man/systemd-journald.service.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2010 Lennart Poettering
  11
  12   systemd is free software; you can redistribute it and/or modify it
  13   under the terms of the GNU Lesser General Public License as published by
  14   the Free Software Foundation; either version 2.1 of the License, or
  15   (at your option) any later version.
  16
  17   systemd is distributed in the hope that it will be useful, but
  18   WITHOUT ANY WARRANTY; without even the implied warranty of
  19   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  20   Lesser General Public License for more details.
  21
  22   You should have received a copy of the GNU Lesser General Public License
  23   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  24 -->
  25
  26 <refentry id="systemd-journald.service">
  27
  28   <refentryinfo>
  29     <title>systemd-journald.service</title>
  30     <productname>systemd</productname>
  31
  32     <authorgroup>
  33       <author>
  34         <contrib>Developer</contrib>
  35         <firstname>Lennart</firstname>
  36         <surname>Poettering</surname>
  37         <email>lennart@poettering.net</email>
  38       </author>
  39     </authorgroup>
  40   </refentryinfo>
  41
  42   <refmeta>
  43     <refentrytitle>systemd-journald.service</refentrytitle>
  44     <manvolnum>8</manvolnum>
  45   </refmeta>
  46
  47   <refnamediv>
  48     <refname>systemd-journald.service</refname>
  49     <refname>systemd-journald.socket</refname>
  50     <refname>systemd-journald-dev-log.socket</refname>
  51     <refname>systemd-journald-audit.socket</refname>
  52     <refname>systemd-journald</refname>
  53     <refpurpose>Journal service</refpurpose>
  54   </refnamediv>
  55
  56   <refsynopsisdiv>
  57     <para><filename>systemd-journald.service</filename></para>
  58     <para><filename>systemd-journald.socket</filename></para>
  59     <para><filename>systemd-journald-dev-log.socket</filename></para>
  60     <para><filename>systemd-journald-audit.socket</filename></para>
  61     <para><filename>/usr/lib/systemd/systemd-journald</filename></para>
  62   </refsynopsisdiv>
  63
  64   <refsect1>
  65     <title>Description</title>
  66
  67     <para><filename>systemd-journald</filename> is a system service
  68     that collects and stores logging data. It creates and maintains
  69     structured, indexed journals based on logging information that is
  70     received from a variety of sources:</para>
  71
  72     <itemizedlist>
  73       <listitem><para>Kernel log messages, via kmsg</para></listitem>
  74
  75       <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
  76       project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  77       call</para></listitem>
  78
  79       <listitem><para>Structured system log messages via the native
  80       Journal API, see
  81       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>4</manvolnum></citerefentry></para></listitem>
  82
  83       <listitem><para>Standard output and standard error of service units. For further details see
  84       below.</para></listitem>
  85
  86       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  87     </itemizedlist>
  88
  89     <para>The daemon will implicitly collect numerous metadata fields
  90     for each log messages in a secure and unfakeable way. See
  91     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  92     for more information about the collected metadata.
  93     </para>
  94
  95     <para>Log data collected by the journal is primarily text-based but can also include binary data where
  96     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
  97
  98     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  99     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
 100     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
 101     to volatile storage otherwise. Use <varname>Storage=</varname> in
 102     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
 103     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
 104
 105     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
 106     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
 107     ensure it has the correct access modes and ownership:</para>
 108
 109     <programlisting>mkdir -p /var/log/journal
 110 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
 111
 112     <para>See
 113     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 114     for information about the configuration of this service.</para>
 115   </refsect1>
 116
 117   <refsect1>
 118     <title>Stream logging</title>
 119
 120     <para>The systemd service manager invokes all service processes with standard output and standard error connected
 121     to the journal by default. This behaviour may be altered via the
 122     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
 123     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 124     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
 125     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
 126
 127     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
 128     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 129     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 130     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 131     write attempts will also result in such process signals being generated, see
 132     <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
 133     systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
 134     default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
 135     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 136     details). After the standard output/standard error streams have been terminated they may not be recovered until the
 137     services they are associated with are restarted. Note that during normal operation,
 138     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
 139     service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
 140     restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
 141     <command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
 142     and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
 143     is not recommended.</para>
 144
 145     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 146     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 147     processes (such as further child processes forked off the main service process), the log records will not reflect
 148     their metadata, but will continue to describe the original process. This is different from the other logging
 149     transports listed above, which are inherently record based and where the metadata is always associated with the
 150     individual record.</para>
 151
 152     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 153     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 154     line tool.</para>
 155
 156     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 157     4096. When this limit is reached further log streams may be established but will receive
 158     <constant>EPIPE</constant> right from the beginning.</para>
 159   </refsect1>
 160
 161   <refsect1>
 162     <title>Signals</title>
 163
 164     <variablelist>
 165       <varlistentry>
 166         <term>SIGUSR1</term>
 167
 168         <listitem><para>Request that journal data from
 169         <filename>/run/</filename> is flushed to
 170         <filename>/var/</filename> in order to make it persistent (if
 171         this is enabled). This must be used after
 172         <filename>/var/</filename> is mounted, as otherwise log data
 173         from <filename>/run</filename> is never flushed to
 174         <filename>/var</filename> regardless of the configuration. The
 175         <command>journalctl --flush</command> command uses this signal
 176         to request flushing of the journal files, and then waits for
 177         the operation to complete. See
 178         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 179         for details.</para></listitem>
 180       </varlistentry>
 181
 182       <varlistentry>
 183         <term>SIGUSR2</term>
 184
 185         <listitem><para>Request immediate rotation of the journal
 186         files. The <command>journalctl --rotate</command> command uses
 187         this signal to request journal file
 188         rotation.</para></listitem>
 189       </varlistentry>
 190
 191       <varlistentry>
 192         <term>SIGRTMIN+1</term>
 193
 194         <listitem><para>Request that all unwritten log data is written
 195         to disk. The <command>journalctl --sync</command> command uses
 196         this signal to trigger journal synchronization, and then waits
 197         for the operation to complete.</para></listitem>
 198       </varlistentry>
 199     </variablelist>
 200   </refsect1>
 201
 202   <refsect1>
 203     <title>Kernel Command Line</title>
 204
 205     <para>A few configuration parameters from
 206     <filename>journald.conf</filename> may be overridden on the kernel
 207     command line:</para>
 208
 209     <variablelist class='kernel-commandline-options'>
 210       <varlistentry>
 211         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 212         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 213         <term><varname>systemd.journald.forward_to_console=</varname></term>
 214         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 215
 216         <listitem><para>Enables/disables forwarding of collected log
 217         messages to syslog, the kernel log buffer, the system console
 218         or wall.
