man/systemd-journald.service.xml

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