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<refentry id="systemd.journal-fields">

  <refentryinfo>
    <title>systemd.journal-fields</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.journal-fields</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.journal-fields</refname>
    <refpurpose>Special journal fields</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>Entries in the journal resemble an environment block in
    their syntax but with fields that can include binary data.
    Primarily, fields are formatted UTF-8 text strings, and binary
    formatting is used only where formatting as UTF-8 text strings
    makes little sense. New fields may freely be defined by
    applications, but a few fields have special meaning. All fields
    with special meanings are optional. In some cases, fields may
    appear more than once per entry.</para>
  </refsect1>

  <refsect1>
    <title>User Journal Fields</title>

    <para>User fields are fields that are directly passed from clients
    and stored in the journal.</para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>MESSAGE=</varname></term>
        <listitem>
          <para>The human-readable message string for this entry. This
          is supposed to be the primary text shown to the user. It is
          usually not translated (but might be in some cases), and is
          not supposed to be parsed for metadata.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MESSAGE_ID=</varname></term>
        <listitem>
          <para>A 128-bit message identifier ID for recognizing
          certain message types, if this is desirable. This should
          contain a 128-bit ID formatted as a lower-case hexadecimal
          string, without any separating dashes or suchlike. This is
          recommended to be a UUID-compatible ID, but this is not
          enforced, and formatted differently. Developers can generate
          a new ID for this purpose with <command>journalctl
          <option>--new-id128</option></command>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PRIORITY=</varname></term>
        <listitem>
          <para>A priority value between 0 (<literal>emerg</literal>)
          and 7 (<literal>debug</literal>) formatted as a decimal
          string. This field is compatible with syslog's priority
          concept.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>CODE_FILE=</varname></term>
        <term><varname>CODE_LINE=</varname></term>
        <term><varname>CODE_FUNC=</varname></term>
        <listitem>
          <para>The code location generating this message, if known.
          Contains the source filename, the line number and the
          function name.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ERRNO=</varname></term>
        <listitem>
          <para>The low-level Unix error number causing this entry, if
          any. Contains the numeric value of
          <citerefentry project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          formatted as a decimal string.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SYSLOG_FACILITY=</varname></term>
        <term><varname>SYSLOG_IDENTIFIER=</varname></term>
        <term><varname>SYSLOG_PID=</varname></term>
        <listitem>
          <para>Syslog compatibility fields containing the facility
          (formatted as decimal string), the identifier string (i.e.
          "tag"), and the client PID. (Note that the tag is usually
          derived from glibc's
          <varname>program_invocation_short_name</varname> variable,
          see
          <citerefentry project='die-net'><refentrytitle>program_invocation_short_name</refentrytitle><manvolnum>3</manvolnum></citerefentry>.)</para>
        </listitem>

      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Trusted Journal Fields</title>

    <para>Fields prefixed with an underscore are trusted fields, i.e.
    fields that are implicitly added by the journal and cannot be
    altered by client code.</para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>_PID=</varname></term>
        <term><varname>_UID=</varname></term>
        <term><varname>_GID=</varname></term>
        <listitem>
          <para>The process, user, and group ID of the process the
          journal entry originates from formatted as a decimal
          string. Note that entries obtained via <literal>stdout</literal> or
          <literal>stderr</literal> of forked processes will contain credentials valid for a parent
          process (that initiated the connection to <command>systemd-journald</command>).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_COMM=</varname></term>
        <term><varname>_EXE=</varname></term>
        <term><varname>_CMDLINE=</varname></term>
        <listitem>
          <para>The name, the executable path, and the command line of
          the process the journal entry originates from.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_CAP_EFFECTIVE=</varname></term>
        <listitem>
          <para>The effective
          <citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
          of the process the journal entry originates from.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_AUDIT_SESSION=</varname></term>
        <term><varname>_AUDIT_LOGINUID=</varname></term>
        <listitem>
          <para>The session and login UID of the process the journal
          entry originates from, as maintained by the kernel audit
          subsystem.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_SYSTEMD_CGROUP=</varname></term>
        <term><varname>_SYSTEMD_SLICE=</varname></term>
        <term><varname>_SYSTEMD_UNIT=</varname></term>
        <term><varname>_SYSTEMD_USER_UNIT=</varname></term>
        <term><varname>_SYSTEMD_SESSION=</varname></term>
        <term><varname>_SYSTEMD_OWNER_UID=</varname></term>

