man/systemd-journald.service.xml

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