, via kmsg</para></listitem>

      <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
      project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
      call</para></listitem>

      <listitem><para>Structured system log messages via the native
      Journal API, see
      <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry></para></listitem>

      <listitem><para>Standard output and standard error of service units. For further details see
      below.</para></listitem>

      <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
    </itemizedlist>

    <para>The daemon will implicitly collect numerous metadata fields
    for each log messages in a secure and unfakeable way. See
    <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    for more information about the collected metadata.
    </para>

    <para>Log data collected by the journal is primarily text-based but can also include binary data where
    necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>

    <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
    volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
    data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
    to volatile storage otherwise. Use <varname>Storage=</varname> in
    <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
    where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>

    <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
    desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
    ensure it has the correct access modes and ownership:</para>

    <programlisting>mkdir -p /var/log/journal
systemd-tmpfiles --create --prefix /var/log/journal</programlisting>

    <para>See
    <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for information about the configuration of this service.</para>
  </refsect1>

  <refsect1>
    <title>Stream logging</title>

    <para>The systemd service manager invokes all service processes with standard output and standard error connected
    to the journal by default. This behaviour may be altered via the
    <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
    journal converts the log byte stream received this way into individual log records, splitting the stream at newline
    (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>

    <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
    services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
    errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
    ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
    write attempts will also result in such process signals being generated, see
    <citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
    To mitigate this issue, systemd service manager explicitly turns off the <constant>SIGPIPE</constant>
    signal for all invoked processes by default (this may be changed for each unit individually via the
    <varname>IgnoreSIGPIPE=</varname> option, see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
    details). After the standard output/standard error streams have been terminated they may not be recovered
    until the services they are associated with are restarted. Note that during normal operation,
    <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in
    the service manager. If <filename>systemd-journald.service</filename> is restarted using
    <command>systemctl restart</command> or equivalent operation instead of a pair of separate
    <command>systemctl stop</command> and <command>systemctl start</command> commands (or equivalent
    operations), these stream connections are not terminated and survive the restart. It is thus safe to
    restart <filename>systemd-journald.service</filename>, but stopping it is not recommended.</para>

    <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
    metadata of the peer the stream was originally created for. If the stream connection is passed on to other
    processes (such as further child processes forked off the main service process), the log records will not reflect
    their metadata, but will continue to describe the original process. This is different from the other logging
    transports listed above, which are inherently record based and where the metadata is always associated with the
    individual record.</para>

    <para>In addition to the implicit standard output/error logging of services, stream logging is also available
    via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
    line tool.</para>

    <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
    4096. When this limit is reached further log streams may be established but will receive
    <constant>EPIPE</constant> right from the beginning.</para>
  </refsect1>

  <refsect1>
    <title>Journal Namespaces</title>

    <para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
    consisting of one or more services from the rest of the system and a mechanism for improving
    performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
    stream managed by its own instance of <command>systemd-journald</command>. Namespaces are independent of
    each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
    exists, managed by <filename>systemd-journald.service</filename> (and its associated socket
    units). Additional namespaces are created by starting an instance of the
    <filename>systemd-journald@.service</filename> service template. The instance name is the namespace
    identifier, which is a short string used for referencing the journal namespace. Service units may be
    assigned to a specific journal namespace through the <varname>LogNamespace=</varname> unit file setting,
    see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
    details. The <option>--namespace=</option> switch of
    <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
    used to view the log stream of a specific namespace. If the switch is not used the log stream of the
    default namespace is shown, i.e. log data from other namespaces is not visible.</para>

    <para>Services associated with a specific log namespace may log via syslog, the native logging protocol
    of the journal and via stdout/stderr; the logging from all three transports is associated with the
    namespace.</para>

    <para>By default only the default namespace will collect kernel and audit log messages.</para>

    <para>The <command>systemd-journald</command> instance of the default namespace is configured through
    <filename>/etc/systemd/journald.conf</filename> (see below), while the other instances are configured
    through <filename>/etc/systemd/journald@<replaceable>NAMESPACE</replaceable>.conf</filename>. The journal
    log data for the default namespace is placed in
    <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable></filename> (see below) while the data
    for the other namespaces is located in
    <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable>.<replaceable>NAMESPACE</replaceable></filename>.</para>
  </refsect1>