        <listitem>
          <para>The control group path in the systemd hierarchy, the
          the systemd slice unit name, the systemd unit name, the
          unit name in the systemd user manager (if any), the systemd
          session ID (if any), and the owner UID of the systemd user
          unit or systemd session (if any) of the process the journal
          entry originates from.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_SELINUX_CONTEXT=</varname></term>
        <listitem>
          <para>The SELinux security context (label) of the process
          the journal entry originates from.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_SOURCE_REALTIME_TIMESTAMP=</varname></term>
        <listitem>
          <para>The earliest trusted timestamp of the message, if any
          is known that is different from the reception time of the
          journal. This is the time in microseconds since the epoch
          UTC, formatted as a decimal string.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_BOOT_ID=</varname></term>
        <listitem>
          <para>The kernel boot ID for the boot the message was
          generated in, formatted as a 128-bit hexadecimal
          string.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_MACHINE_ID=</varname></term>
        <listitem>
          <para>The machine ID of the originating host, as available
          in
          <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_SYSTEMD_INVOCATION_ID=</varname></term>
        <listitem>
          <para>The invocation ID for the runtime cycle of the unit
          the message was generated in, as available to processes
          of the unit in <varname>$INVOCATION_ID</varname> (see
          <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_HOSTNAME=</varname></term>
        <listitem>
          <para>The name of the originating host.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>_TRANSPORT=</varname></term>
        <listitem>
          <para>How the entry was received by the journal service.
          Valid transports are:
          </para>
          <variablelist>
            <varlistentry>
              <term>
                <option>audit</option>
              </term>
              <listitem>
                <para>for those read from the kernel audit subsystem
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>
                <option>driver</option>
              </term>
              <listitem>
                <para>for internally generated messages
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>
                <option>syslog</option>
              </term>
              <listitem>
                <para>for those received via the local syslog socket
                with the syslog protocol
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>
                <option>journal</option>
              </term>
              <listitem>
                <para>for those received via the native journal
                protocol
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>
                <option>stdout</option>
              </term>
              <listitem>
                <para>for those read from a service's standard output
                or error output
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>
                <option>kernel</option>
              </term>
              <listitem>
                <para>for those read from the kernel
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_STREAM_ID=</varname></term>
        <listitem>
          <para>Only applies to <literal>_TRANSPORT=stdout</literal> records: specifies a randomized 128bit ID assigned
          to the stream connection when it was first created. This ID is useful to reconstruct individual log streams
          from the log records: all log records carrying the same stream ID originate from the same stream.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_LINE_BREAK=</varname></term>
        <listitem>
          <para>Only applies to <literal>_TRANSPORT=stdout</literal> records: indicates that the log message in the
          standard output/error stream was not terminated with a normal newline character (<literal>\n</literal>,
          i.e. ASCII 10). Specifically, when set this field is one of <option>nul</option> (in case the line was
          terminated by a NUL byte), <option>line-max</option> (in case the maximum log line length was reached, as
          configured with <varname>LineMax=</varname> in
          <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) or
          <option>eof</option> (if this was the last log record of a stream and the stream ended without a final
          newline character). Note that this record is not generated when a normal newline character was used for
          marking the log line end.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Kernel Journal Fields</title>

    <para>Kernel fields are fields that are used by messages
    originating in the kernel and stored in the journal.</para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>_KERNEL_DEVICE=</varname></term>
        <listitem>
          <para>The kernel device name. If the entry is associated to
          a block device, the major and minor of the device node,
          separated by <literal>:</literal> and prefixed by
          <literal>b</literal>. Similar for character devices but
          prefixed by <literal>c</literal>. For network devices, this
          is the interface index prefixed by <literal>n</literal>. For
          all other devices, this is the subsystem name prefixed by
          <literal>+</literal>, followed by <literal>:</literal>,
          followed by the kernel device name.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_KERNEL_SUBSYSTEM=</varname></term>
        <listitem>
          <para>The kernel subsystem name.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_UDEV_SYSNAME=</varname></term>
        <listitem>
          <para>The kernel device name as it shows up in the device
          tree below <filename>/sys</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_UDEV_DEVNODE=</varname></term>
        <listitem>
          <para>The device node path of this device in
          <filename>/dev</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>_UDEV_DEVLINK=</varname></term>
        <listitem>
          <para>Additional symlink names pointing to the device node
          in <filename>/dev</filename>. This field is frequently set
          more than once per entry.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Fields to log on behalf of a different program</title>

