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306 lines
16 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="systemd-journald.service">
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<refentryinfo>
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<title>systemd-journald.service</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-journald.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-journald.service</refname>
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<refname>systemd-journald.socket</refname>
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<refname>systemd-journald-dev-log.socket</refname>
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<refname>systemd-journald-audit.socket</refname>
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<refname>systemd-journald</refname>
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<refpurpose>Journal service</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-journald.service</filename></para>
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<para><filename>systemd-journald.socket</filename></para>
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<para><filename>systemd-journald-dev-log.socket</filename></para>
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<para><filename>systemd-journald-audit.socket</filename></para>
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<para><filename>/usr/lib/systemd/systemd-journald</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-journald</filename> is a system service
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that collects and stores logging data. It creates and maintains
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structured, indexed journals based on logging information that is
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received from a variety of sources:</para>
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<itemizedlist>
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<listitem><para>Kernel log messages, via kmsg</para></listitem>
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<listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
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project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call</para></listitem>
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<listitem><para>Structured system log messages via the native
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Journal API, see
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<citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry></para></listitem>
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<listitem><para>Standard output and standard error of service units. For further details see
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below.</para></listitem>
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<listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
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</itemizedlist>
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<para>The daemon will implicitly collect numerous metadata fields
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for each log messages in a secure and unfakeable way. See
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<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for more information about the collected metadata.
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</para>
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<para>Log data collected by the journal is primarily text-based but can also include binary data where
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necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
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<para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
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volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
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data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
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to volatile storage otherwise. Use <varname>Storage=</varname> in
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
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where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
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<para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
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desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
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ensure it has the correct access modes and ownership:</para>
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<programlisting>mkdir -p /var/log/journal
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systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
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<para>See
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for information about the configuration of this service.</para>
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</refsect1>
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<refsect1>
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<title>Stream logging</title>
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<para>The systemd service manager invokes all service processes with standard output and standard error connected
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to the journal by default. This behaviour may be altered via the
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<varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
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journal converts the log byte stream received this way into individual log records, splitting the stream at newline
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(<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
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<para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
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services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
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errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
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ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
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write attempts will also result in such process signals being generated, see
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<citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
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systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
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default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
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details). After the standard output/standard error streams have been terminated they may not be recovered until the
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services they are associated with are restarted. Note that during normal operation,
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<filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
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service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
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restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
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<command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
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and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
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is not recommended.</para>
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<para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
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metadata of the peer the stream was originally created for. If the stream connection is passed on to other
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processes (such as further child processes forked off the main service process), the log records will not reflect
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their metadata, but will continue to describe the original process. This is different from the other logging
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transports listed above, which are inherently record based and where the metadata is always associated with the
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individual record.</para>
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<para>In addition to the implicit standard output/error logging of services, stream logging is also available
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via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
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line tool.</para>
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<para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
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4096. When this limit is reached further log streams may be established but will receive
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<constant>EPIPE</constant> right from the beginning.</para>
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</refsect1>
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<refsect1>
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<title>Signals</title>
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<variablelist>
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<varlistentry>
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<term>SIGUSR1</term>
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<listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
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<filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
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after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run</filename> is
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never flushed to <filename>/var</filename> regardless of the configuration. Use the
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<command>journalctl --flush</command> command to request flushing of the journal files, and wait for
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the operation to complete. See
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
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details.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>SIGUSR2</term>
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<listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
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--rotate</command> command to request journal file rotation, and wait for the operation to
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complete.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>SIGRTMIN+1</term>
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<listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
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--sync</command> command to trigger journal synchronization, and wait for the operation to
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complete.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Kernel Command Line</title>
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<para>A few configuration parameters from
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<filename>journald.conf</filename> may be overridden on the kernel
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command line:</para>
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<variablelist class='kernel-commandline-options'>
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<varlistentry>
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<term><varname>systemd.journald.forward_to_syslog=</varname></term>
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<term><varname>systemd.journald.forward_to_kmsg=</varname></term>
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<term><varname>systemd.journald.forward_to_console=</varname></term>
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<term><varname>systemd.journald.forward_to_wall=</varname></term>
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<listitem><para>Enables/disables forwarding of collected log
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messages to syslog, the kernel log buffer, the system console
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or wall.
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</para>
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<para>See
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for information about these settings.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Access Control</title>
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<para>Journal files are, by default, owned and readable by the
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<literal>systemd-journal</literal> system group but are not
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writable. Adding a user to this group thus enables them to read
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the journal files.</para>
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<para>By default, each user, with a UID outside the range of system users,
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dynamic service users, and the nobody user, will get their own set of
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journal files in <filename>/var/log/journal/</filename>. See
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<ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
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for more details about UID ranges. These journal
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files will not be owned by the user, however, in order to avoid
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that the user can write to them directly. Instead, file system
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ACLs are used to ensure the user gets read access only.</para>
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<para>Additional users and groups may be granted access to journal
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files via file system access control lists (ACL). Distributions
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and administrators may choose to grant read access to all members
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of the <literal>wheel</literal> and <literal>adm</literal> system
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groups with a command such as the following:</para>
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<programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
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<para>Note that this command will update the ACLs both for
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existing journal files and for future journal files created in the
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<filename>/var/log/journal/</filename> directory.</para>
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</refsect1>
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<refsect1>
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<title>Files</title>
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<variablelist>
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<varlistentry>
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<term><filename>/etc/systemd/journald.conf</filename></term>
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<listitem><para>Configure <command>systemd-journald</command> behavior. See
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
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<term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
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<term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
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<term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
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<listitem><para><command>systemd-journald</command> writes entries to files in
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<filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
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or
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<filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
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with the <literal>.journal</literal> suffix. If the daemon is
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stopped uncleanly, or if the files are found to be corrupted,
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they are renamed using the <literal>.journal~</literal>
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suffix, and <command>systemd-journald</command> starts writing
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to a new file. <filename>/run</filename> is used when
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<filename>/var/log/journal</filename> is not available, or
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when <option>Storage=volatile</option> is set in the
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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configuration file.</para>
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<para>When <filename>systemd-journald</filename> ceases writing to a journal file,
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it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
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(or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
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Such files are "archived" and will not be written to any more.</para>
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<para>In general, it is safe to read or copy any journal file (active or archived).
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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and the functions in the
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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library should be able to read all entries that have been fully written.</para>
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<para><filename>systemd-journald</filename> will automatically remove the oldest
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archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
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and related settings in
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><filename>/dev/kmsg</filename></term>
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<term><filename>/dev/log</filename></term>
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<term><filename>/run/systemd/journal/dev-log</filename></term>
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<term><filename>/run/systemd/journal/socket</filename></term>
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<term><filename>/run/systemd/journal/stdout</filename></term>
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<listitem><para>Sockets and other paths that
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<command>systemd-journald</command> will listen on that are
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visible in the file system. In addition to these, journald can
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listen for audit events using netlink.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<command>pydoc systemd.journal</command>
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</para>
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</refsect1>
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</refentry>
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