  <refsect1>
    <title>Signals</title>

    <variablelist>
      <varlistentry>
        <term>SIGUSR1</term>

        <listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
        <filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
        after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run</filename> is
        never flushed to <filename>/var</filename> regardless of the configuration. Use the
        <command>journalctl --flush</command> command to request flushing of the journal files, and wait for
        the operation to complete. See
        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>SIGUSR2</term>

        <listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
        --rotate</command> command to request journal file rotation, and wait for the operation to
        complete.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>SIGRTMIN+1</term>

        <listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
        --sync</command> command to trigger journal synchronization, and wait for the operation to
        complete.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Kernel Command Line</title>

    <para>A few configuration parameters from
    <filename>journald.conf</filename> may be overridden on the kernel
    command line:</para>

    <variablelist class='kernel-commandline-options'>
      <varlistentry>
        <term><varname>systemd.journald.forward_to_syslog=</varname></term>
        <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
        <term><varname>systemd.journald.forward_to_console=</varname></term>
        <term><varname>systemd.journald.forward_to_wall=</varname></term>

        <listitem><para>Enables/disables forwarding of collected log
        messages to syslog, the kernel log buffer, the system console
        or wall.
        </para>

        <para>See
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for information about these settings.</para>
        </listitem>

      </varlistentry>
    </variablelist>

    <para>Note that these kernel command line options are only honoured by the default namespace, see
    above.</para>
  </refsect1>

  <refsect1>
    <title>Access Control</title>

    <para>Journal files are, by default, owned and readable by the
    <literal>systemd-journal</literal> system group but are not
    writable. Adding a user to this group thus enables them to read
    the journal files.</para>

    <para>By default, each user, with a UID outside the range of system users,
    dynamic service users, and the nobody user, will get their own set of
    journal files in <filename>/var/log/journal/</filename>. See
    <ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
    for more details about UID ranges. These journal
    files will not be owned by the user, however, in order to avoid
    that the user can write to them directly. Instead, file system
    ACLs are used to ensure the user gets read access only.</para>

    <para>Additional users and groups may be granted access to journal
    files via file system access control lists (ACL). Distributions
    and administrators may choose to grant read access to all members
    of the <literal>wheel</literal> and <literal>adm</literal> system
    groups with a command such as the following:</para>

    <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>

    <para>Note that this command will update the ACLs both for
    existing journal files and for future journal files created in the
    <filename>/var/log/journal/</filename> directory.</para>
  </refsect1>

  <refsect1>
    <title>Files</title>

    <variablelist>
      <varlistentry>
        <term><filename>/etc/systemd/journald.conf</filename></term>

        <listitem><para>Configure <command>systemd-journald</command> behavior. See
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
        <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
        <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
        <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>

        <listitem><para><command>systemd-journald</command> writes entries to files in
        <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
        or
        <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
        with the <literal>.journal</literal> suffix. If the daemon is
        stopped uncleanly, or if the files are found to be corrupted,
        they are renamed using the <literal>.journal~</literal>
        suffix, and <command>systemd-journald</command> starts writing
        to a new file. <filename>/run</filename> is used when
        <filename>/var/log/journal</filename> is not available, or
        when <option>Storage=volatile</option> is set in the
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        configuration file.</para>

        <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
        it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
        (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
        Such files are "archived" and will not be written to any more.</para>

        <para>In general, it is safe to read or copy any journal file (active or archived).
        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        and the functions in the
        <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        library should be able to read all entries that have been fully written.</para>

        <para><filename>systemd-journald</filename> will automatically remove the oldest
        archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
        and related settings in
        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/dev/kmsg</filename></term>
        <term><filename>/dev/log</filename></term>
        <term><filename>/run/systemd/journal/dev-log</filename></term>
        <term><filename>/run/systemd/journal/socket</filename></term>
        <term><filename>/run/systemd/journal/stdout</filename></term>

        <listitem><para>Sockets and other file node paths that <command>systemd-journald</command> will
        listen on and are visible in the file system. In addition to these,
        <command>systemd-journald</command> can listen for audit events using <citerefentry
        project='man-pages'><refentrytitle>netlink</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
      </varlistentry>
    </variablelist>

    <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <command>pydoc systemd.journal</command>
    </para>
  </refsect1>

</refentry>