man/systemd-journald.service.xml

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