 219         </para>
 220
 221         <para>See
 222         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 223         for information about these settings.</para>
 224         </listitem>
 225
 226       </varlistentry>
 227     </variablelist>
 228   </refsect1>
 229
 230   <refsect1>
 231     <title>Access Control</title>
 232
 233     <para>Journal files are, by default, owned and readable by the
 234     <literal>systemd-journal</literal> system group but are not
 235     writable. Adding a user to this group thus enables her/him to read
 236     the journal files.</para>
 237
 238     <para>By default, each logged in user will get her/his own set of
 239     journal files in <filename>/var/log/journal/</filename>. These
 240     files will not be owned by the user, however, in order to avoid
 241     that the user can write to them directly. Instead, file system
 242     ACLs are used to ensure the user gets read access only.</para>
 243
 244     <para>Additional users and groups may be granted access to journal
 245     files via file system access control lists (ACL). Distributions
 246     and administrators may choose to grant read access to all members
 247     of the <literal>wheel</literal> and <literal>adm</literal> system
 248     groups with a command such as the following:</para>
 249
 250     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 251
 252     <para>Note that this command will update the ACLs both for
 253     existing journal files and for future journal files created in the
 254     <filename>/var/log/journal/</filename> directory.</para>
 255   </refsect1>
 256
 257   <refsect1>
 258     <title>Files</title>
 259
 260     <variablelist>
 261       <varlistentry>
 262         <term><filename>/etc/systemd/journald.conf</filename></term>
 263
 264         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 265         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 266         </para></listitem>
 267       </varlistentry>
 268
 269       <varlistentry>
 270         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 271         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 272         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 273         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 274
 275         <listitem><para><command>systemd-journald</command> writes entries to files in
 276         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 277         or
 278         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 279         with the <literal>.journal</literal> suffix. If the daemon is
 280         stopped uncleanly, or if the files are found to be corrupted,
 281         they are renamed using the <literal>.journal~</literal>
 282         suffix, and <command>systemd-journald</command> starts writing
 283         to a new file. <filename>/run</filename> is used when
 284         <filename>/var/log/journal</filename> is not available, or
 285         when <option>Storage=volatile</option> is set in the
 286         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 287         configuration file.</para>
 288
 289         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 290         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 291         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 292         Such files are "archived" and will not be written to any more.</para>
 293
 294         <para>In general, it is safe to read or copy any journal file (active or archived).
 295         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 296         and the functions in the
 297         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 298         library should be able to read all entries that have been fully written.</para>
 299
 300         <para><filename>systemd-journald</filename> will automatically remove the oldest
 301         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 302         and related settings in
 303         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 304         </para></listitem>
 305       </varlistentry>
 306
 307       <varlistentry>
 308         <term><filename>/dev/kmsg</filename></term>
 309         <term><filename>/dev/log</filename></term>
 310         <term><filename>/run/systemd/journal/dev-log</filename></term>
 311         <term><filename>/run/systemd/journal/socket</filename></term>
 312         <term><filename>/run/systemd/journal/stdout</filename></term>
 313
 314         <listitem><para>Sockets and other paths that
 315         <command>systemd-journald</command> will listen on that are
 316         visible in the file system. In addition to these, journald can
 317         listen for audit events using netlink.</para></listitem>
 318       </varlistentry>
 319     </variablelist>
 320   </refsect1>
 321
 322   <refsect1>
 323     <title>See Also</title>
 324     <para>
 325       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 326       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 327       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 328       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 329       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 330       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 331       <citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 332       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
 333       <command>pydoc systemd.journal</command>
 334     </para>
 335   </refsect1>
 336
 337 </refentry>