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 but can also include binary data where
  94     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
  95
  96     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  97     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
  98     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
  99     to volatile storage otherwise. Use <varname>Storage=</varname> in
 100     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
 101     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
 102
 103     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
 104     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
 105     ensure it has the correct access modes and ownership:</para>
 106
 107     <programlisting>mkdir -p /var/log/journal
 108 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
 109
 110     <para>See
 111     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 112     for information about the configuration of this service.</para>
 113   </refsect1>
 114
 115   <refsect1>
 116     <title>Stream logging</title>
 117
 118     <para>The systemd service manager invokes all service processes with standard output and standard error connected
 119     to the journal by default. This behaviour may be altered via the
 120     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
 121     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 122     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
 123     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
 124
 125     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
 126     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 127     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 128     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 129     write attempts will also result in such process signals being generated, see
 130     <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
 131     systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
 132     default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
 133     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 134     details). After the standard output/standard error streams have been terminated they may not be recovered until the
 135     services they are associated with are restarted. Note that during normal operation,
 136     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
 137     service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
 138     restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
 139     <command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
 140     and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
 141     is not recommended.</para>
 142
 143     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 144     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 145     processes (such as further child processes forked off the main service process), the log records will not reflect
 146     their metadata, but will continue to describe the original process. This is different from the other logging
 147     transports listed above, which are inherently record based and where the metadata is always associated with the
 148     individual record.</para>
 149
 150     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 151     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 152     line tool.</para>
 153
 154     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 155     4096. When this limit is reached further log streams may be established but will receive
 156     <constant>EPIPE</constant> right from the beginning.</para>
 157   </refsect1>
 158
 159   <refsect1>
 160     <title>Signals</title>
 161
 162     <variablelist>
 163       <varlistentry>
 164         <term>SIGUSR1</term>
 165
 166         <listitem><para>Request that journal data from
 167         <filename>/run/</filename> is flushed to
 168         <filename>/var/</filename> in order to make it persistent (if
 169         this is enabled). This must be used after
 170         <filename>/var/</filename> is mounted, as otherwise log data
 171         from <filename>/run</filename> is never flushed to
 172         <filename>/var</filename> regardless of the configuration. The
 173         <command>journalctl --flush</command> command uses this signal
 174         to request flushing of the journal files, and then waits for
 175         the operation to complete. See
 176         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 177         for details.</para></listitem>
 178       </varlistentry>
 179
 180       <varlistentry>
 181         <term>SIGUSR2</term>
 182
 183         <listitem><para>Request immediate rotation of the journal
 184         files. The <command>journalctl --rotate</command> command uses
 185         this signal to request journal file
 186         rotation.</para></listitem>
 187       </varlistentry>
 188
 189       <varlistentry>
 190         <term>SIGRTMIN+1</term>
 191
 192         <listitem><para>Request that all unwritten log data is written
 193         to disk. The <command>journalctl --sync</command> command uses
 194         this signal to trigger journal synchronization, and then waits
 195         for the operation to complete.</para></listitem>
 196       </varlistentry>
 197     </variablelist>
 198   </refsect1>
 199
 200   <refsect1>
 201     <title>Kernel Command Line</title>
 202
 203     <para>A few configuration parameters from
 204     <filename>journald.conf</filename> may be overridden on the kernel
 205     command line:</para>
 206
 207     <variablelist class='kernel-commandline-options'>
 208       <varlistentry>
 209         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 210         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 211         <term><varname>systemd.journald.forward_to_console=</varname></term>
 212         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 213
 214         <listitem><para>Enables/disables forwarding of collected log
 215         messages to syslog, the kernel log buffer, the system console
 216         or wall.
 217         </para>
 218
 219         <para>See
 220         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 221         for information about these settings.</para>
 222         </listitem>
 223
 224       </varlistentry>
 225     </variablelist>
 226   </refsect1>
 227
 228   <refsect1>
 229     <title>Access Control</title>
 230
 231     <para>Journal files are, by default, owned and readable by the
 232     <literal>systemd-journal</literal> system group but are not
 233     writable. Adding a user to this group thus enables her/him to read
 234     the journal files.</para>
 235
 236     <para>By default, each logged in user will get her/his own set of
 237     journal files in <filename>/var/log/journal/</filename>. These
 238     files will not be owned by the user, however, in order to avoid
 239     that the user can write to them directly. Instead, file system
 240     ACLs are used to ensure the user gets read access only.</para>
 241
 242     <para>Additional users and groups may be granted access to journal
 243     files via file system access control lists (ACL). Distributions
 244     and administrators may choose to grant read access to all members
 245     of the <literal>wheel</literal> and <literal>adm</literal> system
 246     groups with a command such as the following:</para>
 247
 248     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 249
 250     <para>Note that this command will update the ACLs both for
 251     existing journal files and for future journal files created in the
 252     <filename>/var/log/journal/</filename> directory.</para>
 253   </refsect1>
 254
 255   <refsect1>
 256     <title>Files</title>
 257
 258     <variablelist>
 259       <varlistentry>
 260         <term><filename>/etc/systemd/journald.conf</filename></term>
 261
 262         <listitem><para>Configure
 263         <command>systemd-journald</command>
 264         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
 276         entries to files in
 277         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 278         or
 279         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 280         with the <literal>.journal</literal> suffix. If the daemon is
 281         stopped uncleanly, or if the files are found to be corrupted,
 282         they are renamed using the <literal>.journal~</literal>
 283         suffix, and <command>systemd-journald</command> starts writing
 284         to a new file. <filename>/run</filename> is used when
 285         <filename>/var/log/journal</filename> is not available, or
 286         when <option>Storage=volatile</option> is set in the
 287         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 288         configuration file.</para></listitem>
 289       </varlistentry>
 290
 291       <varlistentry>
 292         <term><filename>/dev/kmsg</filename></term>
 293         <term><filename>/dev/log</filename></term>
 294         <term><filename>/run/systemd/journal/dev-log</filename></term>
 295         <term><filename>/run/systemd/journal/socket</filename></term>
 296         <term><filename>/run/systemd/journal/stdout</filename></term>
 297
 298         <listitem><para>Sockets and other paths that
 299         <command>systemd-journald</command> will listen on that are
 300         visible in the file system. In addition to these, journald can
 301         listen for audit events using netlink.</para></listitem>
 302       </varlistentry>
 303     </variablelist>
 304   </refsect1>
 305
 306   <refsect1>
 307     <title>See Also</title>
 308     <para>
 309       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 310       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 311       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 312       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 313       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 314       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 315       <citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 316       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
 317       <command>pydoc systemd.journal</command>
 318     </para>
 319   </refsect1>
 320
 321 </refentry>