    <para>Fields in this section are used by programs to specify that
    they are logging on behalf of another program or unit.
    </para>

    <para>Fields used by the <command>systemd-coredump</command>
    coredump kernel helper:
    </para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>COREDUMP_UNIT=</varname></term>
        <term><varname>COREDUMP_USER_UNIT=</varname></term>
        <listitem>
          <para>Used to annotate messages containing coredumps from
          system and session units. See
          <citerefentry><refentrytitle>coredumpctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>Privileged programs (currently UID 0) may attach
    <varname>OBJECT_PID=</varname> to a message. This will instruct
    <command>systemd-journald</command> to attach additional fields on
    behalf of the caller:</para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>OBJECT_PID=<replaceable>PID</replaceable></varname></term>
        <listitem>
          <para>PID of the program that this message pertains to.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>OBJECT_UID=</varname></term>
        <term><varname>OBJECT_GID=</varname></term>
        <term><varname>OBJECT_COMM=</varname></term>
        <term><varname>OBJECT_EXE=</varname></term>
        <term><varname>OBJECT_CMDLINE=</varname></term>
        <term><varname>OBJECT_AUDIT_SESSION=</varname></term>
        <term><varname>OBJECT_AUDIT_LOGINUID=</varname></term>
        <term><varname>OBJECT_SYSTEMD_CGROUP=</varname></term>
        <term><varname>OBJECT_SYSTEMD_SESSION=</varname></term>
        <term><varname>OBJECT_SYSTEMD_OWNER_UID=</varname></term>
        <term><varname>OBJECT_SYSTEMD_UNIT=</varname></term>
        <term><varname>OBJECT_SYSTEMD_USER_UNIT=</varname></term>
        <listitem>
          <para>These are additional fields added automatically by
          <command>systemd-journald</command>. Their meaning is the
          same as
          <varname>_UID=</varname>,
          <varname>_GID=</varname>,
          <varname>_COMM=</varname>,
          <varname>_EXE=</varname>,
          <varname>_CMDLINE=</varname>,
          <varname>_AUDIT_SESSION=</varname>,
          <varname>_AUDIT_LOGINUID=</varname>,
          <varname>_SYSTEMD_CGROUP=</varname>,
          <varname>_SYSTEMD_SESSION=</varname>,
          <varname>_SYSTEMD_UNIT=</varname>,
          <varname>_SYSTEMD_USER_UNIT=</varname>, and
          <varname>_SYSTEMD_OWNER_UID=</varname>
          as described above, except that the process identified by
          <replaceable>PID</replaceable> is described, instead of the
          process which logged the message.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1>
    <title>Address Fields</title>

    <para>During serialization into external formats, such as the
    <ulink
    url="https://www.freedesktop.org/wiki/Software/systemd/export">Journal
    Export Format</ulink> or the <ulink
    url="https://www.freedesktop.org/wiki/Software/systemd/json">Journal
    JSON Format</ulink>, the addresses of journal entries are
    serialized into fields prefixed with double underscores. Note that
    these are not proper fields when stored in the journal but for
    addressing metadata of entries. They cannot be written as part of
    structured log entries via calls such as
    <citerefentry><refentrytitle>sd_journal_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    They may also not be used as matches for
    <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry></para>

    <variablelist class='journal-directives'>
      <varlistentry>
        <term><varname>__CURSOR=</varname></term>
        <listitem>
          <para>The cursor for the entry. A cursor is an opaque text
          string that uniquely describes the position of an entry in
          the journal and is portable across machines, platforms and
          journal files.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>__REALTIME_TIMESTAMP=</varname></term>
        <listitem>
          <para>The wallclock time
          (<constant>CLOCK_REALTIME</constant>) at the point in time
          the entry was received by the journal, in microseconds since
          the epoch UTC, formatted as a decimal string. This has
          different properties from
          <literal>_SOURCE_REALTIME_TIMESTAMP=</literal>, as it is
          usually a bit later but more likely to be monotonic.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>__MONOTONIC_TIMESTAMP=</varname></term>
        <listitem>
          <para>The monotonic time
          (<constant>CLOCK_MONOTONIC</constant>) at the point in time
          the entry was received by the journal in microseconds,
          formatted as a decimal string. To be useful as an address
          for the entry, this should be combined with the boot ID in
          <literal>_BOOT_ID=</literal>.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>
  </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>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>coredumpctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>
